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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 1.710-4 210-5 (~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) 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 – 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