Mon. Not. R. Astron. Soc. 000, 1–6 () Printed 10 October 2006 (MN LATEX style file v2.2)

Evidence for a large fraction of Compton-thick quasars at

high-redshift

Alejo Martınez-Sansigre1,2⋆, Steve Rawlings1, David G. Bonfield1, Silvia Mateos3,

Chris Simpson4, Mike Watson3, Omar Almaini5, Sebastien Foucaud5,

Kazuhiro Sekiguchi6, Yoshihiro Ueda7

1Astrophysics, Department of Physics,University of Oxford, Keble Road, Oxford OX1 3RH, UK2Max-Planck-Institut fur Astronomie, Koningstuhl-17, D-69117, Heidelberg, Germany3Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK4Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK5School of Physics and Astronomy, University of Nottingham, University Park, Nottingham NG7 2RD, UK6Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, Hawai’i 96720, USA7Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan

ABSTRACT

Using mid-infrared and radio selection criteria, we pre-select a sample of candidatehigh-redshift type-2 quasars in the Subaru XMM-Newton Deep Field (SXDF). Tofilter out starburst contaminants, we use a bayesian method to fit the spectral energydistributions (SEDs) between 24-µm and B-band, obtain photometric redshifts, andidentify the best candidates for high-z type-2 quasars. This leaves us with 12 zphot >

1.7 type-2 quasar candidates in an area ∼ 0.8 deg2, of which only two have secureX-ray detections. Given the large bolometric luminosities and redshifts of the objects,the lack of X-ray detections suggests extreme absorbing columns NH > 1028 m−2

are typical. We conclude that we have found a population of “Compton-thick” high-redshift type-2 quasars, at least comparable to, and probably larger than the type-1quasar population.

Key words: quasars:general-galaxies:nuclei-galaxies:active-X-rays:galaxies

1 INTRODUCTION

High-redshift quasars with column densities NH ∼> 1028 m−2

are so heavily absorbed that they are barely detectablein even the most sensitive hard X-ray surveys (see, e.g.,Brandt & Hasinger 2005), and are known as “Compton-thick”. While in the local Universe, 40% of active galacticnuclei (AGN) are found to be Compton-thick (Risaliti et al.1999), the fraction of high-redshift quasars with such ab-sorbing columns is currently unkown.

An alternative to (hard) X-ray selection is in the mid-infrared, where the obscuration due to dust becomes small.AGN are powerful mid-infrared emitters due to dust sur-rounding the accretion disk (the torus) reprocessing the UVand X-ray photons from the central engine. AGN invisi-ble in X-rays were indeed found by Donley et al. (2005),who used a mid-infrared and radio excess criterion to toselect a sample of AGN. The majority of these AGN are

⋆ E-mail: martinez@mpia-hd.mpg.de (AMS)

at z ∼< 1 and are better described as Seyfert-2s. The au-thors found that while the sources detected in the X-raywere unlikely to be Compton-thick, the AGN not detectedin the X-ray (∼20% of their sample) could be Compton-thick. The obscured and gravitationally-lensed quasar IRASFSC10214+4724, at z ∼ 2.3 has also been found to beCompton-thick (Alexander et al. 2005a).

Polletta et al. (2006) use two different selection crite-ria, X-ray and infrared, to look for Compton-thick AGN inan area of 0.6 deg2. Their X-ray selection, with a flux limit∼10−17 W m−2 in the 2.5-8 keV band, finds 5 Compton-thick AGN. Of these 2 are spectroscopically-confirmed high-redshift quasars Of the infrared-selected sources, the strictSED criteria imposed on the sources guarantee AGN, but arelikely to exclude heavily obscured (AV ∼> 20) high-redshift(z > 1) sources which do not show power-law mid-infraredspectral energy distributions (SEDs), but have significantcontributions from stellar light at 3.6 and 4.5 µm. It is notsuprising, therefore, that they find a smaller Compton-thickfraction (∼10%) than expected in their infrared-selected

c© RAS

2 A. Martınez-Sansigre et al.

sample, as it is probably biased against heavily obscuredAGN.

In this letter, we present a sample of obscured (type-2) high-redshift quasars, found to be Compton-thick, se-lected from mid-infrared and radio data in the Subaru-XMM-Newton Deep Field (SXDF).

2 SAMPLE SELECTION AND DATASET

Martınez-Sansigre et al. (2005), hereafter MS05, found apopulation of high-redshift type-2 quasars at least as numer-ous as the unobscured (type-1) population and which possi-bly outnumber the type-1s by ∼2-3:1. If these type-2s indeedoutnumber the type-1s then some of these mid-infrared andradio-selected quasars are expected to be Compton-thick,since the ratio of type-2 to type-1 quasars in X-ray selectedsamples is ∼1:1 (e.g. Zheng et al. 2004).

We proceed to select a similar sample to that of MS05,in the Subaru XMM-Newton Deep Field, where deep X-raydata are available. In this work we decrease the lower ra-dio flux density criterion, to increase the number of quasars.This is important as here we have a smaller area (∼0.8 deg2

as opposed to 3.8 deg2 in the MS05 sample). The selectioncriteria used here are S24 µm > 300 µJy, S3.6 µm 6 45 µJy,and 100 µJy 6 S1.4GHz 6 2 mJy, which yields 38 candidates.For a detailed discussion of these criteria see Martınez-Sansigre et al. (2006, MS06).

As a brief summary, the two mid-infrared criteria areable to target z ∼ 2 type-2 quasars with Av ∼> 5, but willalso allow z ∼< 1 ultra-luminous infrared galaxies (ULIRGs)in the sample. These ULIRGs will have radio luminositiesfollowing the FIR-radio correlation (Condon 1992), and soby choosing a high-enough radio flux, one can cut out allbut the most extreme starburst contaminants. The MS05criteria were carefully chosen to avoid starburst galaxies,and lowering the radio criterion has the disadvantage thatless extreme starbursts are allowed in the sample.

The mid-infrared data were obtained by using theSWIRE DR2 (Surace et al. 2005), which covers the SXDF,and has a 5σ flux density limit of ∼250 µJy at 24 µm, and14 µJy and 15 µJy at 3.6 and 4.5 µm respectively. The 1.4GHz catalogue used is described in Simpson et al. (2006)and comes from a B-array (∼5×4 arcsec2 beam) VLA sur-vey with a flux-density limit of 100 µJy (∼5-7 σ).

The spectroscopic completeness of the SXDF are poorat this stage, so we decided to undertake this preliminarystudy using photometric redshifts. This was much helped bythe fact that, for most of our target objects, the optical tonear-infrared light originates from the host galaxy only, asthe quasar is severely obscured at these wavelengths (exceptthe object ID135, see Table 1 and supplementary SEDs1).The dataset for this came from two sources. The opticaldata were obtained from the Subaru B,V,R,i’ and z’ imag-ing (Sekiguchi et al., in prep.). The near-infrared data (J andK) from the UKIDSS ultra-deep survey (UDS) DR1 (War-ren et al. 2006). The X-ray data were obtained with theXMM-Newton observatory, which covered the SXDF fieldin 7 pointings, with exposures of 50 ks each, except for the

1 http://www.mpia.de/homes/martinez/sxdftype2seds.pdf

central pointing, which was observed for 100 ks (Ueda et al.,in prep.).

3 PHOTOMETRIC REDSHIFTS AND

FILTERING OUT ULIRG CONTAMINANTS

In order to disentangle type-2 quasars from ULIRGs and ob-tain photometric redshifts, the SEDs of the candidates werefitted using a model consisting of three components (warmdust, galaxy light and blue light). The normalisations of thecomponents were allowed to vary together with the redshift,z, although for a given AV and z, the normalisation of thewarm dust component was fixed to fit the 24 µm data. Twodifferent models were investigated, and the bayesian odds ra-tio (e.g. Sivia 1996) was used to select between the models.These models differed only in their warm dust propertiesand consisted of a quasar (with dust) and a ULIRG. Theredshift was given a flat prior between 0 6 z 6 7. No realquasars have been detected at z ∼> 7, justifying this cutoff.

The stellar population was modelled by taking the SEDof a z = 0 elliptical galaxy from Coleman et al. (1980).SEDs showing smaller 4000-A breaks were well fitted bythe elliptical galaxy together with the blue component (seebelow). The elliptical was normalised to match the K-bandluminosity of an L∗ galaxy in the local K-band luminosityfunction of Cole et al. (2001). The luminosity of the galaxyrelative to the local L∗

K was then allowed to vary with aprior flat in log-space, between −1.3 6log10(Lgal/L∗

K)6 1.3.This choice brackets the reasonable range of host galaxies fora powerful quasar accounting for passive evolution: from avery faint 0.05L∗

K to a very bright 20L∗K galaxy. The choice

of a prior flat in log-space means that the host galaxy isbelieved, a priori, to be as likely to lie between 0.1L∗

K andL∗

K as between L∗K and 10L∗

K. This is more realistic than aprior flat in real space, which would imply a quasar is aslikely to be hosted by an L∗

K galaxy as by a 10L∗K one.

The blue component, between 912 A and 5000 A, hasa physical motivation as well as serving a practical pur-pose. It can represent the scattered light from the ob-scured quasar, blue light from young stars or a UV-upturnbrighter than that of the template elliptical galaxy used (andtherefore a smaller 4000-A break). The slope was thereforeset to Sν ∝ ν−0.5, which is representative of both type-1 quasars and starforming galaxies. The normalisation ofthe blue component is given as a ratio of the luminosityto that of an L∗

K galaxy at 5000 A, and was allowed tovary independently of the galaxy, with a flat prior between−2 6log10(Lblue/L∗

K,5000A)6 2, spanning the range between

a blue component fainter than an L∗K elliptical galaxy’s UV-

upturn, and as bright as the most powerful type-1 quasar.To avoid overfitting and loosing the ability to discriminatebetween models, neither the stellar or the blue componentare reddened by dust.

The ULIRG dust component was modelled using themodels of Siebenmorgen & Kruegel (2006), hereafter SK06.Only allowing variation of one parameter, AV. For a givenAV and z, the bolometric luminosity of the ULIRG, Lbol waschosen to make the SED go as close as possible to the 24-µm data point. The values of Lbol are only restricted by therange available in the SK06 library: 10.1 6 log10(Lbol/L⊙)6 12.7 for our choice of parameters. Of the other parameters,

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Evidence for high-z Compton-thick quasars 3

Name RA Dec zphot log10[Lbol AV log10[Lgal log10[Lblue ln[OR(q/u)] log10[p(d|q)/ S2−12keV

(J2000) /W] /L∗k] /L∗

k] (S17)12/] / S17

ID052 02 16 17.92 -05 07 18.56 1.90±0.05 42.111 119 0.85 0.55 13.0 -10.0 <0.7ID123 02 19 28.76 -05 09 08.81 1.75±0.05 41.12 72 0.65 0.20 5.2 -9.7 <2.92

ID135 02 19 01.89 -05 11 14.22 4.15±0.05 41.14 2.23 1.20 0.15 26.0 -9.4 <0.3ID142 02 17 23.82 -04 35 13.72 4.05±0.05 41.29 6.7 1.25 0.45 10.0 -8.9 <0.4ID147 02 19 10.31 -05 16 03.00 1.80±0.05 41.01 35.9 0.75 -0.05 28.6 -13.4 <0.3

ID200 02 18 15.71 -05 05 10.34 1.75+0.05−0.07 40.95 72 0.75 0.50 8.4 -9.1 <0.4

ID249 02 19 13.74 -04 56 04.27 1.75±0.05 40.89 35.9 0.50 0.00 25.2 -9.2 <0.6ID342 02 17 05.35 -05 09 24.61 3.85±0.05 40.90 9.0 1.20 0.30 4.6 -15.25 <6.82

ID345 02 16 29.56 -05 03 10.65 2.00+0.08−0.07 40.68 35.94 1.00 0.10 10.4 -11.1 <0.6

ID347 02 18 09.64 -05 18 42.42 1.75±0.05 41.06 72 0.55 -0.30 5.8 -8.0 1.7±0.95

ID386 02 17 25.11 -05 16 17.27 1.90+0.05−0.12 40.26 17.9 0.50 -0.75 6.2 -11.5 <0.1

ID401 02 16 23.02 -05 08 06.76 1.90+0.05−0.12 40.93 35.9 0.70 0.40 26.7 -8.9 3.4±1.35

Table 1. Best-fit parameters and X-ray fluxes for the high-redshift type-2 quasars. The names are from the radio catalogue of Simpsonet al. (2006), and we define S17 = 10−17 W m−2 for convenience. The errors in zphot are estimated from the full-width half-maximumvalues from the marginalised probability distribution functions (PDFs) for zphot. For most objects, the PDF has at least one secondarypeak, always adjacent to the primary one, with peak value between 0.5 and 0.05 of the primary peak. For the objects with zphot ∼ 4,the value is likely to be an overestimate due to the lack of H-band data (i.e. they are likely to be at z ∼ 3). The median value oflog10[Lgal/L∗

k], 0.75, corresponds to a 5.6L∗

k(where L∗

kis the break in the local K-band luminosity function) galaxy at z ∼ 2, which

asuming passive evolution and using the models of Bruzual & Charlot (2003), would become a present-day ∼L∗k

galaxy. 1This highvalue for Lbol is probably due to an overestimation of the AV. 2The reason for these high values for the fluxes is that the noise issignificantly higher (∼100×) in the 4.5-12 keV band than in the 2-4.5 keV one. ID123 and ID342 are not detected in the X-ray data.3This value of AV suggests a reddened type-1 quasar rather than a genuine type-2. 4ID345 is a point source at K-band, which suggeststhe value of AV is an overestimate. 5ID347 and ID401 have safe X-ray detections. The best-fitting SEDs for all 12 objects can be foundat: http://www.mpia.de/homes/martinez/sxdftype2seds.pdf

the nuclear radius was fixed to 1 kpc, the ratio of luminosityof OB stars with hot spots to Ltot was fixed to 0.6, andthe hydrogen number density was fixed to 1010 m−3. Themodels have a discreet set of values of the extinction (AV =2.2, 4.5, 6.7,9, 17.9, 35.9, 72 and 119), so the prior for theAV consists of a set of δ-functions (with equal probability)at these values.

The mid-infrared SED for quasars was modelled usingthe Elvis et al. (1994), hereafter E94, type-1 SED, and ob-scuring it with dust from the models of Pei (1992). Forconsistency with SK06, only Milky Way (MW)-type dustis used. This SED and dust model would allow us, in prin-ciple, to vary the values of AV continuously, but to makethe fitting procedure as fair as possible, we restricted thevalues of AV to those present in the models of SK06 andassigned the same prior to them. This flat prior reflects ourignorance about the range in AV, particularly for quasars.For each object, the quasar bolometric luminosity Lbol wasnot allowed to vary freely, but was fixed for a given AV andz by the observed flux density at 24 µm. There is, however,no upper limit set on Lbol.

For a given model, the likelihood of a given combina-tion of parameters is given by the n-dimensional probabilitydensity function

p(data|model A, z , AV, Lgal, Lblue) =

1

(2π)n/2

n∏i

σi

e−χ2A

/2, (1)

where

χ2A =

n∑i

(model Ai − datai)2/σ2

i , (2)

model Ai is the flux predicted by model A, given the param-eters, over a given waveband, datai is the observed flux atthat band, σi is the measurement error in that band and n isthe number of bands. In the cases where σi was smaller than10% of the flux density of the object, it was set to 10%. Totreat non-detections at a particular band, we followed thefollowing method: when the observed galaxy flux density andthe model fell below the flux density limit (5-σband, whereσband is the rms noise in that band), the band made no con-tribution to χ2. When the model lay above the limit, theobject was assigned a flux density Si and an error σi bothequal to half of the flux density limit (so Si =σi = 2.5σband).An undetected source would therefore only be 1σi away fromzero flux density, and from the limit. This prescription wasappropriate for our sources, selected to be faint and thereforeclose to the limits in most of the bands, to avoid excessivelypenalising nondetections in one band.

To select between models, we follow Sivia (1996) in cal-culating the odds ratio (OR),

OR(A/B) =p(data|model A) p(model A)

p(data|model B) p(model B), (3)

where we called model A, q (for quasar), model B, u (forULIRG) and data, d. We take p(q)/p(u)=1 and the evidencefor model y ∈ (q,u) is defined as

p(data|model y) =

∫e−χ2

y/2p(z)p(AV)p(Lgal)p(Lblue)dzdAVdLgaldLblue, (4)

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4 A. Martınez-Sansigre et al.

Figure 1. Best fit spectral energy distribution (SED) of ID401.The dashed black lines show (from left to right) the quasar (withextinction), galaxy and blue templates normalised by the values

(from Table 1) alongside the best fit to the data (red dots), thesolid yellow line shows the combined best-fit templates. The pur-ple dots and limits are the X-ray data in the 0.3-0.5 keV, 0.5-1keV, 1-2 keV, 2-4.5 keV and 4.5-12 keV bands, from all threeEPIC cameras. For comparison, the E94 type-1 SED has beenoverplotted (dash-dotted line), with the same values of Lbol andzphot as ID401 (from Table 1). The top x-axis and right y-axisshow the rest-frame frequencies and luminosities (in units of L⊙)assuming the best-fit zphot = 1.90.

and p(x) is the normalised prior probability distributionof parameter x and the integral is over the entire priorspace. To make sure that our X-ray analysis did not in-clude any ULIRGs, only objects with OR(q/u)> 100 wereconsidered as quasars. Note this criterion is very similarto the so-called Jeffreys’ criterion (Jeffreys 1961). In addi-tion, to make sure the fits were acceptable, only sourceswith evidence p(d|q)/(S17)

12> 10−17 were accepted, where

S17 = 10−17 W m−2 (S17 is defined to avoid numbers oforder 10−204 or smaller). Our range of evidences in Table 1,1.3 × 10−9

6p(d|q)/(S17)12

6 5.5 × 10−16, corresponds ap-proximately to values of reduced χ2 = 1.9−6.1. Finally, onlysources with zphot > 1.70 were kept, since Ly α is visible inoptical spectroscopy, so the redshifts can be checked obser-vationally. Preliminary spectroscopy of the sample suggestsour values of zphot are in good agreement with spectroscopicredshifts (Martınez-Sansigre et al. in prep.), except for theobjects with zphot ∼ 4, which are likely to lie at z ∼ 3 withzphot an overestimate due to the lack of H-band data.

Of the 38 candidates in the sample, 12 (32%) followour above criteria and are considered our best candidatesfor high-redshift type-2 quasars. Their best-fit parametersare quoted in Table 1, and we proceed to analyse the X-rayproperties of these sources. Visual inspection of the opticalimages showed several cases where the blue light was unre-solved. This fits in well with scattered light from the quasarreaching us and justifies our choice of a “blue” component.Typically the best fitting normalisation for the blue compo-nent is ∼5% of the normalisation of the quasar component,which agrees well with scattering fractions. The K-band im-ages showed resolved sources in 11 cases, while ID345 is apoint-source (suggesting a real AV ∼< 5; a reddened type-1,see MS06). The SED and AV of ID135 (see supplementary

SEDs) also suggest a reddened quasar, rather than a genuinetype-2.

4 X-RAY PROPERTIES OF THE TYPE-2

QUASARS

The type-2 quasars were cross-matched with the X-ray cata-logue of Ueda et al. (in prep.) with a flux limit of ∼3×10−18

W m−2 in the 2-12 keV band, but only 2 out of 12 (17%)were detected: ID347 and ID401.

To obtain meaningful limits for the undetected sources,these had their counts measured directly from the X-rayimage, using the XMM-Newton Science Analysis Software(SAS). The positions of the sources not detected in theXMM-Newton observations were added manually to theeboxdetect source lists of each observation. We then ranthe SAS task emldetect keeping the positions of the sourcesfixed. Emldetect performs a maximum likelihood fit on thedistribution of observed counts of the sources previously ex-tracted by the task eboxdetect. The fit uses the five diferentenergy bands (in keV: 0.2-0.5,0.5-1,1-2,2-4.5,4.5-12) and thethree EPIC cameras (M1,M2,pn) simultaneously. This pro-cedure allowed to fit the sources as multiple componentswith separate point-spread functions (PSFs), and sourceswith no evidence of any X-ray detection were still fitted.

This procedure allowed us to obtain fits to the counts forall sources in the 5 EPIC bands, and these were convertedto fluxes in each band, assuming a photon index Γ = 1.7and an obscuring column of NH = 3 × 1024 m−2 (the de-fault SAS values). These are not the most likely values forobscured quasars, but due to the large uncertainties in thecount rates, the error in flux due to these values of Γ and NH

is small by comparison, and the hardness ratios are meaning-less (except for ID347 and ID401, see discussion). All the 10sources undetected in the Ueda et al. catalogue were foundto have fitted values consistent with background noise, evenin the harder bands (see Table 1). We therefore find that 83%of our sources classified as high-redshift type-2 quasars areundetected in the X-ray image, even down to a flux limit of∼3×10−18 W m−2 in the 2-12 keV band. Note from Figure 1that even X-ray detections are well below the νLν ∼constantexpectation from E94 and are therefore presumably highlyobscured in X-rays.

5 DISCUSSION

We have found that 10 out of 12 of our sources classified astype-2 quasars, are undetected in an X-ray image with anapproximate flux limit ∼3×10−18 W m−2 (in the 2-12 keVband). For the two detections, we use the photometric red-shifts, the hardness ratios between the 0.3-2 and 2-12 keVbands, and the Monte-Carlo models of Wilman & Fabian(1999, WF99) to estimate the absorbing columns. The esti-mates for ID347 and ID401 are log10(NH/m2) = 28.50 and28.75 respectively, and derived gas to dust ratios of NH

AV=

4×1026 and 2×1027 m−2 (similar to those found by Watan-abe et al. 2004, and much larger than the Milky-Way value,often used as “standard”). For the undetected objects, itis not possible to estimate values of NH from the hardnessratios.

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Evidence for high-z Compton-thick quasars 5

Figure 2. Hard X-ray flux (S2−12keV) versus 24-µm flux density(S24 µm) as a function of redshift for a MB = −25.7 model quasarobscured with dust from the models of Pei (1992), assuming two

gas-to-dust ratios (NH = 1.9× 1025 and 5× 1026 m−2×AV) andusing the absorbed X-ray spectra of WF99 (see MS06 for moredetails). The 12 type-2 quasars in our sample are overplotted inblue. The vertical dotted line is the 300 µJy flux density limit im-posed on our sample, the horizontal dotted line is the approximateflux limit of the X-ray map, ∼ 3 × 10−18 W m−2. Both quasartracks have AV = 35.9 (the median of our sample) and have blackdots marking redshifts. The green track has NH = 6× 1026 m−2,and is therefore a Compton-thin obscured (type-2) quasar, whilethe red track has NH = 2×1028 m−2 and is therefore so obscuredit is known as “Compton-thick”. Considering our type-2 quasarsare well matched in luminosity to the fiducial tracks, and theyare selected to be at zphot > 1.70, the lack of X-ray detectionscan only be explained by extreme absorbing columns (NH ∼> 1028

m−2).

Instead, to get a handle of the possible values ofNH,in Figure 2 we show fiducial tracks for a model quasar.The model quasar has MB = −25.7, which from the E94SED corresponds to Lbol∼ 7× 1039 W and is therefore wellmatched to our candidates (see Figure caption for details).ID347 and ID401 have values of S24µm and S2−12keV andestimates for NH (derived only from hardness ratios) all inreasonable agreement with the tracks. Another two objects(ID123 and ID342) have limits for S2−12keV also consistentwith the tracks, suggesting log10(NH/m2) ∼> 28.25.

Eight other quasars at 1.75 6 zphot 6 4.15 have similarvalues of S24µm, Lbol to ID347 and ID401, yet significantlyfainter limits for S2−12keV. If the values of Lbol have beenoverestimated, or the values of zphot have been underesti-mated, then NH could be slightly overestimated, but thederived values are those expected from our selection func-tion: they do not seem gross overestimates. The most likelysource of error is the choice of Γ and NH used to derivethe fluxes (Section 4), which can underestimate S2−12keV.Still, the lack of detections and comparison with the fiducialtracks suggests very high absorbing columns and gas-to-dustratios. The X-ray properties of this sample seem to requireNH > 1028 m−2, so that effectively all objects, even the twodetections, are Compton-thick.

We have therefore found that selection of type-2 quasarsin the mid-infrared unveils a population of Compton-thickobjects, a large fraction of which is undetectable in X-raysurveys due to the lack of surveys covering large areas at

the necessary depth. In the infrared, these objects are oftenconsistent with stellar light dominating the emission at 3.6µm and shorter wavelengths, meaning they were impossibleto find until Spitzer was launched (Werner et al. 2004).

Following MS05, we model the expected number oftype-1 quasars following our 24-µm and radio flux densitycriteria, at z > 1.70, and in 0.8 deg2. We predict 2±0.8type-1 quasars following these criteria, and since Poisson er-rors dominate the number of type-2 quasars, we can easilyestimate the ratio of type-2s to type-1s at z > 1.70 fromthis study as 4.5+5.5

−2.4. This is obviously plagued by small-number statistics, so we simply consider this estimate con-sistent within the error bars with previous results (MS05).What does seem significant is that, since all the type-2s pre-sented here are Compton-thick, at high redshift the numberof Compton-thick quasars is probably larger than the num-ber of type-1 quasars.

With such high values of NH, due to their rare spa-tial density and their high-redshifts, these objects are un-likely to make a significant contribution to the hard X-ray background (energies > 1 keV). However, conclusionson the cosmological evolution and relative fractions of ob-scured and unobscured AGN (e.g. Ueda et al. 2003; Bargeret al. 2005), estimates of the radiative efficency of accretingSMBHs (e.g. Marconi et al. 2004), and searches for AGNburied in dusty objects (e.g. Alexander et al. 2005b), havegenerally been based on X-ray observations and are there-fore likely to have underestimated the number of accretingSMBHs. Mid-infrared selection of quasars seems more suc-cessful at finding heavily obscured objects, although insteadthe difficulty lies in filtering out the starbursts from thequasars.

ACKNOWLEDGMENTS

We gladly thank Richard Wilman for access to his X-rayspectra, Filipe Abdalla for discussions about statistics, andRalf Siebenmorgen for useful comments on the ULIRG tem-plates. SR, DGB and CS thank the UK PPARC for a SeniorResearch Fellowship, a Studentship and an Advanced Fel-lowship respectively.

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