The Growth of Luminous
Red Galaxies from
Mergers
Rosalind Skelton
In collaboration with David Gilbank, Daniél
Groenewald and Sarah Blyth
Evolution of Luminous Red Galaxies
●  Mergers expected to be
important
●  Dramatic growth in size (e.g.
Bezanson+09)
●  Early-types have old
populations, in place at high z
●  Little change in LF since z = 1
(e.g. Cool +08, Wake+08, Brown+07)
but see Skelton, Bell &
Somerville 2009, 2012
Identifying Mergers
●  Merger remnants
o 
Morphological classification
  visual
  automated e.g. CAS, Gini-M20
●  Merger progenitors: Close pairs
o 
o 
Spectroscopic - Δvmax, rp, max criteria to identify bound
pairs
Photometric - correction for estimated
contamination, photo-zs
Mergers of massive galaxies
Merger fraction
Robaina et al. 2010
2%
M* > 5 x 1010 M◉
Combo-17,
COSMOS,
SDSS
0.7 mergers per galaxy with M★ > 1011 M◉ since z = 1.2
Merger Fractions of LRGs
●  Lopez-Sanjuan et al. 2012:
o 
o 
0.89 mergers per M★ > 1011 M◉ galaxy since z=1
Mass growth 30%, half from minors
●  Bundy et al. 2009:
o 
Higher merger fraction for more massive galaxies
●  Masjedi et al. 2006, 2008:
o 
o 
Low LRG-LRG merger rate (<0.6% per Gyr)
Growth of 1.2 - 3.9 % per Gyr for z < 0.36
LRGs in Close Pairs from SDSS DR10
SDSS II Luminous red galaxies (z < 0.35)
SDSS III BOSS LOWZ (z ~ 0.2 - 0.4)
SDSS III BOSS CMASS (z ~ 0.4 - 0.7)
LRGs in Close Pairs from SDSS DR10
Sample selection:
z = 0.28 - 0.4
(g-r)rest > 0.75
Mg < -22.5
SDSS LRGs:
42871
BOSS LOWZ: 58379
Total:
101250
LRGs in Close Pairs
Sample selection:
z = 0.28 - 0.4
(g-r)rest > 0.75
Mg < -22.5
SDSS LRGs:
42871
BOSS LOWZ: 58379
Total:
101250
Median mass log M*/Msun = 11.7
LRGs in Close Pairs: Method
●  Identify companions within 50 kpc of LRGs
●  Statistical (local) background correction
Tal et al.
2012
●  10 large 1 Mpc
apertures, 2-5 Mpc
from LRG
●  20 random 50kpc
regions in annulus
0.1-1 Mpc or 2-5
Mpc from LRG
Luminosity function of LRG regions
●  Distribution of
LRGs with
z = 0.28 - 0.4
●  Convert mr to
Lr, with kcorrection
Luminosity function of LRG companions
●  Distribution of
all galaxies in
50 kpc
●  Convert mr to
Lr, with kcorrection
●  Assume
companions
at LRG z
Luminosity function of LRG companions
●  Distribution
of all
galaxies in
50 kpc
●  Convert mr
to Lr, with kcorrection
●  Assume
companions
at LRG z
Luminosity function of LRG companions
●  Distribution
of all
galaxies in
50 kpc
●  20 random
apertures of
50 kpc
●  All assumed
to be at
LRG z
Luminosity function of LRG companions
●  Clear excess in
LRG regions
compared to
randoms
Luminosity function of LRG companions
●  Clear excess in
LRG regions
compared to
randoms
●  Companion
distribution
peaks at
log Lr/L◉ = 10.7
Luminosity function of LRG companions
●  Good agreement with Tal et al. 2012 on 1
Mpc scales
●  Significant excess over background even
within 50 kpc
●  Luminosity gap between LRGs and
companions
●  Suggestions of strong environmental
dependence
Merger Fraction from Close Pairs
●  Average number of companions per LRG in
50 kpc in excess over random background
●  Major mergers - luminosity ratio of 1:1 - 1:4
●  Minor mergers - 1:4 - 1:10
Merger Fraction from Close Pairs
●  Contamination
correction is factor
of ~1.5 - 2
15%
7%
●  Little evolution over
this z range (~1.7
Gyr)
Mass growth from mergers
Sum the total luminosity in major & minor
companions
Assume a constant M/L
Total mass growth if all mass added onto LRG:
Average 6% from z = 0.28 - 0.4
(4 - 12% per Gyr, depending on timescale)
Comparison to previous work
●  Masjedi et al. 2008 find mass growth 1.2 3.9 % per Gyr for z < 0.36
●  For a constant merger fraction & lowest
mass growth ~ 32% growth since z~1
●  Agrees well with Lopez-Sanjuan et al. 2012
(30% growth for M★ > 1011 M◉ galaxies)
Future directions
●  Extend to higher z with BOSS CMASS
( z~ 0.7) and 3D-HST (z ~ 0.7 - 3)
●  Environmental dependence
●  Explore selection criteria in dense
environments in simulations
(with Chris Power, Daniel Cunnama)
Summary
●  There is a significant excess of galaxies in
close pairs with LRGs on scales of 50 kpc
●  The companion distribution peaks at
log Lr/L◉ = 10.7 ≈ 0.2 x Lr median
●  Little evolution in merger fraction to z = 0.4
●  Mass growth of 6% (4 - 12% per Gyr)