Eddington-limited Accretion in z ∼ 2 WISE-selected Hot, Dust-obscured Galaxies

Abstract

Hot, Dust-Obscured Galaxies, or "Hot DOGs", are a rare, dusty, hyperluminous galaxy population discovered by the WISE mission. Predominantly at redshifts 2-3, they include the most luminous known galaxies in the universe. Their high luminosities likely come from accretion onto highly obscured super massive black holes (SMBHs). We have conducted a pilot survey to measure the SMBH masses of five z ∼ 2 Hot DOGs via broad Hα emission lines, using Keck/MOSFIRE and Gemini/FLAMINGOS-2. We detect broad Hα emission in all five Hot DOGs. We find substantial corresponding SMBH masses for these Hot DOGs (∼ 10 9 M ), and their derived Eddington ratios are close to unity. These z ∼ 2 Hot DOGs are the most luminous AGNs for their BH masses, suggesting they are accreting at the maximum rates for their BHs. A similar property is found for known z ∼ 6 quasars. Our results are consistent with scenarios in which Hot DOGs represent a transitional, high-accretion phase between obscured and unobscured quasars. Hot DOGs may mark a special evolutionary stage before the red quasar and optical quasar phases, and they may be present at other cosmic epochs.

Spectral Classification and Ionized Gas Outflows in z ∼ 2 WISE-selected Hot Dust-obscured Galaxies

“…We used the IDL-based package MPFIT (Markwardt 2009) to perform iterative least χ 2 fitting with a set of initial parameter values determined from the relative strengths of the features. The fit was improved by applying a 2.5-σ Gaussian clipping (Wu et al 2018), and masking some strong outlying values still left on the spectra ( §2.2). Detailed prescriptions are described below, and Table 2 summarizes the broad/narrow nature of the selected lines and the independent/dependent parameters.…”

“…For the rest-frame 3500-7000Å fit, we assumed a powerlaw continuum and Gaussian emission lines for [ Traditionally, Hydrogen Balmer lines in AGN are fit by both broad and narrow components with the dividing line between these components at FWHM = 1000 km s −1 , while forbidden transitions are treated as narrow lines only. However, this standard approach is inappropriate for asymmetric, blueshifted and broadened forbidden line profiles, as observed in some [O III] doublets, and even for some [O II] and Hydrogen Balmer lines in type 2 quasars (e.g., Kang et al 2017;Wu et al 2018). These profiles are often broad, but instead of being broadened by the motions of the broad-line region clouds, they are thought to be broadened by emission from ionized gas outflowing from the AGN at ∼ 10 3 km s −1 (Veilleux et al 1994;Crenshaw & Kraemer 2000).…”

“…The broadband VLT spectra emphasizes the importance of multiple line detections for confident redshift determinations, although our quality "B" redshifts could still be biased by noise and we remove those sources from further analysis. As a side note, the two redshifts (W1136+4236, W2216+0723) from Wu et al (2018) were not flagged in their Table 4 Rest far-UV Emission Line Properties Ferland & Osterbrock 1986), and Hα-to-Hβ ratio (e.g., Osterbrock 1989), to better differentiate between the AGN and star-forming properties of Hot DOGs. According to the narrow-to-broad line flux ratios for Balmer lines (assuming the broad lines come from the BLR, not from outflows), all targets with sufficient emission line S/N (≥ 5) in Hα (N=8) or Hβ (N=4) show a clear narrow component with no pure type 1 (broad-line dominated) classification.…”

“…DATA 2.1. Sample description and observations Our targets, listed in Table 1, were selected to probe restframe UV to optical spectral properties of z ∼ 2 Hot DOGs, complementing our observation programs using Keck and Gemini telescopes to investigate BH accretion using the Hα line (Wu et al 2018). The XSHOOTER sample consists of 10 galaxies, some with existing redshifts at the time of writing the observation proposal (6/10).…”

Luminous WISE-selected Obscured, Unobscured, and Red Quasars in Stripe 82∗

“…These sources also lie well below the relation, and systematically below luminous Type-1 quasars by ∼0.5 dex. Ricci et al (2017a) suggest that the Hot DOGs are underluminous in X-rays owing to their potentially very high Eddington ratios (L/L Edd ∼ 1; Wu et al 2018) and possible saturation of the X-ray-emitting region of the AGN. F2M red quasars have also been shown to have very high Eddington ratios (Urrutia et al 2012;Kim et al 2015), and the same may be the case for the WISEselected sources in this work.…”