---
abstract: |
    We report on a  and  program that has observed a sample of three extremely luminous, heavily obscured -selected AGN at $z \sim 2$ in a broad X-ray band ($0.1 - 79$ keV). The parent sample, selected to be faint or undetected in the  $3.4 \mu$m ($W1$) and $4.6 \mu$m ($W2$) bands but bright at $12 \mu$m ($W3$) and $22 \mu$m ($W4$), are extremely rare, with only $\sim 1000$ so-called “$W1W2$-dropouts” across the extragalactic sky. Optical spectroscopy reveals typical redshifts of $z \sim 2$ for this population, implying rest-frame mid-IR luminosities of $\nu L_\nu(6 \mu{\rm m}) \sim 6 \times
    10^{46}\, {\rm erg}\, {\rm s}^{-1}$ and bolometric luminosities that can exceed $L_{\rm bol} \sim 10^{14}\, L_\odot$. The corresponding intrinsic, unobscured hard X-ray luminosities are $L(2-10\, {\rm keV})
    \sim 4 \times 10^{45}\, {\rm erg}\, {\rm s}^{-1}$ for typical quasar templates. These are amongst the most luminous AGN known, though the optical spectra rarely show evidence of a broad-line region and the selection criteria imply heavy obscuration even at rest-frame $1.5\, \mu{\rm m}$. We designed our X-ray observations to obtain robust detections for gas column densities $N_{\rm H} \leq 10^{24}\,
    {\rm cm}^{-2}$. In fact, the sources prove to be fainter than these predictions. Two of the sources were observed by both and , with neither being detected by  ($f_{\rm 3-24\
    keV} \simlt 10^{-13}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$), and one being faintly detected by  ($f_{\rm 0.5-10\ keV} \sim
    5 \times 10^{-15}\, {\rm erg}\, {\rm cm}^{-2}\, {\rm s}^{-1}$). A third source was observed only with , yielding a faint detection ($f_{\rm 0.5-10\ keV} \sim 7 \times 10^{-15}\, {\rm erg}\, {\rm
    cm}^{-2}\, {\rm s}^{-1}$). The X-ray data require gas column densities $N_{\rm H} \simgt 10^{24}\, {\rm cm}^{-2}$, implying the sources are extremely obscured, consistent with Compton-thick, luminous quasars. The discovery of a significant population of heavily obscured, extremely luminous AGN does not conform to the standard paradigm of a receding torus, in which more luminous quasars are less likely to be obscured. If a larger sample conforms with this finding, then this suggests an additional source of obscuration for these extreme sources.
author:
- 'D. Stern, G. B. Lansbury, R. J. Assef, W. N. Brandt, D. M. Alexander, D. R. Ballantyne, M. Baloković, D. Benford, A. Blain, S. E. Boggs, C. Bridge, M. Brightman, F. E. Christensen, A. Comastri, W. W. Craig, A. Del Moro, P. R. M. Eisenhardt, P. Gandhi, R. Griffith, C. J. Hailey, F. A. Harrison, R. C. Hickox, T. H. Jarrett, M. Koss, S. Lake, S. M. LaMassa, B. Luo, C.-W. Tsai, D. J. Walton, E. L. Wright, J. Wu, L. Yan and W. W. Zhang'
title: |
    NuSTAR and XMM-Newton Observations of Luminous,\
    Heavily Obscured, WISE-Selected Quasars at $z \sim 2$
---

Introduction
============

The [*Wide-field Infrared Survey Explorer*]{} () [@Wright:10] is an extremely capable and efficient black hole finder. As demonstrated in selected fields by  [, @Stern:05; @Donley:12], the same material that obscures AGN at UV, optical and soft X-ray energies is heated by the AGN and emits strongly at mid-IR wavelengths. The all-sky survey identifies millions of obscured and unobscured quasars across the full sky [, @Stern:12; @Assef:13], as well as very rare populations of extremely luminous, heavily obscured AGN.

In terms of the latter, the extragalactic team has been pursuing sources that are faint or undetected in $W1$ (3.4 $\mu$m) and $W2$ (4.6 $\mu$m), but are bright in $W3$ (12 $\mu$m) and $W4$ (22 $\mu$m). We refer to this population as $W1W2$-dropouts [@Eisenhardt:12]. This is a very rare population; selecting to a depth of 1 mJy at 12 $\mu$m, there are only $\sim 1000$ such sources across the extragalactic sky ($\sim 1$ per $30\ {\rm deg}^2$). These objects are undetected by [*ROSAT*]{} and tend to be optically faint ($r \simgt 23$), below the detection threshold of SDSS. We have obtained spectroscopic redshifts for $> 100$ $W1W2$-dropouts thus far, consistently finding redshifts $z \simgt 2$, with our current highest redshift source at $z = 4.6$ [@Eisenhardt:14]. Approximately half of the sources show clear type-2 AGN signatures in the optical spectra, with the other half typically showing only Ly$\alpha$ emission, sometimes extended, which could be due to star formation and/or AGN activity [@Bridge:13]. The lack of a far-IR peak in their broad-band SEDs suggests the dominant energy input for this population comes from a heavily obscured AGN and not extreme starbursts [, @Eisenhardt:12; @Wu:12]. Related high-luminosity sources selected from the  satellite have also recently been reported by @Weedman:12 and @Alexandroff:13, while several teams have identified less rare, less luminous sources from  surveys with less extreme colors [, @Dey:08; @Fiore:09].

Here we report on the first targeted X-ray follow-up of $W1W2$-dropouts. We observed two sources with both the [*Nuclear Spectroscopic Telescope Array*]{} [; @Harrison:13] and  [@Jansen:01]; a third source was only observed by . Unless otherwise specified, we use Vega magnitudes throughout and adopt the concordance cosmology, $\Omega_M = 0.3$, $\Omega_\Lambda
= 0.7$ and $H_0 = 70\, \kmsMpc$.

Sample
======

Fig. \[fig:spectra\] presents the optical spectra of the three $W1W2$-dropouts targeted for X-ray follow-up: WISEA J181417.29+341224.8 (hereafter, ), WISEA J220743.82+193940.1 (hereafter, ) and WISEA J235710.82+032802.8 (hereafter, ). The spectra were all obtained with the Low Resolution Imaging Spectrometer [LRIS; @Oke:95] on the Keck I telescope, between 2010 July and 2010 November. The sources were selected on the basis of having unusually red colors across the  passbands in the initial All-Sky  data release: $W1 > 17.4$, $W4 < 7.7$, and $W2 -
W4 > 8.2$ [for further details on the $W1W2$-dropout selection, see @Eisenhardt:12]. The most likely interpretation of sources with these extreme colors is that they host an extremely luminous, heavily obscured AGN which only becomes evident at observed wavelengths $\simgt 10\mu$m. The $W1$ flux limit essentially constrains the sample to $z \simgt 1.5$ for the host galaxy not to be detected.

Note the diversity of the optical spectra of the three sources targeted for X-ray follow-up (Fig. \[fig:spectra\]). This is representative of the diverse optical spectroscopic properties of the $W1W2$-dropout population in general [@Wu:12; @Eisenhardt:14]. @Eisenhardt:12 discusses  in depth: briefly, the optical spectrum is indistinguishable from an $L^*$ Lyman-break galaxy (LBG) at $z \sim 2.5$ [, @Shapley:03], with no obvious signature of a (buried) AGN. However, typical LBGs have $22 \mu$m flux densities a factor of 1000 lower [@Reddy:06]. The redshift of  is based on a single, asymmetric, high equivalent width emission line which is reliably identified as Ly$\alpha\ \lambda
1216$ [for a detailed discussion of one-line redshifts, see @Stern:00d].  shows a high equivalent width ($\sim
300$ Å, observed), slightly broadened (FWHM $\sim 1500\ {\rm km}\
{\rm s}^{-1}$), self-absorbed line identified as $\lambda
1549$, which is a common strong line in AGN. However, quite unusually, no Ly$\alpha$ emission is evident. The only strong feature blue-ward of the emission line is a continuum break at observed $\sim 3785$ Å, which is consistent with the Ly$\alpha$ forest break for the longer wavelength emission line being . @Hall:04 reports on a detailed investigation of a similar SDSS quasar with broad emission, but lacking broad Ly$\alpha$ emission. They argue that the unusual spectrum cannot be solely due to dust extinction in the broad line region (BLR), and instead suggest that most, but not all, of the spectral properties can be explained by an ununusually high density gas in the BLR ($n_{\rm
H} \sim 10^{15}\, {\rm cm}^{-3}$) with an incident power-law continuum extending to $\geq 200 \mu$m. Clearly the unusual optical spectrum of  is worthy of future study, but such analysis is beyond the scope of the current paper which focuses on the X-ray properties of the $W1W2$-dropout population.

In terms of their radio properties,  has a counterpart offset by 61 in the NRAO/VLA Sky Survey [NVSS; @Condon:98] with a flux density $S_{\rm 1.4\, GHz} = 3.4 \pm 0.5$ mJy. @Eisenhardt:12 report on follow-up radio observations of this source with the Jansky Very Large Array which resolves the NVSS emission into two distinct sources, the fainter of which is associated with . Based on both its rest-frame 1.4 GHz radio luminosity ($L_{\rm 1.4\, GHz} \sim 5 \times 10^{25}\, {\rm W}\,
{\rm Hz}^{-1}$) and its radio-to-optical ratio (rest-frame $L_{\rm
5\, GHz} / L_{\rm 0.44\, \mu m} \sim 200$),  qualifies as radio-loud.  has a counterpart offset by 71 in the NVSS with $S_{\rm 1.4\, GHz} = 5.2 \pm 0.4$ mJy, suggesting that it is also radio loud.  has no radio counterpart in either NVSS or the VLA Faint Images of the Radio Sky at Twenty Centimeters survey [FIRST; @Becker:95]. The radio luminosities of the first two sources further indicate the presence of a powerful AGN in this -selected population.

[lccccccc]{} & &\
(rl)[3-5]{} (rl)[6-8]{} & $z$ & Observation ID & UT Date & Exposure & Observation ID & UT Date & MOS1/MOS2/pn Exposure\
& & & & (ks) & & & (ks)\
& (2) & (3) & (4) & (5) & (6) & (7) & (8)\
& 2.452 & 60001114002 & 2012 Oct 30 & 21.3 & 0693750101 & 2012 Oct 07 & 29.6 / 29.6 / 19.6\
& 2.021 & 60001115002 & 2012 Oct 30 & 20.8 & 0693750201 & 2012 Nov 22 & 18.4 / 18.7 / 11.5\
& 2.113 & & & & 0693750401 & 2013 Jan 02 & 29.1 / 28.7 / 14.4\

— (1): Target name; full name and coordinates are in Table 2. (2): Redshift. (3) and (4):  observation ID and start date. (5): Net on-axis  exposure time. This value applies for both FPMA and FPMB. The target was on-axis for both of the  observations. (6) and (7):  observation ID and start date. (8): Net on-axis exposure time, corrected for flaring and bad events, for the MOS cameras and the pn camera, as indicated.

\[table:obslog\]

Observations and Multiwavelength Data
=====================================

NuSTAR Observations
-------------------

In 2012 October  obtained $\sim 20$ ks observations of  and ; details of the observations, including net exposure times, are provided in Table \[table:obslog\]. We processed the level 1 data using the  Data Analysis Software (NuSTARDAS) v.1.2.0, and produced calibrated and cleaned event files (level 2 data) for both  focal plane modules (FPMA and FPMB) using [nupipeline]{} and the most current available version of the Calibration Database files (CALDB 20130509).

Neither source was detected, though a serendipitous broad-lined AGN at $z = 0.763$ was identified in the  field [@Alexander:13]. We measured gross source counts in 45 radius apertures centered on the  positions and local background counts from an annulus of inner radius 90 and outer radius 150centered on the sources. We performed photometry in the observed-frame 3-24 keV, 3-8 keV and 8-24 keV bands, as well as the rest-frame 10-40 keV band for both FPMs and used binomial statistics to determine the likelihood of the sources being detected. Binomial statistics are more accurate than Poisson statistics at these faint limits since it takes into account uncertainty in the measured background (, it takes the total background counts into account, not just the scaled background counts). We use binomial statistics to calculate the probability that the measured source counts are purely due to background fluctuations [, false, or ‘no-source’ probabilities; for details, see @Lansbury:14]. For both sources the probability of a false  detection based on the binomial statistics is $> 15\%$. As we take a no-source probability $< 1\%$ to indicate a detection, neither source was detected. Since binomial statistics are not amenable to plotting simple tracks, Fig. \[fig:Xraycounts\] shows the X-ray counts with Poisson no-source probabilities. The latter provide a good approximation of binomial probabilities for our sources given the reasonably high background count rates.

Table \[table.phot\] reports the 90% confidence limit upper limits to the flux in the 3-24 keV energy band, calculated using the Bayesian method of @Kraft:91. To convert count rate to source flux, we used XSPEC v12.8.0k, taking into account the Response Matrix File (RMF) and Ancillary Response File (ARF) for each FPM. We assumed a power-law model with $\Gamma = 1.8$. Assuming a harder intrinsic spectrum does not qualitatively change these results; , adopting $\Gamma = 1.0$ only changes the rest-frame 10-40 keV luminosities by $\leq 3\%$.

XMM-Newton Observations
-----------------------

We obtained $\sim 30$ ks observations of the three $W1W2$-dropouts between 2012 October and 2013 January with the  EPIC-MOS [@Turner:01] and EPIC-pn [@Struder:01] cameras. Details of the observations are provided in Table \[table:obslog\]. We used data products from the Pipeline Processing System (PPS), analyzed with the [*Science Analysis Software*]{}[^1] (SAS v.12.0.1). We measured 0.5-10 keV source counts in 15 radius apertures centered on the positions. For most observations, we measured local backgrounds in slightly offset source-free circular apertures with radii of $\sim 70 - 100\arcsec$ selected to avoid serendipitous sources and chip gaps. The exceptions were the EPIC-MOS observations of which had no nearby serendipitous sources and thus allowed for a 30 to 70 radius annulus centered the source position. As above with the  observations, we calculated binomial false probabilities and plot the equivalent using Poisson statistics in Fig. \[fig:Xraycounts\]. Only  is reliably detected, with a binomial no-source probability of 0.04% with EPIC-pn and 0.1% with EPIC-MOS1;  was not reliably detected with EPIC-MOS2.  is weakly detected with EPIC-MOS2 with a no-source probability of 0.2%, and is undetected with the other two cameras. We used XSPEC to convert count rate (or count rate limits) to flux, assuming a power-law model with $\Gamma = 1.8$, and the  RMFs and ARFs for each EPIC camera. Again, adopting a harder intrinsic X-ray spectrum would have a modest quantitative effect on the derived luminosities, but would not affect our broad conclusions. Specifically, were we to instead assume $\Gamma =
1.0$, the rest-frame 2-10 keV luminosities would change by $\leq
18\%$.

Mid-IR Data
-----------

Table \[table.phot\] presents basic source properties and multi-wavelength photometry for the three X-ray-targeted $W1W2$-dropouts. We list mid-IR data from the AllWISE data release[^2] and , where the latter comes from the [*Warm Spitzer*]{} observations reported by @Griffith:12. Comparing the AGN luminosity and reddening of  derived solely from the mid-IR photometry (§4.1) to the values in @Eisenhardt:12 derived from 16-band multi-wavelength photometry, we obtain consistent values within $\sim 10\%$.

[cccc]{} & & &\
R.A. (J2000) & 18:14:17.29 & 22:07:43.82 & 23:57:10.82\
Dec. (J2000) & +34:12:24.8 & +19:39:40.1 & +03:28:02.8\
$z$ & 2.452 & 2.021 & 2.113\
& & &\
$W1$ (3.4 $\mu$m) & $18.861\pm0.440$ & $17.174\pm0.127$ & $>18.142$\
$W2$ (4.6 $\mu$m) & $17.609\pm0.492$ & $16.136\pm0.170$ & $>16.614$\
$W3$ (12 $\mu$m) & $10.410\pm0.061$ & $10.630\pm0.106$ & $10.088\pm0.068$\
$W4$ (22 $\mu$m) & $ 6.863\pm0.071$ & $ 7.135\pm0.101$ & $ 6.942\pm0.112$\
$[3.6]$ & $17.707\pm0.023$ & $17.023\pm0.048$ & $17.487\pm0.076$\
$[4.5]$ & $17.021\pm0.020$ & $16.208\pm0.025$ & $16.544\pm0.036$\
$f_{\rm 0.5-2\, keV}$ & $0.109\pm0.053$ & $<0.264$ & $<0.243$\
$f_{\rm 2-10\, keV}$ & $<1.30$ & $<0.780$ & $1.05\pm0.53$\
$f_{\rm 0.5-10\, keV}$ & $0.521\pm0.141$ & $<0.523$ & $0.73\pm0.31$\
$f_{\rm 3-24\, keV}$ & $<7.55$ & $<6.04$ &\
& & &\
$L_{\rm 6\mu m}$ & $20.10\pm2.40$& $8.28\pm1.62$ & $5.04\pm0.36$\
$E(B-V)_{\rm AGN}$ & $15.1\pm1.1$ & $17.6\pm2.3$ & $5.5\pm0.4$\
$L_{\rm 2-10\, keV}$ & 1.04 & $<1.13$ & $<1.14$\
$L_{\rm 10-40\, keV}$ & $<17.1$ & $<10.1$ &\

— Astrometry and  photometry are from the AllWISE release; mid-IR photometry is all in Vega magnitudes and limits report the 95% confidence lower limit to the apparent magnitude.  photometry, in brackets, is from Griffith et al. (2012). X-ray fluxes, all in the observed frame, are in units of $10^{-14} \ergcm2s$. $L_{\rm 6\mu m}$ is the rest-frame $6 \mu$m luminosity ($\nu L_\nu$) of the AGN in units of $10^{46} \ergs$. Rest-frame X-ray luminosities are in units of $10^{44} \ergs$. We report 68% confidence limit (CL) uncertainties on the X-ray fluxes; upper limits correspond to the 90% CL.

\[table.phot\]

Results
=======

AGN Properties from Mid-IR Data
-------------------------------

We modelled the spectral energy distribution (SED) of each source using the @Assef:10 0.03-30 $\mu$m empirical AGN and galaxy templates. Each SED is modelled as a best-fit, non-negative combination of an elliptical, spiral, and irregular galaxy component, plus an AGN component. Only the AGN component is fit for dust reddening [for further details on the SED models, see @Assef:08; @Assef:10]. The modeling outputs $L_{\rm 6\mu m}$, the derived intrinsic luminosity ($\nu L_\nu$) of the AGN component at rest-frame $6 \mu$m, as well as the reddening of the AGN component, $E(B-V)_{\rm
AGN}$ (see Table \[table.phot\]). For the typical gas to dust ratio observed by @Maiolino:01 for luminous AGN, the nuclear reddening values of $E(B-V)_{\rm AGN} \sim 5 - 20$ imply gas columns of $N_{\rm H} \sim (5 - 20) \times 10^{23}\ {\rm cm}^{-2}$, which reach into the Compton-thick regime ($N_{\rm H} \geq 1.5 \times
10^{24}\ {\rm cm}^{-2}$).

Indirect X-Ray Absorption Constraints
-------------------------------------

The mid-IR properties of $W1W2$-dropouts indicate the presence of extremely luminous, heavily obscured AGN, with bolometric luminosities approaching, or even exceeding $L_{\rm bol} \sim 10^{14}\, L_\odot$ [@Eisenhardt:12; @Wu:12; @Bridge:13; @Assef:14; @Jones:14; @Tsai:14]. Given the heavy obscuration implied by their mid-IR SEDs and optical spectra, it is perhaps unsurprising that the three sources we targeted for X-ray follow-up are either undetected or only faintly detected, despite observing at the penetrating energies above rest-frame 10 keV which are less affected by absorption. But what are their absorption column densities? We can obtain indirect estimates of this from their mid-IR luminosities since unobscured AGN tend to have a fairly tight relation between their mid-IR and X-ray luminosities.

Fig. \[fig:LxLir\] compares the rest-frame $6 \mu$m and X-ray luminosities for our targeted sources. We show local relations from @Lutz:04, @Gandhi:09 and @Fiore:09, as well as non-beamed AGN with $L_X > 10^{43} \ergs$ from the serendipitous survey [@Alexander:13]; the serendipitous sources all lie within the scatter of the published relations. We also show Compton-thick quasars observed at soft energies from @Alexander:08, three SDSS type-2 quasars observed by [@Lansbury:14], and an obscured quasar at $z \approx 2$ detected by  in the Extended  Deep Field South [ECDFS; @DelMoro:14]. The literature obscured AGN and the -selected $W1W2$-dropouts generally lie significantly below the published relations. Assuming the suppression of their X-ray emission is due to absorption rather than intrinsic X-ray weakness, we can estimate their column densities from the dashed lines in Fig. \[fig:LxLir\], which apply columns of $N_{\rm H} = 10^{24}\
{\rm cm}^{-2}$ to the published relations. These were calculated using the MYTorus model [@Murphy:09] with photon index $\Gamma
= 1.8$ and a torus inclination angle $\theta_{\rm obs} = 70\deg$. The implication is that all three targeted sources are obscured, possibly heavily obscured or Compton-thick. For the rest-frame 2-10 keV panel, the one detected source, , has a column of $N_{\rm H} \sim 10^{24}\, {\rm cm}^{-2}$ for the @Fiore:09 relation, and yet higher columns for the other two relations. The undetected sources require minimum columns of $N_{\rm H} \sim
10^{24}\, {\rm cm}^{-2}$ for all but the @Fiore:09 relation. Constraints from the  nondetections are less strict, but are again consistent with heavy absorption columns. Furthermore, two of the three -selected targets are radio-loud. Since optically selected radio-loud quasars tend to have extra X-ray emission quasars as compared to radio-quiet quasars [@Miller:11], this suggests yet larger minimum absorption columns for  and . In addition, the jet-linked X-ray emission is apparently subject to similarly high obscuration as the nuclear X-ray emission.

Discussion
==========

We designed our X-ray integration times to provide robust detections for (i) typical intrinsic AGN SEDs, and (ii) gas column densities $N_{\rm H} \simlt 10^{24}\, {\rm cm}^{-2}$. None of the three sources was strongly detected, implying that at least one of the assumptions in our experimental design does not hold.

 has now observed a range of obscured AGN, from famous, local sources such as Mrk 231 [@Teng:14], Circinus [@Arevalo:14], NGC 424 [@Balokovic:14], and NGC 4945 [@Puccetti:14], to higher redshift obscured quasars at $z \sim 0.5$ from SDSS [@Lansbury:14] and $z \sim 2$ in the ECDFS [@DelMoro:14]. A recurring theme of these observations is that several AGN which are extremely luminous at certain wavelengths, such as in the mid-IR or \[\] $\lambda 5007$, remain faint at X-ray energies. For some objects, this is the case even for the more penetrating hard X-rays $> 10$ keV. For some sources, such as optically bright ($B \simlt
16$) broad-absorption line (BAL) quasars from the Palomar-Green (PG) survey [@Schmidt:83] and Mrk 231, we consider, and sometimes even favor attributing the hard X-ray faintness to intrinsic X-ray weakness [@Luo:13; @Teng:14]. Such intrinsic X-ray weakness seems the most plausible scenario when the AGN appears unobscured at certain wavelengths, through strong UV continuum emission, broad emission lines, and/or weak X-ray spectra that are well described by a moderately absorbed power-law. For a more detailed description of the intrinsically X-ray weak scenario, see @Luo:13 and @Teng:14.

For the three -selected AGN discussed here, we instead favor interpreting the X-ray faintness as being due to a typical AGN seen through extremely high absorbing columns, consistent with their mid-IR SEDs and optical spectra. Indeed, the X-ray column constraints from Fig. \[fig:LxLir\] are broadly consistent with the mid-IR measurements given typical luminous AGN gas-to-dust ratios from @Maiolino:01. What is more surprising is that these sources, amongst the bolometrically most luminous AGN known, appear heavily obscured. Various observations have shown that more luminous AGN are less likely to be obscured [, @Ueda:03; @Simpson:05; @Assef:13]. This is consistent with the “receding torus model”, first proposed by @Lawrence:91, in which the height of the torus is independent of luminosity while the the inner radius of the torus, corresponding to the distance at which dust reaches its sublimation temperature, increases with luminosity. Therefore, in this model, more luminous AGN have more sightlines into the nucleus and thus have a lower likelihood of being obscured.

The $W1W2$-dropout population is a rare population, with a surface density of just one source per $\sim 30\, {\rm deg}^2$ in the extragalactic sky. The mid-IR luminosities imply intrinsic X-ray luminosities of a few $\times 10^{45}\, {\rm erg}\, {\rm s}^{-1}$ from the relations of @Lutz:04 and @Gandhi:09. @Just:07 report on X-ray follow-up of the most luminous quasars in the SDSS available at the time, finding 34 quasars across 4188 deg$^2$, or one source per $\sim 120\, {\rm deg}^2$. The X-ray luminosities of this luminous quasar sample prove comparable to the expected intrinsic X-ray luminosities from the -selected sample, implying the surprising discovery of comparable numbers of obscured and unobscured quasars at the top of the luminosity function. @Assef:14 and @Tsai:14 present more detailed comparisons between the $W1W2$-dropout and luminous unobscured quasar populations.

The discovery of a significant population of heavily obscured, extremely luminous AGN does not conform to the simple receding torus model, suggesting an additional source of obscuration. Indeed, the models of @Draper:10 predict that Compton-thick AGN should be more common at higher redshift because of the high fueling rates of quasars require significant gas reservoirs. This is consistent with models showing that mergers may be more prominent in fueling AGNs at $z \sim 2$ [, @Hopkins:08; @Draper:12]. Indeed, these models also predict that the unified model of AGN will break down for high-luminosity AGNs at $z \simgt 1$ because the obscuration is not confined to the nucleus [see also @Draper:11].

Further investigations into this interesting population are clearly warranted. Deeper X-ray observations, achieving robust detections rather than faint and non-detections, would be valuable. For example, assuming the obscuration is from a nuclear torus, deeper X-ray observations should detect narrow, reflected Fe K$\alpha$ fluorescent emission at rest-frame 6.4 keV [, @Nandra:07]; a significant non-detection could point towards obscuration on larger scales than the torus. Such large-scale obscuration could also be probed by high-resolution imaging, such as far-IR observations with ALMA, and, eventually, mid-IR observations with [*JWST*]{}. Near-IR spectroscopy could also look for reddening in the AGN narrow-line region [, @Brand:07], which is expected to be significantly larger than the torus. A clearer understanding of the geometry of the obscuring region combined with an improved reckoning of the $W1W2$-dropout space density will enable us to better place this population within the context of AGNs and galaxy evolution.

This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the [*NuSTAR*]{} mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the [*NuSTAR*]{} Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the [*NuSTAR*]{} Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). This publication makes use of data products from the [*Wide-field Infrared Survey Explorer*]{}, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. We acknowledge financial support from the Science and Technology Facilities Council (STFC) grants ST/K501979/1 (GBL), ST/I001573/1 (DMA and ADM) and ST/J003697/1 (PG). RJA was supported by Gemini-CONICYT grant number 32120009. RCH acknowledges support from NASA through ADAP award NNX12AE38G and the National Science Foundation through grant number 1211096.

, , , ,

©2014. All rights reserved.

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[^1]: http://xmm.esa.int/sas/

[^2]: http://wise2.ipac.caltech.edu/docs/release/allwise/
