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H. E. Smith

Publications and source records attributed to H. E. Smith.

At least 19 recordsLinked to original sources
arXiv2011

Specific star-formation and the relation to stellar mass from 0<z<2 as seen in the far-infrared at 70 and 160mu

We use the Spitzer Wide-area InfraRed Extragalactic Legacy Survey (SWIRE) to explore the specific star-formation activity of galaxies and their evolution near the peak of the cosmic far-infrared (FIR) background at 70 and 160um. We use a stacking analysis to determine the mean FIR properties of well defined subsets of galaxies at flux levels well below the FIR catalogue detection limits of SWIRE and other Spitzer surveys. We tabulate the contribution of different subsets of galaxies to the FIR background at 70um and 160um. These long wavelengths provide a good constraint on the bolometric, obscured emission. The large area provides good constraints at low z and in finer redshift bins than previous work. At all redshifts we find that the specific FIR Luminosity (sLFIR) decreases with increasing mass, following a trend L_FIR/M* propto M_* ^beta with beta =-0.38\pm0.14. This is a more continuous change than expected from the {Delucia2007} semi-analytic model suggesting modifications to the feedback prescriptions. We see an increase in the sLFIR by about a factor of ~100 from 0<z<2 and find that the sLFIR evolves as (1+z)^alpha with alpha=4.4\pm0.3 for galaxies with 10.5 < log M*/Msun < 12. This is considerably steeper than the {Delucia2007} semi-analytic model (alpha \sim 2.5). When separating galaxies into early and late types on the basis of the optical/IR spectral energy distributions we find that the decrease in sLFIR with stellar mass is stronger in early type galaxies (beta ~ -0.46), while late type galaxies exhibit a flatter trend (beta \sim -0.15). The evolution is strong for both classes but stronger for the early type galaxies. The early types show a trend of decreasing strength of evolution as we move from lower to higher masses while the evolution of the late type galaxies has little dependence on stellar mass. We suggest that in late-type galaxies we are seeing a consistently declining sSFR..

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arXiv2008

MAMBO 1.2mm 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 SWIRE survey. The sources are selected on the basis of bright 24um fluxes (f_24um>0.4mJy) and of stellar dominated near-infrared spectral energy distributions in order to favor z~2 starburst galaxies. The average 1.2mm 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 1.2mm flux (1.9+/-0.3 mJy) is higher than in other fields (1.1+/-0.2 mJy). The analysis of the sources multi-wavelength spectral energy distributions indicates that they are starburst galaxies with far-infrared luminosities ~10^12-10^13.3 Lsun, and stellar masses of ~0.2-6 x10^11 M_sun. Compared to sub-millimeter selected galaxies (SMGs), the SWIRE-MAMBO sources are among those with the largest 24um/millimeter 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 exhibit strong PAH features, and a warm dust continuum. The warm dust continuum contributes to ~34% of the mid-infrared emission, and is likely associated with an AGN component. This constribution is consistent with what is found in SMGs. The large 24um/1.2mm 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 ultra-luminous infrared sources at high-z, a class of objects that is rarely found in SMG samples. Our sample is the largest Spitzer-selected sample detected at millimeter wavelengths currently available.

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arXiv2008

Properties of dusty tori in AGN: I. The Case of SWIRE/SDSS Quasars

We derive the properties of dusty tori in Active Galactic Nuclei (AGN) 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, 2MASS J, H, and K data, whenever available and mid-Infrared (MIR) data from the Spitzer Wide-area InfraRed Extragalactic (SWIRE) Survey. The adopted model is that of Fritz et al., 2006. The fit is performed by standard chi^2 minimisation, the model however can be multi-component comprising a stellar and a starburst components, 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 infrared luminosity, L_IR, integrated in the wavelength range between 1 and 1000 micron, do not show significant variations with redshift, once the observational biases are taken into account. Objects with 70 micron detections, representing 25% 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%.

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arXiv2008

The SCUBA Half-Degree Extragalactic Survey (SHADES) - VIII. The Nature of Faint Submm Galaxies in SHADES, SWIRE and SXDF Surveys

We present the optical-to-submm spectral energy distributions for 33 radio & mid-IR identified submillimetre galaxies discovered via the SHADES 850 micron SCUBA imaging in the Subaru-XMM Deep Field (SXDF). Optical data for the sources comes from the Subaru-XMM Deep Field (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 micron fluxes. We then fit spectral energy distribution (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 infrared template fits are also used to resolve ambiguous identifications and cases of redshift aliasing. The redshift distribution obtained broadly matches previous results for submm sources and on the SHADES SXDF field. Our template fitting finds that AGN, present in about 10% of 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 fit 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 submm 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.

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arXiv2008

The nature of star formation in distant ultraluminous infrared galaxies selected in a remarkably narrow redshift range

We present mid-infrared spectra of thirty two high redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest-frame 1.6um, and an observed-frame 24um flux of >500muJy. Nearly all the sample reside in a redshift range of =1.71+/-0.15, and have rest-frame 1-1000um luminosities of 10^12.9 - 10^13.8 Lsun. Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features, and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is however evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2um/7.7um and 6.2um/11.2um flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10^10-10^11 Lsun rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4Kpc, and (2) star formation in unusually gas-rich regions.

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arXiv2007

Galaxy Counts at 24 Microns in the SWIRE Fields

This paper presents galaxy source counts at 24 microns in the six Spitzer Wide-field InfraRed Extragalactic (SWIRE) fields. The source counts are compared to counts in other fields, and to model predictions that have been updated since the launch of Spitzer. This analysis confirms a very steep rise in the Euclidean-normalized differential number counts between 2 mJy and 0.3 mJy. Variations in the counts between fields show the effects of sample variance in the flux range 0.5-10 mJy, up to 100% larger than Poisson errors. Nonetheless, a "shoulder" in the normalized counts persists at around 3 mJy. The peak of the normalized counts at 0.3 mJy is higher and narrower than most models predict. In the ELAIS N1 field, the 24 micron data are combined with Spitzer-IRAC data and five-band optical imaging, and these bandmerged data are fit with photometric redshift templates. Above 1 mJy the counts are dominated by galaxies at z less than 0.3. By 300 microJy, about 25% are between z ~ 0.3-0.8, and a significant fraction are at z ~ 1.3-2. At low redshifts the counts are dominated by spirals, and starbursts rise in number density to outnumber the spirals' contribution to the counts below 1 mJy.

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arXiv2007

Obscuration in extremely luminous quasars

The spectral energy distributions and infrared (IR) spectra of a sample of obscured AGNs selected in the mid-IR are modeled with recent clumpy torus models to investigate the nature of the sources, the properties of the obscuring matter, and dependencies on luminosity. The sample contains 21 obscured AGNs at z=1.3-3 discovered in the largest Spitzer surveys (SWIRE, NDWFS, & FLS) by means of their extremely red IR to optical colors. All sources show the 9.7micron silicate feature in absorption and have extreme mid-IR luminosities (L(6micron)~10^46 erg/s). The IR SEDs and spectra of 12 sources are well reproduced with a simple torus model, while the remaining 9 sources require foreground extinction from a cold dust component to reproduce both the depth of the silicate feature and the near-IR emission from hot dust. The best-fit torus models show a broad range of inclinations, with no preference for the edge-on torus expected in obscured AGNs. Based on the unobscured QSO mid-IR luminosity function, and on a color-selected sample of obscured and unobscured IR sources, we estimate the surface densities of obscured and unobscured QSOs at L(6micron)>10^12 Lsun, and z=1.3-3.0 to be about 17-22 deg^-2, and 11.7 deg^-2, respectively. Overall we find that ~35-41% of luminous QSOs are unobscured, 37-40% are obscured by the torus, and 23-25% are obscured by a cold absorber detached from the torus. These fractions constrain the torus half opening angle to be ~67 deg. This value is significantly larger than found for FIR selected samples of AGN at lower luminosity (~46 deg), supporting the receding torus scenario. A far-IR component is observed in 8 objects. The estimated far-IR luminosities associated with this component all exceed 3.3x10^12 Lsun, implying SFRs of 600-3000 Msun/yr. (Abridged)

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arXiv2007

Obscured and unobscured AGN populations in a hard-X-ray selected sample of the XMDS survey

Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength coverage. The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical - IR identification. The observed optical and IR spectral energy distributions of sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template and are grouped together in a single class of ``optically obscured'' AGN. They have ``red'' optical colors and generally show significant X-ray absorption from X-ray spectra or hardness ratios (N$_H > 10^{22}$ cm$^{-2}$). Sources with SEDs typical of type 1 AGN have ``blue'' optical colors and exhibit X-ray absorption in about 30% of cases. We performed a stacking analysis for obscured and type 1 AGN. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Gamma = 1.6. The subsample of objects fitted by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 - 1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than ``optical'' obscuration and its incidence is nearly constant with redshift and luminosity. This implies that X-ray absorption is not necessarily related to optical obscuration.

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arXiv2007

Spectral Energy Distributions of Hard X-ray selected AGNs in the XMDS 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.

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arXiv2007

Keck spectroscopy of z=1-3 ULIRGs from the Spitzer SWIRE survey

(Abridged) High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. We present Keck/LRIS optical spectroscopy of 35 z>1.4 luminous IR galaxies in the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) northern fields (Lockman Hole, ELAIS-N1, ELAIS-N2). The primary targets belong to the ``IR-peak'' class of galaxies, having the 1.6 micron (restframe) stellar feature detected in the IRAC Spitzer channels.The spectral energy distributions of the main targets are thoroughly analyzed, by means of spectro-photometric synthesis and multi-component fits (stars + starburst dust + AGN torus). The IR-peak selection technique is confirmed to successfully select objects above z=1.4, though some of the observed sources lie at lower redshift than expected. Among the 16 galaxies with spectroscopic redshift, 62% host an AGN component, two thirds being type-1 and one third type-2 objects. The selection, limited to r'<24.5, is likely biased to optically-bright AGNs. The SEDs of non-AGN IR-peakers resemble those of starbursts (SFR=20-500 Msun/yr) hosted in massive (M>1e11 Msun) galaxies. The presence of an AGN component provides a plausible explanation for the spectroscopic/photometric redshift discrepancies, as the torus produces an apparent shift of the peak to longer wavelengths. These sources are analyzed in IRAC and optical-IR color spaces. In addition to the IR-peak galaxies, we present redshifts and spectral properties for 150 objects, out of a total of 301 sources on slits.

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arXiv2006

AGN and Starburst Classification from Spitzer Mid-Infrared Spectra for High Redshift SWIRE Sources

Spectra have been obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope for 20 sources in the Lockman Hole field of the SWIRE survey. The sample is divided between sources with indicators of an obscured AGN, based primarily on X-ray detections of optically-faint sources, and sources with indicators of a starburst, based on optical and near-infrared spectral energy distributions (SEDs) which show a luminosity peak from stellar photospheric emission. Ten of the 11 AGN sources have IRS spectra which show silicate absorption or are power laws; only one AGN source shows PAH emission features. All 9 of the sources showing starburst SEDs in the near-infrared show PAH emission features in the IRS spectra. Redshifts are determined from the IRS spectra for all 9 starbursts (1.0 < z < 1.9) and 8 of the 11 AGN (0.6 < z < 2.5). Classification as AGN because of an X-ray detection, the classification as AGN or starburst derived from the photometric SED, and the IRS spectroscopic classification as AGN (silicate absorption) or starburst (PAH emission) are all consistent in 18 of 20 sources. The surface density for starbursts which are most luminous in the mid-infrared is less than that for the most luminous AGN within the redshift interval 1.7 < z < 1.9. This result implies that mid-infrared source counts at high redshift are dominated by AGN for f(24micron) > 1.0 mJy.

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arXiv2006

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 Legacy Survey (SWIRE) 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 AGN to optically selected red sequence galaxies. We 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-disk ratio. The (g-r) colour is found to be sensitive to the bulge-to-disk ratio for disk-dominated galaxies, whereas the log(L_{24}/L_{3.6}) colour is more sensitive for bulge-dominated systems. We identify ~18% (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. The number densities of optically red star-forming galaxies are found to correspond to ~13% of the total number density of our sample. In addition, these systems contribute ~13% of the total optical luminosity density, and 28% of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for "dry-mergers".

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arXiv2006

Luminosity functions for galaxies and quasars in the Spitzer Wide-Area Infrared Extra-galactic (SWIRE) Legacy survey

We construct rest-frame luminosity functions at 3.6, 4.5, 5.8, 8 and 24 microns over the redshift range 0<z<2 for galaxies and 0<z<4 for optical QSOs, using optical and infrared data from the Spitzer Wide-area InfraRed Extragalactic survey. The 3.6 and 4.5 micron galaxy LFs show evidence for moderate positive luminosity evolution up to z~1.5, consistent with the passive ageing of evolved stellar populations. Their comoving luminosity density was found to evolve passively, gradually increasing out to z~0.5-1 but flattening, or even declining, at higher redshift. Conversely, the 24 micron galaxy LF, which is more sensitive to obscured star formation and/or AGN activity, undergoes strong positive evolution, with the derived IR energy density and SFR density proportional to (1+z)^gamma with gamma=4.5^{+0.7}_{-0.6} and the majority of this evolution occurring since z~1. Optical QSOs, however, show positive luminosity evolution in all bands, out to the highest redshifts (3<z<4). Modelling as L* proportional to (1+z)^gamma gave gamma=1.3^{+0.1}_{-0.1} at 3.6 micron, gamma=1.0^{+0.1}_{-0.1} at 4.5 micron and stronger evolution at the longer wavelengths (5.8, 8 and 24 micron), of gamma~3. Comparison of the galaxy LFs to predictions from a semi-analytic model based on CDM indicate that an IMF skewed towards higher mass star formation in bursts compared to locally is preferred. As a result the currently inferred massive star formation rates in distant sub-mm sources may require substantial downwards revision.

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arXiv2006

The z=3 QSO Luminosity Function with SWIRE

We use a simple optical/infrared photometric selection of high redshift QSOs which identifies a Lyman Break in the optical and requires a red IRAC color to distinguish QSOs from common interlopers. We find 100 U-dropout (z=3) QSO candidates with r'<22 within 11.2 deg^2 in the ELAIS-N1 & ELAIS-N2 fields in the Spitzer Wide-Area Infrared Extragalactic (SWIRE) Legacy Survey. Spectroscopy of 10 candidates shows that they are all QSOs with 2.83<z<3.44. We use detailed simulations which incorporate variations in QSO SEDs, IGM transmission and imaging depth to derive a completeness of 85-90% between 3.0<z<3.4. The resulting luminosity function is two magnitudes fainter than SDSS and, when combined with those data, gives a faint end slop β= 1.62 \pm 0.18, consistent with measurements at z<2 and steeper than initial measurements at the same redshift.

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arXiv2006

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.

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arXiv2005

Morphological Studies of the SWIRE Galaxy Population in the UGC 10214 HST/ACS field

We present results of a morphological analysis of a small subset of the Spitzer Wide-area InfraRed Extragalactic survey (SWIRE) galaxy population. The analysis is based on public ACS data taken inside the SWIRE N1 field, which are the deepest optical high-resolution imaging available within the SWIRE fields as of today. Our reference sample includes 156 galaxies detected by both ACS and SWIRE. Among the various galaxy morphologies, we disentangle two main classes, spheroids (or bulge-dominated galaxies) and disk-dominated ones, for which we compute the number counts as a function of flux. We then limit our sample to objects with IRAC fluxes brighter than 10 microJy, estimated ~90% completeness limit of the SWIRE catalogues, and compare the observed counts to model predictions. We find that the observed counts of the spheroidal population agree with the expectations of a hierarchical model while a monolithic scenario predicts steeper counts. Both scenaria, however, under-predict the number of late-type galaxies. These observations show that the large majority (close to 80 per cent) of the 3.6 and 4.5 micron galaxy population, even at these moderately faint fluxes, is dominated by spiral and irregular galaxies or mergers.

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arXiv2004

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).

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