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The Environmental Effect on the UV Color-Magnitude
Relation of Early-type Galaxies
Hwihyun Kim
Journal Club
10/24/2008
Schawinski et al. 2007, ApJS 173, 512
Color-Magnitude Relation• Tool for understanding the
formation and evolution of early-type galaxies (Visvanathan
& Sandage,1977)
• 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
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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
1.710-4 210-5 (~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)
• Fiducial value of = 2.0 Mpc
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cz =1+ 0.2n,
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g σ( ) =1
2πσexp −
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σ 2+rz
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cz2σ 2
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• 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 star-
formation
• 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
– 293% of ellipticals and 395% 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(-23.83Mr-22.13) show the strong dependence on environment
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