1. 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 Chiueh
Nottingham Univ. NOAO:
Christopher Conselice Jennifer Lotz

2. Why Study Galaxy Mergers
Wechsler et al. 02
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)

3. Wet Mergers (gas-rich mergers) in Galaxy Evolution
Hopkins et al. 2007

4. Dry Mergers
Van Dokkum 2005
~35% of bulge-dominated galaxies experienced a merger with mass ratio >1:4 in the recent past

5. 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

6. The Redshift Evolution of Various Mergers from Semi-Analytical Models
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
Khochfar & Burkert 03

7. Samples 0.4 < z < 1.2 0.05 < z < 0.4 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: 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

8. Wet, Dry, and Mixed Merger Candidates in the DEEP2 Sample
wet mergers  blue-blue pairs
dry mergers  red-red pairs
mixed mergers  blue-red pairs
Luminosity range:
-21 < MBe <-19
(MBe  MB + Qz with Q=1.3)
Pair criteria:
10 < Dr < 30 h-1kpc
Dv <= 500 km/s

9. Pair Fraction (averaged number of companion per galaxy)
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
Lin et al. 04 ApJ, 617, L9
Lin et al. 08, submitted

10. Nc vs 2-point correlation function
Z=0
Z=1
For non-evolving galaxy number density, if clustering is fixed in comoving space and γ = = > ~(1+z)1.2

11. 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

12. Comoving Merger Rates wet : dry: mixed mergers ~ 9 : 1: 3 at z~1.1
Dry mergers increases from 8% at z~1 to 25% at z~0
Lin et al. 2008

13. Comoving Merger Rates wet : dry: mixed mergers ~ 9 : 1: 3 at z~1.1
Dry mergers increases from 8% at z~1 to 25% at z~0
Lin et al. 2008

14. How Does Galaxies Evolve into the Red Sequence?
Faber et al. 2007
Present 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 galaxies
1*1011 for red galaxies

15. 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?

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

17. Summary
For 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.

18. Thank You!