Galaxy Mergers through Cosmic Time

Jennifer Lotz Space Telescope Science Institute The Role of Mergers in Galaxy Assembly galaxy mergers vs. gas accretion how to galaxies grow in mass?

Lacey & Cole 1993 Ceverino et al. 2009 The Role of Mergers in shaping galaxy properties

How To Find a Galaxy Merger

close galaxy pairs morphology (pre-mergers) (pre- and post-mergers)

not the same!

kinematics (pre- and post-mergers)

Galaxy Merger Fractions disagree!

10 Mstar > 10 M☉ or LB> 0.4 L*

Lotz et al. 2008 Lin et al. 2008 Bundy et al. 2009 de Ravel et al. 2009 Darg et al. 2009 Conselice et al. 2009 Galaxy Merger Fractions disagree!

10 Mstar > 10 M☉ or LB> 0.4 L* Galaxy Merger Fractions disagree!

10 Mstar > 10 M☉ or LB> 0.4 L*

Lotz et al. 2008 Lin et al. 2008 Bundy et al. 2009 de Ravel et al. 2009 Darg et al. 2009 Conselice et al. 2009 How to Calculate the Galaxy Merger Rate the ‘merger’ rate is defined as -1 Rmerg [Gyr ]= fmerg/Tmerg

-3 -1 or Γmerg [Mpc Gyr ]= ϕmerg / Tmerg where fmerg is fraction of galaxies in a merger ϕmerg is number density of merger events (fmerg x ngal) Tmerg is time for merger to complete (few Gyr) How to Calculate the Galaxy Merger Rate but we observe merger events at particular stages (depending on method) with a range of merger parameters (mass ratio, gas fraction... )

-1 Rmerg [Gyr ]= f′merg(z) /〈Tobs(z)〉

-1 -3 Γmerg [Gyr Mpc ]= f′merg(z) ngal(z)/〈Tobs(z)〉

Tobs depends on merger parameters... and

〈Tobs(z)〉depends on distribution of parameters with z Galaxy Merger Simulations

GADGET N-Body/SPH galaxy merger simulations initial galaxies: gas-rich disks (5 models)

(Cox et al. 2006, 2008) Disk-disk mergers: - orientation - orbits (pericentric distance, eccentricity) - gas fraction (20%, 40%, 50% of baryons) - total mass (109 - 1012 Msun) - mass ratio (1:1 -- 1:10) - supernova feedback prescription (No AGN!) - nparticles 1x, 10x with T.J. Cox, P. Jonsson, J. Primack, R. Somerville Galaxy Merger Simulations

Particles (stars, gas) ⇒ Light (dust, λ) SUNRISE: Monte-Carlo radiative transfer code (Jonsson 2006)

☁ ☁ ★ ★★ ☁ ★ ★☁ ★ ☁★ ★

http://archive.stsci.edu/prepds/diggss Equal-Mass Disk-Disk Prograde Merger, face-on

1 pre-merger 2 first pass 3 max. sep.

4 merger 5 post-merger 6 remnant

Lotz et al., 2008b Merger Timescales - Mass Ratio

fixed gas fraction fixed gas fraction

G-M20 finds major and minor mergers Lotz, Jonsson, Cox, & Primack, 2010a Merger Timescales - Gas Fraction

1:1 mass ratio 1:1 mass ratio

Asymmetry timescales depend on gas fraction Lotz, Jonsson, Cox, & Primack, 2010b Cosmological Weighting

〈T(z)〉= ∑ ni(z) Ti

to compute Γmerger (z) need〈T(z)〉 weighted by n(mass ratio, gas fraction...)

⇒ use distributions of merger properties from cosmological models (Somerville et al. 2008; Croton et al. 2006; Stewart et al. 2008) Cosmological Weighting

Gas fraction v. z

Somerville 08Somerville 08 Cosmological Weighting Timescale v. z

Somerville 08

T(pair), T(G-M20) independent of fgas

T(Asym) ~ fgas(z)

Lotz et al., in prep Galaxy Merger Rate at z<1 (z) merg Γ

Lotz et al., in prep How to Measure the Galaxy Merger Rate

fmerg(z) 〈Tobs(z)〉 Rmerg (z) ngal(z) Γmerg (z) -selected α~1.7 ±3.0 α~-0.9 ±3.2 star M

α~2.1± 0.2 α~0.1± 0.4 *-selected B L

2 for major mergers: Rmerg ~ (1+z) Γmerg ~ flat major+minor mergers: 3-5 x major merger; weaker evolution? Galaxy Merger Rate at z<1

Γminor ~ (ΓG-M20 − Γpairs) ⇒ ~ 1-3 minor mergers per >0.5 L* galaxy since z~1

Γmajor ~ Γpairs ~ Γasym ⇒ ~ 0.5 - 1 major merger per >0.5 L* galaxy since z~1 Connecting to Dark Matter Halos

z=0.24 z=3.0

α~2-3

12.5 Mhalo ~10 Msun dark matter halo

2-3 Dark Matter Halos: Rmerg ~ (1+z) for FIXED halo mass merger rate dependsα~3.0± on 0.2 halo mass α~2.9± 1.1 (see also Hopkins et al. 2010) Connecting to Dark Matter Halos ngal-selected

α~3.0± 0.2 α~2.9± 1.1

Galaxy Merger Rate evolution depends on how galaxy sample is selected (because merger rate depends on halo mass, halo ⇔ galaxy mapping?) Peak Epoch of Star-Formation at 2

redshift Bouwens et al. 2009 Merger → Starburst → AGN→ Spheroid

1

3 4 2

5 6 e.g. Sanders & Mirabel 1996 HST WFC3 NIR needed at z~2

need high resolution NIR imaging to probe rest-frame optical structures of z ~ 2.5 galaxies

Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey HST MCT program Galaxy Structures at z~2

~35% disturbed, ~20% spheroids, ~45% disks (30 galaxies, 1.5 < z < 2.5, Mstar > 1010 M☉) Merger v. Clumpy Star-Forming Disk? ⇒ need simulations of z~2 unstable disk galaxies and gas-rich mergers to interpret HST structures (+ALMA + Herschel + ground-based IFU) cold gas stars

Bournaud & Elmegreen 2009 Narayanan et al. 2009 [24μ - R - B] Galaxy Mergers at z< 1 - merger ‘fractions’ disagree (not surprising!) - theoretical timescales needed to compute ‘rate’ - fractional merger rate R(z) ~ (1+z)2 volume-averaged merger rate Γ(z) ~ flat (for constant stellar mass selection) - need halo ⇔ galaxy to compare to halo merger rate

- Merger-driven SF accounts for <12% total ρSFR The Role of Mergers at z~2 - peak epoch of SF, AGN activity; also mergers? - need high-res NIR imaging CANDELS (+ JWST) - mergers v. clumpy disk?? need simulations + kinematics (ALMA?)