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 present the preliminary results from implementing a new software tool that enables inspection of time-tagged photon data for the astronomical sources contained within individual GALEX ultraviolet ...(UV) images of the sky. We have inspected the photon data contained within 1802 GALEX images to reveal rapid, short-term ( unk500 s) UV source variability in the form of stellar "flares." The mean associated change in near-UV (NUV) magnitude due to this flaring activity is 2.7 plus or minus 0.3 mag. A list of 49 new UV variable star candidates is presented, together with their associated Sloan Digital Sky Survey (SDSS) photometric magnitudes. From these data we can associate the main source of these UV flare events with magnetic activity on M dwarf stars. Photometric parallaxes have been determined for 32 of these sources, placing them at distances ranging from approximately 25 to 1000 pc. The average UV flare energy for these flare events is 2.5 x 10 super(30) ergs, which is of a similar energy to that of U-band, X-ray, and EUV flares observed on many local M dwarf stars. We have found that stars of classes M0 to M5 flare with energies spanning a far larger range and with an energy approximately 5 times greater than those of later (M6 to M8) spectral type.
We use GALEX/optical photometry to construct color-color relationships for early-type galaxies sorted by morphological type. We have matched objects in the GALEX GR1 public release and the first ...IR1.1 internal release, with the RC3 early-type galaxies having a morphological type -5.5 less than or equal to T < -1.5, with mean error on T < 1.5 and mean error on (B - V) sub(T) < 0.05. After visual inspection of each match, we are left with 130 galaxies with reliable GALEX pipeline photometry in the far-UV and near-UV bands. This sample is divided into ellipticals (-5.5 less than or equal to T < -3.5) and lenticulars (-3.5 less than or equal to T < -1.5). After correction for Galactic extinction, the color-color diagrams FUV - NUV versus (B - V) sub(To) are plotted for the two subsamples. We find a tight anticorrelation between the FUV - NUV and (B - V) sub(To) colors for ellipticals, with the UV color getting bluer when the (B - V) sub(To) gets redder. This relationship very likely is an extension of the color-metallicity relationship in the GALEX NUV band. We suspect that the main source of the correlation is metal line blanketing in the NUV band. The FUV - NUV versus B - V correlation has larger scatter for lenticular galaxies; we speculate that this reflects the presence of low-level star formation. If the latter objects (i.e., those that are blue both in FUV - NUV and in B - V) are interpreted as harboring recent star formation activity, this would be the case for a few percent ( similar to 4%) of ellipticals and similar to 15% of lenticulars; this would mean about 10% of early-type galaxies have residual star formation in our full sample of 130 early-type galaxies. We also plot FUV - NUV versus the Mg2 index and central velocity dispersion. We find a tight anticorrelation between FUV - NUV and the Mg2 index; we suspect that this reflects blanketing in the NUV band being correlated with overall metallicity. We find a marginal anticorrelation of FUV - V sub(T) with Mg2 for elliptical galaxies.
We characterize the star formation in the low-metallicity galaxy NGC 6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 A) and near-UV (NUV, 1771-2831 A) imaging, and ...ground-based H Delta *a imaging. From the GALEX FUV image, we define 77 star-forming (SF) regions with area >860 pc2, and surface brightness 26.8 mag (AB) arcsec--2, within 02 (1.7 kpc) of the center of the galaxy. We estimate the extinction by interstellar dust in each SF region from resolved photometry of the hot stars it contains: E(B -- V) ranges from the minimum foreground value of 0.22 mag up to 0.66 ? 0.21 mag. The integrated FUV and NUV photometry, compared with stellar population models, yields ages of the SF complexes up to a few hundred Myr, and masses from 2 X 102 to 1.5 X 106 . The derived ages and masses strongly depend on the assumed type of interstellar selective extinction, which we find to vary across the galaxy. The total mass of the FUV-defined SF regions translates into an average star formation rate (SFR) of 1.4 X 10--2 yr--1 over the past 100 Myr, and SFR = 1.0 X 10--2 yr--1 in the most recent 10 Myr. The latter is in agreement with the value that we derive from the H Delta *a luminosity, SFR = 0.008 yr--1. The SFR in the most recent epoch becomes higher if we add the SFR = 0.02 yr--1 inferred from far-IR measurements, which trace star formation still embedded in dust (age a few Myr).
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.
GALEX-SDSS Catalogs for Statistical Studies Budavári, Tamás; Heinis, Sébastien; Szalay, Alexander S ...
The Astrophysical journal,
04/2009, Letnik:
694, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We present a detailed study of the Galaxy Evolution Explorer's (GALEX) photometric catalogs with special focus on the statistical properties of the All-sky and Medium Imaging Surveys. We introduce ...the concept of primaries to resolve the issue of multiple detections and follow a geometric approach to define clean catalogs with well understood selection functions. We cross-identify the GALEX sources (GR2+3) with Sloan Digital Sky Survey (SDSS; DR6) observations, which indirectly provides an invaluable insight into the astrometric model of the UV sources and allows us to revise the band merging strategy. We derive the formal description of the GALEX footprints as well as their intersections with the SDSS coverage along with analytic calculations of their areal coverage. The crossmatch catalogs are made available for the public. We conclude by illustrating the implementation of typical selection criteria in SQL for catalog subsets geared toward statistical analyses, e.g., correlation and luminosity function studies.
We measure the UV-optical colour dependence of galaxy clustering in the local Universe. Using the clean separation of the red and blue sequences made possible by the NUV−r colour–magnitude diagram, ...we segregate the galaxies into red, blue and intermediate ‘green’ classes. We explore the clustering as a function of this segregation by removing the dependence on luminosity and by excluding edge-on galaxies as a means of a non-model dependent veto of highly extincted galaxies. We find that ξ(rp, π) for both red and green galaxies shows strong redshift-space distortion on small scales – the ‘finger-of-God’ effect, with green galaxies having a lower amplitude than is seen for the red sequence, and the blue sequence showing almost no distortion. On large scales, ξ(rp, π) for all three samples show the effect of large-scale streaming from coherent infall. On scales of 1 h−1 Mpc < rp < 10 h−1 Mpc, the projected auto-correlation function wp(rp) for red and green galaxies fits a power law with slope γ∼ 1.93 and amplitude r0∼ 7.5 and 5.3, compared with γ∼ 1.75 and r0∼ 3.9 h−1 Mpc for blue sequence galaxies. Compared to the clustering of a fiducial L* galaxy, the red, green and blue have a relative bias of 1.5, 1.1 and 0.9, respectively. The wp(rp) for blue galaxies display an increase in convexity at ∼ 1 h−1 Mpc, with an excess of large-scale clustering. Our results suggest that the majority of blue galaxies are likely central galaxies in less massive haloes, while red and green galaxies have larger satellite fractions, and preferentially reside in virialized structures. If blue sequence galaxies migrate to the red sequence via processes like mergers or quenching that take them through the green valley, such a transformation may be accompanied by a change in environment in addition to any change in luminosity and colour.
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