Kinematics and Star Formation of High-Redshift Hot Dust-Obscured Quasars as Seen by ALMA

Abstract

Hot, dust-obscured galaxies (Hot DOGs) are a population of hyper-luminous obscured quasars identified by WISE. We present ALMA observations of the [C ii] 158 µm fine-structure line and underlying dust continuum emission in a sample of seven of the most extremely luminous (EL; L bol ≥ 10 14 L ) Hot DOGs, at redshifts z 3.0-4.6. The [C ii] line is robustly detected in four objects, tentatively in one, and likely red-shifted out of the spectral window in the remaining two based on additional data. On average, [C ii] is red-shifted by 780 km s −1 from rest-frame ultraviolet emission lines. EL Hot DOGs exhibit consistently very high ionized gas surface densities, with Σ [CII] 1-2 × 10 9 L kpc −2 ; as high as the most extreme cases seen in other high-redshift quasars. As a population, EL Hot DOG hosts seem to be roughly centered on the main-sequence of star forming galaxies, but the uncertainties are substantial and individual sources can fall above and below. The average, intrinsic [C ii] and dust continuum sizes (FWHMs) are 2.1 kpc and 1.6 kpc, respectively, with a very narrow range of line-to-continuum size ratios, 1.61 ± 0.10, suggesting they could be linearly proportional. The [C ii] velocity fields of EL Hot DOGs are diverse: from barely rotating structures, to resolved hosts with ordered, circular motions, to complex, disturbed systems that are likely the result of ongoing mergers. In contrast, all sources display large line-velocity dispersions, FWHM [CII] 500 km s −1 , which on average are larger than optically and IR-selected quasars at similar or higher redshifts. We argue that a possible hypothesis for the lack of a common velocity structure, the systematically large dispersion of the ionized gas, and the presence of nearby companion galaxies may be that, rather than a continuous single event, the EL Hot DOG phase could be recurrent. The dynamical friction from the frequent in-fall of nearby galaxies and gas clumps, together with the subsequent quasar feedback, would contribute to the high turbulence of the gas within the host in a self-sustaining process that could potentially trigger not only a single, continuous EL event, but instead a number of recurrent shorter-lived episodes as long as external accretion continues.

Imaging Polarization of the Blue-excess Hot Dust-obscured Galaxy WISE J011601.41–050504.0

“…Hence, dust scattering from the former should dominate over free electrons unless the scattering medium is very dustdeficient. We expect dust to be present in significant quantities in the ISM of the host galaxy as, by selection, Hot DOGs have bright dust emission in the mid-IR and are typically well detected in the far-IR (Jones et al 2014;Wu et al 2014;Diaz-Santos et al 2021) 4 . Hence, the Thomson scattering scenario would only be plausible if the scattering medium is within the dust sublimation radius of the accretion disk, which we estimate to be 8.7 pc using equation ( 1) of Nenkova et al (2008) and the bolometric luminosity of W0116-0505 measured by Tsai et al (2015), adjusted for the differences in the cosmological model assumed.…”

“…A similar scenario has been recently suggested by Lansbury et al (2020) and Jun et al (2021) based on estimates of the radiation feedback in luminous obscured quasars. Recently, Diaz-Santos et al (2021) has suggested that the availability of gas at large scales may lead to Hot DOGs being a recurring phase throughout the evolution of massive galaxies, with each episode perhaps leading to a BHD/ERQ/heavily reddened type 1 quasar phase as well, while the massive outflows slowly deplete the gas available in the ISM of their host galaxies.…”

“…A number of studies have identified a hyper-luminous, highlyobscured AGN as the primary source of the luminosity in these objects (e.g., Eisenhardt et al 2012;Assef et al 2015). This obscured AGN component dominates at mid-IR wavelengths (Assef et al 2015), but is sometimes luminous enough to dominate the emission at far-IR wavelengths as well (Jones et al 2014;Díaz-Santos et al 2016;Diaz-Santos et al 2021). Hard X-ray spectra have been obtained for several Hot DOGs, leading to the conclusion that the obscuration is close to, or above, the Compton-thick threshold (Stern et al 2014;Piconcelli et al 2015;Assef et al 2016Assef et al , 2020Vito et al 2018).…”

Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z$\simeq$6.8: A Radio-Loud AGN in the Reionization Era?

“…Eisenhardt et al 2012;Wu et al 2012;Tsai et al 2015). Similar to COS-87259, these lower-redshift hot DOGs are found to possess very large stellar mass (log (M * /M )∼11-12), heavily obscured AGN, and often recent intense starbursts (SFR∼200-3000 M yr −1 ; Assef et al 2015;Diaz-Santos et al 2021). The SED shape of COS-87259 at rest-frame ≈3-100𝜇m is also consistent with that of known hot DOGs (Fig.…”

Co-evolution of massive black holes and their host galaxies at high redshift: discrepancies from six cosmological simulations and the key role of JWST

“…The population of z 6 quasars known to date are mostly non-obscured type I quasars (see discussion in Connor et al 2019;Vito et al 2019;Connor et al 2020;Vito et al 2021). This is likely due to a bias against obscured quasars in the selection process of z > 6 quasars, as observations have identified obscured quasars up to z ∼ 4.6 (e.g., Assef et al 2015;Vito et al 2020;Diaz-Santos et al 2021).…”