The dwarf low surface brightness population in different environments of the Local Universe

Abstract

The nature of the dwarf galaxy population as a function of location in the cluster and within different environments is investigated. We have previously described the results of a search for low surface brightness objects in data drawn from an East-West strip of the Virgo cluster (Sabatini et al., 2003) and have compared this to a large area strip outside of the cluster (Roberts et al., 2004). In this talk I compare the East-West data (sampling sub-cluster A and outward) to new data along a North-South cluster strip that samples a different region (part of sub-cluster A, and the N,M clouds) and with data obtained for the Ursa Major cluster and fields around the spiral galaxy M101. The sample of dwarf galaxies in different environments is obtained from uniform datasets that reach central surface brightness values of ~26 B mag/arcsec^2 and an apparent B magnitude of 21 (M_B=-10 for a Virgo Cluster distance of 16 Mpc). We discuss and interpret our results on the properties and distribution of dwarf low surface brightness galaxies in the context of variuos physical processes that are thought to act on galaxies as they form and evolve. Comment: 10 pages, 3 figures, to appear in "Dark Galaxies and Lost Baryons", IAU244 conference proceedings

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arXiv:0707.4079v1 [astro-ph] 27 Jul 2007
Dark Galaxies and Lost Baryons IAU Symposium
Proceedings IAU Symposium No. 244, 2007
A.C. Editor, B.D. Editor & C.E. Editor, eds.
c ? 2007 International Astronomical Union
DOI: 00.0000/X000000000000000X
The dwarf low surface brightness population
in different environments of the Local
Universe
S. Sabatini1†, J.I. Davies2, S. Roberts2, R. Scaramella1
1INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone,
Roma,Italy
email: sabatini@oa-roma.inaf.it; kosmobob@oa-roma.inaf.it
2School of Physics and Astronomy, Cardiff University, 5 The Parade, Cardiff, CF24 3AA, UK
email: Jonathan.Davies@astro.cf.ac.uk; Sarah.Roberts@faulkes-telescope.com
Abstract. The nature of the dwarf galaxy population as a function of location in the cluster
and within different environments is investigated. We have previously described the results
of a search for low surface brightness objects in data drawn from an East-West strip of the
Virgo cluster (Sabatini et al. (2003)) and have compared this to a large area strip outside of
the cluster (Roberts et al. (2004)). In this talk I compare the East-West data (sampling sub-
cluster A and outward) to new data along a North-South cluster strip that samples a different
region (part of sub-cluster A, and the N,M clouds) and with data obtained for the Ursa Major
cluster and fields around the spiral galaxy M101. The sample of dwarf galaxies in different
environments is obtained from uniform datasets that reach central surface brightness values of
26 B mag/arcsec2and an apparent B magnitude of 21 (MB=-10 for a Virgo Cluster distance of
16 Mpc). We discuss and interpret our results on the properties and distribution of dwarf low
surface brightness galaxies in the context of variuos physical processes that are thought to act
on galaxies as they form and evolve.
Keywords. dwarf galaxies, low suface brightness galaxies, clusters
1. Introduction
Although dwarf galaxies are the most abundant galaxies of the Local Universe, their
number counts in the field (measured by the faint end part of the Luminosity Function,
LF, Blanton et al. (2001), Norberg et al. (2002)) are well below those predicted by the
Mass Function (MF) of ΛCDM hierarchicalgalaxy formation models (Kauffman et al. (1993),Klypin et al.˙(1999),Moore et al. (19
- this is known as the ’missing satellites’ problem. However, recent discoveries of ultra-
faint dwarfs of the Local Group made with SDSS data (Irwin et al. (2007) and references
therein) have doubled the number of known dwarf satellites of the MW, reducing the dis-
crepancy between observations and predictions, even if a factor of 4 is still missing in
order to solve the missing satellites problem (Simon & Geha (2007)). These results prove
that, even in the Local Universe, the census of the dwarf galaxy population is still in-
complete and the slope (α) of the faint end part of the LF is therefore still subject
to revision: selection effects due to the intrinsic faint total magnitude and low surface
brightness of dwarf galaxies, have made them particularly elusive in surveys that were
not purposely designed to select them. On the observational side, the issue is further
clouded by non-uniform datasets: different surveys reach different magnitude and surface
brightness limits and detection methods and selection criteria vary in the way they iden-
tify dwarf galaxies. This can lead to studies of the LF in which different authors report
† Present address: INAF-OAR, via Frascati 33, Monte Porzio Catone, Roma,Italy
1

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2 Sabatini S. et al
extremely different results even for the same region of sky: see for example the Fornax
cluster where an extremely steep LF (α ∼ −2.0) is found by Kambas et al. (2000), while
a flat one (α ∼ −1.1) is reported by Mieske et al (this conf). In this talk we report on our
ongoing work to try and quantify the numbers of Low Surface Brightness (LSB) dwarf
galaxies as a function of the environment in the Local Universe (within 20 Mpc) and to
assess and compare their properties. We reduced as much as possible the observational
biases with the uniformity of our data and by using exactly the same detection algorithm
and selection criteria for identifying dwarf galaxies in all the different environments that
we analysed. To reach this goal we put very strong requirements on the way we carried
out our work:
• the dataset and the data analysis need to be homogeneuos. As already mentioned,
data in the literature are highly inhomogeneous: often the detection methods, candidate
selection and the resulting completeness of the compared samples are different. In order
to avoid these problems, our data are obtained in exaclty the same way for the different
environments (same observing instrumentation and setup, same data analysis, detection
and selection method);
• data need to go deep both in magnitude and surface brightness. Flux limited sur-
veys sample only our neighborhoods for the dwarfs contribution and easily miss further
away denser environments like the Virgo Cluster; surface brightness limited surveys miss
the contribution by LSB galaxies (e.g. the SDSS catalogue misses 50% of galaxies from
µ50,r ≃ 23.5 mag/arcsec2, Blanton et al. (2005)). our method allows the detection of
galaxies with intrinsic properties of 23?µo?26 B mag/arcsec2and -10?MB?-14, where
µoand MBare the B band exponential central surface brightness and absolute magnitude
respectively ;
• data need to cover different environments: inhabiting the shallowest potential wells
and thus being the most sensitive to internal and external physical processes that con-
trol galaxy formation, dwarfs are the best test-beds to investigate how the environment
affects galaxy evolution and to estimate the contribution of different physiscal processes
(photo-ionization, tidal interactions, SN winds, ram pressure stripping, harassment). We
obtained data from different environments of the Local Unieverse (the field, M101, the
Ursa Major Cluster, the Virgo Cluster). Given the magnitude limits of our survey (see
previous point), the galaxies that we analyse are fainter than those typically detected in
the comprehensive survey of the Virgo cluster carried out by Binggeli et al. (1984) and
extends down towards the properties of the ’classical’ dSph galaxies of the Local Group
(excluding the recently discovered ultra-faint dwarfs).
2. Data and data analysis
The optical data were obtained using the Wide Field Camera (WFC) on the Isaac
Newton Telescope, La Palma, Canary Islands as part of the Wide Field Survey (WFS),
a multicolour data survey covering over 200 deg2of sky. The WFC is a mosaic of four
thinned EEV 4K ×2K CCDs with a pixel size of 0.33 arc sec and total sky coverage of
0.29 deg2. Images on CCD 3 were not used due to its vignetting, this reduced our total
field of view to 0.21 deg2. The Virgo data, taken during observing runs in 1999 and 2002,
consists of two perpendicular strips extending from the centre of the Virgo cluster (de-
fined as M87) outwards by 7 deg and covering a total area of about 28 deg2. These 2 data
strips sample different regions of the Virgo cluster, with one lying roughly perpendicular
to the super-galactic plane (E-W) and covering sub-cluster A and outward, and the other
(N-S) almost parallel to it, sampling clouds N,M. The Ursa Major data was obtained in
2002: they consist of 8 fields going along a N-S, E-W cross in the cluster and totalling

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The dwarf LSB population in different environments
3
Figure 1. Position of Virgo Cluster data strips, Ursa Major and MGS data viewd from the
North galactic pole. Also plotted are all galaxies listed in NED with v¡ 4500 km/s.
1.68 deg2. The M101 data was obtained in 2004 and consist of 72 frames covering an
area of just over 15 deg2, in a box around M101. The field region is taken from the
Millennium Galaxy Strip (MGS) and consists of an area of 30 deg2along the celestial
equator (Liske et al. (2003)), passing through filaments and voids and crossing the Virgo
Southern extension at its mid point. The different environments of this survey are shown
in black in Fig 1. All B band exposures were for 750s, whilst the exposure time for the i
band images was 1000s. All data reduction was carried out by the Cambridge Astronom-
ical Survey Unit pipeline (http://www.ast.cam.ac.uk∼wfcsur/index.php). This included
de-biasing, bad pixel replacement, non-linearity correction, flat-fielding, de-fringing, gain
correction and photometric calibration.
2.1. Data analysis
The detection of LSB dwarf galaxies is subject to strong selection effects, due to their in-
trinsic low surface brightness that is, by definition, below the sky level (i.e. they consist of
low signal-to-noise objects on a digital image). For this reason, standard algorithm based
on a connected pixels above a threshold concept (e.g. SExtractor,Bertin& Arnouts (1996))
are not ideal for their detection. The galaxy detection in this work was therefore per-
formed using a purposely built, fully automated, detection algorithm based on the convo-
lution of the original images in the Fourier Space with a set of multi-scale filters of expo-
nential profile. The method and its potentialities are discussed in Scaramella et al (this
conf). The detection algorithm produces a list of candidate LSB galaxies that need to be
selected against the background galaxies. In order to estimate the best selection criteria
(i.e. the one that optimize cluster members detection and minimize background contam-
ination of the sample), we ran numerical simulations of a cone of Universe uniformly
populated by galaxies and of a cluster at the position of the Virgo Cluster. The details
of this detection algorithm and the selection criteria have been extensively discussed in
Sabatini et al. (2003) and Roberts et al. (2004). The result of this process is that we
selected galaxies that, at the distance of the Virgo cluster (16 Mpc, Jerjen et al. (2004)),
satisfy the following criteria: a central surface brightness of 23 ? µo? 26 B mag/arcsec2,
and an exponential scale size (h) in the range: 4 ? h ? 9 arcsec. † Our final sample con-
† This actually corresponds to galaxies with intrinsic scale sizes of 3 arcsec as it can be shown
that convolution of the typical seeing function of our data with a 3 arcsec scale length resulted
in a measured scale length of about 4 arcsec (Sabatini et al. (2003)).

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4 Sabatini S. et al
sist of 596 dwarf LSB galaxies (roughly 2/3 out of which were previously uncatalogued)
found over an area of 70 deg2in environments of increasing density.
3. Results
3.1. Number density and Dwarf-to-Giant Ratio
In the EW strip of the Virgo Cluster, we found on average about 18 galaxies per deg2
but ranging from about 40 per deg2at the cluster centre to approximateley 4 per deg2
at the cluster edge (Sabatini et al. (2003)). This decline in numbers with cluster radius
is a good indicator that we have selected a predominately cluster population rather than
background galaxies. In a related paper (Roberts et al. (2004)) we compared the Virgo
cluster result with field data from the MGS. Over this region the average number of
galaxies detected were only 4 per deg2, consistent with the numbers we detected at the
edge of the Virgo cluster, again indicating that our cluster data consists predominately
of cluster galaxies. Comparing the numbers of dwarf galaxies to that of the giant galaxy
population over the same region of sky gave a Dwarf-to-Giant Ratio (DGR) of about 14
(DGR=Num(MB? −19)/Num(−14 ? MB? −10)) for the Virgo E-W strip and 22 for
the N-S strip. The DGR of the MGS strip was at most 6, consistent with the numbers of
dwarf and giant galaxies found in the Local Group (Mateo, 1998) if we imagine to move
the Local Group to the Virgo Cluster distance and apply the above discussed selection
criteria. The Virgo cluster clearly supports many more dwarf galaxies per giant than do
galaxies in less rich environments. Values of DGR are shown in table 1 (column 3) for
the environments where it was possible to calculate them. Average number density of
dwarf galaxies is also shown (column 2) and can be compared in different environments.
The MGS survey clearly shows that there is no large field population of LSB low lu-
minosity galaxies that would steepen the luminosity functions of Blanton et al. (2001)
and Norberg et al. (2002) at fainter magnitudes, to account for the discrepancy with
theoretical predictions. In the special environment of the Virgo cluster, consistently with
previous shallower surveys, we found relatively large numbers of dwarf galaxies, that
provide a little closer match to the theory (Moore et al., 1999).
3.2. Properties of the dwarf galaxies in our sample
In Sabatini et al. (2005) we followed up the galaxies of our catalogue and analysed the B-
I colours and gas content (through observations of our candidates carried out at Arecibo,
with a mass limit of ∼ 1.5 × 107M⊙at the distance of the Virgo Cluster and a column
density limit of ∼ 5 × 1018atoms/cm2). Table 1 shows average values of the B-I colours
(column 4). Interestingly colours of the two different regions analysed in Virgo are similar,
but the NS strip shows a higher dispersion: this could be due to the strip being populated
by galaxies from further away infalling clouds (implying fainter magnitudes and therefore
larger errors in the colour)†. On average dwarfs in Virgo are bluer than typical giant
ellipticals in the cluster ((B-I)= 2.1 ± 0.1, Michard (2000)) and wihin the errors have
similar (B-I) colour to typical globular clusters (GC) around M87 ((B-I)= 1.65 ± 0.25,
Conture et al. (1990)). It’s also worth noticing that (B-I) colours of galaxies in the 2
cluster strips does not depend upon distance from the cluster centre.
HI detection rate for a subsample of galaxies in our surveys of the Virgo Cluster and
† This picture is also supported by the comparison of the total magnitude distributions of the
two sample and the surface brightness (SB) distributions: galaxies in the NS strip have fainter
total apparent magnitude and than galaxies in the EW strip similar SB, suggesting that they
are at a higher distance (Roberts et al. (2007))

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The dwarf LSB population in different environments
5
10987
0
0.5
1
1.5
Zwaan et al.
Virgo
Figure 2.
Virgo HIPASS data, the dashed line is the derived fit to the field galaxy mass function from
Zwaan et al. (2005), arbitrarily normalised to the peak on the Virgo data at MHI = 8.5. On the
right: Virgo faint end part of the LF. Open circles are counts for the raw data and filled points
are those corrected for detection efficiency and background contamination. Squares are points
from the VCC LF with a slope of -1.3.
On the left: HI mass function. The solid line shows the mass function from the
Table 1. Comparison of dwarf galaxy’s properties in different environments
Region < Density > DGR < B − I > HI
(N/deg2)
%dE
%dIrr %Other
(det. rate)
MGS
M101*
M101
UMa
Virgo EW 18
Virgo NS
4
3.6
0.1
4
< 6
−
1
−
14
22
1.2±0.3
−
−
−
1.8±0.4
1.8±0.7
23%
−
−
−
5%
−
24 %
40 %
100 %
33 %
62 %
54 %
33 %
38 %
−
17 %
26 %
22 %
43 %
22 %
−
50 %
12 %
24% 22
the field that were followed up are shown in table 1 (column 5): galaxies in the cluster
tend to be gas poor compared with those in the field. In aggrement with this picture,
although still under intense discussion at the low masses (see this conf), we find that the
HI Mass Function (HIMF) in the cluster is shallower than in the field (see Fig 2). This
has an opposite trend to the LF, that we find steep in Virgo and flat in the field (Fig
2), suggesting that the cluster plays an important role in promoting star formation and
therefore in converting atomic gas into stars.
To summarise: in the comparison of Virgo cluster dwarf galaxies to MGS field dwarf
galaxies, consistent with previous observations, we find that the cluster population is
redder and gas poor.
Concerning the morphology of galaxies in the different environments analysed, again
consistent with previous results in the literature, we find that dE are more abundant in
denser environments and tend to be a small percentage in the field (table 1, columns 6
to 8).
4. Discussion
In what follows we will discuss the properties of these extremely faint low surface
brightness galaxies in relation to the environment in which they reside and evaluate
which of the analysed physical mechanisms can be the main driver for their evolution.
The main issue that we are trying to assess is wheter these galaxies are fossils of the