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

1
The Role of Galaxy Mergers in Forming the
Red-Sequence Galaxies

• Lihwai Lin
• ASIAA, Taiwan

Collaborators Trent Univ. UCO/LICK ASIAA D
avid 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 gt14 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 lt z lt 1.2
• DEEP2 Redshift Survey
• 4 Fields each 30by 120 (15by 120 for EGS)
• 50,000 galaxies
• Photometry
• BRI from CFHT/12K
• i z from CFHT/MegaCam (CosPA OIR project)
• Grating and Spectra 1200/mm _at_ 6000A9000A
• OII doublet is visible at 0.7ltzlt1.4
• Resolution 1.0 slits FWHM1.7A68/(1z) km/s
  (R5000)
• TKRS in GOODS-N
• 0.05 lt z lt 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 lt MBe lt-19
• (MBe ? MB Qz with Q1.3)
• Pair criteria
• 10 lt Dr lt 30 h-1kpc
• Dv lt 500 km/s

9
Pair Fraction(averaged number of companion per
galaxy)
Nc(1z)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
For non-evolving galaxy number density, if
clustering is fixed in comoving space and ? 1.8
gt (1z)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 0ltzlt1.2.
• 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 z1.1
• wet dry mixed mergers 6 5 9 at z0.1
• Dry mergers increases from 8 at z1 to 25 at
  z0

Lin et al. 2008
13
Comoving Merger Rates

• wet dry mixed mergers 9 1 3 at z1.1
• wet dry mixed mergers 6 5 9 at z0.1
• Dry mergers increases from 8 at z1 to 25 at
  z0

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 z1
Typical stellar mass in our sample 21010 for
blue galaxies 11011 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 lt z lt 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 deg2, 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
  gt 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
  z1.1 but changes to 659 at z0.1
• 22 to 54 of todays galaxies have experienced
  major mergers since z1.2 24 of red galaxies at
  the present epoch have had dry mergers since z1.
  
• 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!