Major Mergers Host the Most Luminous Red Quasars at z ~ 2: A Hubble
  Space Telescope WFC3/IR Study
  We used the Hubble Space Telescope WFC3 near-infrared camera to image the
host galaxies of a sample of eleven luminous, dust-reddened quasars at z ~ 2 --
the peak epoch of black hole growth and star formation in the Universe -- to
test the merger-driven picture for the co-evolution of galaxies and their
nuclear black holes. The red quasars come from the FIRST+2MASS red quasar
survey and a newer, deeper, UKIDSS+FIRST sample. These dust-reddened quasars
are the most intrinsically luminous quasars in the Universe at all redshifts,
and may represent the dust-clearing transitional phase in the merger-driven
black hole growth scenario. Probing the host galaxies in rest-frame visible
light, the HST images reveal that 8/10 of these quasars have actively merging
hosts, while one source is reddened by an intervening lower redshift galaxy
along the line-of-sight. We study the morphological properties of the quasar
hosts using parametric Sersic fits as well as the non-parametric estimators
(Gini coefficient, M_{20} and asymmetry). Their properties are heterogeneous
but broadly consistent with the most extreme morphologies of local merging
systems such as Ultraluminous Infrared galaxies. The red quasars have a
luminosity range of log(L_bol) = 47.8 - 48.3 (erg/s) and the merger fraction of
their AGN hosts is consistent with merger-driven models of luminous AGN
activity at z=2, which supports the picture in which luminous quasars and
galaxies co-evolve through major mergers that trigger both star formation and
black hole growth.
