The Mass-Dependent Role of Galaxy Mergers Kevin Bundy (UC Berkeley) Hubble Symposium March, 2009 Masataka Fukugita, Richard Ellis, Tom Targett Sirio Belli, Tadayuki Kodama

Hierarchical CDM Assembly z=18 z=6 z=1.4 z=0 Millennium, Springel et al. 2005

Hierarchical CDM Assembly z=18 z=6 z=1.4 z=0 redshift fraction M * >10 10 M * >10 11 de Lucia et al Millennium, Springel et al. 2005

Downsizing of Star Formation The sites of star formation appear to shift from including high-mass galaxies at early epochs (z~1-2) to only lower-mass galaxies at later epochs. Top-Down Formation of Ellipticals The most massive galaxies transform into ellipticals first, with less massive galaxies following later.

Downsizing: Red Growth Mass Increasing abundance Bundy et al. 2006

Downsizing: Red Growth Mass Increasing abundance Bundy et al. 2006

Downsizing: Red Growth Mass Increasing abundance Morphological spheroidals have a similar formation pattern. (Bundy et al. 2005) Bundy et al. 2006

Merger Simulations Springel, Hernquist, Hopkins

Trigger Mechanism Maintenance Star formation Quenching (gas) Spheroidal Formation (morphology) Radio Mode AGN Feedback Major Merger AGN? Major Merger N-body Simulation Prevent cold disk formation ? Starburst (z < 1)

Conflict? Hierarchical Assembly - Increasing Mass Scale Early-Type/Red-sequence Formation - Decreasing Mass Scale Mergers

Must distinguish major mergers. Must probe the mass dependence. Need complete, mass-limited samples. What is the merger rate? Are there enough mergers to make spherdoial galaxies? Challenges Merger Rate? Lin et al. 2004, 2008 (DEEP2); de Ravel et al (VVDS), Kartaltepe et al (COSMOS), Bell et al (COMBO17), Lotz et al (morph.), Patton & Atfield 2008 (SDSS)

Observed Near-IR Galaxy Merger Rate Subaru K~22 Observations in GOODS-N + ISAAC in GOODS-S Bundy, Fukugita, Ellis, Targett, Belli, Kodama arXiv: , ApJ accepted

Observations MOIRCS K~22 (Vega), 0.5’’, GOODS N+S, 0.08 deg 2 K Catalog (17155 sources) matched to ACS data and spec-z surveys. ~3000 hosts (60% spec-z), M * >10 10 M , 0.4 < z < 1.4 Additional GOODS-S photo-zs from Grazian et al (dz/1+z ~ 0.03). BPZ in GOODS-N (dz/1+z ~ 0.09). Iichi Tanaka and MOIRCS

Pair Fraction Count the fraction of galaxies with a fainter companion. Companion Criteria: 5 < r sep < 20 kpc no fainter than K host + 1.5, ensures major mergers defined as M comp /M host > 1/4 example IR pairs

Contamination Correction Correction 1: Subtract background number density. Correction 2: Use redshift information,  z 2 <  host 2 +  comp 2. Then randomize the x,y positions 100 times, subtract the average remainder.

Mass Dependent Pair Fraction Background field correction Redshift pair correction. Log M* ~ 11.3 Log M* ~ 10.3 Bundy et al. 2009

Mass Dependent Pair Fraction Background field correction Redshift pair correction. Log M* ~ 11.3 Log M* ~ 10.3 Lin et al Bundy et al. 2009

Mass Dependent Pair Fraction Background field correction Redshift pair correction. Log M* ~ 11.3 Log M* ~ 10.3 Lotz et al Bundy et al. 2009

Deriving the Merger Rate Merger efficiency and timescale. Kitzbichler & White Find  decreases as M * -0.3 Patton & Atfield  = 0.5 Gyr, less efficient at high M * We use our sample to determine volumetric merger rate.

Volumetric Merger Rate Log Merger Rate Density, Gyr -1 Mpc -3 Log M * Bundy et al. 2009

Volumetric Merger Rate Log Merger Rate Density, Gyr -1 Mpc -3 Log M * Wet/Mixed Mergers Bundy et al. 2009

Volumetric Merger Rate Log Merger Rate Density, Gyr -1 Mpc -3 Log M * Too few mergers! Bundy et al. 2009

Volumetric Merger Rate Log Merger Rate Density, Gyr -1 Mpc -3 Log M * Bundy et al. 2009

Volumetric Merger Rate Log Merger Rate Density, Gyr -1 Mpc -3 Log M * Halo-based Predictions (Stewart et al. 2008) Halo-based Predictions (Stewart et al. 2008) Bundy et al. 2009

Summary and Conclusions We find an increase in the (major) merger fraction for more massive galaxies. Roughly 30% experience of M sun galaxies a major merger since z~1, but less than 10% at lower masses where star formation is more important in building stellar mass. Our results are in good agreement with halo based predictions. Comparing to the growing abundance of spheroidals, major mergers appear unable to fully account for morphological evolution. Merger timescales remain an important uncertainty -- a potential for resolving some of the discrepancy? Time to consider other processes? Disk instabilities, psuedo- bulges, minor mergers…??