The Role of Galaxy Mergers in Forming the Red-Sequence Galaxies

Lihwai Lin

ASIAA, Taiwan

Collaborators:

Trent Univ.: UCO/LICK: ASIAA:David Patton David Koo Bau-Ching Hsieh Kevin Casteels + DEEP2 team NTU:

Tzi-hong ChiuehNottingham Univ. NOAO:Christopher Conselice Jennifer Lotz

Why Study Galaxy Mergers

To understand the formation and evolution of the galaxy properties Wet mergers (gas-rich mergers) Dry mergers (gas-poor mergers) Mixed mergers

Understanding when and how the galaxies are assembled

Providing knowledge for models of galaxy formation (e.g. SAM)

Wechsler et al. 02

Wet Mergers (gas-rich mergers) in Galaxy Evolution

Hopkins et al. 2007

Dry Mergers

Van Dokkum 2005

~35% of bulge-dominated galaxies experienced a merger with mass ratio >1:4 in the recent past

How Does Galaxies Evolve into the Red Sequence?

Faber et al. 2007

Wet mergers might be responsible for quenching star formations in the blue cloud; while dry mergers produce more massive red galaxies

The Redshift Evolution of Various Mergers from Semi-Analytical Models

Khochfar & Burkert 03

For both field and cluster galaxies, the merger events are dominated by Sp-Sp mergers; but the fraction of Sp-E mergers and E-E mergers increases over time

0.4 < z < 1.2 DEEP2 Redshift Survey

4 Fields: each 30’by 120’ (15’by 120’ for EGS) ~ 50,000 galaxies Photometry:

BRI from CFHT/12K i’ z’ from CFHT/MegaCam (CosPA OIR project)

Grating and Spectra: 1200/mm @ 6000A~9000A [OII] doublet is visible at 0.7<z<1.4 Resolution: 1.0” slits; FWHM=1.7A~68/(1+z) km/s (R=5000)

TKRS in GOODS-N

0.05 < z < 0.4 Millennium Galaxy Catalog CNOC2 Redshift Survey

Samples

Wet, Dry, and Mixed Merger Candidates in the DEEP2 Sample

Luminosity range:-21 < MB

e <-19(MB

e MB + Qz with Q=1.3)

Pair criteria:10 < r < 30 h-1kpc v <= 500 km/s

wet mergers blue-blue pairsdry mergers red-red pairsmixed mergers blue-red pairs

Pair Fraction(averaged number of companion per galaxy)

Lin et al. 04 ApJ, 617, L9 Lin et al. 08, submitted

Nc~(1+z)m

m = 0.4 for full sample = 1.3 for b-b pairs = -0.9 for r-r pairs = -1.5 for mixed pairs

Nc vs 2-point correlation function

γ-

0

)r

r(rξ

drr4πrξ)(fractionpair 2R/a

0 zn

γ330γ30 z13

)(4)a

R(

3)(4

R

rzn

rzn

For non-evolving galaxy number density, if clustering is fixed in comoving space and γ = 1.8 = > ~(1+z)1.2

zxa

xr 1length comoving:r

Z=0

Z=1

Comoving Major Merger Rates

The averaged rate for galaxies brighter than 0.4L* involved in major mergers is nearly constant at ~ 10-3h3Mpc-

3Gyr-1 over 0<z<1.2.

The merger rates are dominated by wet mergers at all epoch, followed by mixed mergers and then dry mergers

Lin et al. 2008

Comoving Merger Rateswet : dry: mixed mergers ~ 9 : 1: 3 at z~1.1wet : dry: mixed mergers ~ 6 : 5: 9 at z~0.1Dry mergers increases from 8% at z~1 to 25% at z~0

Lin et al. 2008

Comoving Merger Rateswet : dry: mixed mergers ~ 9 : 1: 3 at z~1.1wet : dry: mixed mergers ~ 6 : 5: 9 at z~0.1Dry mergers increases from 8% at z~1 to 25% at z~0

Lin et al. 2008

How Does Galaxies Evolve into the Red Sequence?

Faber et al. 2007Present day red galaxies have experienced 0.71, 0.24, and 0.36 wet, dry, and mixed mergers respectively since z~1

Typical stellar mass in our sample:

2*1010 for blue galaxies1*1011 for red galaxies

Unanswered Questions

Small statistics? The above results are based on ~ 50 dry mergers

(rare populations!) with L ~ L* at 0.4 < z < 1.2

Where do the mergers occur? Which environment (field, groups, or clusters) do wet, dry, and mixed mergers reside?

Clustering properties of galaxy pairs?

The role of minor mergers?

Future Prospects from HSC Survey

Pin down robust dry (wet, mixed) major merger rate evolution down to galaxies 3 mag fainter than M* galaxiesAllow to study the minor merger history as well => build up complete pictures of assembling histories of red-sequence galaxiesProbe the environment of mergers as a function of merger types and redshiftStudy the clustering properties of various merger types to connect the galaxy properties at low redshift and their high-z merger projenitors.

HSC moderate deep survey (exptime in i’ ~ 20 mins) will cover ~ 700 deg^2, sample thousands of galaxy groups/clusters, and tens of thousands of red-red pairs (dry mergers)

SummaryFor galaxies brighter than 0.4L*, we find the pair fraction evolves differently for blue-blue, red-red, and mixed pairs:

m~ 0.4 for all types of pairs m~ 1.3 for blue-blue pairs m~-0.9 for red-red pairs m~ -1.5 for blue-red pairs

The ratio of galaxy merger rates (# of merger events per unit comoving volume per unit time) for wet, dry, and mixed mergers ~ 9 : 1 : 3 at z~1.1 but changes to 6:5:9 at z~0.1

22% to 54% of today’s galaxies have experienced major mergers since z~1.2; 24% of red galaxies at the present epoch have had dry mergers since z~1.

Wet and mixed mergers may be responsible for producing red galaxies with intermediate masses while the most massive red galaxies are assembled through dry mergers at later time.

The HSC Survey will be an excellent data set to enhance our understanding on the major & minor merger histories, environment of different merger types, and clustering properties of close pairs, as well as their roles in forming the red-sequence galaxies.

Thank You!