Hubble Space Telescope Images of Post-Starburst Quasars

1
Hubble Space Telescope Images of Post-Starburst
Quasars

• Michael S. Brotherton, S. Cales, R. Ganguly, Z.
  Shang (University of Wyoming)
• G. Canalizo (University of California at
  Riverside)

ABSTRACT We report the results of a Hubble Space
Telescope snapshot survey of 29 post-starburst
quasars at redshifts between 0.25 and 0.4. The
broadband F606W ACS images resolve morphological
detail in nearly all targets and indicate that
interactions and merger activity are common for
this class. Some spectacular images include
double nuclei, starbursting ring systems, and
nuclear dust rings. The images will be used to
constrain spectral modeling to separate AGN and
stellar components. We are investigating the
hypothesis that post-starburst quasars represent
the stage of massive galaxy evolution known as
"blow out."
Introduction and Observations The rare class of
post-starburst quasars show composite spectra
with both the broad lines of an AGN and the
Balmer jumps and absorption lines that are the
signatures of a massive luminous starburst a few
100 Myr old, A striking case is UN J1025-0040 at
z0.634 (Fig. 1 Brotherton et al. 1999). The
starburst component has an age of 400 Myr, and a
bolometric luminosity of 1011.6 solar, equal to
that of the quasar. Keck and HST imaging shows a
merger remnant (Brotherton et al. 1999, 2002). A
nearby companion galaxy is also in a
post-starburst phase (Canalizo et al. 2000). UN
J1025-0040 is consistent being an evolved Ultra
Luminous IR Galaxy (ULIRG). The new large quasar
surveys like the Sloan Digital Sky Survey (SDSS,
e.g., Schneider et al. 2002), are revealing
post-starburst quasars in significant numbers for
the first time. We have spectroscopically
selected some 600 post-starburst quasars in SDSS
data release 3 (DR3) using an automated algorithm
based on one that Zabludoff et al. (1996) used to
select post-starburst galaxies based on Balmer
jumps and high-order absorption lines. SDSS
images show that about 40 have close companions,
and many single sources show fuzz or tidal
features, reminiscent of the post-starburst
galaxies which are uniformly interacting
systems. We recently completed an ACS SNAP
program obtaining F606W images of 29
post-starburst SDSS DR3 quasars, a subsample of a
catalog selected with r lt 18.6, H-delta
absorption equivalent widths gt 1A, 0.25 lt z lt 0.4
(ensuring high luminosity and similar size
scales, resolving structures a half kpc across).
Using the F606W filter longward of the Balmer
jump maximizes the contrast of the stellar
component against the AGN core, and reveals the
morphology of the host galaxies. Fig. 2 shows our
HST images. Fig. 3 compares HST and SDSS images
for one example post-starburst quasar.
Fig.3. A comparison of ground-based SDSS imaging
(left) with HST imaging (right). The SDSS
rest-frame spectrum is shown below.
Relevance Recently evidence for an
AGN-starburst'' connection has become
quantitative and compelling in the form of the
MBH-sigma correlation (e.g. Tremaine et al.
2002). Stated more physically, essentially every
large galaxy harbors a black hole some 0.15 of
the mass of its spheroidal component. Theorists
have proposed various models, with those
involving hierarchical galaxy evolution via
merger activity, and feedback from the central
AGN to control star formation becoming popular.
Sophisticated simulations of galaxy mergers now
exist (e.g. di Matteo et al. 2005) and the
process proceeds through observable phases.
Hopkins et al. (2008) shows this evolutionary
sequence isolated systems, small group,
interaction/merger, coalescence/(U)LIRG,
Blowout, Quasar, Decay (post-starburst galaxy),
ending with a dead elliptical. Hopkins
identifies IRAS-selected quasars as objects at
blow out, and some do have suggestive properties.
Were investigating the alternative notion that
post-starburst quasars are objects at blow
out. Results Discussion The first striking
result for this spectroscopically selected sample
is that interactions and merging are extremely
common. Close inspection reveals that most of
the isolated systems show asymmetries, tidal
tails, shells, or other features indicative of
mergers. These objects represent the most extreme
examples of recent star formation and black hole
growth in the semi-local universe. They appear
consistent with a hierarchical evolutionary
scheme, but indicate that the simplest versions
require modification. The AGN activity can
appear at different epochs, all the way from
pre-merger to post-merger. There are a few
isolated and apparently undisturbed systems
(e.g., a few barred spirals) that may represent
the transition to Seyfert mode fueling that
creates both stars and nuclear activity, but at
the lower levels more consistent with the local
neighborhood. We acknowledge support from the US
National Science Foundation through grant AST
05-07781 and NASA through grant NNG05GE84G.
References Brotherton et al. 1999, ApJ, 540,
L87 Brotherton et al. 2002, PASP, 114,
593 Canalizo et al. 2000, AJ, 119, 59 Di Matteo
et al. 2005, Nature, 433, 604 Hopkins et al.
2008, ApJS, 175, 356 Schneider et al. 2002, AJ,
123, 567 Tremaine et al. 2002, ApJ, 574,
740 Zabludoff et al. 1996, ApJ, 466, 104
Figure 2 Our ACS F606W snapshot images, 10
arcseconds on a side (which is about 50 kpc at
these redshifts). Note double nuclei,
star-forming knots, and other structures too
small to resolve from the ground such as nuclear
dust lanes. Faint tidal tails and shells are
apparent in many images, too.
Fig.1. UN J1025-0040 from Brotherton et al.
(1999). The Keck spectrum is modeled as an
instantaneous starburst plus an AGN. The Keck
K-band image shows extended, asymmetric fuzz with
a nearby companion (verified also at z0.634).