We present results from a pilot HST ACS deep imaging study in broad-band V of
five low-redshift QSO host galaxies classified in the literature as
ellipticals. The aim of our study is to determine whether these early-type
hosts formed at high redshift and have since evolved passively, or whether they
have undergone relatively recent mergers that may be related to the triggering
of the nuclear activity. We perform two-dimensional modeling of the light
distributions to analyze the host galaxies' morphology. We find that, while
each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the
majority of them (4/5) reveal significant fine structure such as shells and
tidal tails. These structures contribute between ~5% and 10% to the total
V-band luminosity of each host galaxy within a region of r ~ 3 r_eff and are
indicative of merger events that occurred between a few hundred Myr and a Gyr
ago. These timescales are comparable to starburst ages in the QSO hosts
previously inferred from Keck spectroscopy. Our results thus support a
consistent scenario in which most of the QSO host galaxies suffered mergers
with accompanying starbursts that likely also triggered the QSO activity in
some way, but we are also left with considerable uncertainty on physical
mechanisms that might have delayed this triggering for several hundred Myr
after the merger.Comment: 22 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
