Download Hwihyun Kim

The Environmental Effect on
the UV Color-Magnitude
Relation of Early-type Galaxies
Schawinski et al. 2007, ApJS 173, 512
Hwihyun Kim
Journal Club
10/24/2008
Color-Magnitude Relation
• Tool for understanding the
formation and evolution of
early-type galaxies (Visvanathan
& Sandage,1977)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Optical CMR displays a small
intrinsic scatter as a result of
a small age dispersion (Bower
et al. 1992)
• Massive early-type galaxies
– Initial, intense star-formation at
high-z
– No significant evolution on CMR
Monolithic collapse
vs. Hierarchical Merger
• Monolithic collapse mode;
– Simple evolutionary model for early-type galaxies
– Rapid collapse of a gas cloud, forming all of its stars in
an initial burst (duration < 1 Gyr)
• Hierarchical merger scenario
– Small galaxies form first and later assemble into larger
objects
– Denser environment affects galaxy evolution
– Complicated than the pure monolithic collapse model
– Kaviraj et al. (2005) : Optical CMR of early-type galaxies
only if monolithic => we are not probing the entire star
formation history of early-types
• Why we need the UV CMR?
– To study the recent star formation of early-type
galaxies
– Optical filters are not sufficiently sensitive to detect
a low-level star-forming activity
• Why we need the GALEX?
– Capable of detecting even a small (~1% of mass
fraction) young stellar population
– Ideal for tracing the recent SF history
Sample Selection
•
•
•
•
•
Early-type galaxy selection in SDSS
Cross-match to GALEX detections
Visual inspection of galaxy morphology
Volume-limited sample
AGN contamination was removed
Early-type galaxy selection in SDSS DR3
• Catalog of Bernardi et al. (2003)
– ~9000 galaxies
– Biased strongly against star-forming elliptical galaxies
– Contaminated by late-type interlopers
• Morphology driven criteria (this paper)
– Bulge-dominated galaxies: inclusive sample
– de Vaucouleurs’ surface brightness profile (r1/4) in g, r
and i bands
– SED quality: S/N larger than 10
==> total 89248 early-type galaxies in SDSS DR3 with no
constraints on luminosity or redshift
Matching to GALEX-MIS
• GALEX Medium Imaging Survey
– Single orbit exposures (1500sec) of 1000 square
degrees in positions
• Matching to GALEX
– All early-type galaxies within GALEX field of view
– Within 4” angular resolution limit of GALEX of
each SDSS early-type
• Visual inspection of galaxy morphology
– Reliable limits: z (redshift) < 0.1 and r < 16.8 mag
• Availability of SDSS spectroscopic data
– Incomplete for z < 0.05
• 10% of the sample do not have GALEX detections
• For z=[0.05, 0.1] and r < 16.8 (Mr=-21.5 mag at z=0.1)
– 847 ellipticals, 112 lenticulars, 126 others
• By a BPT analysis, AGN contamination was removed from
the sample with S/N > 3 (11% removed)
• Removed all strong radio sources by the VLA FIRST survey
• Volume-limited CMR
– Some of UV blue galaxies are not genuine early-types
– Late-types and AGN candidates are bluer
• 839 early-type galaxies
Classification of environment
• Two-dimensional projected number densities
– Use SDSS spectroscopic redshift with accuracy of
1.710-4  210-5 (~0.5 Mpc)
– Adaptive volume
• Count all neighbors within a certain radius  (n  10)
• Gaussian distribution gives more weight to closer
neighbors (g: adaptive environment parameter)
c z 1 0.2n,
g   
• Fiducial value of  = 2.0 Mpc
2 
 1 r 2

r
1
z
a
exp  2  2 2 
c z  
2
 2 
• Cube size = 100 Mpc
• Green sphere: galaxies Mr < -20.5
• UV-upturn
– Unusually strong UV flux rise
in 1000-2500Å
– Due to presence of low-mass
HB stars (Yi et al. 1997)
– Dominant at NUV-r > 5.4
• NUV-r < 5.4 mag
– Recent episode of starformation
• NUV-r > 5.4 mag
– Either forming stars or
exhibiting UV-upturn
– Can’t distinguish using GALEX
alone
• More massive early-type galaxies in denser environment
• Most luminous galaxies reside in the most high-density
environment (Hogg et al. 1984)
• In this figure,
– The higher density CMR extends to more massive galaxies
– The low-density CMR extends to bluer colors than the high-density one
• Dependence of NUV-r
color on environment
– More blue galaxies in
low-density
– Medium and high
density curves are
indistinguishable
• Dependence of mass on
environment
– Brighter galaxies in
higher density
environment
• more RSF galaxies in low density
• 30%3% RSF of 839 early-type galaxies
– 293% of ellipticals and 395% of S0s
Summary
• Volume-limited sample of z=0.05~0.1 and Mr<-21.5
• Fraction of RSF = 30%2%
– Residual star formation is common among the present
day early-type galaxy population
• UV CMR varies more clearly with environment
• RSF history of early-types also varies with
environment
• The most massive galaxies(-23.83Mr-22.13)
show the strong dependence on environment