D. L. Shupe

Spectral Energy Distributions of Hard X-Ray Selected Active Galactic Nuclei in the XMM-Newton Medium Deep Survey

We present the spectral energy distributions (SEDs) of a hard X-ray selected sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and 132 are AGNs. The sources are detected in a 1 square degree area of the XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys, and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2 (50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E>10 keV are too faint to be detected even in the deepest current radio surveys.We present the SEDs of a hard X-ray selected sample containing 136 sources with F_(2-10 keV) > 10^(-14) erg cm^(-2) s^(-1); 132 are AGNs. The sources are detected in a 1 deg^2 area of the XMM-Newton Medium Deep Survey where optical data from the VVDS and CFHTLS and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with σ(1 + z) = 0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17^(+9)_(-6)% of AGNs show star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33^(+6)_(-1)%) and AGN2 (50^(+6)_(-11)%). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the infrared SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low L_(3-20 μm)/L^(corr)_(0.5-10 keV) ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E > 10 keV are too faint to be detected even in the deepest current radio surveys.

SWIRE: The SIRTF Wide-Area Infrared Extragalactic Survey

The largest of the SIRTF Legacy programs, SWIRE will survey 65 sq. deg. in seven high latitude fields selected to be the best wide low-extinction windows into the extragalactic sky. SWIRE will detect millions of spheroids, disks and starburst galaxies to z>3 and will map L* and brighter systems on scales up to 150 Mpc at z∼0.5–1. It will also detect ∼104 low extinction AGN and large numbers of obscured AGN. An extensive program of complementary observations is underway. The data are non-proprietary and will be made available beginning in Spring 2004.

Obscured and unobscured active galactic nuclei in the Spitzer Space Telescope First Look Survey

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2 extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_(8.0) ~ 1 mJy. We have investigated the properties of 43 objects with S_(8.0) ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_(8.0) ~ 1 mJy.

Absolute Calibration and Characterization of the Multiband Imaging Photometer for Spitzer. I. The Stellar Calibrator Sample and the 24 μm Calibration

We present the stellar calibrator sample and the conversion from instrumental to physical units for the 24 μm channel of the Multiband Imaging Photometer for Spitzer (MIPS). The primary calibrators are A stars, and the calibration factor based on those stars is MJy sr^−1 (DN s^−1)^−1, with a nominal uncertainty of 2%. We discuss the data reduction procedures required to attain this accuracy; without these procedures, the calibration factor obtained using the automated pipeline at the Spitzer Science Center is lower. We extend this work to predict 24 μm flux densities for a sample of 238 stars that covers a larger range of flux densities and spectral types. We present a total of 348 measurements of 141 stars at 24 μm. This sample covers a factor of 460 in 24 μm flux density, from 8.6 mJy up to 4.0 Jy. We show that the calibration is linear over that range with respect to target flux and background level. The calibration is based on observations made using 3 s exposures; a preliminary analysis shows that the calibration factor may be 1% and 2% lower for 10 and 30 s exposures, respectively. We also demonstrate that the calibration is very stable: over the course of the mission, repeated measurements of our routine calibrator, HD 159330, show a rms scatter of only 0.4%. Finally, we show that the point-spread function (PSF) is well measured and allows us to calibrate extended sources accurately; Infrared Astronomy Satellite (IRAS) and MIPS measurements of a sample of nearby galaxies are identical within the uncertainties.

HerMES: The SPIRE confusion limit

We report on the sensitivity of SPIRE photometers on the Herschel Space Observatory. Specifically, we measure the confusion noise from observations taken during the Science Demonstration Phase of the Herschel Multi-tiered Extragalactic Survey. Confusion noise is defined to be the spatial variation of the sky intensity in the limit of infinite integration time, and is found to be consistent among the different fields in our survey at the level of 5.8, 6.3 and 6.8 mJy/beam at 250, 350 and 500 microns, respectively. These results, together with the measured instrument noise, may be used to estimate the integration time required for confusion-limited maps, and provide a noise estimate for maps obtained by SPIRE.

HerMES : SPIRE galaxy number counts at 250, 350, and 500 μm

Emission at far-infrared wavelengths makes up a significant fraction of the total light detected from galaxies over the age of Universe. Herschel provides an opportunity for studying galaxies at the peak wavelength of their emission. Our aim is to provide a benchmark for models of galaxy population evolution and to test pre-existing models of galaxies. With the Herschel Multi-tiered Extra-galactic survey, HerMES, we have observed a number of fields of different areas and sensitivity using the SPIRE instrument on Herschel. We have determined the number counts of galaxies down to ~20 mJy. Our constraints from directly counting galaxies are consistent with, though more precise than, estimates from the BLAST fluctuation analysis. We have found a steep rise in the Euclidean normalised counts <100 mJy. We have directly resolved ~15% of the infrared extra-galactic background at the wavelength near where it peaks.

Photometric redshifts in the SWIRE Survey

We present the SWIRE Photometric Redshift Catalogue 1 025 119 redshifts of unprecedented reliability and of accuracy comparable with or better than previous work. Our methodology is based on fixed galaxy and quasi-stellar object templates applied to data at 0.36–4.5 μm, and on a set of four infrared emission templates fitted to infrared excess data at 3.6–170 μm. The galaxy templates are initially empirical, but are given greater physical validity by fitting star formation histories to them, which also allows us to estimate stellar masses. The code involves two passes through the data, to try to optimize recognition of active galactic nucleus (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. The full reduced χ^2_ν (z) distribution is given for each source, so the full error distribution can be used, and aliases investigated. We use a set of 5982 spectroscopic redshifts, taken from the literature and from our own spectroscopic surveys, to analyse the performance of our method as a function of the number of photometric bands used in the solution and the reduced χ^2_ν . For seven photometric bands (5 optical + 3.6, 4.5 μm), the rms value of (z_(phot)−z_(spec)/(1 +z_(spec) is 3.5 per cent, and the percentage of catastrophic outliers [defined as >15 per cent error in (1 +z)], is ∼1 per cent. These rms values are comparable with the best achieved in other studies, and the outlier fraction is significantly better. The inclusion of the 3.6- and 4.5-μm IRAC bands is crucial in supression of outliers. We discuss the redshift distributions at 3.6 and 24 μm. In individual fields, structure in the redshift distribution corresponds to clusters which can be seen in the spectroscopic redshift distribution, so the photometric redshifts are a powerful tool for large-scale structure studies. 10 per cent of sources in the SWIRE photometric redshift catalogue have z > 2, and 4 per cent 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) . For cirrus galaxies this is a measure of the mean extinction in the interstellar medium of the galaxy. There is a population of ultraluminous galaxies with cool dust and we have shown SEDs for some of the reliable examples. For starbursts, we estimate the specific star formation rate, φ_*/M_* . Although the very highest values of this ratio tend to be associated with Arp220 starbursts, by no means all ultraluminous galaxies are. We discuss an interesting population of galaxies with elliptical-like spectral energy distributions in the optical and luminous starbursts in the infrared. 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 per cent. Our infrared templates also allow us to estimate dust masses for all galaxies with an infrared excess.

Visual and Near-Infrared Imaging of Ultraluminous Infrared Galaxies: The IRAS 2 Jy Sample

New near-infrared and visual images at 2.2 μm and 6550 A are presented for 46 galaxies having infrared luminosities of L_(IR) > 8.5 x 10^(11) L_⊙, 60 μm flux densities greater than 1.94 Jy, and declinations greater than -35°. These galaxies make up a significant fraction of a complete, northern hemisphere sample of ultraluminous infrared galaxies. Visual and/or near-infrared imaging data now exist for 56 ultraluminous infrared galaxies out to nearly 50 000 km s^(-1). Of these 56 galaxies, 53 (95%) show evidence for current or past interactions. Among these systems, there are a large variety of visual morphologies, including strongly interacting pairs with apparent tidal tails, as well as single, distorted galaxies with close double nuclei. There are three galaxies which, to the limits of the imaging data, do not appear to have suffered a recent interaction or merger. Approximately 47% (25/53) of the interacting systems have double nuclei, with projected nuclear separations ranging from 0.3 to 48 kpc. Seven systems have nuclear separations larger than 10 kpc. If the 53 interacting galaxies are viewed as stages in the evolution of pairs of interacting spiral galaxies to a single, luminous AGN or starburst, the present imaging data can be used to estimate the lifetime of the bright infrared phase. Including only those sample galaxies with morphological evidence for interactions, we calculate a lower and an upper limit to the lifetime of the ultraluminous infrared phase of the sample as a whole to be 2 x 10^8 and 2 x 10^9 yr, respectively. Comparison of these dynamical estimates to models of the mergers of gas-rich galaxies and the rates at which fuel is exhausted by starbursts or AGN suggests the lifetime of the ultraluminous phase lies much closer to the smaller of these two values. Selecting galaxies based upon luminous infrared activity clearly biases the sample towards merging galaxies with small physical separations. However, the existence of pairs with large separations indicates that the ultraluminous phase may in some cases start early during the merger process. Alternatively, these systems may contain unresolved third nuclei responsible for triggering the ultraluminous activity. We briefly compare our results to recent models of merging spiral galaxies.

Mid- and far-infrared luminosity functions and galaxy evolution from multiwavelength Spitzer observations up to z ~ 2.5

Context. Studies of the infrared (IR) emission of cosmic sources have proven essential to constraining the evolutionary history of cosmic star formation and the gravitational accretion of nuclear black holes, because many of these events occur inside heavily dust-extinguished environments. Aims. The Spitzer Space Telescope has provided a large amount of data to constrain the nature and cosmological evolution of infrared source populations. In the present paper we exploit a large homogeneous dataset to derive a self-consistent picture of IR emission based on the time-dependent λ_(eff) = 24, 15, 12, and 8 μm monochromatic and bolometric IR luminosity functions (LF) over the full 0 < z < 2.5 redshift range. Methods. Our present analysis is based on a combination of data from deep Spitzer surveys of the VIMOS VLT Deep Survey (VVDS-SWIRE) and GOODS fields. To our limiting flux of S_(24) = 400 μJy, our sample derived from VVDS-SWIRE includes 1494 sources, and 666 and 904 sources brighter than S_(24) = 80 μJy are catalogued in GOODS-S and GOODS-N, respectively, for a total area of ~0.9 square degrees. Apart from a few galaxies, we obtain reliable optical identifications and redshifts for all these sources, providing a rich and robust dataset for our luminosity function determination. The final combined reliable sample includes 3029 sources, the fraction with photometric redshifts being 72% over all redshifts and almost all galaxies at z > 1.5. Based on the multiwavelength information available in these areas, we constrain the LFs at 8, 12, 15, and 24 μm. We also infer the total IR luminosities from our best-fit model of the observed SEDs of each source, and use this to derive the bolometric (8–1000 μm) LF and comoving volume emissivity to z ~ 2.5. Results. In the redshift interval 0 1. The mean redshift of the peak in the source number density shifts with luminosity: the brightest IR galaxies appear to form stars at earlier cosmic times (z > 1.5), while star formation in the less luminous galaxies continues until more recent epochs (z ~ 1 for L_(IR) < 10^(11)_☉), in overall agreement with similar analyses in the literature. Conclusions. Our results are indicative of a rapid increase in the galaxy IR comoving volume emissivity up to z ~ 1 and a constant average emissivity at z > 1. We also appear to measure a difference in the evolutionary rate of the source number densities as a function of luminosity, which is consistent with the downsizing evolutionary patterns reported for other samples of cosmic sources.

HerMES: Far infrared properties of known AGN in the HerMES fields

Nuclear and starburst activity are known to often occur concomitantly. Herschel-SPIRE provides sampling of the FIR SEDs of type 1 and type 2 AGN, allowing for the separation between the hot dust (torus) and cold dust (starburst) emission. We study large samples of spectroscopically confirmed type 1 and type 2 AGN lying within the Herschel Multi-tiered Extragalactic Survey (HerMES) fields observed during the science demonstration phase, aiming to understand their FIR colour distributions and constrain their starburst contributions. We find that one third of the spectroscopically confirmed AGN in the HerMES fields have 5-sigma detections at 250um, in agreement with previous (sub)mm AGN studies. Their combined Spitzer-MIPS and Herschel-SPIRE colours - specifically S(250)/S(70) vs. S(70)/S(24) - quite clearly separate them from the non-AGN, star-forming galaxy population, as their 24-um flux is dominated by the hot torus emission. However, their SPIRE colours alone do not differ from those of non-AGN galaxies. SED fitting shows that all those AGN need a starburst component to fully account for their FIR emission. For objects at z > 2, we find a correlation between the infrared luminosity attributed to the starburst component, L(SB), and the AGN accretion luminosity, L(acc), with L(SB) propto L(acc)^0.35. Type 2 AGN detected at 250um show on average higher L(SB) than type 1 objects but their number is still too low to establish whether this trend indicates stronger star-formation activity.