Transcript Slide 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)
Introduction and Observations
ABSTRACT: We report the results of a Hubble Space Telescope snapshot survey of 29 poststarburst 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."
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 z=0.634 (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).
Fig.3. A comparison of ground-based SDSS
imaging (left) with HST imaging (right). The SDSS
rest-frame spectrum is shown below.
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.
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.
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.
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 (poststarburst galaxy), ending with a dead elliptical. Hopkins
identifies IRAS-selected quasars as objects at blow out,
and some do have suggestive properties. We’re
investigating the alternative notion that post-starburst
quasars are objects at blow out.
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 < 18.6, H-delta
absorption equivalent widths > 1A, 0.25 < z < 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.
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 semilocal 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 premerger 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
Fig.1. UN J1025-0040 from Brotherton et al.
(1999). The Keck spectrum is modeled as an
instantaneous starburst plus an AGN. The Keck Kband image shows extended, asymmetric fuzz with
a nearby companion (verified also at z=0.634).
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.
Di Matteo et al. 2005, Nature, 433, 604
Hopkins et al. 2008, ApJS, 175, 356
Schneider et al. 2002, AJ, 123, 567
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