Dave Shupe

Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE 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. nWe 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.

First Insights into the Spitzer Wide-Area Infrared Extragalactic Legacy Survey (SWIRE) Galaxy Populations

We characterize the Spitzer Wide-area Infrared Extragalactic Legacy Survey (SWIRE) galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24 μm Spitzer data and deep U, g, r, i optical imaging within an area ~1/3 deg2 for ~16,000 Spitzer SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6 μm and almost 350,000 at 24 μm; ~70,000 of these will be five-band 3.6-24 μm detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations; however, significant samples of unusually blue objects in the [3.6]-[4.5] color are found, as well as many objects very red in the 3.6-24 μm mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts, and active galactic nuclei (AGNs) from z = 0.37 to 2.8, with luminosities from LIR = 1010.3 to 1013.7 L.We characterize the SWIRE galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24μ Spitzer data and deep U,g,r,i optical imaging within an area ∼1/3 sq. deg for ∼16,000 Spitzer-SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6μm and almost 350,000 at 24μm; ∼ 70,000 of these will be 5-band 3.6-24μ detections. The colors cover a broad range, generally well represented by redshifted spectral

Spectral Energy Distributions and Luminosities of Galaxies and Active Galactic Nuclei in the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey

We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24, 70, 160 mu) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The spectral energy distributions of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, 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 7%. The photometric redshifts are used to derive the 3.6 and 24 mu redshift distribution and to compare this with the predictions of models. For those sources with a clear mid infrared excess, relative to the galaxy starlight model used for the optical and near infrared, the mid and far infrared data are modelled in terms of the same dust emission templates. The proportions found of each template type are: cirrus 31%, M82 29%, Arp 220 10%, AGN dust tori 29%. 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.We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24, 70, 160 mu) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The spectral energy distributions of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, 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 7%. The photometric redshifts are used to derive the 3.6 and 24 mu redshift distribution and to compare this with the predictions of models. For those sources with a clear mid infrared excess, relative to the galaxy starlight model used for the optical and near infrared, the mid and far infrared data are modelled in terms of the same dust emission templates. The proportions found of each template type are: cirrus 31%, M82 29%, Arp 220 10%, AGN dust tori 29%. 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.We discuss optical associations, spectral energy distributions (SEDs), and photometric redshifts for Spitzer Wide-Area Infrared Extragalactic (SWIRE) Survey sources in the European Large-Area ISO Survey (ELAIS) N1 area and the Lockman Validation Field (VF). The band-merged Infrared Array Camera (IRAC) (3.6, 4.5, 5.8, and 8.0 μm) and Multiband Imaging Photometer for Spitzer (24, 70, and 160 μm) data have been associated with optical UgriZ data from the Isaac Newton Telescope Wide Field Survey in ELAIS N1 and with our own optical Ugri data in Lockman-VF. Criteria for eliminating spurious infrared sources and for carrying out star-quasar-galaxy separation are discussed, and statistics of the identification rate are given. Thirty-two percent of sources in the ELAIS N1 field are found to be optically blank (to r = 23.5) and 16% in Lockman-VF (to r = 25). The SEDs of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, are modeled in terms of a simple set of galaxy and quasar templates in the optical and near-infrared (NIR), and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and active galactic nucleus [AGN] dust torus) in the mid-infrared. The optical data, together with the IRAC 3.6 and 4.5 μm 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 7%. Although further spectroscopic data are needed to confirm this result, the prospect of determining good photometric redshifts for much of the SWIRE survey, expected to yield over 2 million extragalactic objects, is excellent. Some modifications to the optical templates were required in the previously uninvestigated wavelength region 2–5 μm. The photometric redshifts are used to derive the 3.6 and 24 μm redshift distribution and to compare this with the predictions of models. For those sources with a clear mid-infrared excess, relative to the galaxy starlight model used for the optical and NIR, the mid- and far-infrared data are modeled in terms of the same dust emission templates (cirrus, M82, Arp 220, and AGN dust torus). The proportions found of each template type are cirrus, 31%; M82, 29%; Arp 220, 10%; and AGN dust tori, 29%. The distribution of the different infrared SED types in the LIR/Lopt versus LIR plane, where LIR and Lopt are the infrared and optical bolometric luminosities, respectively, is discussed. There is an interesting population of luminous cool cirrus galaxies with LIR > Lopt, implying a substantial dust optical depth. Galaxies with Arp 220–like SEDs, of which there are a surprising preponderance compared with preexisting source count models, tend to have high ratios of infrared to optical bolometric luminosity, consistent with having very high extinction. There is also a high proportion of galaxies whose mid-infrared SEDs are fitted by an AGN dust torus template (29%). Of these only 8% of these are type 1 AGNs according to the optical-NIR template fitting, whereas 25% are fitted with galaxy templates in the optical-NIR and have LIR > Lopt and so have to be type 2 AGN. The remainder have LIR < Lopt and so can be Seyfert galaxies, in which the optical AGN fails to be detected against the light of the host galaxy. The implied dust covering factor, ≥75%, is much higher than that inferred for bright optically selected quasars.

The SCUBA Half-Degree Extragalactic Survey (SHADES) – VIII. The nature of faint submillimetre galaxies in SHADES, SWIRE and SXDF surveys

We present the optical-to-submillimetre spectral energy distributions (SEDs) for 33 radio and mid-infrared (mid-IR) identified submillimetre galaxies discovered via the SHADES 850-μm SCUBA imaging in the Subaru-XMM Deep Field (SXDF). Optical data for the sources come from the SXDF and mid- and far-IR fluxes from SWIRE. We obtain photometric redshift estimates for our sources using optical and IRAC 3.6- and 4.5-μm fluxes. We then fit SED templates to the longer wavelength data to determine the nature of the far-IR emission that dominates the bolometric luminosity of these sources. The IR template fits are also used to resolve ambiguous identifications and cases of redshift aliasing. The redshift distribution obtained broadly matches previous results for submillimetre sources and on the SHADES SXDF field. Our template fitting finds that active galactic nuclei, while present in about 10 per cent of our sources, do not contribute significantly to their bolometric luminosity. Dust heating by starbursts, with either Arp220 or M82 type SEDs, appears to be responsible for the luminosity in most sources (23/33 are fitted by Arp220 templates, 2/33 by the warmer M82 templates). 8/33 sources, in contrast, are fitted by a cooler cirrus dust template, suggesting that cold dust has a role in some of these highly luminous objects. Three of our sources appear to have multiple identifications or components at the same redshift, but we find no statistical evidence that close associations are common among our SHADES sources. Examination of rest-frame K-band luminosity suggests that ‘downsizing’ is underway in the submillimetre galaxy population, with lower redshift systems lying in lower mass host galaxies. Of our 33 identifications six are found to be of lower reliability but their exclusion would not significantly alter our conclusions.

A Complete Multiwavelength Characterization of Faint Chandra X-Ray Sources Seen in the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Survey

We exploit deep combined observations with Spitzer and Chandra of the Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) in the ELAIS N1 region to investigate the nature of the faint X-ray and IR sources in common, to identify active galactic nucleus (AGN)/starburst diagnostics, and to study the sources of the X-ray and IR cosmic backgrounds (XRB and CIRB). In the 17 × 17 area of the Chandra ACIS-I image there are approximately 3400 SWIRE near-IR sources with 4 σ detections in at least two Infrared Array Camera (IRAC) bands and 988 sources detected at 24 μm with the Multiband Imaging Photometer (MIPS) brighter than S24 sime 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 S0.5–8 keV > 10-15 ergs cm-2 s-1. We have constructed spectral energy distributions (SEDs) for each source using data from the four IRAC wavebands, Chandra fluxes in the hard (2–8 keV) and soft (0.5–2 keV) X-rays, 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 IR 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 IR spectral slopes, and optical morphologies. Although we have spectroscopic redshifts for only a minority of the Chandra sources (10 type 1 QSOs or Seyfert sources and three galaxies), 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 (QSOs or Seyfert 1), 23% display optical/IR spectra typical of type 2 AGNs, while the remaining 38% show starburst-like or even normal galaxy spectra (including five passively evolving early-type galaxies). Since we prove that all these galaxies are dominated by AGN emission in X-rays (considering their large 0.5–8 keV rest-frame X-ray luminosities and their high X-ray to IR flux ratios), this brings the fraction of type 1 AGNs to 80% of the type 2 AGNs; 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. Our analysis of the mid-IR MIPS 24 μm–selected sources, making up ~50% of the CIRB, shows that the fraction of those dominated by an AGN (either type 1 or type 2) is relatively constant with the IR flux and around 10%–15%. Our combined IR and hard X-ray observations allow us to verify that the dust covering fraction in type 1 AGNs is widely distributed between ~10% and 100%. A significant fraction, from 15% to 30% or more, of the sources of the XRB are hosted in galaxies whose optical/IR spectra are dominated by starburst (or normal galaxy) emission and for which only the hard X-ray spectra reveal the presence of a moderately luminous hidden AGN.

Properties of dusty tori in active galactic nuclei – I. The case of SWIRE/SDSS quasars

We derive the properties of dusty tori in active galactic nuclei from the comparison of observed spectral energy distributions (SEDs) of SDSS quasars and a precomputed grid of torus models. The observed SEDs comprise SDSS photometry, Two-Micron All-Sky Survey J, H and K data, whenever available, and mid-infrared (mid-IR) data from the Spitzer Wide-area InfraRed Extragalactic Survey. The adopted model is that of Fritz, Franceschini & Hatziminaoglou. The fit is performed by standard chi(2)-minimization; the model, however, can be a multicomponent comprising a stellar and a starburst component, whenever necessary. Models with low equatorial optical depth, tau(9.7), were allowed as well as traditional models with tau(9.7) >= 1.0, corresponding to A(V) >= 22 and the results were compared. Fits using high optical depth tori models only produced dust more compactly distributed than in the configuration where all tau(9.7) models were permitted. Tori with decreasing dust density with the distance from the centre were favoured while there was no clear preference for models with or without angular variation of the dust density. The computed outer radii of the tori are of some tens of parsecs large but can reach, in a few cases, a few hundreds of parsecs. The mass of dust, M-Dust, and IR luminosity, L-IR, integrated in the wavelength range between 1 and 1000 mu m, do not show significant variations with redshift, once the observational biases are taken into account. Objects with 70-mu m detections, representing 25 per cent of the sample, are studied separately and the starburst contribution (whenever present) to the IR luminosity can reach, in the most extreme but very few cases, 80 per cent.

MAMBO 1.2 mm OBSERVATIONS OF LUMINOUS STARBURSTS AT z ∼ 2 IN THE SWIRE FIELDS

We report on-off pointed MAMBO observations at 1.2 mm of 61 Spitzer-selected star-forming galaxies from the Spitzer Wide Area Infrared Extragalactic Legacy survey (SWIRE). The sources are selected on the basis of bright 24 μm fluxes (f _(24μm) > 0.4 mJy) and of stellar dominated near-infrared spectral energy distributions in order to favor z ~ 2 starburst galaxies. The average 1.2 mm flux for the whole sample is 1.5 ± 0.2 mJy. Our analysis focuses on 29 sources in the Lockman Hole field where the average 1.2 mm flux (1.9 ± 0.3 mJy) is higher than in other fields (1.1 ± 0.2 mJy). The analysis of the multiwavelength spectral energy distributions indicates that these sources are starburst galaxies with far-infrared luminosities from 10^(12) to 10^(13.3)L_☉ , and stellar masses of ~0.2-6 × 10^(11)M_☉ . Compared to submillimeter selected galaxies (SMGs), the SWIRE-MAMBO sources are among those with the largest 24 μm/1.2 mm flux ratios. The origin of such large ratios is investigated by comparing the average mid-infrared spectra and the stacked far-infrared spectral energy distributions of the SWIRE-MAMBO sources and of SMGs. The mid-infrared spectra, available for a handful of sources, exhibit strong polycyclic aromatic hydrocarbon (PAH) features, and a warm dust continuum. The warm dust continuum contributes ~34% of the mid-infrared emission, and is likely associated with an AGN component. This contribution is consistent with what is found in SMGs. The large 24 μm/1.2 mm flux ratios are thus not due to AGN emission, but rather to enhanced PAH emission compared to SMGs. The analysis of the stacked far-infrared fluxes yields warmer dust temperatures than typically observed in SMGs. Our selection favors warm ultraluminous infrared sources at high-z, a class of objects that is rarely found in SMG samples. Indeed SMGs are not common among bright 24 μm sources (e.g., only about 20% of SMGs have f_(24 μm) > 0.4 mJy). Our sample is the largest Spitzer-selected sample detected at millimeter wavelengths currently available.

Mid-infrared spectroscopy of Spitzer-selected ultra-luminous starbursts at z ~ 2

Context. Spitzers wide-field surveys and followup capabilities have allowed a new breakthrough in mid-IR spectroscopy up to red- shifts ≥2, especially for 24 μm detected sources. Aims. We want to study the mid-infrared properties and the starburst and AGN contributions, of 24 μm sources at z ∼ 2, through analysis of mid-infrared spectra combined with millimeter, radio, and infrared photometry. Mid-infrared spectroscopy allows us to recover accurate redshifts. Methods. A complete sample of 16 Spitzer-selected sources (ULIRGs) believed to be starbursts at z ∼ 2( 5.8μm-peakers) was selected in the (0.5 deg 2 ) J1064+56 SWIRE Lockman Hole field (Lockman-North). These sources have S 24 μm > 0.5 mJy, a stellar emission peak redshifted to 5.8 μm, and ra > 23. The entire sample was observed with the low resolution units of the Spitzer/IRS infrared spectrograph. These sources have 1.2 mm observations with IRAM 30 m/MAMBO and very deep 20 cm observations from the VLA. Nine of our sources also benefit from 350 μm observation and detection from CSO/SHARC-II. All these data were jointly analyzed. Results. The entire sample shows good quality IRS spectra dominated by strong PAH features. The main PAH features at 6.2, 7.7, 8.6, and 11.3 μ mh ave highS /N average luminosities of 2.90 ± 0.31, 10.38 ± 1.09, 3.62 ± 0.27, and 2.29 ± 0.26 × 10 10 L� , respec- tively. Thanks to their PAH spectra, we derived accurate redshifts spanning from 1.750 to 2.284. The average of these redshifts is 2.017 ± 0.038. This result confirms that the selection criteria of 5.8 μm-peakers associated with a strong detection at 24 μ ma re reliable to select sources at z ∼ 2. We have analyzed the different correlations between PAH emission and infrared, millimeter, and radio emissions. Practically all our sources are strongly dominated by starburst emission, with only one source showing an important AGN contribution. We have also defined two subsamples based on the equivalent width at 7.7 μm to investigate AGN contributions. Conclusions. Our sample contains strong starbursts and represents a particularly 24 μm-bright class of SMGs. The very good cor- relation between PAH and far-IR luminosities is now confirmed in high-z starburst ULIRGs. These sources show a small AGN contribution to the mid-IR, around ∼20% or less in most cases.

Optical and infrared diagnostics of SDSS galaxies in the SWIRE survey

We present the rest-frame optical and infrared colours of a complete sample of 1114 z < 0.3 galaxies from the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star-forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies. n nWe propose that the optical (g-r) colour and infrared log(L-24/L-3.6) colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The (g-r) colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the log (L-24/L-3.6) colour is more sensitive for bulge-dominated systems. n nWe identify similar to 18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star-forming galaxies with blue optical colours. Using emission-line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star-forming systems. The red (g-r) colour of the star-forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component. n nThe number densities of optically red star-forming galaxies are found to correspond to similar to 13 per cent of the total number density of our sample. In addition, these systems contribute similar to 13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for dry mergers.

SPIRE point source photometry: within the Herschel interactive processing environment (HIPE)

The different algorithms appropriate for point source photometry on data from the SPIRE instrument on-board the Herschel Space Observatory, within the Herschel Interactive Processing Environment (HIPE) are compared. Point source photometry of a large ensemble of standard calibration stars and dark sky observations is carried out using the 4 major methods within HIPE: SUSSEXtractor, DAOphot, the SPIRE Timeline Fitter and simple Aperture Photometry. Colour corrections and effective beam areas as a function of the assumed source spectral index are also included to produce a large number of photometric measurements per individual target, in each of the 3 SPIRE bands (250, 350, 500μm), to examine both the accuracy and repeatability of each of the 4 algorithms. It is concluded that for flux densities down to the level of 30mJy that the SPIRE Timeline Fitter is the method of choice. However, at least in the 250 and 350μm bands, all 4 methods provide photometric repeatability better than a few percent down to at approximately 100mJy. The DAOphot method appears in many cases to have a systematic offset of ∼8 % in all SPIRE bands which may be indicative of a sub-optimal aperture correction. In general, aperture photometry is the least reliable method, i.e. largest scatter between observations, especially in the longest wavelength band. At the faintest fluxes, <30mJy, SUSSEXtractor or DAOphot provide a better alternative to the Timeline Fitter.