We present the first results of an ongoing spectroscopic survey of galaxies selected in the rest frame ultraviolet (UV). The source catalogue has been constructed from a flux-limited sample of stars, ...galaxies and QSOs imaged at 2000 Å in Selected Area 57 with the FOCA balloon-borne imaging camera. Accurate positions for the UV sources have been obtained by matching with optical counterparts using APM scans of the Palomar Sky Survey limited at B 20.5. Here we present results derived from optical spectroscopy conducted with the WIYN telescope and the WHT for 142 faint sources. The redshift distribution for this UV-selected sample extends over 0 < z < 0.5, and a high fraction of the sources show intense nebular emission lines and UV-optical colours bluer than normal Hubble sequence galaxies. Such UV-selected surveys are thus a very efficient way to locate and study intermediate-redshift galaxies undergoing intense star formation. Although our sample is currently small, we derive a rest frame UV luminosity function with a steep faint-end slope consistent with that found for late-type galaxies in optical samples. However, the integrated luminosity density derived implies a volume-averaged star formation rate higher than other recent estimates, assuming a normal initial mass function. If representative of other UV fields, as suggested by UV number count studies, our data imply that the local abundance of star-forming galaxies may have been underestimated, and consequently claims for strong evolution in the global star formation rate in the range 0 < z < 1 overstated. An intensive study of a large UV-selected sample is likely to reveal important information on the declining population of star-forming galaxies of all types.
We present a source catalog from a deep 26 ks Galaxy Evolution Explorer (GALEX) observation of the Coma cluster in the far-UV (FUV; 1530 A) and near-UV (NUV; 2310 A) wavebands. The observed field is ...centered ~09 (1.6 Mpc) southwest of the Coma core in a well-studied region of the cluster known as 'Coma-3.' The entire field is located within the apparent virial radius of the Coma cluster, and has optical photometric coverage with Sloan Digital Sky Survey (SDSS) and deep spectroscopic coverage to r ~ 21. We detect GALEX sources to NUV = 24.5 and FUV = 25.0, which corresponds to a star formation rate of ~10--3 M yr--1 for galaxies at the distance of Coma. We have assembled a catalog of 9700 galaxies with GALEX and SDSS photometry, including 242 spectroscopically confirmed Coma member galaxies that span a large range of galaxy types from giant spirals and elliptical galaxies to dwarf irregular and early-type galaxies. The full multi-wavelength catalog (cluster plus background galaxies) is ~80% complete to NUV = 23 and FUV = 23.5. The GALEX images presented here are very deep and include detections of many resolved cluster members superposed on a dense field of unresolved background galaxies. This required a two-fold approach to generating a source catalog: we used a Bayesian deblending algorithm to measure faint and compact sources (using SDSS coordinates as position prior), and used the GALEX pipeline catalog for bright and/or extended objects. We performed simulations to assess the importance of systematic effects (e.g., object blends, source confusion, Eddington Bias) that influence the source detection and photometry when using both methods. The Bayesian deblending method roughly doubles the number of source detections and provides reliable photometry to a few magnitudes deeper than the GALEX pipeline catalog. This method is free from source confusion over the UV magnitude range studied here; we estimate that the GALEX pipeline catalogs are confusion limited at NUV ~ 23 and FUV ~ 24. We have measured the UV field galaxy counts using our catalog and report a ~50% (30%) excess of counts across FUV = 22-23.5 (NUV = 21.5-23) relative to other GALEX studies. Our number counts are a better match to deeper UV galaxy counts measured with Hubble Space Telescope.
We test the empirical relation between ultraviolet color and attenuation as derived for starburst galaxies with a wide assortment of galaxy types detected by the Galaxy Evolution Explorer and find ...that it systematically overestimates the far-ultraviolet attenuation of our sample by ~0.5 mag. Our efforts to find an additional parameter that could improve the starburst reddening relation were unsuccessful. In particular, UV-Ks colors (in nonmatching apertures) show no correlation with the offset from the starburst reddening relation, suggesting either that UV-Ks colors are a poor tracer of present to past average star formation history (the ``b'' parameter) or that the intrinsic dust distribution/geometry may be responsible for moving galaxies off the correlation. It is possible to reduce the systematic overestimate of AFUV by using the linear correlation derived from our sample, which simply lowers the starburst predicted values of AFUV by 0.58 mag. The scatter, however, remains large at 0.89 mag.
Measuring the build-up of stellar mass is one of the main objectives of studies of galaxy evolution. Traditionally, the mass in stars and the star formation rates have been measured by different ...indicators, such as photometric colours, emission lines, and the UV and IR emission. We intend to show that it is possible to derive the physical parameters of galaxies from their broad-band spectral energy distribution out to a redshift of 1.2. This method has the potential to yield the physical parameters of all galaxies in a single field in a homogeneous way, thus overcoming problems with the sample size that particularly plague methods relying on spectroscopy. We use an extensive dataset, assembled in the context of the VVDS survey, which reaches from the UV to the IR and covers a sample of 84 073 galaxies over an area of 0.89 deg2. We also use a library of 100 000 model galaxies with a wide variety of star formation histories (in particular including late bursts of star formation). We find that we can determine the physical parameters stellar mass, age, and star formation rate with good confidence. We validate the star formation rate determination in particular by comparing it to a sample of spectroscopically observed galaxies with an emission-line measurement. While the attenuation in the galaxies shows more scatter, the mean over the sample is unbiased. Metallicity, however, cannot be measured from rest-frame optical photometry alone. As a first application we use our sample to build the number density function of galaxies as a function of stellar mass, specific star formation rate, and redshift. We are then able to study whether the stellar mass function at a later time can be predicted from the stellar mass function and star formation rate distribution at an earlier time. We find that, between redshifts of 1.02 and 0.47, the predicted growth in stellar mass from star formation agrees with the observed one. However, the predicted stellar mass density for massive galaxies is lower than observed, while the mass density of intermediate mass galaxies is overpredicted. This apparent discrepancy can be explained by major and minor mergers. Indeed, when comparing with a direct measurement of the major merger rate from the VVDS survey, we find that major mergers can account for about half of the mass build-up at the massive end. Minor mergers are very likely to contribute the missing fraction.
We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a sample of intermediate-redshift (0.2 less than or equal to z < 0.7) UV-selected galaxies from the ELAIS N1 and ELAIS N2 fields ...by fitting a multi-wavelength data set to a library of GRASIL templates. Star formation related properties of the galaxies are derived from the library of models by using Bayesian statistics. We find a decreasing presence of galaxies with low attenuation and low total luminosity as redshift decreases, which does not hold for high total luminosity galaxies. In addition, the dust attenuation of low-mass galaxies increases as redshift decreases, and this trend seems to disappear for galaxies with unk greater than or equal to 10 super(11) M unk. This result is consistent with a mass-dependent evolution of the dust-to-gas ratio, which could be driven by a mass-dependent efficiency of star formation in star-forming galaxies. The specific star formation rates (SSFR) decrease with increasing stellar mass at all redshifts, and for a given stellar mass the SSFR decreases with decreasing redshift. The differences in the slope of the M*-SSFR relation found between this work and others at similar redshift could be explained by the adopted selection criteria of the samples, which for a UV-selected sample, favors blue, star-forming galaxies.
We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey Data Release 1 with the GALEX Medium and All-sky Imaging ...Surveys. We obtain a working sample of 1032 early-type galaxies with GALEX FUV detections, SDSS spectroscopy, and $z<0.2$. Using the SDSS spectra to identify galaxies with even weak star formation or evidence of AGN, and further removing galaxies with any evidence of non early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies. We find that the $FUV-r$ color has a full range of 5 mag. Plotting against the $FUV-r$ color the metallicity sensitive Lick $\\rm Mg_2$ and D4000 indices, and the stellar velocity dispersion, we find no correlation between our measurement of UV rising flux, and any parameter sensitive to metallicity.
We present Far Ultraviolet Spectroscopic Explorer observations of the star-forming galaxy Mrk 54 at $z = 0.0448$. The Lyman continuum radiation is not detected above the Hi absorption edge in our ...Galaxy. An upper limit is evaluated by comparison with the background measured in regions of the detector adjacent to the observed spectrum. A spectral window of 16 Å, reasonably free of additional Hi Lyman series line absorption, is used. No correction is needed for molecular hydrogen absorption in our Galaxy but a foreground extinction of 0.29 mag is accounted for. An upper limit of $6.15 \times 10^{-16}$ erg cm-2 s-1 A-1 is obtained for the flux at ≈900 Å in the rest frame of Mrk 54. By comparison with the number of ionizing photons derived from the Hα flux, this limit translates into an upper limit of $f_{\mathrm{esc}}< 0.062$ for the fraction of Lyman continuum photons that escape the galaxy without being absorbed by interstellar material. This limit compares with the limits obtained in three other nearby galaxies and is compatible with the escape fractions predicted by models. The upper limits obtained in nearby galaxies contrasts with the detection of Lyman continuum flux in the composite spectrum of Lyman-break galaxies at $z \approx 3.4$. The difficulties and implications of a comparison are discussed.
We have built two samples of galaxies selected at 0.2 mu m (hereafter UV) and 60 mu m (hereafter FIR) covering a sky area of 35.36 deg super(2). The UV selected sample contains 25 galaxies brighter ...than AB sub(0.2) = 17. All of them, but one elliptical, are detected at 60 mu m with a flux density larger or equal to 0.2 Jy. The UV counts are significantly lower than the Euclidean extrapolation towards brighter fluxes of previous determinations. The FIR selected sample contains 42 galaxies brighter than f sub(60) = 0.6 Jy. Except four galaxies, all of them have a UV counterpart at the limiting magnitude AB sub(0.2) = 20.3 mag. The mean extinction derived from the analysis of the FIR to UV flux ratio is similar to 1 mag for the UV selected sample and similar to 2 mag for the FIR selected one. For each sample we compare several indicators of the recent star formation rate (SFR) based on the FIR and/or the UV emissions. We find linear relationships with slopes close to unity between the different SFR indicator, which means that, over the whole converting offset. Various absolute calibrations for both samples are discussed in this paper. A positive correlation between extinction and SFR is found when both samples are considered together although with a considerable scatter. A similar result is obtained when using the SFR normalized to the optical surface of the galaxies.
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