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A. Del Moro,
D. M. Alexander,
J. R. Mullaney,
E. Daddi,
M. Pannella,
F. E. Bauer, A. Pope,
M. Dickinson,
D. Elbaz,
P. D. Barthel, [......],
R. C. Hickox,
H. S. Hwang,
R. J. Ivison,
S. Juneau,
E. Le Floc'h,
B. Luo,
G. E. Morrison,
E. Rovilos,
M. T. Sargent,
Y. Q. Xue
[show abstract]
[hide abstract]
ABSTRACT: We present here a new spectral energy distribution (SED) fitting approach
that we adopt to select radio-excess sources amongst distant star-forming
galaxies in the GOODS-Herschel (North) field and to reveal the presence of
hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies
with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest
Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to
date, we have robustly identified a sample of 51 radio-excess AGN (~1300
deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower
far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming
galaxies (q~2.2). We find that ~45% of these radio-excess sources have a
dominant AGN component in the mid-IR band, while for the remainders the excess
radio emission is the only indicator of AGN activity. The fraction of
radio-excess AGN increases with X-ray luminosity reaching ~60% at
Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN
population. However, almost half (24/51) of these radio-excess AGN are not
detected in the deep Chandra X-ray data, suggesting that some of these sources
might be heavily obscured. We also find that the specific star formation rates
(sSFRs) of the radio-excess AGN are on average lower that those observed for
X-ray selected AGN hosts, indicating that our sources are forming stars more
slowly than typical AGN hosts, and possibly their star formation is
progressively quenching.
10/2012;
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C. D. Wilson,
B. E. Warren,
F. P. Israel,
S. Serjeant,
D. Attewell,
G. J. Bendo,
H. M. Butner,
P. Chanial,
D. L. Clements,
J. Golding, [......],
E. Rosolowsky,
E. R. Seaquist,
K. Spekkens,
N. Tanvir,
J. M. van der Hulst,
P. van der Werf,
C. Vlahakis,
T. M. Webb,
B. Weferling,
G. J. White
[show abstract]
[hide abstract]
ABSTRACT: The James Clerk Maxwell Telescope Nearby Galaxies Legacy Survey (NGLS)
comprises an HI-selected sample of 155 galaxies spanning all morphological
types with distances less than 25 Mpc. We describe the scientific goals of the
survey, the sample selection, and the observing strategy. We also present an
atlas and analysis of the CO J=3-2 maps for the 47 galaxies in the NGLS which
are also part of the Spitzer Infrared Nearby Galaxies Survey. We find a wide
range of molecular gas mass fractions in the galaxies in this sample and
explore the correlation of the far-infrared luminosity, which traces star
formation, with the CO luminosity, which traces the molecular gas mass. By
comparing the NGLS data with merging galaxies at low and high redshift which
have also been observed in the CO J=3-2 line, we show that the correlation of
far-infrared and CO luminosity shows a significant trend with luminosity. This
trend is consistent with a molecular gas depletion time which is more than an
order of magnitude faster in the merger galaxies than in nearby normal
galaxies. We also find a strong correlation of the L(FIR)/L(CO3-2) ratio with
the atomic to molecular gas mass ratio. This correlation suggests that some of
the far-infrared emission originates from dust associated with atomic gas and
that its contribution is particularly important in galaxies where most of the
gas is in the atomic phase.
06/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: To optimally identify candidates of the Compton-thick (CT) active galactic
nuclei (AGN) that contribute to the unresolved X-ray background in infrared
surveys, a tracer of column density is desirable in addition to an AGN
indicator. In a recent study, we aimed to test whether the 9.7 micron silicate
absorption feature can be used for this purpose when seen at high optical
depths. We found that the extreme criterion of optical thickness at 9.7 micron
is efficient in identifying CT objects among local AGN. Having identified six
of the nine CT AGN in the 12 micron sample with Spitzer and X-ray spectra, we
expanded this analysis at intermediate/high z, using all GOODS and FLS sources
with Spitzer and X-ray observations. We found 12 sources with tau_9.7>1 that
host an AGN between 0.8<z<2.7. Four of them are likely to be CT according to
their low X-ray to 6 micron luminosity ratio. Surveys with complete coverage in
both mid-infrared spectra and X-ray data can provide large populations of such
sources, as at least 5-9% of all infrared bright galaxies in the GOODS and FLS
samples are tau_9.7>1 AGN.
10/2011;
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Georgios E. Magdis,
D. Elbaz,
M. Dickinson,
H. S. Hwang,
V. Charmandaris,
L. Armus,
E. Daddi,
E. Le Floc'h,
H. Aussel,
H. Dannerbauer, [......], A. Pope,
M. Pannella,
D. Burgarella,
M. Bethermin,
K. Dasyra,
J. Kartaltepe,
R. Leiton,
B. Magnelli,
P. Popesso,
I. Valtchanov
[show abstract]
[hide abstract]
ABSTRACT: Using extremely deep PACS 100- and 160um Herschel data from the
GOODS-Herschel program, we identify 21 infrared bright galaxies previously
missed in the deepest 24um surveys performed by MIPS. These MIPS dropouts are
predominantly found in two redshift bins, centred at z ~0.4 and ~1.3. Their
S_100/S_24 flux density ratios are similar to those of local LIRGs and ULIRGs,
whose silicate absorption features at 18um (at z ~ 0.4) and 9.7um (at z ~ 1.3)
are shifted into the 24um MIPS band at these redshifts. The high-z sub-sample
consists of 11 infrared luminous sources, accounting for ~2% of the whole
GOODS-Herschel sample and putting strong upper limits on the fraction of
LIRGs/ULIRGs at 1.0<z<1.7 that are missed by the 24um surveys. We find that a
S_100/S_24 > 43 colour cut selects galaxies with a redshift distribution
similar to that of the MIPS dropouts and when combined with a second colour
cut, S_16/S_8 > 4, isolates sources at 1.0 < z < 1.7. We show that these
sources have elevated specific star formation rates (sSFR) compared to main
sequence galaxies at these redshifts and are likely to be compact starbursts
with moderate/strong 9.7um silicate absorption features in their mid-IR
spectra. Herschel data reveal that their infrared luminosities extrapolated
from the 24um flux density are underestimated, on average, by a factor of ~3.
These silicate break galaxies account for 16% (8%) of the ULIRG (LIRG)
population in the GOODS fields, indicating a lower limit in their space density
of 2.0 \times 10^(-5) Mpc^(-3). Finally, we provide estimates of the fraction
of z < 2 MIPS dropout sources as a function of the 24-, 100-, 160-, 250- and
350um sensitivity limits, and conclude that previous predictions of a
population of silicate break galaxies missed by the major 24um extragalactic
surveys have been overestimated.
08/2011;
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D. Elbaz,
M. Dickinson,
H. S. Hwang,
T. Diaz-Santos,
G. Magdis,
B. Magnelli,
D. Le Borgne,
F. Galliano,
M. Pannella,
P. Chanial, [......],
D. Frayer,
M. Giavalisco,
M. Huynh,
A. M. Koekemoer,
C. Papovich,
N. Reddy,
C. Surace,
H. Teplitz,
M. S. Yun,
G Wilson
[show abstract]
[hide abstract]
ABSTRACT: We present the deepest far-IR observations obtained with Herschel and examine
the 3-500um SEDs of galaxies at 0<z<2.5, supplemented by a local reference
sample from IRAS, ISO, Spitzer and AKARI data. We find that the ratio of total
IR luminosity to rest-frame 8um luminosity, IR8 (=Lir/L8), follows a Gaussian
distribution centered on IR8=4 and defines an IR main sequence (MS). A minority
population (<20 %) of outliers producing a tail skewed toward higher values of
IR8 consist of starbursts (SB) with compact projected star formation densities.
IR8 can be used to separate galaxies with normal and extended modes of star
formation from compact SBs with high-IR8, high projected IR surface brightness
(>3x10^10 Lsun kpc^-2) and a high specific SFR (i.e., SBs). The rest-frame,
UV-2700A size of these distant SBs is typically half that of MS galaxies,
supporting the correlation between star formation density and SB activity that
is measured for the local sample. Locally, (U)LIRGs are systematically in the
SB mode, whereas most distant (U)LIRGs form stars in the "normal" MS mode. This
confusion between two modes of star formation is the cause of the so-called
"mid-IR excess" population of galaxies found at z>1.5 by previous studies. MS
galaxies have strong PAH emission line features, a broad far-IR bump resulting
from a combination of dust temperatures (Tdust~15-50 K), and an effective
Tdust~31 K, as derived from the peak wavelength of their IR SED. Galaxies in
the SB regime instead exhibit weak PAH EW and a sharper far-IR bump with an
effective Tdust~40 K. Finally, we present evidence that the mid-to-far IR
emission of X-ray AGNs is predominantly produced by star formation and that
candidate dusty AGNs with a power-law emission in the mid-IR systematically
occur in compact, dusty SBs. After correcting for the effect of SBs on IR8, we
identify new candidates for extremely obscured AGNs.
05/2011;
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I. Georgantopoulos,
K. M. Dasyra,
E. Rovilos, A. Pope,
Y Wu,
M. Dickinson,
A. Comastri,
R. Gilli,
D. Elbaz,
L. Armus,
A. Akylas
[show abstract]
[hide abstract]
ABSTRACT: Spitzer/IRS has revealed many sources with very deep Si features at 9.7micron
(tau>1). We set out to investigate whether a strong Si absorption feature is a
good indicator for the presence of a heavily obscured AGN. We compile X-ray
spectroscopic observations available in the literature on the
optically-thick,tau(9.7)>1 sources from the IRAS Seyfert sample. We find that
the majority of the high-tau optically confirmed Seyferts (6/9) in this sample
are probably CT. Thus we provide direct evidence for a connection between
mid-IR optically-thick galaxies and CT AGN, with the success rate being close
to 70% in the local Universe. This is at least comparable, if not better, than
other rates obtained with photometric information in the mid to far-IR, or even
mid-IR to Xray. However, this technique cannot provide complete CT AGN
samples,ie there are many CT AGN which do not show significant Si absorption,
with the most notable example being N1068. Having assessed the validity of the
high 9.7micron technique locally, we attempt to construct a sample of candidate
CT AGN at higher redshifts. We compile a sample of 7 high-tau sources in the
GOODS and 5 in the Spitzer FLS. All these have been selected to have no PAH
features EW(6.2)<0.3 in order to maximize the probability that they are AGN. 6
out of 7 sources in the GOODS have been detected in X-rays, while for the five
FLS sources only X-ray flux upper limits are available. The high X-ray
luminosities of the detected GOODS sources corroborates that these are AGN. For
FLS, ancillary optical spectroscopy reveals hidden nuclei in two more sources.
SED fitting can support the presence of an AGN in the vast majority of sources.
We cannot derive useful X-ray spectroscopy constraints on whether these are CT.
However, the low LX/L6 ratios, suggest that at least 4 out of the 6 detected
sources in GOODS may be associated with CT AGN.
05/2011;
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[show abstract]
[hide abstract]
ABSTRACT: (Abridged) We report on an accounting of the star formation and accretion
driven energetics of 24um detected sources in GOODS North. For sources having
infrared (IR; 8-1000um) luminosities >3x10^12 L_sun when derived by fitting
local SEDs to 24um photometry alone, we find these IR luminosity estimates to
be a factor of ~4 times larger than those estimated when the SED fitting
includes additional 16 and 70um data (and in some cases mid-infrared
spectroscopy and 850um data). This discrepancy arises from the fact that high
luminosity sources at z>>0 appear to have far- to mid-infrared ratios, as well
as aromatic feature equivalent widths, typical of lower luminosity galaxies in
the local Universe. Using our improved estimates for IR luminosity and AGN
contributions, we investigate the evolution of the IR luminosity density versus
redshift arising from star formation and AGN processes alone. We find that,
within the uncertainties, the total star formation driven IR luminosity density
is constant between 1.15 < z < 2.35, although our results suggest a slightly
larger value at z>2. AGN appear to account for <18% of the total IR luminosity
density integrated between 0< z < 2.35, contributing <25% at each epoch. LIRG
appear to dominate the star formation rate (SFR) density along with normal
star-forming galaxies (L_IR < 10^11 L_sun) between 0.6 < z < 1.15. Once beyond
z >2, the contribution from ultraluminous infrared galaxies ULIRGs becomes
comparable with that of LIRGs. Using our improved IR luminosity estimates, we
find existing calibrations for UV extinction corrections based on measurements
of the UV spectral slope typically overcorrect UV luminosities by a factor of
~2, on average, for our sample of 24um-selected sources; accordingly we have
derived a new UV extinction correction more appropriate for our sample.
02/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We present deep {\it Spitzer} mid-infrared spectroscopy, along with 16, 24, 70, and 850\,$\micron$\ photometry, for 22 galaxies located in the Great Observatories Origins Deep Survey-North (GOODS-N) field. The sample spans a redshift range of $0.6\la z \la 2.6$, 24~$\mu$m flux densities between $\sim$0.2$-$1.2 mJy, and consists of submillimeter galaxies (SMGs), X-ray or optically selected active galactic nuclei (AGN), and optically faint ($z_{AB}>25$\,mag) sources. We find that infrared (IR; $8-1000~\micron$) luminosities derived by fitting local spectral energy distributions (SEDs) with 24~$\micron$ photometry alone are well matched to those when additional mid-infrared spectroscopic and longer wavelength photometric data is used for galaxies having $z\la1.4$ and 24~$\micron$-derived IR luminosities typically $\la 3\times 10^{12}~L_{\sun}$. However, for galaxies in the redshift range between $1.4\la z \la 2.6$, typically having 24~$\micron$-derived IR luminosities $\ga 3\times 10^{12}~L_{\sun}$, IR luminosities are overestimated by an average factor of $\sim$5 when SED fitting with 24~$\micron$ photometry alone. This result arises partly due to the fact that high redshift galaxies exhibit aromatic feature equivalent widths that are large compared to local galaxies of similar luminosities. Through a spectral decomposition of mid-infrared spectroscopic data, we are able to isolate the fraction of IR luminosity arising from an AGN as opposed to star formation activity. This fraction is only able to account for $\sim$30\% of the total IR luminosity among the entire sample. Comment: To appear in "Reionization to Exoplanets: Spitzer's Growing Legacy", ASP Conference Series, ed: P. Ogle
05/2010;
-
K. Coppin, A. Pope,
K. Menéndez-Delmestre,
D. M. Alexander,
J. S. Dunlop,
E. Egami,
J. Gabor,
Edo Ibar,
R. J. Ivison,
J. E. Austermann, [......],
D. H. Hughes,
A. M. J. Mortier,
M. J. Page,
M. Rowan-Robinson,
D. Scott,
C. Simpson,
Ian Smail,
A. M. Swinbank,
M. Vaccari,
and M. S. Yun
[show abstract]
[hide abstract]
ABSTRACT: Spitzer spectroscopy has revealed that 80% of submm galaxies (SMGs) are starburst (SB)-dominated in the mid-infrared. Here we focus on the remaining 20% that show signs of harboring powerful active galactic nuclei (AGNs). We have obtained Spitzer-InfraRed Spectrograph spectroscopy of a sample of eight SMGs that are candidates for harboring powerful AGNs on the basis of IRAC color selection (S 8 μm/S 4.5 μm>2, i.e., likely power-law mid-infrared spectral energy distributions). SMGs with an AGN dominating (50%) their mid-infrared emission could represent the "missing link" sources in an evolutionary sequence involving a major merger. First of all, we detect polycyclic aromatic hydrocarbon (PAH) features in all of the SMGs, indicating redshifts from 2.5 to 3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Second, we see signs of both star formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGNs dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGNs are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S 8 μm/S 4.5 μm>1.65 works well at selecting mid-infrared energetically dominant AGNs in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.
The Astrophysical Journal 03/2010; 713(1):503. · 6.02 Impact Factor
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K. Coppin, A. Pope,
K. Menendez-Delmestre,
D. M. Alexander,
J. S. Dunlop,
E. Egami,
J. Gabor,
Edo Ibar,
R. J. Ivison,
J. E. Austermann, [......],
D. H. Hughes,
A. M. J. Mortier,
M. J. Page,
M. Rowan-Robinson,
D Scott,
C. Simpson,
Ian Smail,
A. M. Swinbank,
M. Vaccari,
M. S. Yun
[show abstract]
[hide abstract]
ABSTRACT: Spitzer spectroscopy has revealed that ~80% of submm galaxies (SMGs) are starburst (SB) dominated in the mid-infrared. Here we focus on the remaining ~20% that show signs of harboring powerful active galactic nuclei (AGN). We have obtained Spitzer-IRS spectroscopy of a sample of eight SMGs which are candidates for harboring powerful AGN on the basis of IRAC color-selection (S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN dominating (>50%) their mid-infrared emission could represent `missing link' sources in an evolutionary sequence involving a major merger. First of all, we detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Secondly, we see signs of both star-formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGN dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGN are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S8/S4.5>1.65 works well at selecting mid-infrared energetically dominant AGN in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence. Comment: Accepted for publication in ApJ. 17 pages, 10 figures
03/2010;
-
[show abstract]
[hide abstract]
ABSTRACT: We have employed the Giant Metre-wave Radio Telescope and the Very Large Array to map the Lockman Hole. At 610 and 1400 MHz, we reach noise levels of 15 and 6 μJy beam−1, respectively, with well-matched resolutions (∼5 arcsec). At this depth, we obtained reliable detections for about half of the known sub-mm galaxies (SMGs) in the field. For radio-identified SMGs, which are typically at z∼ 2, we measure a mean radio spectral index of α1400610=−0.75 ± 0.06 (where Sν∝να) and standard deviation of 0.29, between approximate rest-frame frequencies of 1.8 and 4.2 GHz. The slope of their continuum emission is indistinguishable from that of local star-forming galaxies and suggests that extended optically thin synchrotron emission dominates the radio output of SMGs. Cooling effects by synchrotron emission and Inverse Compton scattering off the cosmic microwave background do not seem to affect their radio spectral energy distributions. For those SMGs judged by Spitzer mid-infrared colours and spectroscopy to host obscured active galactic nuclei (AGN), we find a clear deviation from the rest of the sample – they typically have steeper radio spectral indices, α1400610≲−1.0. These findings suggest these mid-IR-/AGN-selected SMGs may have an intrinsically different injection mechanism for relativistic particles, or they might reside in denser environments. This work provides a reliable spectral template for the estimation of far-IR/radio photometric redshifts, and will enable accurate statistical K-corrections for the large samples of SMGs expected with SCUBA-2 and Herschel.
Monthly Notices of the Royal Astronomical Society Letters 12/2009; 401(1):L53 - L57.
-
C. M. Casey,
S. C. Chapman,
E. Daddi,
H. Dannerbauer, A. Pope,
D Scott,
F. Bertoldi,
R. J. Beswick,
A. W. Blain,
P. Cox,
R. Genzel,
T. R. Greve,
R. J. Ivison,
T. W. B. Muxlow,
R. Neri,
A. Omont,
I. Smail,
L. J. Tacconi
[show abstract]
[hide abstract]
ABSTRACT: Using the IRAM Plateau-de-Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon (CI(2->1), nu_rest = 809 GHz) and 12CO(J=7->6) (nu_rest=806GHz) towards the submillimetre galaxies (SMGs) GN20 (SMM J123711.9+622212, z = 4.055) and GN20.2 (SMM J123708.8+622202, z = 4.051). The far-infrared (FIR) continuum is detected at 8sigma significance in GN20, with a flux density of S_1.8mm = 1.9+-0.2mJy, while no continuum is detected in GN20.2. Both sources are statistically undetected in both CI and CO(7-6) lines; we derive line luminosity limits for both CI and CO of L'<~2x10^10K km s^-1 pc^2. Assuming carbon excitation temperatures of T_ex = 30K (the galaxies' measured dust temperatures), we infer CI mass limits of M_CI < 5.4x10^6 Msun (GN20) and M_CI < 6.8x10^6 Msun (GN20.2). The derived CI abundance limits are <1.8x10^-5 for GN20 and <3.8x10^-5 for GN20.2 implying that the systems have Milky Way level neutral carbon enrichment (X[CI]/X[H2]) or lower, similar to high-redshift carbon-detected systems (at 5x10^-5) but about 50 times less than the neutral carbon enrichment of local starburst galaxies. Observations of GN20 and GN20.2 in high-resolution MERLIN+VLA radio maps of GOODS-N are used to further constrain the sizes and locations of active regions. We conclude that the physical gas properties of young rapidly evolving systems like GN20 and GN20.2 are likely significantly different than starburst/ULIRG environments in the local Universe yet similar to z~2 SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN20 will require the order of magnitude increase in sensitivity of the Atacama Large Millimetre Array (ALMA) to constrain their CI and high-J CO content, despite the fact that they are the brightest systems at z~4. Comment: 7 pages, 3 figures; accepted for publication in MNRAS
08/2009;
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J. E. Austermann,
J. S. Dunlop,
T. A. Perera,
K. S. Scott,
G W Wilson,
I. Aretxaga,
D. H. Hughes,
O. Almaini,
E. L. Chapin,
S. C. Chapman, [......],
I. Roseboom,
M. Rowan-Robinson,
D Scott,
S. Serjeant,
I. Smail,
A. M. Swinbank,
J. A. Stevens,
M. Velazquez,
J. Wagg,
M. S. Yun
[show abstract]
[hide abstract]
ABSTRACT: We present the first results from the largest deep extragalactic millimetre-wavelength survey undertaken to date. These results are derived from maps covering over 0.7 deg^2, made at 1.1mm, using the AzTEC continuum camera mounted on the James Clerk Maxwell Telescope. The maps were made in the two fields originally targeted at 0.85mm with SCUBA in the SHADES project, namely the Lockman Hole East (mapped to a depth of 0.9-1.3 mJy rms) and the Subaru XMM Deep Field (1.0-1.7 mJy rms). The wealth of existing and forthcoming deep multi-frequency data in these two fields will allow the bright mm source population revealed by these images to be explored in detail in subsequent papers. Here we present the maps themselves, a catalogue of 114 high-significance sub-millimetre galaxy detections, and a thorough statistical analysis leading to the most robust determination to date of the 1.1mm source number counts. Through careful comparison, we find that both the COSMOS and GOODS North fields, also imaged with AzTEC, contain an excess of mm sources over the new 1.1mm source-count baseline established here. In particular, our new AzTEC/SHADES results indicate that very luminous high-redshift dust enshrouded starbursts (S_{1.1} > 3 mJy) are 25-50% less common than would have been inferred from these smaller surveys, thus highlighting the potential roles of cosmic variance and clustering in such measurements. We compare number count predictions from recent models of the evolving mm/sub-mm source population to these SMG surveys, which provide important constraints for the ongoing refinement of semi-analytic and hydrodynamical models of galaxy formation, and find that all recent models over-predict the number of bright sub-millimetre galaxies found in this survey. Comment: 19 pages, 10 figures, 7 tables - Very minor revisions; accepted for publication in MNRAS
07/2009;
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E. L. Chapin, A. Pope,
D Scott,
I. Aretxaga,
J. E. Austermann,
R. -R. Chary,
K. Coppin,
M. Halpern,
D. H. Hughes,
J. D. Lowenthal,
G. E. Morrison,
T. A. Perera,
K. S. Scott,
G W Wilson,
M. S. Yun
[show abstract]
[hide abstract]
ABSTRACT: We present results from a multi-wavelength study of 29 sources (false detection probabilities <5%) from a survey of the Great Observatories Origins Deep Survey-North field at 1.1mm using the AzTEC camera. Comparing with existing 850um SCUBA studies in the field, we examine differences in the source populations selected at the two wavelengths. The AzTEC observations uniformly cover the entire survey field to a 1-sigma depth of ~1mJy. Searching deep 1.4GHz VLA, and Spitzer 3--24um catalogues, we identify robust counterparts for 21 1.1mm sources, and tentative associations for the remaining objects. The redshift distribution of AzTEC sources is inferred from available spectroscopic and photometric redshifts. We find a median redshift of z=2.7, somewhat higher than z=2.0 for 850um-selected sources in the same field, and our lowest redshift identification lies at a spectroscopic redshift z=1.1460. We measure the 850um to 1.1mm colour of our sources and do not find evidence for `850um dropouts', which can be explained by the low-SNR of the observations. We also combine these observed colours with spectroscopic redshifts to derive the range of dust temperatures T, and dust emissivity indices $\beta$ for the sample, concluding that existing estimates T~30K and $\beta$~1.75 are consistent with these new data. Comment: 18 pages, 8 figures, 3 tables. Accepted for publication in MNRAS. AzTEC map available at http://www.astro.umass.edu/AzTEC/
06/2009;
-
E. Daddi,
M. Dickinson,
G. Morrison,
R. Chary,
A. Cimatti,
D. Elbaz,
D. Frayer,
A. Renzini, A. Pope,
D. M. Alexander,
F. E. Bauer,
M. Giavalisco,
M. Huynh,
J. Kurk,
and M. Mignoli
[show abstract]
[hide abstract]
ABSTRACT: Examining a sample of massive galaxies at 1.4 < z < 2.5 with KVega < 22 from GOODS, we compare photometry from Spitzer at mid- and far-IR to submillimeter, radio, and rest-frame UV wavelengths, to test the agreement between different tracers of star formation rates (SFRs) and to explore the implications for galaxy assembly. For z ~ 2 galaxies with moderate luminosities (L8 μm < 1011 L), we find that the SFR can be estimated consistently from the multiwavelength data based on local luminosity correlations. However, 20%-30% of massive galaxies, and nearly all those with L8 μm > 1011 L, show a mid-IR excess that is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24 μm-detected galaxies. For a given mass, the SFR at z = 2 was larger by a factor of ~4 and ~30 relative to that in star-forming galaxies at z = 1 and 0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z = 2 are relatively "transparent" to ultraviolet light, and their activity is long lived (400 Myr), unlike that in local ULIRGs and high-redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z = 2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity. Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4 and the space density of ULIRGs by an order of magnitude but give better agreement for z > 1.4 quiescent galaxies.
The Astrophysical Journal 12/2008; 670(1):156. · 6.02 Impact Factor
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E. Daddi,
D. M. Alexander,
M. Dickinson,
R. Gilli,
A. Renzini,
D. Elbaz,
A. Cimatti,
R. Chary,
D. Frayer,
F. E. Bauer,
W. N. Brandt,
M. Giavalisco,
N. A. Grogin,
M. Huynh,
J. Kurk,
M. Mignoli,
G. Morrison, A. Pope,
and S. Ravindranath
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ABSTRACT: Approximately 20%-30% of 1.4 z 2.5 galaxies with KVega < 22 detected with Spitzer MIPS at 24 μm show excess mid-IR emission relative to that expected based on the rates of star formation measured from other multiwavelength data. These galaxies also display some near-IR excess in Spitzer IRAC data, with an SED peaking longward of 1.6 μm in the rest frame, indicating the presence of warm dust emission usually absent in star-forming galaxies. Stacking Chandra data for the mid-IR excess galaxies yields a significant hard X-ray detection at rest-frame energies >6.2 keV. The stacked X-ray spectrum rises steeply at >10 keV, suggesting that these sources host Compton-thick AGNs with column densities NH 1024 cm-2 and an average, unobscured X-ray luminosity L2-8 keV ≈ (1-4) × 1043 ergs s-1. Their sky density (~3200 deg-2) and space density (~2.6 × 10-4 Mpc-3) are twice those of X-ray-detected AGNs at z ≈ 2, and much larger than those of previously known Compton-thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching ~50%-60% for M ~ 1011 M☉, an effect likely connected with downsizing in galaxy formation. The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z = 2 is similar to the mass ratio of black holes to stars in local spheroids, implying concurrent growth of both within the precursors of today's massive galaxies.
The Astrophysical Journal 12/2008; 670(1):173. · 6.02 Impact Factor
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D. Iono,
A. B. Peck, A. Pope,
C. Borys,
D. Scott,
D. J. Wilner,
M. Gurwell,
P. T. P. Ho,
M. S. Yun,
S. Matsushita,
G. R. Petitpas,
J. S. Dunlop,
M. Elvis,
A. Blain,
and E. Le Floc'h
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ABSTRACT: We present high-resolution 890 μm images of two 20 mJy submillimeter galaxies, SMM J123711+622212 and MIPS J142824.0+352619, obtained using the Submillimeter Array (SMA). Using submillimeter interferometric observations with an angular resolution of 25, the coordinates of these high-redshift sources are determined with an accuracy of 02. The new SMA data on SMM J123711+622212 reveal an unresolved submillimeter source offset to the east by 08 from an optical galaxy found in deep HST images, suggesting either a large galaxy with a dusty central region or an interacting galaxy system. The SMA image of hyperluminous (LFIR = 3.2 × 1013 L☉) source MIPS J142824.0+352619 provides a firm upper limit to the source size of 12. This constraint provides evidence that the foreground lens is only weakly affecting the observed high far-infrared luminosity.
The Astrophysical Journal 12/2008; 640(1):L1. · 6.02 Impact Factor
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E. Daddi,
M. Dickinson,
R. Chary, A. Pope,
G. Morrison,
D. M. Alexander,
F. E. Bauer,
W. N. Brandt,
M. Giavalisco,
H. Ferguson,
K.-S. Lee,
B. D. Lehmer,
C. Papovich,
and A. Renzini
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ABSTRACT: We investigate the multiwavelength emission of BzK-selected star-forming galaxies at z ~ 2 in the Great Observatories Origins Deep Survey (GOODS) North region. Most (82%) of the sources are individually detected at 24 μm in the Spitzer MIPS imaging, and one-fourth (26%) in the VLA radio data. Significant detections of the individually undetected objects are obtained through stacking in the radio, submillimeter, and X-ray domains. The typical star-forming galaxy with stellar mass ~1011 M☉ at z = 2 is an ultraluminous infrared galaxy (ULIRG), with LIR ~ (1-2) × 1012 L☉ and star formation rate SFR ≈ 200-300 M☉ yr-1, implying a comoving density of ULIRGs at z = 2 at least 3 orders of magnitude above the local one. SFRs derived from the reddening-corrected UV luminosities agree well, on average, with the longer wavelength estimates. The high 24 μm detection rate suggests a relatively large duty cycle for the BzK star-forming phase, which is consistent with the available independent measurements of the space density of passively evolving galaxies at z > 1.4. If the IMF at z = 2 is similar to the local one, and in particular if it is not top-heavy, this suggests that a substantial fraction of the high-mass tail (1011 M☉) of the galaxy stellar mass function was completed by z ≈ 1.4.
The Astrophysical Journal 12/2008; 631(1):L13. · 6.02 Impact Factor
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ABSTRACT: (abridged) We present deep Spitzer mid-infrared spectroscopy, along with 16,
24, 70, and 850um photometry, for 22 galaxies located in GOODS-N. The sample
spans a redshift range of 0.6 < z < 2.6, 24um flux densities between ~0.2-1.2
mJy, and consists of SMGs, AGN, and optically faint (z_AB > 25) sources. We
find that IR luminosities derived by fitting local SEDs with 24um photometry
alone are well matched to those when additional mid-infrared spectroscopic and
longer wavelength photometric data is used for galaxies having z < 1.4 and
24um-derived IR luminosities typically > 3x10^12 L_sun. However, for galaxies
in the redshift range between 1.4 < z < 2.6, typically having 24um-derived IR
luminosities > 3x10^12 L_sun, IR luminosities are overestimated by an average
factor of ~5 when SED fitting with 24um photometry alone. This result arises
partly due to the fact that high redshift galaxies exhibit PAH EQWs that are
large compared to local galaxies of similar luminosities. Using improved
estimates for the IR luminosities of these sources, we investigate whether
their IR emission is found to be in excess relative to that expected based on
extinction corrected UV SFRs, possibly suggesting the presence of an obscured
AGN. Through a spectral decomposition of IRS spectroscopic data, we are able to
isolate the fraction of IR luminosity arising from an AGN as opposed to star
formation activity. This fraction is only ~30% of the total IR luminosity among
the entire sample, on average. Of the sources identified as having mid-infrared
excesses, half are accounted for by using proper bolometric corrections while
half show the presence of obscured AGN. We do not find evidence for evolution
in the FIR-radio correlation over this redshift range, although the SMGs have
IR/radio ratios which are, on average, ~3 times lower than the nominal value.
12/2008;
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ABSTRACT: We present the serendipitous discovery of z=4.05 molecular gas CO emission
lines with the IRAM Plateau de Bure interferometer coincident with GN20 and
GN20.2, two luminous submillimeter galaxies (SMGs) in the Great Observatories
Origins Deep Survey North field (GOODS-N). These are among the most distant
submillimeter-selected galaxies reliably identified through CO emission and
also some of the most luminous known. In terms of CO to bolometric
luminosities, stellar mass and star formation rates (SFRs), these newly
discovered z>4 SMGs are similar to z~1.5-3 SMGs studied to date. These z~4 SMGs
have much higher specific SFRs than typical B-band dropout Lyman break galaxies
at the same redshift. The stellar mass-SFR correlation for normal galaxies does
not seem to evolve much further, between z~2 and z~4. A significant z=4.05
spectroscopic redshift spike is observed in GOODS-N, and a strong spatial
overdensity of B-band dropouts and IRAC selected z>3.5 galaxies appears to be
centered on the GN20 and GN20.2 galaxies. This suggests a proto-cluster
structure with total mass ~10^14 Msun. Using photometry at mid-IR, submm and
radio wavelengths, we show that reliable photometric redshifts (Dz/(1+z)~0.1)
can be derived for SMGs over 1<z<4. This new photometric redshift technique has
been used to provide a first estimate of the space density of 3.5<z<6
hyper-luminous starburst galaxies, and to show that they contribute
substantially to the SFR density at early epochs. Many of these high-redshift
starbursts will be within reach of Herschel. We find that the radio to mid-IR
flux density ratio can be used to select z>3.5 starbursts, regardless of their
submm/mm emission [abridged].
10/2008;
