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 (r 1/4 ) 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 (M r =-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 M r < -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 M r <-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  M r  -22.13) show the strong dependence on environment