We derive a variety of physical parameters including star formation rates (SFRs), dust attenuation and burst mass fractions for 6472 galaxies observed by the Galaxy Evolution Explorer (GALEX) and ...present in the SDSS DR1 main spectroscopic sample. Parameters are estimated in a statistical way by comparing each observed broad-band SED (two GALEX and five SDSS bands) with an extensive library of model galaxy SEDs, which cover a wide range of star formation histories and include stochastic starbursts. We compare the constraints derived using SDSS bands only with those derived using the combination of SDSS and GALEX photometry. We find that the addition of the GALEX bands leads to significant improvement in the estimation of both the dust optical depth and the star formation rate over timescales of 100 Myr to 1 Gyr in a galaxy. We are sensitive to SFRs as low as 10^{-3} M_sun/yr, and we find that low levels of star formation (SF) are mostly associated with early-type, red galaxies. The least massive galaxies have ratios of current to past-averaged SF rates (b-parameter) consistent with constant SF over a Hubble time. For late-type galaxies, this ratio on average decreases with mass. We find that b correlates tightly with NUV-r color, implying that the SF history of a galaxy can be constrained on the basis of the NUV-r color alone. The fraction of galaxies that have undergone a significant starburst episode within the last 1 Gyr steeply declines with mass-from ~20% for galaxies with ~10^8 M_sun to ~5% for ~10^11 M_sun galaxies.
We have used the GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of image8 Aa in the far-ultraviolet (FUV; 1350-1750 Aa) and image20 Aa in the near-ultraviolet (NUV; ...1950-2750 Aa) for a systematic search of Ly alpha -emitting galaxies at low redshift. Our aim is to fill a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in five deep exposures (5.65 deg super(2)) has resulted in 96 Ly alpha - emitting galaxy candidates in the FUV domain after accounting for broad-line AGNs. The Ly alpha equivalent widths (EWs) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, with the exception of a possible decrease in the most luminous objects that may be due to small-number statistics. The objects' distribution in EW is similar to that at image, but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a subsample of 66 objects in the range image has been used to build a Ly alpha luminosity function (LF). The incompleteness due to objects with significant Ly alpha emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with H alpha LFs in the same redshift domain is consistent with an average Ly alpha /H alpha of image1 in about 15% of the star-forming galaxies. A comparison with high-redshift Ly alpha LFs implies an increase of the Ly alpha luminosity density by a factor of about 16 from image to image. By comparison with the factor of 5 increase in the UV luminosity density in the same redshift range, this suggests an increase of the average Ly alpha escape fraction with redshift.
We analyze a volume-limited sample of massive bulge-dominated galaxies with data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX) satellite. The galaxies have central ...velocity dispersions greater than 100 km s super(-1) and stellar surface mass densities that lie above the value where galaxies transition from actively star-forming to passive systems. The sample is limited to redshifts 0.03 < z < 0.07. At these distances, the SDSS spectra sample the light from the bulge-dominated central regions of the galaxies. The GALEX NUV data provide high sensitivity to low rates of global star formation in these systems. Our sample of bulge-dominated galaxies exhibits a much larger dispersion in NOT - r color than in optical g - r color. The dispersion increases for galaxies with smaller central velocity dispersions, and nearly all of the galaxies with bluer NUV - r colors are active galactic nuclei (AGNs). Both GALEX images and SDSS color profiles demonstrate that the excess UV light is nearly always associated with an extended disk. When comparing fiber-based estimates of stellar age to global ones, we find that galaxies with red outer regions almost never have a young bulge or a strong AGN. Galaxies with blue outer regions have bulges and black holes that span a wide range in age and accretion rate. Galaxies with young bulges and strongly accreting black holes almost always have blue outer disks. The black hole growth rate correlates much more strongly with the age of the stars in the bulge than in the disk. Our suggested scenario is one in which the source of gas that builds the bulge and black hole is a low-mass reservoir of cold gas in the disk. The presence of this gas is a necessary but not sufficient condition for bulge and black hole growth. Some mechanism must transport this gas inward in a time variable way. The disk gas itself is likely to be the result of the accretion of gas from an external source. As the gas in the disk is converted into stars, galaxies will turn red, but further inflow can bring them back into the blue NUV - r sequence.
The meaningful comparison of models of galaxy evolution to observations is critically dependent on the accurate treatment of dust attenuation. To investigate dust absorption and emission in galaxies ...we have assembled a sample of similar to 1000 galaxies with UV through IR photometry from GALEX, SDSS, and Spitzer, and optical spectroscopy from SDSS. The ratio of IR to UV emission (IRX) is used to constrain the dust attenuation in galaxies. We use the 4000 AA break as a robust and useful, although coarse, indicator of star formation history (SFH). We examine the relationship between IRX and the UV spectral slope (a common attenuation indicator at high redshift) and find little dependence of the scatter on D sub(n)(4000). we construct average UV through far-IR spectral energy distributions (SEDs) for different ranges of IRX, D sub(n)(4000) and stellar mass (M unk) to show the variation of the entire SED with these parameters. When binned simultaneously by IRX, D sub(n)(4000), and M unk these SEDs allow us to determine a low-resolution average attenuation curve for different ranges of M unk. The attenuation curves thus derived are consistent with a lambda super(-0.7) attenuation law, and we find no significant variations with M unk. Finally, we show the relationship between IRX and the global stellar mass surface density and gas-phase metallicity. Among star-forming galaxies we find a strong correlation between IRX and stellar mass surface density, even at constant metallicity, a result that is closely linked to the well-known correlation between IRX and star formation rate.
The color of galaxies is a fundamental property, easily measured, that constrains models of galaxies and their evolution. Dust attenuation and star formation history (SFH) are the dominant factors ...affecting the color of galaxies. Here we explore the empirical relation between SFH, attenuation, and color for a wide range of galaxies, including early types. These galaxies have been observed by GALEX, SDSS, and Spitzer, allowing the construction of measures of dust attenuation from the ratio of infrared (IR) to ultraviolet (UV) flux and measures of SFH from the strength of the 4000 AA break. The empirical relation between these three quantities is compared to models that separately predict the effects of dust and SFH on color. This comparison demonstrates the quantitative consistency of these simple models with the data and hints at the power of multiwavelength data for constraining these models. The UV color is a strong constraint; we find that a Milky Way extinction curve is disfavored, and that the UV emission of galaxies with large 4000 AA break strengths is likely to arise from evolved populations. We perform fits to the relation between SFH, attenuation, and color. This relation links the production of starlight and its absorption by dust to the subsequent reemission of the absorbed light in the IR. Galaxy models that self-consistently treat dust absorption and emission as well as stellar populations will need to reproduce these fitted relations in the low-redshift universe.
Using a sample of galaxies from the Sloan Digital Sky Survey spectroscopic catalog with measured star formation rates (SFRs) and ultraviolet (UV) photometry from the GALEX Medium Imaging Survey, we ...derived empirical linear correlations between the SFR to UV luminosity ratio and the UV-optical colors of blue-sequence galaxies. The relations provide a simple prescription to correct UV data for dust attenuation that best reconciles the SFRs derived from UV and emission-line data. The method breaks down for the red-sequence population as well as for very blue galaxies such as the local "supercompact" UV luminous galaxies and the majority of high-redshift Lyman break galaxies, which form a low-attenuation sequence of their own.
We use a new stacking technique to obtain mean mid-IR and far-IR to far-UV flux ratios over the rest-frame near-UV, near-IR color-magnitude diagram. We employ COMBO-17 redshifts and COMBO-17 optical, ...GALEX far- and near-UV, and Spitzer IRAC and MIPS mid-IR photometry. This technique permits us to probe the infrared excess (IRX), the ratio of far-IR to far-UV luminosity, and the specific star formation rate (SSFR) and their coevolution over 2 orders of magnitude of stellar mass and over redshift 0.1 < z < 1.2. We find that the SSFR and the characteristic mass (M sub(0)) above which the SSFR drops increase with redshift (downsizing). At any given epoch, the IRX is an increasing function of mass up to M sub(0). Above this mass the IRX falls, suggesting gas exhaustion. In a given mass bin below M sub(0), the IRX increases with time in a fashion consistent with enrichment. We interpret these trends using a simple model with a Schmidt-Kennicutt law and extinction that tracks gas density and enrichment. We find that the average IRX and SSFR follow a galaxy age parameter xi , which is determined mainly by the galaxy mass and time since formation. We conclude that blue-sequence galaxies have properties which show simple, systematic trends with mass and time such as the steady buildup of heavy elements in the interstellar media of evolving galaxies and the exhaustion of gas in galaxies that are evolving off the blue sequence. The IRX represents a tool for selecting galaxies at various stages of evolution.
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