Abstract
Hot Dust-Obscured Galaxies (Hot DOGs) are hyperluminous ($L_{\mathrm{8-1000\,\mu m}}>10^{13}\,\mathrm{L_\odot}$) infrared galaxies with extremely high (up to hundreds of K) dust temperatures. The sources powering both their extremely high luminosities and dust temperatures are thought to be deeply buried and rapidly accreting supermassive black holes (SMBHs). Hot DOGs could therefore represent a key evolutionary phase in which the SMBH growth peaks. X-ray observations can be used to study their obscuration levels and luminosities. In this work, we present the X-ray properties of the 20 most-luminous ($L_{\mathrm{bol}}\gtrsim10^{14}\, L_\odot$) known Hot DOGs at $z=2-4.6$. Five of them are covered by long-exposure ($10-70$ ks) Chandra and XMM-Newton observations, with three being X-ray detected, and we study their individual properties. One of these sources (W0116$-$0505) is a Compton-thick candidate, with column density $N_H=(1.0-1.5)\times10^{24}\,\mathrm{cm^{-2}}$ derived from X-ray spectral fitting. The remaining 15 Hot DOGs have been targeted by a Chandra snapshot (3.1 ks) survey. None of these 15 is individually detected; therefore we applied a stacking analysis to investigate their average emission. From hardness-ratio analysis, we constrained the average obscuring column density and intrinsic luminosity to be log$N_H\,\mathrm{[cm^{-2}]}>23.5$ and $L_X\gtrsim10^{44}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$, which are consistent with results for individually detected sources. We also investigated the $L_X-L_{6\mu\mathrm{m}}$ and $L_X-L_{bol}$ relations, finding hints that Hot DOGs are typically X-ray weaker than expected, although larger samples of luminous obscured QSOs are needed to derive solid conclusions.
Highlights
According to a widely discussed framework of supermassive black hole (SMBH) and galaxy co-evolution (e.g., Hopkins et al 2008; Alexander & Hickox 2012), the most-luminous (Lbol 1047erg s−1) quasars (QSOs), typically with black hole masses of 109 M, are expected to be triggered by major “wet” mergers.The large gas and dust reservoirs feed both high-rate accretion, close to or even exceeding the Eddington limit, onto supermassive black holes (SMBHs), and starburst events in the host galaxies
We computed the constraints on the net counts in the 0.5 − 2 and 2 − 7 keV bands for each individual object by deriving the probability distribution function of net counts with the method of Weisskopf et al (2007, see their Appendix A3), which correctly accounts for the Poisson nature of both source and background counts
If it was emitted by that galaxy, the average emission from our sample would be even harder than we found, reinforcing our conclusions. 8 Throughout this paper, whenever we consider the different areas in which source and background photometry are evaluated, we account for the different average exposure times
Summary
The large gas and dust reservoirs feed both high-rate accretion, close to or even exceeding the Eddington limit, onto SMBHs, and starburst events in the host galaxies. This phase of extremely fast SMBH growth is hidden by the same accreting and star-forming material, which absorbs the optical-to-X-ray radiation emitted in the nuclear region, and reprocesses it into the infrared (IR) bands. During the so-called “blow-out phase”, radiative feedback mechanisms sweep away the material surrounding the SMBHs, which eventually shine as blue, unobscured QSOs. Recently, a population of rare hyper-luminous (L8−1000 μm > 1013 L ) infrared galaxies (HyLIRGs) at z = 1.6 − 4.6 has been discovered in the Wide-Field Infrared Survey Explorer (WISE, Wright et al 2010) all-sky survey (Eisenhardt et al 2012; Wu et al 2012; Tsai et al 2015; Assef et al 2016). The selected objects are similar to Dust-Obscured Galaxies (DOGs; Dey et al 2008), but with higher dust temperatures (up to hundreds vs. 30 − 40 K; e.g. Pope et al 2008; Melbourne et al 2012; Wu et al 2012; Jones et al 2014; Tsai et al 2015), and are named Hot DOGs
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