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The Environmental Effect on the UV Color-Magnitude Relation of Early-type Galaxies

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)

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The Environmental Effect on the UV Color-Magnitude Relation of Early-type Galaxies

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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) • Fiducial value of  = 2.0 Mpc

  11. Cube size = 100 Mpc • Green sphere: galaxies Mr < -20.5

  12. 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

  13. 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

  14. Dependence of NUV-r color on environment • More blue galaxies in low-density • Medium and high density curves are indistinguishable

  15. Dependence of mass on environment • Brighter galaxies in higher density environment

  16. more RSF galaxies in low density • 30%3% RSF of 839 early-type galaxies • 293% of ellipticals and 395% of S0s

  17. 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

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