20 research outputs found
The spatial clustering of ultraluminous infrared galaxies over 1.5 < z < 3
We present measurements of the spatial clustering of galaxies with stellar masses 1011 M, infrared luminosities 1012 L, and star formation rates 200 M yr-1 in two redshift intervals: 1.5 < z < 2.0 and 2 < z < 3. Both samples cluster moderately strongly, with spatial correlation lengths of r0 = 6.14 B1 0.84 h-1 Mpc for the 2 < z < 3 sample and r0 = 5.36 B1 1.28 h-1 Mpc for the 1.5 < z < 2.0 sample. These clustering amplitudes are consistent with both populations residing in dark matter halos with masses of 7 C 1012 M, which is comparable to that seen for optical QSOs at the same epochs. We infer that a minimum dark matter halo mass is an important factor for all forms of luminous, obscured activity in galaxies at z > 1, both starbursts and active galactic nuclei. Adopting plausible models for the growth of dark matter halos with redshift, the halos hosting the 2 < z < 3 sample will likely host poor to rich clusters of galaxies at z = 0, whereas the halos hosting the 1.5 < z < 2.0 sample will likely host L* elliptical galaxies or poor clusters at z = 0. We conclude that ultraluminous infrared galaxies (ULIRGs) at z 2.5 likely signpost stellar buildup in galaxies that will reside in clusters at z = 0 and that ULIRGs at z 1.7 signpost stellar buildup in sources that will either become L* elliptical galaxies or reside in poor clusters at z = 0
Understanding Infrared Galaxy Populations: the SWIRE Legacy Survey
We discuss spectral energy distributions, photometric redshifts, redshift
distributions, luminosity functions, source-counts and the far infrared to
optical luminosity ratio for sources in the SWIRE Legacy Survey. The spectral
energy distributions of selected SWIRE sources are modelled in terms of a
simple set of galaxy and quasar templates in the optical and near infrared, and
with a set of dust emission templates (cirrus, M82 starburst, Arp 220
starburst, and AGN dust torus) in the mid infrared. The optical data, together
with the IRAC 3.6 and 4.5 mu data, have been used to determine photometric
redshifts. For galaxies with known spectroscopic redshifts there is a notable
improvement in the photometric redshift when the IRAC data are used, with a
reduction in the rms scatter from 10% in (1+z) to 5%. While further
spectroscopic data are needed to confirm this result, the prospect of
determining good photometric redshifts for the 2 million extragalactic objects
in SWIRE is excellent. The distribution of the different infrared sed types in
the L{ir}/L{opt} versus L{ir} plane, where L{ir} and L{opt} are the infrared
and optical bolometric luminosities, is discussed. Source-counts at 24, 70 and
160 mu are discussed, and luminosity functions at 3.6 and 24 mu are presented.Comment: 8 pages, 14 figures, to appear in proceedings of 'Spitzer IR
Diagnostics Conference, Nov 14-16, 2005
IR and UV Galaxies at z=0.6 -- Evolution of Dust Attenuation and Stellar Mass as Revealed by SWIRE and GALEX
We study dust attenuation and stellar mass of star-forming
galaxies using new SWIRE observations in IR and GALEX observations in UV. Two
samples are selected from the SWIRE and GALEX source catalogs in the
SWIRE/GALEX field ELAIS-N1-00 ( deg). The UV selected sample
has 600 galaxies with photometric redshift (hereafter photo-z) and NUV (corresponding to \rm L_{FUV} \geq 10^{9.6} L_\sun).
The IR selected sample contains 430 galaxies with mJy
(\rm L_{dust} \geq 10^{10.8} L_\sun) in the same photo-z range. It is found
that the mean ratios of the z=0.6 UV galaxies are
consistent with that of their z=0 counterparts of the same . For
IR galaxies, the mean ratios of the z=0.6 LIRGs (\rm
L_{dust} \sim 10^{11} L_\sun) are about a factor of 2 lower than local LIRGs,
whereas z=0.6 ULIRGs (\rm L_{dust} \sim 10^{12} L_\sun) have the same mean
ratios as their local counterparts. This is consistent
with the hypothesis that the dominant component of LIRG population has changed
from large, gas rich spirals at z to major-mergers at z=0. The stellar
mass of z=0.6 UV galaxies of \rm L_{FUV} \leq 10^{10.2} L_\sun is about a
factor 2 less than their local counterparts of the same luminosity, indicating
growth of these galaxies. The mass of z=0.6 UV lunmous galaxies (UVLGs: \rm
L_{FUV} > 10^{10.2} L_\sun) and IR selected galaxies, which are nearly
exclusively LIRGs and ULIRGs, is the same as their local counterparts.Comment: 27 pages, 8 figures, to be published in the Astrophysical Journal
Supplement series dedicated to GALEX result
Parametric modelling of the 3.6um to 8um colour distributions of galaxies in the SWIRE Survey
We fit a parametric model comprising a mixture of multi-dimensional Gaussian
functions to the 3.6 to 8um colour and optical photo-z distribution of galaxy
populations in the ELAIS-N1 and Lockman Fields of SWIRE. For 16,698 sources in
ELAIS-N1 we find our data are best modelled (in the sense of the Bayesian
Information Criterion) by the sum of four Gaussian distributions or modes (C_a,
C_b, C_c and C_d). We compare the fit of our empirical model with predictions
from existing semi-analytic and phenomological models. We infer that our
empirical model provides a better description of the mid-infrared colour
distribution of the SWIRE survey than these existing models. This colour
distribution test is thus a powerful model discriminator and complementary to
comparisons of number counts. We use our model to provide a galaxy
classification scheme and explore the nature of the galaxies in the different
modes of the model. C_a consists of dusty star-forming systems such as ULIRG's.
Low redshift late-type spirals are found in C_b, where PAH emission dominates
at 8um. C_c consists of dusty starburst systems at intermediate redshifts. Low
redshift early-type spirals and ellipticals dominate C_d. We thus find a
greater variety of galaxy types than one can with optical photometry alone.
Finally we develop a new technique to identify unusual objects, and find a
selection of outliers with very red IRAC colours. These objects are not
detected in the optical, but have very strong detections in the mid-infrared.
These sources are modelled as dust-enshrouded, strongly obscured AGN, where the
high mid-infrared emission may either be attributed to dust heated by the AGN
or substantial star-formation. These sources have z_ph ~ 2-4, making them
incredibly infrared luminous, with a L_IR ~ 10^(12.6-14.1) L_sun.Comment: 44 pages, 10 figures, 6 tables. Accepted for publication in the
Astronomical Journa
Extending PLE models into the mid-IR, far-IR & sub-mm
Simple pure luminosity evolution (PLE) models, in which galaxies brighten at
high redshift due to increased star-formation rates (SFRs), are known to
provide a good fit to the colours and number counts of galaxies throughout the
optical and near-infrared. We show that optically defined PLE models, where
dust reradiates absorbed optical light into infrared spectra composed of local
galaxy templates, fit galaxy counts and colours out to 8um and to at least
z=2.5. At 24-70um, the model is able to reproduce the observed source counts
with reasonable success if 16% of spiral galaxies show an excess in mid-IR flux
due to a warmer dust component and a higher SFR, in line with observations of
local starburst galaxies. There remains an under-prediction of the number of
faint-flux, high-z sources at 24um, so we explore how the evolution may be
altered to correct this. At 160um and longer wavelengths, the model fails, with
our model of normal galaxies accounting for only a few percent of sources in
these bands. However, we show that a PLE model of obscured AGN, which we have
previously shown to give a good fit to observations at 850um, also provides a
reasonable fit to the Herschel/BLAST number counts and redshift distributions
at 250-500um. In the context of a LCDM cosmology, an AGN contribution at
250-870um would remove the need to invoke a top-heavy IMF for high-redshift
starburst galaxies, although the excellent fit of the galaxy PLE model at
shorter wavelengths would still need to be explained.Comment: 14 pages, 11 figures; submitted to MNRA
High-Redshift QSOs in the SWIRE Survey and the z~3 QSO Luminosity Function
We use a simple optical/infrared (IR) photometric selection of high-redshift
QSOs that identifies a Lyman Break in the optical photometry and requires a red
IR color to distinguish QSOs from common interlopers. The search yields 100 z~3
(U-dropout) QSO candidates with 19<r'<22 over 11.7 deg^2 in the ELAIS-N1 (EN1)
and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic
(SWIRE) Legacy Survey. The z~3 selection is reliable, with spectroscopic
follow-up of 10 candidates confirming they are all QSOs at 2.83<z<3.44. We find
that our z~4$ (g'-dropout) sample suffers from both unreliability and
incompleteness but present 7 previously unidentified QSOs at 3.50<z<3.89.
Detailed simulations show our z~3 completeness to be ~80-90% from 3.0<z<3.5,
significantly better than the ~30-80% completeness of the SDSS at these
redshifts. The resulting luminosity function extends two magnitudes fainter
than SDSS and has a faint end slope of beta=-1.42 +- 0.15, consistent with
values measured at lower redshift. Therefore, we see no evidence for evolution
of the faint end slope of the QSO luminosity function. Including the SDSS QSO
sample, we have now directly measured the space density of QSOs responsible for
~70% of the QSO UV luminosity density at z~3. We derive a maximum rate of HI
photoionization from QSOs at z~3.2, Gamma = 4.8x10^-13 s^-1, about half of the
total rate inferred through studies of the Ly-alpha forest. Therefore,
star-forming galaxies and QSOs must contribute comparably to the
photoionization of HI in the intergalactic medium at z~3.Comment: Accepted for publication in ApJ. emulateapj format. 23 pages, 17
figure
Complete Multiwavelength Characterization of Faint Chandra X-ray Sources Seen in the Spitzer Wide-Area IR Extragalactic (SWIRE) Survey
We exploit deep combined observations with Spitzer and Chandra of the SWIRE
survey in the ELAIS-N1 region, to investigate the nature of the faint X-ray and
IR sources in common, to identify AGN/starburst diagnostics, and to study the
sources of the X-ray and IR cosmic backgrounds. In the 17'x17' area of the
Chandra ACIS-I image there are 3400 SWIRE near-IR sources with 4 sigma
detections in at least 2 IRAC bands and 988 sources detected at 24micron with
MIPS brighter than 0.1 mJy. Of these, 102 IRAC and 59 MIPS sources have Chandra
counterparts, out of a total of 122 X-ray sources present in the area with
S(0.5-8 kev)>10^(-15) erg/cm^2/s. We have constructed SEDs for each source
using data from the 4 IRAC wavebands, Chandra fluxes, and optical follow-up
data in the wavebands U, g', r', i', Z, and H. We fit a number of spectral
templates to the SEDs at optical and infrared wavelengths to determine
photometric redshifts and spectral categories, and also make use of diagnostics
based on the X-ray luminosities, hardness ratios, X-ray to infrared spectral
slopes and optical morphologies. Although we have spectroscopic redshifts for
only a minority of the Chandra sources, the available SEDs constrain the
redshifts for most of the sample sources, which turn out to be typically at
0.5<z<2. We find that 39% of the Chandra sources are dominated by type-1 AGN
emission, 23% display optical/IR spectra typical of type-2 AGNs, while the
remaining 38% fraction show starburst-like or even normal galaxy spectra. Since
we prove that all these galaxies are dominated by AGN emission in X-rays this
brings the fraction of type-1 AGNs to be 80% of the type-2: even assuming that
all the Chandra sources undetected by Spitzer are type-2 AGNs, the type-1
fraction would exceed 1/3 of the total population (abridged).Comment: Accepted for publication in AJ, March 2005 issu
SWIRE: The SIRTF WideâArea Infrared Extragalactic Survey
The SIRTF Wide-Area Infrared Extragalactic Survey (SWIRE), the largest SIRTF Legacy program, is a wide-area imaging survey to trace the evolution of dusty, star-forming galaxies, evolved stellar populations, and active galactic nuclei (AGNs) as a function of environment, from redshifts to the current z ⌠3 epoch. SWIRE will survey seven high-latitude fields, totaling 60â65 deg2 in all seven SIRTF bands: Infrared Array Camera (IRAC) 3.6, 4.5, 5.6, and 8 mm and Multiband Imaging Photometer for SIRTF (MIPS) 24, 70, and 160 mm. Extensive modeling suggests that the Legacy Extragalactic Catalog may contain in excess of 2 million IR-selected galaxies, dominated by (1) âŒ150,000 luminous infrared galaxies (LIRGs; LFIR 1 1011 L,) detected by MIPS (and significantly more detected by IRAC), âŒ7000 of these with ; (2) 1 million IRAC- z 1 2 detected early-type galaxies (⌠with and âŒ10,000 with ); and (3) âŒ20,000 classical AGNs 5 2 # 10 z 1 1 z 1 2 detected with MIPS, plus significantly more dust-obscured quasi-stellar objects/AGNs among the LIRGs. SWIRE will provide an unprecedented view of the evolution of galaxies, structure, and AGNs.
The key scientific goals of SWIRE are (1) to determine the evolution of actively star forming and passively evolving galaxies in order to understand the history of galaxy formation in the context of cosmic structure formation; (2) to determine the evolution of the spatial distribution and clustering of evolved galaxies, starbursts, and AGNs in the key redshift range over which much of cosmic evolution has occurred; and (3) to 0.5 ! z ! 3 determine the evolutionary relationship between ânormal galaxiesâ and AGNs and the contribution of AGN accretion energy versus stellar nucleosynthesis to the cosmic backgrounds. The large area of SWIRE is important to establish statistically significant population samples over enough volume cells that we can resolve the star formation history as a function of epoch and environment, i.e., in the context of structure formation. The large volume is also optimized for finding rare objects.
The SWIRE fields are likely to become the next generation of large âcosmic windowsâ into the extragalactic sky. They have been uniquely selected to minimize Galactic cirrus emission over large scales. The Galaxy Evolution Explorer will observe them as part of its deep 100 deg2 survey, as will Herschel. SWIRE includes âŒ9 deg2 of the unique large-area XMM Large Scale Structure hard X-ray imaging survey and is partly covered by the UKIDSS deep J and K survey. An extensive optical/near-IR imaging program is underway from the ground. The SWIRE data are nonproprietary; catalogs and images will be released twice yearly, beginning about 11 months after SIRTF launch. Details of the data products and release schedule are presented
Photometric redshifts in the SWIRE Survey
We present the SWIRE Photometric Redshift Catalogue, 1025119 redshifts of
unprecedented reliability and accuracy. Our method is based on fixed galaxy and
QSO templates applied to data at 0.36-4.5 mu, and on a set of 4 infrared
emission templates fitted to infrared excess data at 3.6-170 mu. The code
involves two passes through the data, to try to optimize recognition of AGN
dust tori. A few carefully justified priors are used and are the key to
supression of outliers. Extinction, A_V, is allowed as a free parameter. We use
a set of 5982 spectroscopic redshifts, taken from the literature and from our
own spectroscopic surveys, to analyze the performance of our method as a
function of the number of photometric bands used in the solution and the
reduced chi^2. For 7 photometric bands the rms value of
(z_{phot}-z_{spec})/(1+z_{spec}) is 3.5%, and the percentage of catastrophic
outliers is ~1%.
We discuss the redshift distributions at 3.6 and 24 mu. In individual fields,
structure in the redshift distribution corresponds to clusters which can be
seen in the spectroscopic redshift distribution. 10% of sources in the SWIRE
photometric redshift catalogue have z >2, and 4% have z>3, so this catalogue is
a huge resource for high redshift galaxies.
A key parameter for understanding the evolutionary status of infrared
galaxies is L_{ir}/L_{opt}, which can be interpreted as the specific
star-formation rate for starbursts. For dust tori around Type 1 AGN,
L_{tor}/L_{opt} is a measure of the torus covering factor and we deduce a mean
covering factor of 40%.Comment: 22 pages, 23 figures. Accepted for publication in MNRAS. Revised
28/2/08. Version with figures at full resolution at
http://astro.ic.ac.uk/~mrr/swirephotzcat/swirephotz5.pdf.g
Chandra and Spitzer unveil heavily obscured quasars in the SWIRE/Chandra Survey
Using the large multi-wavelength data set in the chandra/SWIRE Survey (0.6
square degrees in the Lockman Hole), we show evidence for the existence of
highly obscured (Compton-thick) AGN, estimate a lower limit to their surface
density and characterize their multi-wavelength properties. Two independent
selection methods based on the X-ray and infrared spectral properties are
presented. The two selected samples contain 1) 5 X-ray sources with hard X-ray
spectra and column densities > 10^24 cm-2, and 2) 120 infrared sources with red
and AGN-dominated infrared spectral energy distributions (SEDs). We estimate a
surface density of at least 25 Compton-thick AGN per square degree detected in
the infrared in the chandra/SWIRE field of which ~40% show distinct AGN
signatures in their optical/near-infrared SEDs, the remainings being dominated
by the host-galaxy emission. Only ~33% of all Compton-thick AGN are detected in
the X-rays at our depth (F(0.3-8 keV)>10^-15 erg/cm2/s.
We report the discovery of two sources in our sample of Compton-thick AGN,
SWIRE_J104409.95+585224.8 (z=2.54) and SWIRE_J104406.30+583954.1 (z=2.43),
which are the most luminous Compton-thick AGN at high-z currently known. The
properties of these two sources are discussed in detail with an analysis of
their spectra, SEDs, luminosities and black-hole masses.Comment: ApJ accepted (to appear in May 2006 issue, vol. 642, of ApJ) Figures
2, 3, and 14 have been degraded due to space consideration