2019MNRAS.489..497Z

We present Hubble Space Telescope 1.4-1.6 µm images of the hosts of 10 extremely red quasars (ERQs) and six type 2 quasar candidates at z = 2-3. ERQs, whose bolometric luminosities range between 10⁴⁷ and 10⁴⁸ erg s^–1, show spectroscopic signs of powerful ionized winds, whereas type 2 quasar candidates are less luminous and show only mild outflows. After performing careful subtraction of the quasar light, we clearly detect almost all host galaxies. The median rest-frame B-band luminosity of the ERQ hosts in our sample is 10^11.2 L_☉, or ∼4L* at this redshift. Two of the 10 hosts of ERQs are in ongoing mergers. The hosts of the type 2 quasar candidates are 0.6 dex less luminous, with 2/6 in likely ongoing mergers. Intriguingly, despite some signs of interaction and presence of low-mass companions, our objects do not show nearly as much major merger activity as do high-redshift radio-loud galaxies and quasars. In the absence of an overt connection to major ongoing gas-rich merger activity, our observations are consistent with a model in which the near-Eddington accretion and strong feedback of ERQs are associated with relatively late stages of mergers resulting in early-type remnants. These results are in some tension with theoretical expectations of galaxy formation models, in which rapid black hole growth occurs within a short time of a major merger. Type 2 quasar candidates are less luminous, so they may instead be powered by internal galactic processes.