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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• 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