Table Of ContentDRAFT VERSION NOVEMBER 16, 2011
Pre print typeset using B-'IE;X style emulateapj v. 11/lO/09
THE INFRARED PROPERTIES OF SOURCES MATCHED IN THE WISE ALL-SKY AND HERSCHEL ATLAS
SURVEYS
NICHOLAS A. BOND, DOMINIC J. BENFORD, JONATHAN P. GARDNER1, ALEXANDRE AMBLARD2, SIMONE FLEUREN3,
ANDREW W. BLAIN\ LORETTA DUNNE5, DANIEL J. B. SMITHG, STEVE J. MADDOX7, CARLOS HOYOS8, ROBBIE AULD9,
6 I2
MAARTEN BAESlO, DAVID BONFIELD , NATHAN BOURNE7, CARRIE BRIDGEII, SARA BUTTIGLIONE , ANTONIO CAVAI3,
I5 I6 I7 I8
DAVID CLEMENTSI4, ASANTHA COORAy , ALI DARlUSH , GIANFRANCO DE ZOTTI , SIMON DRIVER ,19, SIMON DYE7,
STEVE EALES9, PETER EISENHARDT20,JACOPO FRITZ21, ROSALIND HOPWOODI6, EDO IBAR22, ROB J. IVISON22,23, MATT J.
JARVIS6,2\ LEE KELVINI8,19, ENZO PASCALE7, MICHAEL POHLEN7, EMMA E. RIGBy7, AARON S. G. ROBOTHAMI8.19,
PASQUALE TEMI2, MARK THOMPSONG, CHAO-WEI TSAI25, PAUL VAN DER WERF26, EDWARD L. WRIGHT27, JINGWEN vVUI9,
28
LIN YAN
Dmft version November 16, 2011
ABSTRACT
We describe the infrared properties of sources detected over ~ 36 deg2 of sky in the GAMA 15-hr
equatorial field, using data from both the Herschel Astrophysical Terahertz Large-Area Survey (H
ATLAS) and Wide-field Infrared Survey (WISE). With 50' point-source depths of 34 and 0.048 mJy
at 250/Lm and 3.4 /Lm, respectively, we are able to identify 50.6% of the H-ATLAS sources in the
WISE survey, corresponding to a surface density of rv 630 deg-2. Approximately two-thirds of these
sources have measured spectroscopic or opticaljnear-IR photometric redshifts of z < 1. For sources
with spectroscopic redshifts at z < 0.3, we find a linear correlation between the infrared luminosity
at 3.4 /Lm and that at 250 /Lm, with ±50% scatter over ~ 1.5 orders of magnitude in luminosity,
~109 _lOlO.5 1,0' By contrast, the matched sources without previously measured redshifts (r 2: 20.5)
have 250-350/Lm flux density ratios that suggest either high-redshift galaxies (z 2: 1.5) or optically
faint low-redshift galaxies with unusually low temperatures (T ;S 20). Their small 3.4-250/Lm flux
ratios favor a high-redshift galaxy population, as only the most actively star-forming galaxies at low
redshift (e.g., Arp 220) exhibit comparable flux density ratios. Furthermore, we find a relatively large
AGN fraction (~30%) in a 12/Lm flux-limited subsample of H-ATLAS sources, also consistent with
there being a significant population of high-redshift sources in the no-redshift sample.
Subject headings: surveys - cosmology: observations - galaxies: high-redshift - infrared radiation
galaxies: statistics - galaxies: general
1 Cosmology Laboratory (Code 665), NASA Goddard Space
FI!fht Center, Greenbelt, MD 20771, USA
Astrophysics Branch, NASA/Ames Research Center, MS
245-6, Moffett Field, CA 94035, USA
3 School of Mathematical Sciences, Queen Mary, University of
London, Mile End Road, London, E1 4NS, UK
4 Department of Physics & Astronomy, University of Leices
ter University Road, Leicester LEI 7RH, UK
5 School of Physics and Astronomy, University of Nottingham,
University Park, Nottingham, NG7 2RD, UK vatorio 5, 1-35122 Padova, Italy, and SISSA, Via Bonomea 265,
6 Centre for Astrophysics Research, Science & Technology Re 1-34136 Trieste, Italy
search Institute, University of Hertfordshire, Herts ALlO 9AB, 18 International Centre for Radio Astronomy Research
UK (ICRAR), University of Western Australia, Crawley, WA 6009,
7 School of Physics & Astronomy, University of Nottingham, Australia
University Park, Nottingham NG7 2RD, UK 19 SUPA, School of Physics and Astronomy, University of St.
8 School of Physics and Astronomy, The University of Not Andrews, North Haugh, St. Andrews, KY169SS, UK
ti'lfham, University Park, Nottingham, NG7 2RD, UK. 20 Jet Propulsion Laboratory, California Institute of Technol
School of Physics and Astronomy, Cardiff University, The o~ Pasadena, CA 91109, USA
Parade, Cardiff, CF24 3AA, UK i Sterrenkundig Observatorium, Universiteit Gent, Kri
10 Sterrenkundig Observatorium, Universiteit Gent, Kri jgslaan 281 S9, 9000 Gent, Belgium
jgslaan 281-S9, 8-9000 Gent, Belgium 22 UK ,J\stronomy Technology Centre, Royal Observatory,
11 Division of Physics, Mathematics, and Astronomy, Califor Blackford Hill, Edinburgh EH9 3HJ, UK
nia Institute of Technology, Pasadena, CA 91125, USA 23 Institute for Astronomy, University of Edinburgh, Royal
12 INAF _ Osservatorio Astronomico di Padova. Vicolo Osser- Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
vatorio 5, 35122 Padova, Italy . 24 Physics Department, University of the Western Cape, Cape
13 Departamento de Astroffsica, Facultad de CC. Fisicas, Uni Town, 7535, South Africa
versidad CoOmplutense de Madrid, E-28040 Madrid, Spain 25 IPAC, California Institute of Technology, Pasadena, CA
14 Imperial College, Astrophysics Group, Blackett Lab, Prince 91125, USA
Consort Road, London, SW7 2AZ, UK 26 Leiden Observatory, Leiden University, P.O. Box 9513,2300
15 Department of Physics & Astronomy, University of Califor RA Leiden, The Netherlands
nia, Irvine, California 92697, USA 27 UCLA ,J\stronomy, P.O. Box 951.547, Los Angeles, CA
16 Physics Department, Imperial College London, South Kens 90095-1547, USA
in~on Campus, London SW7 2AZ, UK 28 Spitzer Science Center, California Institute of Technology,
7 INAF-Osservatorio Astronomico di Padova, Vicolo Osser- 1200 E. California Blvd., Pasadena CA 91125, USA
2 Bond et al.
1. INTRODUCTION alogue (Rigby et al. 2011). It contains 27,481 sources
detected at > 50- in any of the three SPIRE bands and
The advent of submillimeter astronomy has opened a
reaches point-source depths of 34, 40, and 44 mJy at 250,
new window into the Universe, allowing us to probe dusty
star-forming galaxies at high redshift, as well as cold 350, and 500 p,m, respectively.
The WISE first public data release contains four-band
dust in nearby galaxies. Along with the recent success
coverage of the entire G 15 field to 50-point-source depths
of the balloon-borne BLAST experiment (Pascale et al.
of 0.048, 0.10, 0.73, and 5.9 mJy at 3.4, 4.6, 12, and
2008; Eales et al. 2009), the launch of the Herschel Space
Observatory1 (Pilbratt et al. 2010) allows us to probe 22 p,m. The angular resolution in these bands is 6~/1, ~/4,
wavelengths from 55 to 672 p,m to a spatial resolution of 6%, and 12~/0, respectively (Wright et al. 2010). There
;S 10". Its largest open-time key project, the Herschel are ~ 240,000 WISE sources within G 15 detected at. > 70-
in at least one of the four WISE bands, correspondlllg to
Astrophysical Terahertz Large-Area Survey (H-ATLAS,
'" 0.14 WISE sources per lO"-radius aperture. In the
Eales et al. 2010a), will observe'" 550 deg2 of sky and
WISE preliminary release catalog, close pairs of sources
detect more than 300,000 galaxies.
are not deblended for separations ;S 9", so when a back
Based upon a preliminary cross-identification with the
ground or foreground source appears near an H-ATLAS
Galaxy and Mass Assembly survey (GAMA, Driver e~.aL source position, it will often be blended with the true
2011) and Sloan Digital Sky Survey (SDSS, AbazaJIan
counterpart.
et aL 2009), ~ 50% of the sources detected in the H In addition to the infrared data from WISE and H
ATLAS survey are at z < 1 (Smith et aL 2011). In
ATLAS, the G15 region has spectroscopic redshifts from
addition, an analysis of the FIR colors of the remaining
the GAMA survey (r ;S 19.4, Driver et al. 2011), and
H-ATLAS sources with flux densities above 35 mJy at
photometric redshifts obtained using opticaljNIR pho
350 p,m and > 30- detections at 250 p,m and 500 p,m sug
tometry from SDSS, VIKING, and the UK Infrared Deep
gests an average redshift of z ~ 2 (Amblard et al. 2010; Sky Survey Large Area Survey (UKIDSS-LAS, Lawrence
Lapi et al. 2011).
et aL 2007). Of the 27,481 H-ATLAS sources in the re
Another method of constraining the redshift distribu
gion, 16% have reliable spectroscopic redshifts and 39%
tion of FIR-selected sources is to look for counterparts
have photometric redshifts.
in the near and mid infrared (NIR and MIR). Fleuren
As a result of the high space density of 3.4 p,m sources,
et al. (in prep) have performed source matching to sur
a naive matching to the H-ATLAS source positions will
vey data from the VISTA Kilo-degree Infrared Galaxy
result in a non-negligible number of misidentifications.
survey (VIKING, Sutherland et al., in prep), but these
A significant fraction of these misidentifications will be
data only extend to '" 2 p,m. Starting in December 2009,
foreground stars that are blue in all WISE bands and
the Wide-field Infrared Survey Explorer2 ( WISE, Wright undetectable in H-ATLAS, so we can reduce the con
et aL 2010) began its mission to observe the entire sky
tamination rate of our matched catalog by considering
in four bands, ranging from 3.4 to 22 p,m, at ;S 12" res
only WISE sources with [3.4]-[4.6] > O. The magnitude
olution. Following the first public data release in April
distribution of all background/foreground WISE sources
2011 ~ 24000 deg2 of IR images and source catalogs be-
came, avail,a ble to the public, including 36 de g 2 0 f s k y I.I I tiso s<uc 1h% th oaft s"o,u 2r0c%es awreit bhliune 1r 0th" aonf athni sH l-iAmTitL, AasS c poomspitaiorend.
the equatorial plane covered by the H-ATLAS survey.
To estimate the fraction of H-ATLAS sources with
The purpose of this letter is to describe the infrared
detectable counterparts in the remainder of the WISE
properties of identified WISE counterparts to H-ATLAS
source catalog, we use the method of Fleuren et aL, in
sources within the GAMA 15-hr field (G15). Through
prep, where it is shown that by counting non-matches
out we will assume a concordance cosmology with Ho =
71 km s-1 Mpc-1, f2m = 0.27, and nA = 0.73 (Spergel cwainth einst iam fiaxteed t hdeis dtaenteccet iforonm r atthee, H-ATLAS positions, we
et al. 2007). In a subsequent paper, we will report WISE
cross-identifications over the entire H-ATLAS area and S
perform spectral energy distribution (SED) fits to the Qo = 1 (1)
13'
matched sources.
where S is the fraction of unmatched positions in the
2. DATA AND METHODOLOGY
H-ATLAS catalog and 13 is the fraction of unmatched
Herschel observations of the G15 field include imaging random positions. This expression holds under the con
data at 250 350 and 500 p,m from the SPIRE instru- dition that there are a negligible number of true WISE
ment, with r"e spective beam FW-HM of 18I.1f, 2"5.2 , and matches outside of the matching radius. Matching all
36% (Griffin et al. 2010), as well as 100 and 160 p,m imag WISE sources within 10" (within which we expect> 99%
ing from the PACS instrument (Po~li~sc~ et al. 201.0). of the true matches to lie) and using 105 random posi
The field subtends approximately 12 III nght ascensIon tions within the G15 field, we find Qo = 0.632 ± 0.004.
and 3° in declination and is centered on the equatorial
This is larger than the value found in the SDSS galaxy
=
plane at a 14h30m. The H-ATLAS G15 source cat
catalog (Qo = 0.583, Smith et al. 2011), but smaller than
alogue (Dunne et al., in prep) is constructed using the in the VIKING Ks band (Qo = 0.75, Fleuren et aI., in
same method as the Science Demonstration Phase cat-
prep). By contrast, we find Qo = 0.012±0.002 within the
subset of WISE sources with [3.4] [4.6J < 0, suggesting
1 Herschel is an ESA space observatory with science instruments that this color cut was effective in removing stars.
provided by European-led Principal Investigator consortia and with
important participation from NASA. In Fig. 1, we use Eq. 1 to estimate the IR detection rate
2 For a description of the WISE mission, see as a function of FIR flux density. We find that WISE
http://wise2.ipac.caltech.edu/docs/release/prelim/expsup/ 3.4 p,m detection rates are > 95% for sources brighter
Sources in both H-ATLAS and WISE 3
(/i'" ____I IRIIeIdIIsIhI_ittl IIm:lB __ O,5
01
0,8 0,8 0.1 0.2 0.3 0.4 0.5
0,6 06 !/
I
0,' 0,'
0,2 0,2
0 0
0 100 200 300 0 100 200 300
0,8 0,8
0,6 Ii/ i 06 lI
c1
0,' 04 J
;'
I
0.2 0,2
0 0
0 100 200 300 0 100 200 300 0,2
Flux (mJy) f (Jy)
250
FIG. 1.- NIR and MIR detection rates plotted as a function FIG. 2.- Distribution of the 3.4-250I1m flux density ratio as a
of limiting FIR flux density at 250l1m (solid), 350l1m (dashed), function of 250l1m flux density for H-ATLAS sources with WISE
and 500 11m (dotted). Detection rates are shown for 3.411m (upper 3.4l1m IDs. Individual points are plotted for sources with spec
left panel), 4.611m (upper right), 1211m (lower left), and 2211m troscopic redshifts from the GAMA survey, with the color of the
(lower right) sources, where the corresponding 50" detection limits point indicating the redshift. The sources without either photo
are 0.06, 0.9, and 3.6 mJy, respectively. Note that the large sky metric or spectroscopic redshifts are indicated by black points.
density of 3.4 11m sources can lead to ambiguity in the identification Dashed lines indicate the approximate limits of the H-ATLAS and
of some sources, so identification rates are typically ~ 10-20% lower WISE surveys. The majority of sources with spectroscopic red
than the corresponding detection rates. shifts (r ;S 19.4) are well detected by WISE, while the no-redshift
sources have small flux density ratios that are inconsistent with the
than 100 mJy at 250 !Jm. The vast majority of these majority of "normal" low-redshift galaxies.
sources (78%) have measured spectroscopic or photomet
ric redshifts < 0.5, suggesting that the low-redshift mode We measure n(m) directly from the WISE prelim
of star-forming galaxies dominates sub-mm sources above inary release catalog, while we determine q(m) by
this flux density. Below this FIR flux density, the NIR first measuring the magnitude distribution of 3.4!Jm
and MIR detection rates drop rapidly as the high-redshift sources within 10" of the H-ATLAS source positions
mode accounts for an increasing fraction of the sub-mm and then subtracting the magnitude distribution of
sources (see Fig. 2). background/foreground sources normalized by the area
By contrast, ~ 50% of sources brighter than 100 mJy within this radius.
at 500!Jm are expected to be lensed z > 1 galaxies (Ne Following SI1, we assume the astrometric uncertainty
grello et al. 2010; Hopwood et al. 2011). Of the 46 to be circularly symmetric and to have a dependence on
such sources in the G 15 region, ~ 20% are undetected the 250!Jm signal-to-noise ratio (SNR) given by,
at 12 and 22 !Jm, most likely because the 500!Jm band
FWHM
pass shifts higher in the blackbody curve at high redshift, (fpos = 0.655 SNR ' (4)
while the rest-frame MIR flux density declines blueward
of 22!Jm (e.g., Rieke et al. 2009). where FWHM= 18.1" is the Herschel beam width. \\lbile
The identification of individual NIR and MIR counter the WISE random astrometric uncertainties are negli
parts is more subtle, as we want to eliminate as many of gible (~(y"3), the WISE positions in the preliminary
the false matches as possible. Here, we use the likelihood release catalog are known to suffer from highly nOIl
ratio technique of Sutherland & Saunders (1992), which Gaussian systematic offsets that can be as large as I", so
was implemented for the H-ATLAS survey in Smith et al. we apply a lower limit of (fpos > 1.5".
(2011, hereafter SI1). Because> 99.9% of sources in the Once we have computed the likelihood, Lj , for a pos
WISE catalog are detected at 3.4!Jm at > 7(f, we use sible counterpart j, we determine the reliability by sum
this band for source identification. The likelihood that ming over all possible counterparts,
a given WISE source is a counterpart to an H-ATLAS
source is a function of the radial probability distribu
tion, the 3.4!Jm magnitude distribution of non
counterparts, n(m), and the 3.4!Jm magnitude distribu
We consider any 3.4!Jm source with R > 0.8 to be a
tion of detected H-ATLAS counterparts,
reliable counterpart. After performing this procedure
separately for WISE sources with star- and gala.xy-like
[3.4J [4.6J colors, we obtain a combined total of 13.898
WISE counterparts, or 50.6 ± 0.4% of the H-ATLAS cat
where is given by alog. The expected number of false matches can be ob
(r2 ) tained with
= --1- exp --- . (3)
2r.(f~os 2(f~os
4 Bond et al.
ing their FIR flux density ratios (e.g., Amblard et al.
2010; Schulz et al. 2010), which give an indication of the
location of the peak of the FIR dust emission. In Fig. 3,
we compare the SPIRE colors of our matched catalog to
a suite of 106 modified blackbody spectral energy distri
butions (including uncertainties) with temperatures uni
formly distributed between 10 and 60 K and emissivitiy
parameters between 0 and 2 (Amblard et al. 2010). We
indicate on the plot the median flux density ratios of the
4,677 H-ATLAS sources with WISE identifications but
no redshifts ("no-redshift" sample, dashed lines), as well
as the individual flux density ratios of 311 such sources
with > 30' detections in all three SPIRE bands.
The no-redshift sample has median FIR colors suggest
ing z ~ 2, while the subset with three-band SPIRE de
tections may have even higher redshifts, perhaps as large
as z ~ 3.5. The redshifts derived in this way are highly
uncertain, as redshift is degenerate with dust temper
ature in the FIR and it is possible that these galaxies
FZ50/F350 have typical dust temperatures different than 35 K. How
FIG. 3.- FIR flux density ratio distribution for 311 H-ATLAS ever, in order for the no-redshift sample to be at z ~ 0,
sources with WISE identifications, but without measured redshifts. they would need to have very cold dust temperatures
Colored points indicate the median redshift of a set of modified
blackbody models with temperatures uniformly distributed be (T ~ 12 K) in addition to being very faint in the optical
tween 10 and 60 K and emissivity parameters between 0 and 2. (r ;:::: 20.5). Although temperatures as low as 10 K have
Dashed lines indicate the median flux density ratios of the sources been seen in H-ATLAS galaxies at z < I, T ~ 25 - 30 K
lacking redshifts. The majority of the sources without redshifts ap is more typical (Dye et al. 2010).
pear to be at z ~ 2. Inset: Combined distribution of spectroscopic Further evidence that the no-redshift sources are at
and photometric redshifts for H-ATLAS sources with detections at
3.4J1.m (solid histogram), 12J1.m (dashed), and 22J1.m (hatched). z » 0 can be found in the 3.4-250 J-lm ratios (see Fig. 2).
There are 73 sources at z > 1 not shown. H-ATLAS sources with GAMA redshifts have NIR-FIR
\Ve estimate that there are 369 false identifications in flux density ratios that decrease from ~ 0.009 at z ~ 0 to
our sample, corresponding to a contamination rate of ~ 0.006 at z ~ 0.35. This trend, which continues toward
2.7%. The majority of our IDs have galaxy-like colors, higher redshift, is due primarily to the larger k-correction
but we were able to identify 14 objects with star-like at 3.5J-lm than at 250J-lm (e.g., Rieke et al. 2009). The
colors ([3.4] - [4.6] < 0) that weren't already identified median flux density ratio for the no-redshift sources, by
as galaxies with SDSS matching. This corresponds to an comparison, is 0.0023. Although low-redshift galaxies
upper limit of 0.05% for the fraction ofH-ATLAS sources will occasionally exhibit such small NIR-FIR flux density
identified as stars and detected in WISE. ratios - Arp 220, for example, has 13.4/1250 = 0.0019 -
Comparing the individual WISE identifications to such objects are rare, actively star-forming, and unlikely
SDSS cross-identifications, we find that 84.7% of the to have dust temperatures as small as ~ 15 K.
10,709 G15 H-ATLAS sources with R > 0.8 SDSS identi We can shed even further light on the properties of
fications also had R > 0.8 WISE identifications. The me the no-redshift sample by examining their position in
dian separation between the WISE and SDSS sources is WISE color space (see Fig. 4). The technique is simi
11.'6, consistent with the astrometric uncertainties in the lar to those developed for a set of lRAC filters at com
current version of the WISE catalog. In addition, ~ 6% parable wavelengths (Sajina et al. 2005; Stern et al.
of these sourcess have separations > 3", consistent with 2005). It was demonstrated that the diagram can be
the expected contamination rates of the WISE (~3%) divided into stars and early-type galaxies (lower left cor
and SDSS (~5%) matched samples. ner, [4.6J [12J;;:; 1), star-forming galaxies (lower right
corner), and active galactic nuclei (AGNs), where AGNs
3. DEMOGRAPHICS are selected largely based on their power-law emission in
The redshift distribution of 250 J-lm-selected sources the NIR and MIR. This was adapted for WISE by Jarrett
is likely bimodal, with populations of both moderately et al. (2011),
star-forming galaxies at z < 1 and high-redshift starburst
galaxies at z ~ 2 (Smith et al. 2011; Lapi et al. 2011). [4.6] [12] > 2.2
Because the latter are typically faint in the observed [4.6] [12] < 4.2
frame optical and UV, the vast majority of objects with
[3.4J - [4.6] < 1.7
measured spectroscopic or photometric redshifts are in
the low-redshift mode. Of the H-ATLAS sources with [3.4]- > 0.1([4.6]- + 0.38.
spectroscopic and photometric redshifts the inset of
Fig. 3), we successfully identify 85.6% in WISE. Of the 4,959 H-ATLAS sources with WISE identifica
Although most of the WISE identifications with kno'\Vll tions, Z < 0.5, and detections in 3.4, 4.6, and 12 J-lm, the
redshifts are at < 0.8 (> 98%), such a sample is biased majority are star-forming galaxies, with < 0.2% main
toward low redshift because redshift measurements re sequence stars (which is consistent with the SDSS/H
quire detectable flux in the UV, optical. or near-infrared. ATLAS matching done by Thompson et al. 2010), and a
For the remaining sources, we can estimate redshifts us- small AGN fraction (0.057),
Sources in both H-ATLAS and WISE 5
Redshift
'f :1 05
0.1 0.2 0.3 0.4 0.5
2 11
9
o Redshift
8
LJ-L~~~~~~~~~~~~~
o 2 4 8 9 10 11
[4.6]-[ 12] log VLz50 (1..0)
FIG. 4.- IR color distribution of ATLAS-WISE matches with FIG. 5.- Luminosity at 3.4/lm as a function of luminosity at
three-baud WISE photometry, including sources with z < 0.5 250/lm for H-ATLAS sources with spectroscopic redshifts, 0.05 <
(color coded by redshift) and no-redshift sources (black points). z < 0.3. Both luminosities are rest-frame k-corrected. Small
The diagram can be divided into stars (lower left corner), ~tar squares are colored according to their redshift. Red stars indi
forming galaxies (lower right corner), and AGNs (d~hed r;gIon). cate AGN, selected with the criteria given in Eq. 7. Dashed lines
The color distribution of all three-band WISE detectIOns (mclud indicate the approximate selection limits for sources at z 0.05
ing sources unmatched to H-ATLAS) is indicated by cc:ntours. ~he and 0.2. The majority of the AGN lie above the relationship, sug
majority of identified sources are normal star-formmg galaxIes, gesting that L3.4 contains a non-negligible contribution from hot
with a small fraction of AGNs. Sources without measured red dust emitting near the central black hole.
shifts tend to be redder in [3.4] [4.6] and have a larger AGN
fmraecatsiuorne mtheannts ,s oz u<rc e0s.5 .w ith spectroscopic or photometric redshift that GAMA misses only ~ 5% of matched z < 0.3 galax
ies based upon the deeper (r ;S 20.5) subsample of H
For the 9% of no-redshift sources with three-band
ATLAS galaxies with photometric redshifts.
WISE photometry, we find a much larger AGN fraction
We find that the majority of the IR-color-selected AGN
(0.30), but are subject to a selection bias toward AGNs
(red points) lie above the relationship for star-forming
due to their additional flux from warm dust at 12/-Lm galaxies, as L3.4 will contain a non-negligible contribu
(Hainline et al. 2010). Coppin et al. (2010) performed
tion from hot dust emitting near the central black hole
SED fits on a set of blank-field SMG samples and esti
in addition to emission from low-mass stars. However, if
mated that ~ 15% of SMGs are dominated by an AGN we exclude AGNs, we find an approximately linear cor
in the mid-infrared.
relation, with a best-fit power law index, ex = 0.98~g:g~.
The quoted systematic uncertainties on the power law
4. FIR-NIR PROPERTIES
index were determined by allowing for a range of possi
Emission from dust in the FIR is often used as an in ble dust temperatures, 18 < T < 34 K, when performing
dicator of the total star formation rate in galaxies (e.g.,
250/-Lm k-corrections. The intrinsic scatter about this
Kennicutt 1998), under the assumption that most of the relationship is 0.18 ± 0.01 dex, or ~ 50%.
dust heating is provided by young stars and that the star
The existence of a linear correlation between L and
forming regions are optically thick. The 250/-Lm band 250
pass samples the cooler dust emission, which can arise L3.4 across one and a half decades in luminosity suggests
a close relationship between the cold dust probed by the
from both star-forming clouds and diffuse regions in the
FIR and the stellar mass probed by the NIR. Previous
ISM (e.g., Eales et al. 201Ob; Dunne et al. 2011). By
indications with Herschel have shown the 250/-Lm lumi
contrast, 3.4 /-Lm emission is dominated by stellar sources
nosity density to tightly correlate with both the 24/-Lm
and is a probe of the stellar mass for galaxies without a
luminosity density, a star formation rate indicator, and
strong AGN contribution (e.g., Stern et al. 2005).
the total infrared luminosity (Elbaz et al. 2010). This
In Fig. 5, we plot 3.4 /-Lm luminosity as a function of
fact, coupled with a linear correlation between stellar
250/-Lm luminosity for H-ATLAS sources with spectro
mass and star formation rate for the general star-forming
scopic redshifts, 0.05 < z < 0.3. We k-correct 3.4 /-Lm
gala.xy population (Daddi et al. 2007; Elbaz et al. 2007;
using a power law interpolation of the [3.4] [4.6] color.
Noeske et al. 2007, Donoso et al., in prep), suggests that
For the 250/-Lm fllLX densities, the majority of the
the majority of the low-redshift H-ATLAS sources are
ies in our sample do not have sufficient FIR filLX
actually "normal" star-forming galaxies.
to obtain reliable dust temperatures, as is needed for a
proper k correction. Instead, we use the median dust
temperature of 26 K found by Dye et al. (2010) using This publication makes use of data products from the
PACS and SPIRE data from the H-ATLAS SDP of 1,346 Wide-field Infrared Survey Explorer, which is a joint
0.1 < z < 0.5 sources. Although the r < 19.4 limit of project of the University of California, Los Angeles,
the GAMA spectroscopic survey restricts the range of and the Jet Propulsion Laboratory/California Institute
luminosities that can appear on the plot, we estimate of Technology, funded by the National Aeronautics and
6 Bond et al.
Space Administration. U.S. participants in Herschel ATLAS acknowledge sup
Herschel is an ESA space observatory with science in port provided by NASA through a contract issued from
struments provided by European-led Principal Investiga JPL.
tor consortia with sig-nificant participation from NASA. In addition, we thank Dan Stern and Roberto Assef
for helpful discussions about the quasar selection.
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