Where do quasar hosts lie with respect to the size-mass relation of
  galaxies?
  The evolution of the galaxy size - stellar mass (Mstellar) relation has been
a puzzle for over a decade. High redshift galaxies are significantly more
compact than galaxies observed today, at an equivalent mass, but how much of
this apparent growth is driven by progenitor bias, minor mergers, secular
processes, or feedback from AGN is unclear. To help disentangle the physical
mechanisms at work by addressing the latter, we study the galaxy size -
Mstellar relation of 32 carefully-selected broad-line AGN hosts at 1.2 < z <
1.7 (7.5 < log M_BH < 8.5; L_bol/L_Edd > 0.1). Using HST with multi-band
photometry and state-of-the-art modeling techniques, we measure half-light
radii while accounting for uncertainties from subtracting bright central point
sources. We find AGN hosts to have sizes ranging from 1 to 6 kpc at Mstellar ~
0.3 - 1 x 10^11 Msun. Thus, many hosts have intermediate sizes as compared to
equal-mass star-forming and quiescent galaxies. While inconsistent with the
idea that AGN feedback may induce an increase in galaxy sizes, this finding is
consistent with hypotheses in which AGNs preferentially occur in systems with
prior concentrated gas reservoirs, or are involved in secular compaction
processes perhaps responsible for simultaneously building bulges and shutting
down star formation. If driven by minor mergers, which do not grow central
black holes as fast as they do bulge-like stellar structures, such a process
would explain both the galaxy size - mass relation observed here and the
evolution in the black hole, bulge mass relation described in a companion
paper.
