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

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“

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?

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)

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

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)

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)

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 0.6-2.2 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)

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?

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

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