1. RESULTS AND ANALYSIS Mass determination Kauffmann et al. determined masses using SDSS spectra (Hdelta & D4000) Comparison with our determination: Relative scatter 0.11 dex Agrees with individual errors (0.08 dex) Dust attenuation in the UV Blue points: exp profiles (late type) Red points: de Vac profiles (early type) Low-Mass => less attenuation High-Mass => large range of A(FUV) Increases with color for late type Decreases with color for early type Star formation and SF History SFR over the last 100 Myr Lowest SFRs – early types Our SFRs compare well with those derived from H-alpha SF History – the “b-parameter” b = (current SFR)/(past-averaged SFR) log b = 0 means constant SFR log b > 1 ongoing bursts low-Mass: b is close to unity Tight correlation: log b vs. NUV-r color => use color to get SFH Recent starbursts Select galaxies which had >5% mass formed in bursts over 100 Myr and over 1 Gyr Fraction of galaxies (of a given mass) which experienced bursts declines with the mass of a galaxy Full line – over 1 Gyr Dashed line – over 100 Myr Star Formation Histories of z <0.25 Galaxies from GALEX Samir Salim (UCLA) and the GALEX Science Team DATA AND SAMPLE GALEX Data GALEX launched in April 2003 Surveys the sky in 1.2 deg FOV in far- and near- UV bands Currently have 143 sq. deg overlap with SDSS Obtain FUV (1530 Å) and NUV (2310 Å) photometry SDSS Data SDSS DR1 counterparts of GALEX objects Only objects with “Galaxy” spectroscopic classification Get redshifts, ugriz photometry Also morphology data (concentration, profile) Sample 50-80% of SDSSsp recovered by GALEX (red end incomplete) Limit to z<0.25 (most at z~0.1) Sample size: 6500 galaxies We derive physical parameters: stellar mass, SFR, dust attenuation and burst mass fractions, for 6500 galaxies observed in the UV by the Galaxy Evolution Explorer (GALEX) and present in the SDSS DR1 spectroscopic sample. Parameters are estimated by comparing each observed broad-band SED (2 GALEX + 5 SDSS bands) with a library of model galaxy SEDs. We are sensitive to SFRs as low as 10 -3 M ⊙/ yr. The least massive galaxies have ratios of current to past-averaged SF rates (b- parameter) consistent with constant SF over a Hubble time. b correlates tightly with NUV − r color, implying that the SF history of a galaxy can be constrained on the basis of the UV/optical color alone. Also, the fraction of galaxies that have undergone a starburst episode within the last 1 Gyr steeply declines with mass. METHOD DATA: GALEX UV photometry SDSS ugriz photometry SDSS redshifts COMPARE WITH: Library of model galaxies (model colors) GET: Star Formation History, Mass, Attenuation NGC 5806 GALEX UV image NGC 5806 SDSS optical Library of models Bruzual & Charlot population synthesis code Get libraries of model SF histories (100,000 models) Exponentially declining SF + random starbursts Two-component dust attenuation model Range of metallicities and galaxy ages Deriving parameters Compare observed 7-band SED to model SEDs Find chi 2 goodness of fit for every model galaxy Based on it determine the weight for the set of model parameters (mass, SFR, age, metallicity, attenuation, burst mass fraction…) Determine best parameter values and their errors Results significantly improved compared to fitting using only SDSS 5 bands