Article

Planck early results. XVII. Origin of the submillimetre excess dust emission in the Magellanic Clouds

Astronomy and Astrophysics (Impact Factor: 4.38). 12/2011; 536:A17. DOI: 10.1051/0004-6361/201116473

ABSTRACT

The integrated spectral energy distributions (SED) of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) appear significantly flatter than expected from dust models based on their far-infrared and radio emission. The still unexplained origin of this millimetre excess is investigated here using the Planck data. The integrated SED of the two galaxies before subtraction of the foreground (Milky Way) and background (CMB fluctuations) emission are in good agreement with previous determinations, confirming the presence of the millimetre excess. In the context of this preliminary analysis we do not propose a full multi-component fitting of the data, but instead subtract contributions unrelated to the galaxies and to dust emission. The background CMB contribution is subtracted using an internal linear combination (ILC) method performed locally around the galaxies. The foreground emission from the Milky Way is subtracted as a Galactic Hi template, and the dust emissivity is derived in a region surrounding the two galaxies and dominated by Milky Way emission. After subtraction, the remaining emission of both galaxies correlates closely with the atomic and molecular gas emission of the LMC and SMC. The millimetre excess in the LMC can be explained by CMB fluctuations, but a significant excess is still present in the SMC SED. The Planck and IRAS-IRIS data at 100 mum are combined to produce thermal dust temperature and optical depth maps of the two galaxies. The LMC temperature map shows the presence of a warm inner arm already found with the Spitzer data, but which also shows the existence of a previously unidentified cold outer arm. Several cold regions are found along this arm, some of which are associated with known molecular clouds. The dust optical depth maps are used to constrain the thermal dust emissivity power-law index (beta). The average spectral index is found to be consistent with beta = 1.5 and beta = 1.2 below 500mum for the LMC and SMC respectively, significantly flatter than the values observed in the Milky Way. Also, there is evidence in the SMC of a further flattening of the SED in the sub-mm, unlike for the LMC where the SED remains consistent with beta = 1.5. The spatial distribution of the millimetre dustexcess in the SMC follows the gas and thermal dust distribution. Different models are explored in order to fit the dust emission in the SMC. It is concluded that the millimetre excess is unlikely to be caused by very cold dust emission and that it could be due to a combination of spinning dust emission and thermal dust emission by more amorphous dust grains than those present in our Galaxy. Corresponding author: J.-P. Bernard, e-mail: jean-philippe.bernard@cesr.fr

Full-text

Available from: Philip Lubin
A&A 536, A17 (2011)
DOI: 10.1051/0004-6361/201116473
c
ESO 2011
Astronomy
&
Astrophysics
Planck early results
Special feature
Planck
early results. XVII. Origin of the submillimetre
excess dust emission in the Magellanic Clouds
Planck Collaboration: P. A. R. Ade
73
, N. Aghanim
48
,M.Arnaud
59
, M. Ashdown
57,4
, J. Aumont
48
, C. Baccigalupi
71
,
A. Balbi
30
,A.J.Banday
77,7,64
,R.B.Barreiro
54
, J. G. Bartlett
3,55
, E. Battaner
79
, K. Benabed
49
, A. Benoît
47
,
J.-P. Bernard
77,7
, M. Bersanelli
27,42
,R.Bhatia
5
,J.J.Bock
55,8
,A.Bonaldi
38
,J.R.Bond
6
, J. Borrill
63,74
,C.Bot
69
,
F. R. Bouchet
49
, F. Boulanger
48
, M. Bucher
3
,C.Burigana
41
,P.Cabella
30
, J.-F. Cardoso
60,3,49
, A. Catalano
3,58
,
L. Cayón
20
, A. Challinor
51,57,9
,A.Chamballu
45
,L.-YChiang
50
,C.Chiang
19
, P. R. Christensen
67,31
,D.L.Clements
45
,
S. Colombi
49
, F. Couchot
62
,A.Coulais
58
, B. P. Crill
55,68
, F. Cuttaia
41
, L. Danese
71
,R.D.Davies
56
,R.J.Davis
56
,
P. de Bernardis
26
,G.deGasperis
30
,A.deRosa
41
,G.deZotti
38,71
, J. Delabrouille
3
, J.-M. Delouis
49
, F.-X. Désert
44
,
C. Dickinson
56
,K.Dobashi
14
,S.Donzelli
42,52
,O.Doré
55,8
,U.Dörl
64
, M. Douspis
48
,X.Dupac
34
, G. Efstathiou
51
,
T. A. Enßlin
64
, F. Finelli
41
, O. Forni
77,7
, M. Frailis
40
, E. Franceschi
41
, Y. Fukui
18
,S.Galeotta
40
, K. Ganga
3,46
,
M. Giard
77,7
, G. Giardino
35
, Y. Giraud-Héraud
3
, J. González-Nuevo
71
,K.M.Górski
55,81
, S. Gratton
57,51
,
A. Gregorio
28
, A. Gruppuso
41
, D. Harrison
51,57
,G.Helou
8
, S. Henrot-Versillé
62
,D.Herranz
54
,S.R.Hildebrandt
8,61,53
,
E. Hivon
49
, M. Hobson
4
,W.A.Holmes
55
,W.Hovest
64
,R.J.Hoyland
53
,K.M.Huenberger
80
,A.H.Jae
45
,
W. C. Jones
19
,M.Juvela
17
,A.Kawamura
18
,E.Keihänen
17
, R. Keskitalo
55,17
,T.S.Kisner
63
, R. Kneissl
33,5
,L.Knox
22
,
H. Kurki-Suonio
17,36
, G. Lagache
48
, A. Lähteenmäki
1,36
, J.-M. Lamarre
58
,A.Lasenby
4,57
, R. J. Laureijs
35
,
C. R. Lawrence
55
, S. Leach
71
, R. Leonardi
34,35,23
,C.Leroy
48,77,7
, M. Linden-Vørnle
11
, M. López-Caniego
54
,
P. M. Lubin
23
, J. F. Macías-Pérez
61
,C.J.MacTavish
57
,S.Madden
59
,B.Maei
56
, N. Mandolesi
41
,R.Mann
72
,
M. Maris
40
, E. Martínez-González
54
, S. Masi
26
, S. Matarrese
25
,F.Matthai
64
, P. Mazzotta
30
, P. R. Meinhold
23
,
A. Melchiorri
26
, L. Mendes
34
, A. Mennella
27,40
, M.-A. Miville-Deschênes
48,6
, A. Moneti
49
,L.Montier
77,7
,
G. Morgante
41
, D. Mortlock
45
, D. Munshi
73,51
, A. Murphy
66
, P. Naselsky
67,31
,F.Nati
26
,P.Natoli
29,2,41
,
C. B. Netterfield
13
, H. U. Nørgaard-Nielsen
11
, F. Noviello
48
, D. Novikov
45
, I. Novikov
67
,T.Onishi
15
, S. Osborne
76
,
F. Pajot
48
,R.Paladini
75,8
, D. Paradis
77,7
, F. Pasian
40
, G. Patanchon
3
, O. Perdereau
62
, L. Perotto
61
, F. Perrotta
71
,
F. Piacentini
26
,M.Piat
3
, S. Plaszczynski
62
, E. Pointecouteau
77,7
,G.Polenta
2,39
, N. Ponthieu
48
,T.Poutanen
36,17,1
,
G. Prézeau
8,55
, S. Prunet
49
, J.-L. Puget
48
, W. T. Reach
78
,R.Rebolo
53,32
, M. Reinecke
64
, C. Renault
61
, S. Ricciardi
41
,
T. Riller
64
, I. Ristorcelli
77,7
,G.Rocha
55,8
, C. Rosset
3
, M. Rowan-Robinson
45
, J. A. Rubiño-Martín
53,32
, B. Rusholme
46
,
M. Sandri
41
,G.Savini
70
,D.Scott
16
,M.D.Seiert
55,8
,G.F.Smoot
21,63,3
, J.-L. Starck
59,10
, F. Stivoli
43
, V. Stolyarov
4
,
R. Sudiwala
73
, J.-F. Sygnet
49
,J.A.Tauber
35
, L. Terenzi
41
,L.Toolatti
12
, M. Tomasi
27,42
, J.-P. Torre
48
, M. Tristram
62
,
J. Tuovinen
65
,G.Umana
37
, L. Valenziano
41
,J.Varis
65
,P.Vielva
54
, F. Villa
41
, N. Vittorio
30
,L.A.Wade
55
,
B. D. Wandelt
49,24
, A. Wilkinson
56
, N. Ysard
17
,D.Yvon
10
, A. Zacchei
40
, and A. Zonca
23
(Aliations can be found after the references)
Received 8 January 2011 / Accepted 31 May 2011
ABSTRACT
The integrated spectral energy distributions (SED) of the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) appear significantly
flatter than expected from dust models based on their far-infrared and radio emission. The still unexplained origin of this millimetre excess is
investigated here using the Planck data. The integrated SED of the two galaxies before subtraction of the foreground (Milky Way) and background
(CMB fluctuations) emission are in good agreement with previous determinations, confirming the presence of the millimetre excess. In the context
of this preliminary analysis we do not propose a full multi-component fitting of the data, but instead subtract contributions unrelated to the galaxies
and to dust emission.
The background CMB contribution is subtracted using an internal linear combination (ILC) method performed locally around the galaxies. The
foreground emission from the Milky Way is subtracted as a Galactic H i template, and the dust emissivity is derived in a region surrounding the
two galaxies and dominated by Milky Way emission. After subtraction, the remaining emission of both galaxies correlates closely with the atomic
and molecular gas emission of the LMC and SMC. The millimetre excess in the LMC can be explained by CMB fluctuations, but a significant
excess is still present in the SMC SED. The Planck and IRAS–IRIS data at 100 μm are combined to produce thermal dust temperature and optical
depth maps of the two galaxies.
Corresponding author: J.-P. Bernard,
e-mail: jean-philippe.bernard@cesr.fr
Article published by EDP Sciences A17, page 1 of 17
Page 1
A&A 536, A17 (2011)
The LMC temperature map shows the presence of a warm inner arm already found with the Spitzer data, but which also shows the existence of a
previously unidentified cold outer arm. Several cold regions are found along this arm, some of which are associated with known molecular clouds.
The dust optical depth maps are used to constrain the thermal dust emissivity power-law index (β). The average spectral index is found to be
consistent with β =1.5 and β =1.2 below 500 μm for the LMC and SMC respectively, significantly atter than the values observed in the Milky
Way. Also, there is evidence in the SMC of a further flattening of the SED in the sub-mm, unlike for the LMC where the SED remains consistent
with β =1.5. The spatial distribution of the millimetre dust excess in the SMC follows the gas and thermal dust distribution. Dierent models are
explored in order to fit the dust emission in the SMC. It is concluded that the millimetre excess is unlikely to be caused by very cold dust emission
and that it could be due to a combination of spinning dust emission and thermal dust emission by more amorphous dust grains than those present
in our Galaxy.
Key words. Magellanic Clouds – dust, extinction – ISM: structure – galaxies: ISM – infrared: galaxies – submillimeter: galaxies
1. Introduction
Star formation and the exchange and evolution of materials be-
tween the stars and the interstellar medium (ISM) are continuous
processes that drive the evolution of galaxies. As stars evolve,
die, and renew the life cycle of dust and gas, the amount of dust
and its distribution in a galaxy has important consequences for
its subsequent star formation. Knowing the dust content through-
out cosmic history would thus provide clues to the star forma-
tion history of the universe as the metallicities evolve. One of
the puzzling results that has emerged from the studies of dust
SEDs in the early universe is finding very large dust masses in
high-redshift galaxies. For example, recent Herschel observa-
tions of submillimetre galaxies (SMGs) find excessively large
dust masses, and high dust-to-gas mass ratios (D/G) (Santini
et al. 2010). These findings contradict the low metallicities mea-
sured in the gas. Large dust masses have also been measured for
a range of low-metallicity, local universe dwarf galaxies (Dumke
et al. 2003; Galliano et al. 2003, 2005; Bendo et al. 2006;
Galametz et al. 2009; Grossi et al. 2010; O’Halloran et al. 2010).
Our understanding of how dust masses are estimated and what
observational contraints are necessary is called into question by
the discovery that the dust masses measured in these galaxies ap-
pear significantly larger than expected from their metal content.
Excessively large dust masses only seem to be found in low
metallicity galaxies to date. The evidence is found in the be-
haviour of the submillimetre (submm) emission, beyond about
400500 μm. From a study of a broad range of metal-rich and
metal-poor galaxies, observed with wide wavelength coverage
that included submm observations beyond 500 μm, a submm ex-
cess, beyond the normal dust SED models, was found only for
low metallicity systems (Galametz et al. 2010). Such a submm
excess is particularly evident in the Magellanic Clouds (Israel
et al. 2010; Bot et al. 2010b). The origin of such an excess is
still uncertain but several suggestions have been put forward:
The submm excess has been modelled as a cold dust compo-
nent with a submm emissivity index (β)ofβ = 1, which sug-
gests a low dust temperature of 10 K (Galliano et al. 2003,
2005; Galametz et al. 2009, 2010). There are few observa-
tional constraints in the submm-wavelength window, hence
this cold component is a rather ad hoc solution. This descrip-
tion often results in discrepantly large D/G ratios being found
for the low metallicity galaxies if the estimates of the total
gas reservoir are well known.
Meny et al. (2007) show that emission by amorphous grains
is expected to produce excess emission in the submm wave-
length range. As a result, the spectral shape of emission by
amorphous grains cannot be reproduced by a single emis-
sivity index over the whole far-infrared (FIR) to millime-
tre (mm) range, and is expected to flatten at long wave-
lengths. The intensity of the excess is also predicted to
depend strongly on the temperature of the dust grains.
Modifying the optical properties of the dust to incorporate
the eects of the disordered structure of the amorphous
grains shows that the e
ective submm emissivity index de-
creases (thus flattening the submm Rayleigh-Jeans slope) as
the dust temperature increases. Similarly, an inverse corre-
lation between the dust temperature and the spectral index
has been observed in data from the FIR to the submm (e.g.
Dupac et al. 2003; Paradis et al. 2010). However, these au-
thors, as well as Shetty et al. (2009a), advise caution in
the interpretation of the observed inverse T β relationship,
which is aected by the natural correlation between these
two parameters, and requires careful treatment of observa-
tional uncertainties. Shetty et al. (2009b) also studied the
eect of temperature variations along the line-of-sight on
the observed T β relation. Paradis et al. (2009), using the
DIRBE, Archeops and WMAP data, showed that the FIR-
submm SEDs of galactic molecular clouds and their neu-
tral surroundings do indeed show a flattening beyond λ
500 μm.
Spinning dust emitted in the ionised gas of galaxies has
been suggested as the explanation for the radio emission of-
ten seen in galaxies (Ferrara & Dettmar 1994). This idea
was further explored by Anderson & Watson (1993)and
Draine & Lazarian (1998b,a), who characterised the 10 to
100 GHz anomalous foreground emission component as ra-
diation from very small spinning dust particles such as poly-
cyclic aromatic hydrocarbons (PAHs). Recent improvements
to the model suggest that the peak frequency, which could
occur in the submm, depends on many parameters, includ-
ing the radiation field intensity, the dust size distribution,
the dipole moment distribution, and the physical parameters
of the gas phases (Hoang et al. 2010; Silsbee et al. 2011;
Ysard et al. 2010; Ysard & Verstraete 2010). Spinning dust
has been a preferred explanation for the submm excess ob-
served on the global scale in the Magellanic Clouds (Israel
et al. 2010; Bot et al. 2010b).
Serra Díaz-Cano & Jones (2008) have suggested hydro-
genated amorphous carbon as the most likely carbonaceous
grain material, rather than graphite. The advantage of using
amorphous carbon, particularly in the cases where a submm
excess is found, is that amorphous carbons are more emis-
sive and result in a flatter submm slope and smaller dust
mass. This has been used with new Herschel observations to
model some low metallicity galaxies (Galametz et al. 2010;
O’Halloran et al. 2008; Meixner et al. 2010
).
It has also been suggested that abundance increase of the
hot, very small, stochastically heated grains with a low dust
emissivity could be the cause of the submm emission when
there is a submm excess present (Lisenfeld et al. 2001; Zhu
et al. 2009).
Some of the possible causes of the submm excess could be ruled
out or constrained if the gas mass estimates were better estab-
lished. However, estimates of the total gas reservoirs in low
A17, page 2 of 17
Page 2
Planck Collaboration: Planck early results. XVII.
metallicity environments have been uncertain, particularly the
molecular gas component. Observations of CO in low metallic-
ity galaxies have been a challenge, and the dearth of detected
CO has often been interpreted as meaning there is very little
mass in molecular gas present in galaxies which are otherwise
actively forming massive stars (Leroy et al. 2009). However,
relying on CO alone to trace the H
2
mass could potentially
miss a large reservoir of molecular gas residing outside the CO-
emitting region, as indicated by the FIR fine structure observa-
tions in the Magellanic Clouds and other low metallicity galaxies
(e.g. Poglitsch et al. 1995; Madden et al. 1997; Madden 2000).
The presence of a hidden gas mass in low metallicity galaxies
is also suggested by dust observations of the Magellanic Clouds
(e.g. Bernard et al. 2008; Dobashi et al. 2008; Leroy et al. 2007;
Roman-Duval et al. 2010; Bot et al. 2010a).
Most of these studies which find a submm excess in low-
metallicity galaxies are derived from models on global galaxy
scales, and use data with a wavelength coverage that is limited
to λ<500 μm with Herschel or to λ<870 μm for ground-based
observations. The Galametz et al. (2010) study demonstrated
that dust masses can dier by an order of magnitude depending
on the availability of submm constraints on the SED modelling.
These require wider wavelength coverage of the submm and mil-
limetre (mm) region and better spatial information, to map out
the submm excess and determine the local physical conditions.
The closest low-metallicity galaxies are our neighbouring
Large Magellanic Cloud (LMC) and Small Magellanic Cloud
(SMC), which have metallicities of 50% and 20% solar metal-
licity (Z
) respectively. They are ideal laboratories for studying
the ISM and star formation in dierent low metallicity environ-
ments. Recent studies with Spitzer (SAGE: Meixner et al. 2006;
Bolatto et al. 2007)andHerschel (HERITAGE: Meixner et al.
2010) have mapped out the the temperature and dust mass dis-
tribution (Bernard et al. 2008; Gordon et al. 2009, 2010; Leroy
et al. 2007). Recent Herschel studies of the LMC already point
to a possible submm band excess and an excessive dust mass,
which may be suggesting the presence of molecular gas not
probed by CO. This could be explained by the presence of amor-
phous carbon grains (Roman-Duval et al. 2010; Meixner et al.
2010; Gordon et al. 2010). It was dicult to be conclusive about
the cause of the excess seen in the Herschel observation be-
cause of the lack of longer wavelength coverage. Global ex-
cess mm and submm emission in the LMC and SMC was in-
disputably shown by Bot et al. (2010b)andIsrael et al. (2010),
using broader wavelength coverage that included the submm to
cm observations of the Top-Hat balloon experiment, WMAP and
COBE. Their results favour spinning dust as the explanation for
the submm and mm excess, but require additional spatial cover-
agetobeconclusive.Planck
1
observations have both wide wave-
length coverage and the spatial resolution to locate the submm
emission within the LMC and SMC, which will help determine
the origin of the submm excess. In the framework of the pre-
liminary analysis carried out here, we focus our interest on dust
emission from the galaxies themselves, which we isolate by sub-
tracting other unrelated components, instead of carrying out a
full multi-component analysis exploiting all spatial information
available in the data.
1
Planck (http://www.esa.int/Planck) is a project of the European
Space Agency (ESA) with instruments provided by two scientific con-
sortia funded by ESA member states (in particular the lead countries:
France and Italy) with contributions from NASA (USA), and telescope
reflectors provided in a collaboration between ESA and a scientific con-
sortium led and funded by Denmark.
Tab l e 1. Characteristics of the FIR/submm data used in this study.
Data λ
ref
ν
ref
θσ
II
σ
abs
[ μm] [ GHz] [arcmin] [MJy sr
1
][%]
IRAS 100 2997.92 4.30 0.06 13.6
†‡
HFI 349.82 857 3.67 0.12
7%
HFI 550.08 545 3.80 0.12
7%
HFI 849.27 353 4.43 0.08
2%
HFI 1381.5 217 4.68 0.08
2%
HFI 2096.4 143 7.04 0.08
2%
HFI 2997.9 100 9.37 0.07
2%
LFI 4285.7 70.3 13.01 5.0
LFI 6818.2 44.1 27.92 5.0
LFI 10 000 28.5 32.65 5.0
WMAP 3200 93.69 13.2 1.76
1.0
WMAP 4900 61.18 21.0 0.36
1.0
WMAP 7300 41.07 30.6 0.11
1.0
WMAP 9100 32.94 39.6 0.05
1.0
WMAP 13 000 23.06 52.8 0.02
1.0
Notes.
()
Assumed to be for the average IRAS coverage. σ
II
computed
by rescaling this value to actual coverage.
()
From Miville-Deschênes &
Lagache (2005).
()
1σ average value in one beam scaled from Planck
HFI Core Team (2011b). We actually use the internal variance maps
for σ
II
.
()
From Bennett et al. (2003).
2. Obser vations
2.1. Planck data
The Planck first mission results are presented in Planck
Collaboration (2011a) and the in-flight performances of the two
focal plane instruments, the HFI (High Frequency Instrument)
and the LFI (Low Frequency Instrument), are given in Planck
HFI Core Team (2011a)andMennella et al. (2011) respectively.
The data processing and calibration of the HFI and LFI data used
here is described in Planck HFI Core Team (2011b)andZacchei
et al. (2011) respectively.
Figure 1 shows the total intensity maps, observed toward the
LMC and SMC at three HFI and LFI frequencies. Both galax-
ies are well detected at high frequencies. Around 100 GHz, their
emission can barely be distinguished from CMB fluctuations. At
lower frequencies, the contrast between the galaxies’ emission
and the CMB fluctuations becomes larger again. Note that the
apparent variation of the noise level at a constant right-ascension
value across the LMC is real and is due to the LMC being
positioned at the edge of the Planck deep field.
The Planck DR2 data have had the CMB fluctuations re-
moved in a way which is inappropriate for detailed examina-
tion of foreground sources like the LMC and SMC. We there-
fore use the original data before CMB subtraction (version DX4
of the Planck-HFI and Planck-LFI data) and perform our own
subtraction of the CMB fluctuations, based on a local ILC (see
Sect. 2.3.2).
We use the internal variance (σ
2
II
) provided with the Planck
data, which represents the white noise on the intensity. The LMC
and SMC have been observed many times, as the Planck scan-
ning strategy (Planck Collaboration 2011a) covers the region
close to the ecliptic pole repeatedly. In particular, the LMC is
located at the boundary of the Planck deep field. Its eastern half
has a much higher signal to noise ratio than its western half.
We assume the absolute uncertainties from calibration given in
Planck HFI Core Team (2011b)andZacchei et al. (2011)for
HFI and LFI respectively and summarised in Table 1.
A17, page 3 of 17
Page 3
A&A 536, A17 (2011)
Fig. 1. Planck total intensity data for the LMC (left)andSMC(right) at 857 (top), 100 (middle) and 28.5 GHz(bottom) at full resolution. The top
panels are shown in log scale. The circle in the top panels shows the region used to extract average SEDs.
2.2. Ancillary data
2.2.1. H i LMC/SMC data
To trace the atomic gas in the Magellanic Clouds, we used
H i maps in the 21 cm line, obtained by combining data from
the Australia Telescope Compact Array (ATCA) and the Pa rkes
single dish telescope. For the LMC, this data was obtained by
Kim et al. (2003)andStaveley-Smith et al. (2003) and covers
11.1
× 12.4
on the sky. The spatial resolution is 1
, correspond-
ing to a physical resolution of about 14.5 pc at the distance of the
LMC. For the SMC, the data were obtained by Staveley-Smith
et al. (1997)andStanimirovic et al. (1999). The area covered is a
4.5
× 4.5
region. Hi observations of the SMC tail (7
× 6
)were
obtained by Muller et al. (2003) and combined with the obser-
vations taken in the direction of the SMC. The spatial resolution
is 98

, corresponding to 30 pc at the distance of the SMC.
2.2.2. H i Galactic data
The LMC and SMC are located at galactic latitudes b
II
= 34
and b
II
= 44
respectively. They can therefore suer from sig-
nificant contamination by Galactic foreground emission, which
has to be removed from the IR data. In order to account for the
Galactic foreground emission, we used Galactic H i column den-
sity maps.
A17, page 4 of 17
Page 4
Planck Collaboration: Planck early results. XVII.
For the LMC, the H i foreground map was constructed by
Staveley-Smith et al. (2003) by integrating the Parkes H i data
in the velocity range from 64 <v
hel
< 100 km s
1
,which
excludes all LMC and SMC associated gas (v>100 km s
1
)
but includes essentially all Galactic emission. The spatial reso-
lution is 14
. For the SMC, a map of combined ATCA and Parkes
data was build by integrating the Galactic velocities by E. Muller
(priv. comm.). The resolution is 98

.
These maps show that the Galactic foreground across the
LMCisasstrongasN
H
= 1.3 × 10
21
Hcm
2
, with signifi-
cant variation across the LMC, particularly in a wide filamen-
tary structure oriented southwest to northeast. The Galactic fore-
ground across the SMC is weaker, with values N
H
3.2 ×
10
20
Hcm
2
but the associated FIR-submm emission is non-
negligible since the SMC emission is weaker than that of the
LMC, due to its lower dust and gas content.
These foreground maps are used to subtract the foreground
IR emission from the IR maps, using the emissivity SED de-
scribed in Sect. 2.3.3.
2.2.3. CO data
The
12
CO(J = 1 0) molecular data used in this work was ob-
tained using the NANTEN telescope, a 4-m radio telescope of
Nagoya University at Las Campanas Observatory, Chile (see
Fukui et al. 2008). The observed region covers about 30 square
degrees where CO clouds were detected in the NANTEN first
survey (e.g. Fukui et al. 1999; Mizuno et al. 2001; Fukui et al.
2008). The observed grid spacing was 2
, corresponding to about
30 and 35 pc at the distance of the LMC and SMC, while the
half-power beam width was 2.6
at 115 GHz.
We used the CO maps of the Magellanic Clouds to construct
an integrated intensity map (W
CO
), integrating over the full v
lsr
range of the data (100 <v
lsr
< 400 km s
1
for about 80 % of
the data, while the remaining 20 % had a velocity range of about
100 km s
1
covering the H i emitting regions.)
2.2.4. Hα data
In order to estimate the free-free contribution to the millime-
tre fluxes, we used the continuum-subtracted H
α
maps from the
Southern H-Alpha Sky Survey Atlas (SHASSA, Gaustad et al.
2001) centred on the Magellanic Clouds.
2.2.5. FIR-submm data
We used the following FIR-submm ancillary data.
IRAS–IRIS (improved reprocessing of the IRAS survey)
100 μm data, in order to constrain the dust temperature. The
characteristics of these data, including the noise properties,
were taken from Miville-Deschênes & Lagache (2005)and
are summarised in Table 1
WMAP 7 yr data. The characteristics of these data, including
the noise properties, were taken from Bennett et al. (2003)
and Jarosik et al. (2011) and are summarised in Table 1.
2.3. Additional data processing
2.3.1. Common angular resolution and pixelisation
For data already available in the HEALPix (Górski et al. 2005)
format (e.g., WMAP), we obtained the data from the Lambda
web site (http://lambda.gsfc.nasa.gov/). For data not
Fig. 2. Error due to the ILC CMB subtraction derived from Monte-Carlo
simulations for both LMC and SMC at 100 GHz, as a function of the
patch size. The values shown are the dierence between the recovered
and the input CMB divided by the simulated LMC and SMC, both inte-
grated in a 4
ring.
originally presented in the HEALPix format, the ancillary data
were brought to the HEALPix pixelisation, using a method
where the area of the surface of intersection between each
HEALPix and FITS pixel of the original data was used as a
weight to regrid the data. The HEALPix resolution was chosen
so as to match the Shannon sampling of the original data at res-
olution θ, with a HEALPix resolution set so that the pixel size is
<θ/2.4. The ancillary data and the description of their processing
will be presented in Paradis et al. (in prep.).
All ancillary data were then smoothed to an appropriate res-
olution by convolution with a Gaussian smoothing function with
appropriate FWHM, using the smoothing HEALPix function.
They were brought to a pixel size matching the Shannon sam-
pling of the final resolution.
2.3.2. CMB subtraction
The upper panel in Fig. 3 shows the total intensity maps ob-
served toward the LMC and SMC before CMB subtraction in
the LFI 70.3 GHz chanel. The amplitude of CMB fluctuations is
of the order of that of the diuse emission of the galaxies and
it is clear that, in order to study the integrated SED of the LMC
and SMC galaxies, an ecient CMB subtraction has to be per-
formed.
The standard CMB–subtracted maps produced by the data
processing centre (DPC) (Planck HFI Core Team 2011b)were
not used in this analysis. They were processed by a needlet inter-
nal linear combination method (NILC) (Planck HFI Core Team
2011b) that left a significant amount of foreground emission in
the CMB estimate towards the LMC and the SMC.
For this reason we have subtracted an estimate of the CMB
optimised locally for the LMC and SMC regions, as described
below. This CMB component was reconstructed through a clas-
sical ILC by means of Lagrange multipliers (Eriksen et al. 2004).
12
× 12
patches around the LMC and SMC were extracted
from the HFI CMB frequency channels maps (100, 143, 217 and
353 GHz) reduced to a common resolution (10 arcmin), in units
of K
CMB
. The CMB component obtained on these patches clearly
contains less LMC and SMC residual than the standard HFI DPC
CMB and therefore will aect the SED determination less. The
A17, page 5 of 17
Page 5
A&A 536, A17 (2011)
Fig. 3. SMC (left)andLMC(right) total intensity maps before (top)andafter(bottom) CMB subtraction in the 70.3 GHz band at the 13.01 arcmin
resolution.
Fig. 4. Average foreground SED in the direction of the LMC and SMC
(lower curve) including IRAS–IRIS (light blue), Planck-LFI (red),
Planck-LFI (green) and WMAP data (dark blue), compared to the SED
of the high latitude low column-density MW SED (upper curve) derived
in Planck Collaboration (2011o), which has been scaled up by a factor
of 10 for clarity.
lower panel in Fig. 3 shows the maps after CMB subtraction in
the LFI 70.3 GHz channel.
We performed Monte-Carlo simulations in order to esti-
mate the error induced on the SED by our CMB removal. The
LMC and SMC were simulated as a sum of two correlated
components. The spatial template for the first component is the
IRAS–IRIS 100 μm map. The spatial template for the second
component is the 545 GHz LMC or SMC Planck map. Their cor-
relation coecients are 83% for the LMC and 92% for the SMC.
We normalised their fluxes inside a 4
ring to the value of a typ-
ical LMC or SMC dust component and a typical millimetre ex-
cess at each HFI CMB frequency, respectively. 200 independent
realisations of a WMAP 7 yr best-fit CMB (Komatsu et al. 2011)
and of nominal inhomogeneous HFI white noise were added to
the synthetic LMC and SMC on patches of varying sizes. For
each of these simulations, for each size, our ILC is performed
and compared with the input CMB. We estimate the error due to
the CMB subtraction in both the LMC and the SMC by compar-
ing the residual in the CMB map to the emission of the two com-
ponents at a given frequency. Results at 100 GHz are displayed
in Fig. 2. For both the LMC and the SMC, the error increases
at small and large patch sizes and an optimal patch size with re-
spect to the CMB subtraction can be found at 5
for the LMC
and 13
for the SMC. The narrower the patch, the lower is the
contribution of the CMB to the total variance which is to be min-
imised in the ILC. On the other hand, when their sizes increase,
A17, page 6 of 17
Page 6
Planck Collaboration: Planck early results. XVII.
Fig. 5. Integrated SEDs of the LMC (left)andSMC(right) before and after CMB subtraction. The black points and model are taken from Botetal.
(2010b). The red symbols show the SEDs derived from the DIRBE, IRAS, and WMAP data before CMB subtraction. The blue symbols show the
same after CMB subtraction.
patches include foreground emission which is uncorrelated with
the galaxies and has a dierent spectrum, and thus both small
and large patches contribute to the total variance which must be
minimised.
For the 12
× 12
patches used in the following analysis, we
estimate the error due to CMB subtraction as 10.38 μK
CMB
and
28.2 μK
CMB
for the LMC and SMC respectively. In terms of the
fraction of the total galaxy brightness, these correspond to 8.1,
6.2, 2.3 and 0.3% for the LMC and 12.1, 10.7, 6.4 and 1.4% for
the SMC at 100, 143, 217and 353 GHz, respectively.
2.3.3. Galactic foreground subtraction
The Milky Way (MW) emission is non-negligible compared to
the emission of the LMC and SMC. We remove this contribu-
tion at all wavelengths using the MW H i template described
in Sect. 2.2.2. We first computed the correlation of all FIR-
submm data with this template, in the region where both the
MW H i template and the CMB estimate are available. We ex-
cluded a circular region centred on each galaxy (centre coordi-
nates taken as α
2000
= 05
h
18
m
14.8
s
, δ
2000
= 68
26
34.5

and
α
2000
= 00
h
53
m
59.6
s
, δ
2000
= 72
40
16.1

for the LMC and
SMC respectively) with radius 4.09
and 2.38
for the LMC
and SMC respectively. The spectral distribution of this corre-
lation factor, taken to represent the SED of the MW foreground
is shown in Fig. 4. The SED is compared to that of the high
galactic latitude reference region used in Planck Collaboration
(2011o)inFig.4. It can be seen that the two SEDs are similar,
although the MW foreground towards the LMC and SMC ap-
pears slightly colder and has a relatively stronger non-thermal
component in the millimetre wavelength range. We subtracted
the MW foreground from all the data using this SED multiplied
by the MW H i template over the full map extent. Note that the
median foreground H i integrated intensities over the LMC and
SMC are 297 K km s
1
and 172 K km s
1
, which correspond
to a brightnesses of 3.0 MJy sr
1
and 1.7 MJy sr
1
respectively
at 857 GHz. This is of the same order as the average brightness
of the galaxies at that frequency (see Table 2). However, most
of the MW emission is canceled when subtracting a local back-
ground around the galaxies and the dierential correction due
to the spatial structure of the MW foreground then accounts for
about 0.4% and 21% of the LMC and the SMC brightness re-
spectively.
3. Integrated SEDs
The integrated SEDs of the LMC and SMC before and after
CMB subtraction are shown in Fig. 5 (red and blue diamonds,
respectively). They were computed by averaging values in the
circular area around each galaxy defined in Sect. 2.3.3, and sub-
tracting an estimate of the sky o the source taken in an annulus
with radius 1
. They are compared to the data taken from Israel
et al. (2010)andBot et al. (2010b), which were integrated over
the same region. It can be seen that the flux observed in the HFI
and LFI bands is consistent with that obtained by previous stud-
ies in this wavelength range before subtraction of the CMB fluc-
tuations. A model of thermal dust, free-free and synchrotron
emission is fitted to the data as follows. As in Botetal.(2010b),
the thermal dust emission is adjusted according to the Draine &
Li (2007) dust model. The free-free emission is deduced from
the H
α
integrated flux, using the expression from Hunt et al.
(2004), assuming an electronic temperature T
e
= 10
4
K, the ratio
of ionised helium to hydrogen n
He
+
/n
+
H
= 0.087, and no extinc-
tion. The synchrotron emission was fitted to the radio data from
the literature as in Israel et al. (2010). The combined dust, free-
free and synchrotron emission is shown by the blue line in Fig. 5.
It can be seen from Fig. 5 that the SED after subtraction of the
CMB fluctuations is in fact compatible with no millimetre emis-
sion excess for the LMC, for this particular model. However,
the CMB-subtracted SED still shows a significant excess for the
SMC, with most data points above λ 1 mm being in excess
over the model by more than 5σ, leading to an overall signifi-
cance of the excess of about 50σ.
The CMB subtraction removes part of the millimetre excess
in both galaxies. This shows that CMB fluctuations behind the
LMC and the SMC average out to a small but positive contri-
bution when integrated over the extent of the galaxies. We note
that some excess emission remains in the SMC regardless of the
dust model used and the assumptions made on the free-free or
synchrotron emission. The shape and intensity of the millime-
tre excess can change, but we could not find a solution where
the SMC SED is explained purely by thermal dust emission,
free-free and synchrotron radiation. We emphasise also that the
dust model, used to reproduce the dust emission up to the sub-
millimetre wavelength range, assumes components heated by a
radiation field 10 times lower than the solar neighbourhood ra-
diation field. Compared to other nearby galaxies, this result is
rather extreme (Draine et al. 2007; Bot et al. 2010b).
A17, page 7 of 17
Page 7
A&A 536, A17 (2011)
Tab l e 2. LMC (Cols. 2–4) and SMC (Cols. 5–7) SEDs averaged in a circular region for each galaxy.
λ I
tot
ν
I
noCMB
ν
I
sub
ν
I
tot
ν
I
noCMB
ν
I
sub
ν
[ μm] [MJy sr
1
][MJysr
1
][MJysr
1
][MJysr
1
][MJysr
1
][MJysr
1
]
IRAS:
12 (2.36 ± 0.12) × 10
1
(2.36 ± 0.12) × 10
1
(2.34 ± 0.13) × 10
1
(1.37 ± 0.11) × 10
2
(1.37 ± 0.11) × 10
2
(2.79 ± 0.27) × 10
2
25 (6.28 ± 0.95) × 10
1
(6.28 ± 0.95) × 10
1
(6.25 ± 0.96) × 10
1
(7.01 ± 1.10) × 10
2
(7.01 ± 1.10) × 10
2
(5.30 ± 0.98) × 10
2
60 5.79 ± 0.60 5.79 ± 0.60 5.78 ± 0.61 1.52 ± 0.16 1.52 ± 0.16 1.56 ± 0.18
100 (1.13 ± 0.15) × 10
1
(1.13 ± 0.15) × 10
1
(1.13 ± 0.15) × 10
1
2.56 ± 0.35 2.56 ± 0.35 2.82 ± 0.39
Planck:
349.82 4.97 ± 0.35 4.97 ± 0.35 4.96 ± 0.35 1.18 ± 0.08 1.18 ± 0.08 1.46 ± 0.11
550.08 1.79 ± 0.13 1.79 ± 0.13 1.78 ± 0.13 (5.02 ± 0.37) × 10
1
(4.99 ± 0.37) × 10
1
(5.96 ± 0.45) × 10
1
849.27 (4.61 ± 0.10) × 10
1
(4.58 ± 0.10) × 10
1
(4.51 ± 0.10) × 10
1
(1.63 ± 0.04) × 10
1
(1.47 ± 0.04) × 10
1
(1.69 ± 0.05) × 10
1
1381.5 (1.16 ± 0.02) × 10
1
(1.10 ± 0.03) × 10
1
(1.04 ± 0.03) × 10
1
(7.12 ± 0.18) × 10
2
(4.30 ± 0.19) × 10
2
(4.63 ± 0.20) × 10
2
2096.4 (3.56 ± 0.08) × 10
2
(3.13 ± 0.10) × 10
2
(2.51 ± 0.09) × 10
2
(3.71 ± 0.10) × 10
2
(1.54 ± 0.11) × 10
2
(1.54 ± 0.11) × 10
2
2997.9 (1.81 ± 0.05) × 10
2
(1.53 ± 0.06) × 10
2
(8.96 ± 0.48) × 10
3
(2.25 ± 0.07) × 10
2
(8.29 ± 0.71) × 10
3
(7.09 ± 0.69) × 10
3
4285.7 (1.14 ± 0.07) × 10
2
(9.87 ± 0.68) × 10
3
(3.34 ± 0.36) × 10
3
(1.24 ± 0.09) × 10
2
(4.55 ± 0.61) × 10
3
(3.16 ± 0.54) × 10
3
6818.2 (9.24 ± 0.52) × 10
3
(8.57 ± 0.51) × 10
3
(1.72 ± 0.17) × 10
3
(6.37 ± 0.49) × 10
3
(2.94 ± 0.34) × 10
3
(1.57 ± 0.28) × 10
3
10 000 (8.43 ± 0.61) × 10
3
(8.17 ± 0.60) × 10
3
(1.05 ± 0.25) × 10
3
(3.87 ± 0.55) × 10
3
(2.53 ± 0.49) × 10
3
(1.10 ± 0.41) × 10
3
WMAP:
3200 (1.61 ± 0.04) × 10
2
(1.36 ± 0.05) × 10
2
(7.23 ± 0.43) × 10
3
(2.10 ± 0.08) × 10
2
(8.28 ± 0.80) × 10
3
(7.07 ± 0.79) × 10
3
4900 (1.04 ± 0.02) × 10
2
(9.13 ± 0.23) × 10
3
(2.51 ± 0.17) × 10
3
(1.06 ± 0.03) × 10
2
(4.32 ± 0.33) × 10
3
(2.92 ± 0.31) × 10
3
7300 (9.15 ± 0.13) × 10
3
(8.56 ± 0.15) × 10
3
(1.66 ± 0.09) × 10
3
(6.12 ± 0.15) × 10
3
(3.09 ± 0.16) × 10
3
(1.67 ± 0.15) × 10
3
9100 (9.38 ± 0.12) × 10
3
(8.98 ± 0.13) × 10
3
(1.93 ± 0.06) × 10
3
(5.10 ± 0.11) × 10
3
(3.04 ± 0.11) × 10
3
(1.63 ± 0.10) × 10
3
13 000 (9.38 ± 0.11) × 10
3
(9.19 ± 0.12) × 10
3
(1.88 ± 0.05) × 10
3
(3.62 ± 0.08) × 10
3
(2.52 ± 0.09) × 10
3
(1.07 ± 0.07) × 10
3
H
α
(R):
–(1.93 ± 0.19) × 10
1
7.00 ± 0.72
W
HI
MW
(Kkms
1
):
–1.01± 0.13 (2.57 ± 0.26) × 10
+1
Notes. The integration region is centred on α
2000
= 05
h
18
m
14.8
s
, δ
2000
= 68
26
34.5

, with radius R
LMC
= 4.09
for the LMC and centred on
α
2000
= 00
h
53
m
59.6
s
, δ
2000
= 72
40
16.1

, with radius R
SMC
= 2.38
for the SMC. A common background was subtracted in a 1
annulus around
this region. Brightness values are in MJy sr
1
in the νI
ν
= cste flux convention. The last two lines give the average H
α
emission (in Rayleigh),
Galactic HI emission (in K km s
1
) in the same area. The table lists the total SED (I
tot
ν
), the CMB subtracted SED (I
noCMB
ν
)andtheCMB,MW
foreground and free-free subtracted SED (I
sub
ν
) and their associated 1σ uncertainties.
The integrated SEDs of the LMC and SMC after CMB and
foreground subtraction are compared in Fig. 6. These SEDs were
computed in the same integration region as those in Fig. 5.The
comparison in Fig. 6 shows that the SMC SED is flatter than the
LMC SED in the submm, while when the SEDs are normalised
in the FIR, they are similar above 10 mm, where the emission
is presumably dominated by free-free and/or spinning dust (see
Sect. 5).
The SEDs at various stages of the background and fore-
ground subtraction are given in Table 2 for the LMC and SMC.
The uncertainties given include the contribution from the data
variance combined for the integration region, the data variance
combined for the background region, and the absolute calibra-
tion uncertainties, using the values given in Table 1.Theyalso
include the noise resulting from the background (CMB) subtrac-
tion and from the foreground MW subtraction and the free-free
removal. All uncertainty contributions were added quadratically.
4. Dust temperature and emissivity
4.1. Temperature determination
As shown by several previous studies (e.g. Reach et al. 1995;
Finkbeiner et al. 1999; Paradis et al. 2009; Planck Collaboration
2011o,t), the dust emissivity spectrum in our Galaxy cannot be
represented by a single dust emissivity index β over the full
FIR-submm domain. The data available indicate that β is usu-
ally steeper in the FIR and flatter in the submm, with a transition
around 500 μm(seeParadis et al. 2009; Planck Collaboration
2011t). This is likely to be the case also for the LMC and the
SMC. The dust temperature derived will depend on the assump-
tions made about β, since these two parameters are somewhat
degenerate in χ
2
space. Note that some of the past attempts at
constraining the equilibrium temperature of the large grains in
the LMC used the IRAS 60 μm emission. Emission at 60 μmis
highly contaminated by out-of-equilibrium emission from very
small grains (VSGs) and this is even more the case in the
Magellanic Clouds, due to the presence of the 70 μm excess (Bot
et al. 2004; Bernard et al. 2008). Combining the IRAS 60 μm
and 100 μm data therefore strongly over-estimates the tempera-
ture and accordingly under-estimates the abundances of all types
of dust particles. A good sampling at frequencies dominated by
big grain (BG) emission became possible using the combination
of the IRAS and Spitzer data (Leroy et al. 2007; Bolatto et al.
2007; Bernard et al. 2008; Sandstrom et al. 2010).
As the dust temperature is best derived from the FIR data,
we limit the range of frequencies used in the determination to
the FIR, which limits the impact of potential changes of the
dust emissivity index β with frequency. In the determination
of the dust temperature (T
D
), we used the IRAS–IRIS 100 μm
map and the two highest HFI frequencies at 857 and 545 GHz.
Temperature maps were derived at the common resolution of
the 3 bands used (5
) and at lower resolution for further analy-
sis. In each case, the emission was computed in the photometric
channels of the instruments used (IRAS, Planck,andWMAP),
including the colour corrections, using the actual transmission
profiles for each instrument, and following the flux convention
description given in the respective explanatory supplements.
In order to derive the thermal dust temperature, we use the
same strategy as described in Planck Collaboration (2011o). To
A17, page 8 of 17
Page 8
Planck Collaboration: Planck early results. XVII.
Tab l e 3 . Dust temperature, β values and t reduced χ
2
for various meth-
ods experimented to derive the temperature maps.
Method T
D
± ΔT
D
β ± Δβχ
2
[K]
LMC:
free β 21.0 ± 1.9 1.48 ± 0.25 1.91
fixed β 20.7 ± 1.7 1.5
1.73
fixed β 19.2 ± 1.6 1.8 1.63
fixed β 18.3 ± 1.6 2.0 1.57
dustem 17.7 ± 1.6 1.60
SMC:
free β 22.3 ± 2.3 1.21 ± 0.27 2.28
fixed β 21.6 ± 1.9 1.2
1.90
fixed β 18.8 ± 1.7 1.8 9.94
fixed β 17.9 ± 1.6 2.0 14.0
dustem 17.3 ± 1.6 12.66
Notes. The values listed are median values in the SED integration re-
gion for each galaxy.
()
Fixed β model used in this paper.
Fig. 6. Integrated SEDs of the LMC (solid) and SMC (dashed) after
CMB and galactic foreground subtraction, including data from Planck-
HFI (red), Planck-LFI (green), IRAS–IRIS (light blue) and WMAP
(dark blue). The uncertainties shown are ±3σ.TheSMCSEDwas
scaled by a factor 4 providing normalization at 100 μm.
minimise computation time, the predictions of the model were
tabulated for a large set of parameters (T
D
, β). For each map
pixel, the χ
2
was computed for each entry of the table and the
shape of the χ
2
distribution around the minimum value was used
to derive the uncertainty on the free parameters. This includes
the eect of the data variance σ
2
II
and the absolute uncertainties.
We explored several options for deriving maps of the appar-
ent dust temperature, which are summarised in Table 3.Wefirst
fitted each pixel of the maps with a modified black body of the
form I
ν
ν
β
B
ν
(T
D
) in the above spectral range (method referred
to as “free β”inTable3). The median values of T
D
and β de-
rived using this method are given in the first line of Table 3.This
ledtomedianβ values of β
LMC
1.5andβ
SMC
1.2forthe
LMC and SMC respectively. Note that the value for the LMC
is consistent with that derived using a combination of the IRAS
and Herschel data by Gordon et al. (2010). However, inspection
of the corresponding maps show correlated variations of the two
parameters which were also correlated with the noise level in the
maps. This suggests that the spurious values probably originated
from the correlation in parameter space and the presence of noise
in the data, particularly in low brightness regions of the maps.
We then performed fits of the FIR emission using the fixed
β values, with the β values given above for the two galaxies
(method referred to as “fixed β” and marked with a
in Table 3).
Although the median reduced χ
2
was slightly higher than for the
“free β method, the temperature maps showed fewer spurious
values, in particular in low brightness regions. This resulted in
a more coherent distribution of the temperature values than in
the “free β case. Since we later used the temperature maps to
investigate the spectral distribution of the dust optical depth, and
the dust temperature is a source of uncertainty, we adopted the
“fixed β” maps in what follows. The corresponding temperature
maps for the LMC and SMC are shown in Fig. 7.
The calculations were also carried out for β = 1.8, which
is the average Galactic value (Planck Collaboration 2011o,t), in
order to be able to derive dust emissivity values under the same
assumption as in the MW. We also experimented using the “stan-
dard” β = 2value.
4.2. Angular distri bution of dust temperature
4.2.1. LMC
The temperature map derived here for the LMC shows a simi-
lar distribution to the one derived from IRAS and Spitzer data
by Bernard et al. (2008). The highest temperatures are observed
toward 30-Dor and are of the order of 24 K. The large scale
distribution shows the existence of an inner warm arm, which
follows the distribution of massive star formation as traced by
known H ii regions. Figure 8 shows that the warm dust at T
D
>
20 K in the LMC is reasonably well correlated with H
α
emis-
sion, indicating that it is heated by the increased radiation field
induced by massive star formation. Two regions departing from
the correlation are visible to the SW of the 30-Dor at α
2000
=
05
h
40
m
, δ
2000
= 70
30
and α
2000
= 05
h
40
m
, δ
2000
= 72
30
.
The first region was already identified in Bernard et al. (2008)
who showed that this low column density region was unexpect-
edly warm given that no star formation is taking place in this
area. They proposed that this could be an artefact of the IRAS–
IRIS 100 μm used in the analysis, possibly due to IRAS gain
variations across the bright 30-Dor source. However, the analysis
carried out here and the comparison between temperature maps
obtained with and without the MW background subtraction sug-
gests that the temperature measurement has been aected by the
low surface brightness. This emphasises the necessity of correct-
ing for temperature bias at low brightness. The second region is
close to the edge of the map and is probably also aected by low
surface brightness.
The Planck data however reveals cold areas in the outer re-
gions of the LMC, which were not seen using the Spitzer data.
This is essentially due to the large coverage of the Planck data
with respect to the limited region which was covered by the
Spitzer data. The south of the LMC exhibits a string of cold re-
gions with T
D
< 20 K. These regions correspond without excep-
tion to known molecular clouds when they fall in the region cov-
ered by the CO survey. They also correspond to peaks of the dust
optical depth as derived in the following sections. Similarly, a set
of cold regions exist at the northwest periphery of the LMC. The
comparison with the distribution of CO clouds is shown in Fig. 8.
As already noticed in Bernard et al. (2008), there is no system-
atic correlation between cold dust and the presence of molecu-
lar material, at least in the inner regions of the LMC. This was
confirmed by the statistical characteristics of the IR properties
of LMC molecular clouds established by Paradis et al. (2010),
which showed no systematic trend for molecular regions to be
A17, page 9 of 17
Page 9
A&A 536, A17 (2011)
Fig. 7. Upper panels: dust temperature maps for the LMC (left )andSMC(right) computed from the foreground subtracted maps using the
IRAS–IRIS 100 μm, HFI 857 and 545 GHz maps, using a fixed β
LMC
= 1.5andβ
SMC
= 1.2. Lower panels: relative uncertainties on the dust
temperature at the same resolution, expressed as percentages.
colder than their surrounding neutral material. However, toward
the outer regions of the LMC, molecular clouds systematically
appear to show a decrease in the dust temperature. This dier-
ence may be due to the absence of star formation activity in the
outer regions and/or to less mixing along the line of sight.
Gordon et al. (2010) derived a dust temperature map from
the Herschel data (HERITAGE program) for a fraction of the
LMC observed during the science demonstration phase (SDP).
They used the 100 μm to 350 μmIRASandHerschel bands to
constrain the temperature. They found that the temperatures de-
rived this way only dier from those derived by Bernard et al.
(2008) by up to 10%. The SDP field covered an elongated re-
gion across the LMC roughly oriented north–south. This strip
crossed the warm inner arm, which is also clearly seen in their
map. However, we stress that, in their study, a gradient was re-
moved along the strip based on the values at the edges. The fact
that we detect significantly colder than average dust in the outer
cold arm of the LMC underlines the need to take those varia-
tions into account when subtracting background emission in the
Herschel data.
4.2.2. SMC
The temperature map derived for the SMC shows more mod-
erate temperature variations than those observed in the LMC
but peaks corresponding to well known HII regions are clearly
identified, and the overall dust temperature correlates with
H
α
emission, as shown in Fig. 9. In particular, the mas-
sive star forming region SMC-N66 (Henize 1956)(α
2000
=
00
h
59
m
27.40
s
, δ
2000
= 72
10
11

) corresponds to the high-
est temperature (20 K) in the SMC. Other well known star
forming regions like SMC-N83/84 (01
h
14
m
21.0
s
, 73
17
12

),
N81 (01
h
09
m
13.6
s
, 73
11
41

), N88/89/90 (in the wing:
01
h
24
m
08.1
s
, 73
08
55

) and DEM S54 (at the centre of
A17, page 10 of 17
Page 10
Planck Collaboration: Planck early results. XVII.
Fig. 8. Comparison between the dust temperature map of the LMC with
H
α
(top) and CO emission (bottom). The CO contours are at 0.5, 2, 4 and
10 K km s
1
.H
α
contours are at 1, 10, 50, 100, 500 and 1000 Rayleigh.
The thick line shows the edge of the available CO surveys.
the main bar: 00
h
50
m
25.9
s
, 72
53
10

) appear as temperature
peaks compared to the surroundings. In contrast, the infrared-
bright star forming region N 76 (located at the northeast of N 66:
01
h
04
m
01.2
s
, 72
01
52

) and southwest star-forming complex
do not stand out. This could be due to the presence of an ex-
tended warm component that we observe in the main bar and
that seems spatially related to the diuse H
α
emission.
4.3. Optical depth determination
The optical depth is derived from:
τ(ν) =
I
ν
B
ν
(T
d
)
, (1)
where B
ν
is the Planck function. We used resolution-matched
maps of T
D
and I
ν
and derived τ maps at the various resolutions
of the data used here. The uncertainty on τ (Δτ) is computed as:
Δτ(ν) = τ
σ
2
II
I
2
ν
+
δB
ν
(T
D
)
δT
ΔT
D
B
ν
(T
D
)
2
1/2
. (2)
Fig. 9. Comparison between the dust temperature map of the SMC with
H
α
(top) and CO emission (bottom). The CO contours are at 0.5, 1 and
1.5 K km s
1
.H
α
contours are at 1, 5, 30 and 100 Rayleigh. The thick
line shows the edge of the available CO surveys.
The optical depth and optical depth uncertainty maps derived at
857 GHz are shown in Figs. 10 and 11 for the LMC and the SMC
respectively.
5. Discussion
5.1. The millimetre excess
Botetal.(2010b)(seeFig.5) found that the integrated SED of
the LMC and more noticeably of the SMC showed excess mil-
limetre emission with respect to a dust and free-free/synchrotron
model based respectively on the FIR and the radio data. They in-
vestigated several causes for this excess. They noticed that the
excess had precisely the colours of CMB fluctuations. They per-
formed simulations by placing the LMC and SMC structure at
various positions over a CMB simulated sky, and concluded that
a CMB origin was unlikely, but not excluded. Other proposed
origins for the excess included the presence of very cold dust,
spinning dust or large modifications of the optical properties of
thermal dust.
A17, page 11 of 17
Page 11
A&A 536, A17 (2011)
Fig. 10. Upper panel: map of the dust optical depths of the LMC at
HFI 217 GHz. Units are 10
4
× τ. Lower panel: map of the dust optical
depth relative uncertainty of the LMC at HFI 217 GHz in percent. Black
pixels in the maps are masked and have relative uncertainties larger than
50%.
Very cold dust (T
D
57 K) has been advocated to explain
the flattening of the millimetre emission observed in more dis-
tant low metallicity galaxies (e.g. Galliano et al. 2005). However,
this has usually led to very large masses, and the existence of
such very cold dust remains controversial and dicult to under-
stand in low metallicity systems where the stronger star forma-
tion rate and the lower dust abundances prevent ecient screen-
ing from UV photons.
Bot et al. (2010b) applied spinning dust models to fit the
SMC and LMC SEDs and found a plausible match. However,
since the observed excess peaked at a significantly higher fre-
quency than observed for spinning dust in other regions (e.g.
Planck Collaboration 2011p), their fit required extreme density
and excitation conditions for the small dust particles.
Using the two level system (TLS) model by Meny et al.
(2007) for the long wavelength emission of amorphous solids
also proved plausible for the LMC, but a convincing fit could
not be found for the SMC, essentially because the model could
not reproduce the shape of the excess.
Fig. 11. Upper panel: map of the dust optical depths of the SMC at
HFI 217 GHz. Units are 10
4
× τ. Lower panel: map of the dust optical
depth relative uncertainty at HFI 217 GHz in percent. Black pixels in
the maps are masked and have relative uncertainties larger than 50%.
The study carried out here, which takes advantage of the
Planck measurements to constrain the CMB foreground fluctu-
ations towards the two galaxies, shows that part of the excess
observed toward the SMC cannot be accounted for by the fluctu-
ations of the background CMB, as discussed in Sect. 2.3.2,but
the intensity of the excess has been greatly reduced compared to
that found by Botetal.(2010b).
To give more insight into this excess, we built a map to trace
the spatial distribution of the excess emission in the SMC. To
do this, we applied the fitting procedure performed for the in-
tegrated SEDs of the Magellanic Cloud, to each point of the
SMC at the angular resolution of the LFI lowest frequency chan-
nel. The SEDs were build at each point using the CMB and
foreground–subtracted data. To model the SEDs, we assumed
that, at each point, the SEDs are dominated by free-free and dust
emission up to the longest wavelengths covered by Planck,fol-
lowing what is observed for the integrated emission of the SMC
(see Fig. 5). The free-free component at each wavelength was
extrapolated from the H
α
emission (as in Sect. 3), assuming that
A17, page 12 of 17
Page 12
Planck Collaboration: Planck early results. XVII.
Fig. 12. Map of the millimetre excess in the SMC at 3 mm computed at
the resolution of the LFI–28.5 GHz channel, after free-free subtraction.
The contours show the H
α
distribution.
the extinction in the SMC is negligible. The thermal dust emis-
sion was then fitted to the data points at λ
ref
< 550 μm, using
the Draine et al. (2007) dust model. The millimetre excess was
then defined as the dierence between the data and the dust-and-
free-free model. The resulting spatial distribution of the excess
emission at 100 GHz (3 mm) is shown in Fig. 12. This shows
that the peak of the excess is located at the southwest tip of the
bar. This region also corresponds to the maximum of the optical
depth derived from the FIR and shown in Fig. 11, and the overall
excess spatial distribution is consistent with being proportional
to the dust column density.
5.2. FIR dust emissivity
The wavelength dependence of the average dust optical depth in
the LMC and SMC is shown in Fig. 13. The spectral index of the
emissivity in the FIR is consistent with β = 1.5andβ = 1.2for
the LMC and the SMC respectively. This value for the LMC
is consistent with findings by Gordon et al. (2010)usingthe
Herschel data. We see no hint of a change of the spectral index
with wavelength for the LMC. In contrast, the SMC SED clearly
flattens at λ>800 μm to reach extremely flat β values around
λ = 3 mm. The dust emissivities as interpolated using the power
laws shown in Fig. 13 can be compared to the reference value
for the solar neighbourhood of τ/N
H
= 10
25
cm
2
at 250 μm
(Boulanger et al. 1996). This comparison indicates lower than
solar dust abundances for the two galaxies, by about 1/2.4 and
1/13 respectively. This is in rough agreement with the metallic-
ity for the LMC and significantly lower than the metallicity for
the SMC.
Gordon et al. (2010) found hints of such excess emission
around 500 μmintheLMCHerschel data, but this remained
within the current calibration uncertainties of the Herschel
Spectral and Photometric Imaging REceiver (SPIRE) instru-
ment. We also observe that the 500 μm optical depth of the LMC
is slightly higher (by 8.4%) than the power law extrapolation
showninFig.13, but this is only marginally larger than the 1σ
uncertainty at this wavelength. In any case, data points at longer
Fig. 13. Average dust optical depth of the LMC (black) and SMC (blue)
obtained from the CMB, foreground and free-free subtracted SEDs. The
average is taken in the same regions as Fig. 5. The SMC SED was nor-
malised (multiplied by 6.31) to that of the LMC at 100 μm. The square
symbol (green) shows the reference value for the solar neighbourhood
by Boulanger et al. (1996). The dashed lines show τ ν
1.5
and τ ν
1.2
normalised at 100 μm. Error bars are ±3-σ.
wavelengths do not support the existence of excess emission in
addition to a β = 1.5 power law for the LMC.
5.3. Possible interpretation of the excess
In this section, we attempt to fit the SED of the SMC using vari-
ous models which have been proposed to explain excess submm
emission, in addition to a single dust emissivity power-law for
thermal dust. Figure 14 shows several fits to the FIR-Submm
SED of the SMC.
The first fit uses the Finkbeiner et al. (1999) model 7
(see their Table 3), which was designed to explain the flatter-
than-expected emission spectrum of our Galaxy as observed by
FIRAS. Here we use this model to assess the possibility that the
millimetre excess is due to very cold dust. The model was fit-
ted using the same β values as in Finkbeiner et al. (1999)for
the two dust components (β
warm
= 2.6, β
cold
= 1.5). It also
assumes the same type of relationship between the cold and
warm dust temperatures, which in the model reflects the fact
that both dust species are subjected to the same radiation field.
However, we allow the IR/optical opacity ratio (q
cold
/q
warm
)to
vary, so that the ratio between the warm and cold dust temper-
atures is free to vary. We also leave the cold dust component
abundance ( f
cold
) and the warm dust temperature (T
warm
) as free
parameters. The best fit values are found for T
warm
= 16.4K
q
cold
/q
warm
= 168.2, f
cold
= 8.7 × 10
3
. For these parameters,
the temperature of the cold component is T
cold
= 5.9K. This
cold temperature is needed to reproduce part of the millime-
tre excess but is much colder than that obtained by Finkbeiner
et al. (1999) for the Galaxy (T
cold
= 9.6 K). It is obtained at
the expense of strongly increasing the IR/optical opacity ratio
for the cold component (q
cold
/q
warm
increased by a factor 15),
which controls the T
cold
/T
warm
ratio. The mass fraction of the
cold component derived here, f
cold
, is about 4 times lower than
that derived for the MW in Finkbeiner et al. (1999), which com-
pensates for the increased q
cold
/q
warm
. It is apparent that, despite
invoking a much larger IR/optical opacity ratio for the cold par-
ticles, such a model has diculties producing the submm excess
above about λ = 2 mm.
A17, page 13 of 17
Page 13
A&A 536, A17 (2011)
Fig. 14. Fit of the SMC SED using the Finkbeiner model (upper left), the TLS model (upper right), the spinning dust model (lower left)anda
combination of TLS and spinning dust models (lower right). In the lower panels, the squares represent the flux in each band predicted by the best
model (the blue line).
The second fit employs the TLS model developed by Meny
et al. (2007). The fit was obtained by minimizing χ
2
in the range
100 μm <λ<5 mm against the following 3 parameters: T
D
the dust temperature, l
cor
the correlation length of defects in the
material and A the density of TLS sites in the material com-
posing the grains. The values derived for these parameters are
T
D
= 18.9K,l
cor
= 12.85 nm and A = 7.678. The reduced
χ
2
for these parameters is χ
2
= 2.56. Compared to the best
values found from Paradis (2007)fortheMW(T
D
= 17.9K,
l
cor
= 12.85 nm, A = 2.42), this indicates dust material with
more TLS sites (more mechanical defects) but a similar de-
fect correlation length compared with the dust dominating the
MW emission. Note that a grey-body fit of the same SED over
the same frequency range leads to T
D
= 18.9Kandβ = 0.74 and
a reduced χ
2
= 6.12, showing that the TLS model reproduces the
data better than a single grey-body fit.
The third fit uses spinning dust. In this case, the thermal
dust emission was reproduced using the Draine & Li (2007)
dust model. The remaining excess was then fitted with the spin-
ning dust model described in Silsbee et al. (2011). Spinning
dust emission has been proved to be sensitive to the neutral and
ionised gas densities and to the size distribution of dust grains.
The radiation field and the size distributions were taken to be the
same as for the Draine & Li (2007) model. The relevant gas pa-
rameters were computed with CLOUDY (Ferland et al. 1998):
we took the parameters from the optically thin zone of iso-
choric simulations. In both cases, we took into account the lower
metallicity and dust grains abundance with respect to the MW.
The electric dipole moment distribution of grains was taken as
in Draine & Lazarian (1998b)
2
. The fit was obtained by adding
two components according to the PDR fraction inferred from the
thermal emission model: a diuse medium with n
H
= 30 cm
3
and 100% of the PAH mass expected from the IR modelling, and
a denser medium with n
H
= 5000 cm
3
and 82% of the expected
PAH mass.
The fourth fit is a combination of the TLS model and the
spinning dust model presented above.
It is clear from Fig. 14 that the model combining TLS and
spinning dust is the only one of the proposed models giving a
satisfactory fit to the millimetre excess observed in the SMC
over the whole spectral range. In addition, it alleviates the need
for using more PAH than allowed by the NIR emission to pro-
duce the required level of spinning dust emission observed.
Alternative models, however, cannot be excluded. For instance,
very large grains (a > 30 μm) that would be ecient radiators
(Rowan-Robinson 1992) could create emission in the millimetre
wavelengths without too much mass. Exploring this possibility
would require specific modelling. This is beyond the scope of
the current article but should be explored in future studies.
2
Ysard & Verstraete (2010) showed that it is in good agreement with
the anomalous emission extracted from the WMAP data.
A17, page 14 of 17
Page 14
Planck Collaboration: Planck early results. XVII.
6. Conclusions
We assessed the existence and investigated the origin of mil-
limetre excess emission in the LMC and the SMC using the
Planck data. In the framework of this preliminary analysis, we
focused our interest on the thermal dust emission from the galax-
ies, which we isolated by subtracting other unrelated compo-
nents such as galactic foreground, CMB fluctuations and free-
free emission. More detailed studies in the future will involve
full multi-component analysis or the use of more sophisticated
component separation algorithms exploiting all spatial and spec-
tral information available in the data.
The integrated SED of the two galaxies before subtraction
of the foreground (Milky Way) and background (CMB fluctua-
tions) emission are in good agreement with previous determina-
tions.
The background CMB contribution was first subtracted using
an ILC method performed locally around the two galaxies. The
uncertainty of this contribution was measured through a detailed
Monte-Carlo simulation. We subtracted the foreground emis-
sion from the Milky Way using a Galactic H i template and the
proper dust emissivity derived in a region surrounding the two
galaxies and dominated by MW emission. We also subtracted
the free-free contribution from ionised gas in the galaxies, us-
ing the H
α
emission, taking advantage of the low extinction in
those galaxies. The remaining emission of both galaxies corre-
lates with the gas emission of the LMC and SMC.
We showed that the excess previously reported in the LMC
can be fully explained by CMB fluctuations. For the SMC, sub-
tracting the CMB fluctuations decreases the intensity of the ex-
cess but a significant millimetre emission above the expected
thermal dust, free-free and synchrotron emission remains.
We combined the Planck and IRAS–IRIS data at 100 μmto
produce thermal dust temperature and optical depth maps of the
two galaxies. The LMC temperature map shows the presence of
a warm inner arm already found with the Spitzer data, but also
shows the existence of a previously unidentified cold outer arm.
Several cold regions were found along this arm, some of which
are associated to known molecular clouds.
We used the dust optical depth maps to constrain the thermal
dust emissivity spectral index (β). The average spectral index in
the FIR (λ<500 μm) is found to be consistent with β = 1.5
and β = 1.2 for the LMC and the SMC respectively. This is
significantly flatter than is observed in the Milky Way. The ab-
solute values of the emissivities in the FIR, when compared to
that in our solar neighbourhood, are compatible with D/Gmass
ratios of 1/2.4 and 1/13 for the LMC and SMC. This is com-
patible with the metallicity of the LMC but significantly lower
than the metallicity of the SMC. In the submm, the LMC SED
remains consistent with β = 1.5, while the SED of the SMC is
even flatter.
The spatial distribution of the mm excess in the SMC appears
to follow the general pattern of the gas distribution. It therefore
appears unlikely that the excess could originate from very cold
dust. Indeed, this is confirmed by attempts to fit the SMC emis-
sion SED with models with two dust components, which led to
poor fits. Alternative models, such as emission excess due to the
amorphous nature of large grains, are likely to provide a natu-
ral explanation for the observed SEDs, although spinning dust is
needed to explain the SED above λ = 3 mm.
Acknowledgements. A description of the Planck Collaboration and a list
of its members can be found at http://www.rssd.esa.int/index.php?
project=PLANCK&page=Planck_Collaboration
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1
Aalto University Metsähovi Radio Observatory, Metsähovintie 114,
02540 Kylmälä, Finland
2
Agenzia Spaziale Italiana Science Data Center, c/oESRIN,via
Galileo Galilei, Frascati, Italy
3
Astroparticule et Cosmologie, CNRS (UMR7164), Université Denis
Diderot Paris 7, Bâtiment Condorcet, 10 rue A. Domon et Léonie
Duquet, Paris, France
4
Astrophysics Group, Cavendish Laboratory, University of
Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, UK
5
Atacama Large Millimeter/submillimeter Array, ALMA Santiago
Central Oces, Alonso de Cordova 3107, Vitacura, Casilla
763 0355, Santiago, Chile
6
CITA, University of Toronto, 60 St. George St., Toronto,
ON M5S 3H8, Canada
7
CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse
Cedex 4, France e-mail: Jean-Philippe.Bernard@cesr.fr
8
California Institute of Technology, Pasadena, California, USA
9
DAMTP, University of Cambridge, Centre for Mathematical
Sciences, Wilberforce Road, Cambridge CB3 0WA, UK
10
DSM/Irfu/SPP, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
11
DTU Space, National Space Institute, Juliane Mariesvej 30,
Copenhagen, Denmark
12
Departamento de Física, Universidad de Oviedo, Avda. Calvo Sotelo
s/n, Oviedo, Spain
13
Department of Astronomy and Astrophysics, University of Toronto,
50 Saint George Street, Toronto, Ontario, Canada
14
Department of Astronomy and Earth Sciences, Tokyo Gakugei
University, Koganei, Tokyo 184-8501, Japan
15
Department of Physical Science, Graduate School of Science, Osaka
Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-
8531, Japan
16
Department of Physics & Astronomy, University of British
Columbia, 6224 Agricultural Road, Vancouver, British Columbia,
Canada
17
Department of Physics, Gustaf Hällströmin katu 2a, University of
Helsinki, Helsinki, Finland
18
Department of Physics, Nagoya University, Chikusa-ku, Nagoya,
464-8602, Japan
19
Department of Physics, Princeton University, Princeton, New Jersey,
USA
20
Department of Physics, Purdue University, 525 Northwestern
Avenue, West Lafayette, Indiana, USA
21
Department of Physics, University of California, Berkeley,
California, USA
22
Department of Physics, University of California, One Shields
Avenue, Davis, California, USA
23
Department of Physics, University of California, Santa Barbara,
California, USA
24
Department of Physics, University of Illinois at Urbana-Champaign,
1110 West Green Street, Urbana, Illinois, USA
25
Dipartimento di Fisica G. Galilei, Università degli Studi di Padova,
via Marzolo 8, 35131 Padova, Italy
26
Dipartimento di Fisica, Università La Sapienza, P. le A. Moro 2,
Roma, Italy
27
Dipartimento di Fisica, Università degli Studi di Milano, via Celoria,
16, Milano, Italy
28
Dipartimento di Fisica, Università degli Studi di Trieste, via A.
Valerio 2, Trieste, Italy
29
Dipartimento di Fisica, Università di Ferrara, Via Saragat 1, 44122
Ferrara, Italy
30
Dipartimento di Fisica, Università di Roma Tor Vergata, via della
Ricerca Scientifica, 1, Roma, Italy
31
Discovery Center, Niels Bohr Institute, Blegdamsvej 17,
Copenhagen, Denmark
32
Dpto. Astrofísica, Universidad de La Laguna (ULL), 38206
La Laguna, Tenerife, Spain
33
European Southern Observatory, ESO Vitacura, Alonso de Cordova
3107, Vitacura, Casilla 19001, Santiago, Chile
34
European Space Agency, ESAC, Planck Science Oce, Camino
bajo del Castillo, s/n, Urbanización Villafranca del Castillo,
Villanueva de la Cañada, Madrid, Spain
35
European Space Agency, ESTEC, Keplerlaan 1, 2201 AZ
Noordwijk, The Netherlands
36
Helsinki Institute of Physics, Gustaf Hällströmin katu 2, University
of Helsinki, Helsinki, Finland
37
INAF Osservatorio Astrofisico di Catania, via S. Sofia 78, Catania,
Italy
38
INAF Osservatorio Astronomico di Padova, vicolo
dell’Osservatorio 5, Padova, Italy
39
INAF Osservatorio Astronomico di Roma, via di Frascati 33,
Monte Porzio Catone, Italy
40
INAF Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11,
Trieste, Italy
41
INAF/IASF Bologna, via Gobetti 101, Bologna, Italy
42
INAF/IASF Milano, via E. Bassini 15, Milano, Italy
43
INRIA, Laboratoire de Recherche en Informatique, Université
Paris-Sud 11, Bâtiment 490, 91405 Orsay Cedex, France
44
IPAG: Institut de Planétologie et d’Astrophysique de Grenoble,
Université Joseph Fourier, Grenoble 1/CNRS-INSU, UMR 5274,
38041 Grenoble, France
45
Imperial College London, Astrophysics group, Blackett Laboratory,
Prince Consort Road, London, SW7 2AZ, UK
46
Infrared Processing and Analysis Center, California Institute of
Technology, Pasadena, CA 91125, USA
47
Institut Néel, CNRS, Université Joseph Fourier Grenoble I, 25 rue
des Martyrs, Grenoble, France
48
Institut d’Astrophysique Spatiale, CNRS (UMR8617) Université
Paris-Sud 11, Bâtiment 121, Orsay, France
49
Institut d’Astrophysique de Paris, CNRS UMR7095, Université
Pierre & Marie Curie, 98bis boulevard Arago, Paris, France
50
Institute of Astronomy and Astrophysics, Academia Sinica, Taipei,
Taiwan
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Planck Collaboration: Planck early results. XVII.
51
Institute of Astronomy, University of Cambridge, Madingley Road,
Cambridge CB3 0HA, UK
52
Institute of Theoretical Astrophysics, University of Oslo, Blindern,
Oslo, Norway
53
Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, La Laguna,
Tenerife, Spain
54
Instituto de Física de Cantabria (CSIC-Universidad de Cantabria),
Avda. de los Castros s/n, Santander, Spain
55
Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Drive, Pasadena, California, USA
56
Jodrell Bank Centre for Astrophysics, Alan Turing Building, School
of Physics and Astronomy, The University of Manchester, Oxford
Road, Manchester, M13 9PL, UK
57
Kavli Institute for Cosmology Cambridge, Madingley Road,
Cambridge, CB3 0HA, UK
58
LERMA, CNRS, Observatoire de Paris, 61 avenue de
l’Observatoire, Paris, France
59
Laboratoire AIM, IRFU/Service d’Astrophysique CEA/DSM
CNRS – Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-
sur-Yvette Cedex, France
60
Laboratoire Traitement et Communication de l’Information, CNRS
(UMR 5141) and Télécom ParisTech, 46 rue Barrault, 75634 Paris
Cedex 13, France
61
Laboratoire de Physique Subatomique et de Cosmologie,
CNRS/IN2P3, Université Joseph Fourier Grenoble I, Institut
National Polytechnique de Grenoble, 53 rue des Martyrs, 38026
Grenoble Cedex, France
62
Laboratoire de l’Accélérateur Linéaire, Université Paris-Sud 11,
CNRS/IN2P3, Orsay, France
63
Lawrence Berkeley National Laboratory, Berkeley, California, USA
64
Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1,
85741 Garching, Germany
65
MilliLab, VTT Technical Research Centre of Finland, Tietotie 3,
Espoo, Finland
66
National University of Ireland, Department of Experimental
Physics, Maynooth, Co. Kildare, Ireland
67
Niels Bohr Institute, Blegdamsvej 17, Copenhagen, Denmark
68
Observational Cosmology, Mail Stop 367-17, California Institute of
Technology, Pasadena, CA, 91125, USA
69
Observatoire Astronomique de Strasbourg, CNRS, UMR 7550,
67000 Strasbourg, France
70
Optical Science Laboratory, University College London, Gower
Street, London, UK
71
SISSA, Astrophysics Sector, via Bonomea 265, 34136, Trieste, Italy
72
SUPA, Institute for Astronomy, University of Edinburgh, Royal
Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
73
School of Physics and Astronomy, Cardi University, Queens
Buildings, The Parade, Cardi, CF24 3AA, UK
74
Space Sciences Laboratory, University of California, Berkeley,
California, USA
75
Spitzer Science Center, 1200 E. California Blvd., Pasadena,
California, USA
76
Stanford University, Dept of Physics, Varian Physics Bldg, 382 via
Pueblo Mall, Stanford, California, USA
77
Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4,
France
78
Universities Space Research Association, Stratospheric Observatory
for Infrared Astronomy, MS 211-3, Moett Field, CA 94035, USA
79
University of Granada, Departamento de Física Teórica y del
Cosmos, Facultad de Ciencias, Granada, Spain
80
University of Miami, Knight Physics Building, 1320 Campo Sano
Dr., Coral Gables, Florida, USA
81
Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478
Warszawa, Poland
A17, page 17 of 17
Page 17
  • [Show abstract] [Hide abstract] ABSTRACT: We present the Sunyaev-Zeldovich (SZ) signal-to-richness scaling relation (Y500 - N200) for the MaxBCG cluster catalogue. Employing a multi-frequency matched filter on the Planck sky maps, we measure the SZ signal for each cluster by adapting the filter according to weak-lensing calibrated mass-richness relations (N200 - M500). We bin our individual measurements and detect the SZ signal down to the lowest richness systems (N200 = 10) with high significance, achieving a detection of the SZ signal in systems with mass as low as M500 &ap; 5 × 1013 M&sun;. The observed Y500 - N200 relation is well modeled by a power law over the full richness range. It has a lower normalisation at given N200 than predicted based on X-ray models and published mass-richness relations. An X-ray subsample, however, does conform to the predicted scaling, and model predictions do reproduce the relation between our measured bin-average SZ signal and measured bin-average X-ray luminosities. At fixed richness, we find an intrinsic dispersion in the Y500 - N200 relation of 60% rising to of order 100% at low richness. Thanks to its all-sky coverage, Planck provides observations for more than 13000 MaxBCG clusters and an unprecedented SZ/optical data set, extending the list of known cluster scaling laws to include SZ-optical properties. The data set offers essential clues for models of galaxy formation. Moreover, the lower normalisation of the SZ-mass relation implied by the observed SZ-richness scaling has important consequences for cluster physics and cosmological studies with SZ clusters. Corresponding author: J. G. Bartlett, e-mail: bartlett@apc.univ-paris7.fr
    No preview · Article · Jan 2011
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    [Show abstract] [Hide abstract] ABSTRACT: The European Space Agency's Planck satellite was launched on 14 May 2009, and has been surveying the sky stably and continuously since 13 August 2009. Its performance is well in line with expectations, and it will continue to gather scientific data until the end of its cryogenic lifetime. We give an overview of the history of Planck in its first year of operations, and describe some of the key performance aspects of the satellite. This paper is part of a package submitted in conjunction with Planck's Early Release Compact Source Catalogue, the first data product based on Planck to be released publicly. The package describes the scientific performance of the Planck payload, and presents results on a variety of astrophysical topics related to the sources included in the Catalogue, as well as selected topics on diffuse emission.
    Full-text · Article · Dec 2011 · Astronomy and Astrophysics
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    [Show abstract] [Hide abstract] ABSTRACT: Planck's all sky surveys at 30-857 GHz provide an unprecedented opportunity to follow the radio spectra of a large sample of extragalactic sources to frequencies 2-20 times higher than allowed by past, large area, ground-based surveys. We combine the results of the Planck Early Release Compact Source Catalog (ERCSC) with quasi-simultaneous ground-based observations, as well as archival data, at frequencies below or overlapping Planck frequency bands, to validate the astrometry and photometry of the ERCSC radio sources and study the spectral features shown in this new frequency window opened by Planck. The ERCSC source positions and flux density scales are found to be consistent with the ground-based observations. We present and discuss the spectral energy distributions (SEDs) of a sample of "extreme" radio sources to illustrate the richness of the ERCSC for the study of extragalactic radio sources. Variability is found to play a role in the unusual spectral features of some of these sources.
    Full-text · Article · Jan 2011 · Astronomy and Astrophysics
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