1. Fueling QSOs: The Relevance of Mergers
Nicola Bennert
University of California Riverside
in collaboration with
Gabriela Canalizo (UCR), Bruno Jungwiert (UCR/Prague), Alan Stockton (IfA),
Francois Schweizer (Carnegie), Mark Lacy (SSC), Chien Peng (HIA)
The Central Kiloparsec: AGNs and Their Hosts June 6th, 2008, Ierapetra, Crete

2. The Relevance of Mergers for Fueling AGNs
What mechanism triggers AGN activity? - ubiquity of SMBHs
Steep evolution of activity with z: - accretion onto BH more common
- triggering mechanism more common
Mergers?
ULIRGs: close connection between mergers and QSO activity
(e.g. Canalizo & Stockton 2001; Surace et al. 1998, 2000)
What is the relevance of mergers for fueling AGNs?
– helpful (e.g. Toomre & Toomre 1972, Stockton 1982)
- not sufficient (e.g. any catalogue of interacting galaxies)
- necessary for QSOs only?
(e.g. Malkan et al. 1998, Combes et al. 2006, Woo et al. 2004)
also: Moshe‘s „final speculations“

3. The Nature of QSO Host Galaxies
Most QSOs: - begin life as mergers (e.g. Sanders et al. 1988) ?
- reside in old ellipticals (e.g. Dunlop et al. 2003) ?
Dunlop et al. (2003): 33 AGNs (RQQs, RLQs, RGs, 0.1<z<0.25; WFPC2, 30min)
hosts are massive ellipticals that are
„indistinguishable from quiescent, evolved,
low-redshift ellipticals at comparable mass“
Evolutionary merger sequence: ULIRGs – QSOs – Ellipticals?
(e.g. Sanders et al. 1988, Surace et al. 1998, 2000)
QSO activity triggered by merger, activity can outlast the signs of interactions?

4. Deep HST/ACS Images of QSO Pilot Sample (z~0.2)
Signs of recent merger events (shells, tidal tails…) in 4 of the 5 QSOs
(Bennert et al. 2008, ApJ, 677, 846; Canalizo, Bennert, et al. 2007, ApJ, 669, 801)

5. Comparison with Numerical Simulations
Minor merger remnant?
here: dwarf + spiral
(1:8 in total mass)
(time in Gyr)
gas
stars
thanks to TJ Cox

6. Comparison with Numerical Simulations
Plummer sphere (r_eff = 7.6 kpc; M=10^11 M_sun)
(Canalizo et al. 2007, Bennert et al. 2008, Jungwiert et al. 2008)
Host galaxy subtraction
(Galfit, Peng et al. 2002)

7. Spectacular Shells in MC2-1635+119
Minor merger: - regular inner shell structure (merger age ~few hundred Myr)
Major merger: - total light contribution from inner shells (~6%)
- outer arcs/tidal tails (merger age up to 1.7 Gyr)
- inner shell structure formed by material raining back in?
Canalizo, Bennert et al. (2007)

8. Stellar Populations
Deep Keck spectroscopy of 14 QSO host galaxies (z~0.2):
- starburst component
in all but one of the hosts
- best fit typically 10 Gyr pop. +
intermediate age starburst
- major starburst episodes (10-90% of mass)
with ages Gyr
(Canalizo et al. 2006, 2007, 2008)
Hosts of most luminous AGNs:
- bulge-dominated but significantly bluer
than inactive ellipticals
- evidence for starburst in relatively recent
past (1-2 Gyr)
(e.g. Kauffmann et al. 2003, Sanchez et al. 2004,
Jahnke et al. 2007)
Canalizo et al. (2008)

9. Recent Merger Events in at least some QSO Hosts
Signs of recent merger events (shells, tidal tails…) in 4 of the 5 QSOs
(Bennert et al. 2008, Canalizo et al. 2007)
Comparison with numerical simulations: ~Gyr or less ago
(Bennert et al. 2008, Canalizo et al. 2007)
Comparable to starburst ages from Keck spectroscopy (Canalizo & Stockton 2008)
but longer than AGN duty cycle (e.g. Yu & Tremaine 2002)
Time delay between interaction and fueling of BH?
(e.g. Barnes 1998, Springel et al. 2005, Hopkins et al. 2006)
Or episodic QSO activity?
(e.g. Norman & Scoville 1988, Hopkins et al. 2006)
How can we „prove“ that merger triggered QSO activity?

10. A Control Sample of Inactive Ellipticals
z=0.279
z=0.160
z=0.182
z=0.287
z=0.245
z=0.220
z=0.190
Preliminary results: No comparable fine structure

11. Outlook: 13 more QSOs with HST/WFPC2
Fine structure in at least some hosts