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Star Formation Histories of z <0.25 Galaxies from GALEX Samir Salim (UCLA) and the GALEX Science Team. RESULTS AND ANALYSIS Mass determination Kauffmann et al. determined masses using SDSS spectra (Hdelta & D4000) Comparison with our determination: Relative scatter 0.11 dex

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Star Formation Histories of z <0.25 Galaxies from GALEX

Samir Salim (UCLA) and the GALEX Science Team

  • 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 burstsdeclines with the mass of a galaxy

  • Full line – over 1 Gyr

  • Dashed line – over 100 Myr

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.

  • 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

NGC 5806

GALEX

UV image

NGC 5806

SDSS

optical

METHOD

  • 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 chi2 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


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