The distribution of neutral hydrogen in the intergalactic medium (IGM) is currently explored at low-z by means of UV spectroscopy of quasars. We propose an alternative approach based on UV colours of quasars as observed from GALEX surveys. We built a NUV-selected sample of 9033 quasars with (FUV-NUV) colours. The imprint of HI absorption in the observed colours is suggested qualitatively by their distribution as a function of quasar redshift. Because broad band fluxes lack spectral resolution and are sensitive to a large range of N_HI a Monte Carlo simulation of IGM opacity is required for quantitative analysis. It was performed with absorbers randomly distributed along redshift and column density distributions, assumed to be a broken power law with index beta1 (10^15 < N_HI <10^17.2 cm^-2) and beta2 (10^17.2 < N_HI <10^19 cm^-2). The redshift distribution is proportional to the redshift evolution law of the number density of Lyman limit systems (LLS) per unit redshift as determined by spectroscopic surveys.The simulation is run with different assumptions on the spectral index alpha_nu of the quasar ionising flux. The fits between the simulated and observed distribution of colours require an LLS redshift density larger than that derived from spectroscopic counting. This result is robust in spite of difficulties in determining the colour dispersion other than that due to HI absorption. We provide arguments to retain alpha_nu = - 2, a value already extreme with respect to those measured with HST/COS. Further fitting of power law index beta1 and beta2 leads to a higher density by a factor of 1.7 (beta1 = -1.7, beta2 = -1.5), possibly 1.5 (beta1 = -1.7, beta2 = -1.7). Beyond the result in terms of density the analysis of UV colours of quasars reveals a tension between the current description of IGM opacity at low z and the published average ionising spectrum of quasars.

We present two luminous UV/optical flares from the nuclei of apparently inactive early-type galaxies at z = 0.37 and 0.33 that have the radiative properties of a flare from the tidal disruption of a star. In this paper we report the second candidate tidal disruption event discovery in the UV by the GALEX Deep Imaging Survey and present simultaneous optical light curves from the CFHTLS Deep Imaging Survey for both UV flares. The first few months of the UV/optical light curves are well fitted with the canonical t(-5/3) power-law decay predicted for emission from the fallback of debris from a tidally disrupted star. Chandra ACIS X-ray observations during the flares detect soft X-ray sources with T-bb = (2-5) x 10(5) K or Gamma > 3 and place limits on hard X-ray emission from an underlying AGN down to L-X(2-10 keV) less than or similar to 10(41) ergs s(-1). Blackbody fits to the UV/optical spectral energy distributions of the flares indicate peak flare luminosities of greater than or similar to 10(44) - 10(45) ergs s(-1). The temperature, luminosity, and light curves of both flares are in excellent agreement with emission from a tidally disrupted main-sequence star onto a central black hole of several times 10(7) M-circle dot. The observed detection rate of our search over similar to 2.9 deg(2) of GALEX Deep Imaging Survey data spanning from 2003 to 2007 is consistent with tidal disruption rates calculated from dynamical models, and we use these models to make predictions for the detection rates of the next generation of optical synoptic surveys.

We use GALEX near-UV (NUV) photometry of a sample of early-type galaxies selected in the SDSS (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). NUV-r color is an excellent tracer of even small amounts (similar to 1% mass fraction) of recent (less than or similar to 1 Gyr) star formation, and so the NUV-r CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually inspected early-type galaxies in the redshift range 0.05 < z < 0.10 brighter than M-r of -21.5 with any possible emission-line or radio-selected active galactic nuclei (AGNs) removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly 30% +/- 3%. This suggests that residual star formation is common even among the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies, however, shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.

We present a study of the morphological nature of redshift z 0.7 star-forming galaxies using a combination of HST/ACS, GALEX and ground-based images of the COSMOS field. Our sample consists of 8,146 galaxies, 5,777 of which are detected in the GALEX near-ultraviolet band down to a limiting magnitude of 25.5 (AB). We make use of the UV to estimate star formation rates, correcting for the effect of dust using the UV-slope, and compute, from the ACS F814W images, the C,A,S,G,M20 morphological parameters for all objects in our sample. We observe a morphological bimodality in the galaxy population and show that it has a strong correspondence with the FUV - g color bimodality. We conclude that UV-optical color predominantly evolves concurrently with morphology. We observe many of the most star-forming galaxies to have morphologies approaching that of early-type galaxies, and interpret this as evidence that strong starburst events are linked to bulge growth and constitute a process through which galaxies can be brought from the blue to the red sequence while simultaneously modifying their morphology accordingly. We conclude that the red sequence has continued growing at z <0.7. We also observe z 0.7 galaxies to have physical properties similar to that of local galaxies, except for higher star formation rates. Whence we infer that the dimming of star-forming galaxies is responsible for most of the evolution in the star formation rate density of the Universe since that redshift, although our data are also consistent with a mild number evolution. [abridged]

We have studied 2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range 0 < z < 0.11. Combining GALEX UV photometry with corollary optical data from the SDSS, we find that, at a 95 percent confidence level, at least 30 percent of galaxies in this sample have UV to optical colours consistent with some recent star formation within the last Gyr. In particular, galaxies with a NUV - r colour less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semi-analytical LCDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 < z < 0.11 have 1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is 300 to 500 Myrs. We also find that monolithically evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colours (NUV - r < 5.5) seen in a significant fraction ( 30 percent) of our observed sample.

We select far-infrared (FIR: 60 μm) and far-ultraviolet (FUV: 530 Å) samples of nearby galaxies in order to discuss the biases encountered by monochromatic surveys (FIR or FUV). Very different volumes are sampled by each selection, and much care is taken to apply volume corrections to all the analyses. The distributions of the bolometric luminosity of young stars are compared for both samples: they are found to be consistent with each other for galaxies of intermediate luminosities, but some differences are found for high (>5 × 1010 L☉) luminosities. The shallowness of the IRAS survey prevents us from securing a comparison at low luminosities (<2 × 109 L☉). The ratio of the total infrared (TIR) luminosity to the FUV luminosity is found to increase with the bolometric luminosity in a similar way for both samples up to 5 × 1010 L☉. Brighter galaxies are found to have a different behavior according to their selection: the LTIR/LFUV ratio of the FUV-selected galaxies brighter than 5 × 1010 L☉ reaches a plateau, whereas LTIR/LFUV continues to increase with the luminosity of bright galaxies selected in FIR. The volume-averaged specific star formation rate (SFR per unit galaxy stellar mass, SSFR) is found to decrease toward massive galaxies within each selection. The mean values of the SSFR are found to be larger than those measured for optical and NIR-selected samples over the whole mass range for the FIR selection, and for masses larger than 1010 M☉ for the FUV selection. Luminous and massive galaxies selected in FIR appear as active as galaxies with similar characteristics detected at z ∼ 0.7.

We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a sample of intermediate-redshift (0.2 ≤ 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 M* ≥ 1011 M☉. 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 combine data from the Sloan Digital Sky Survey (SDSS), the Galaxy Evolution Explorer (GALEX), and the Spitzer Space Telescope to create a sample of galaxies observed homogeneously from the UV to the far-IR. This sample, consisting of ~460 galaxies observed spectroscopically by the SDSS, provides us with a multiwavelength (0.15-24 μm) view of obscured and unobscured star formation in nearby (z < 0.3) galaxies with star formation rates (SFRs) ranging from 0.01 to 100 M☉ yr-1. We calculate a robust dust measure from the infrared-to-UV ratio (or infrared excess [IRX]) and explore the influence of star formation history (SFH) on the dust-UV color relation (i.e., the IRX-β relation). We find that the UV colors of galaxies are only weakly dependent on their SFH as measured by the 4000 Å break. However, we find that the contributions of dust and SFH are distinguishable when colors at widely separated wavelengths (e.g., 0.23-3.6 μm) are introduced. We show this explicitly by recasting the IRX-β relation as a more general IRX-SFH-color relation, which we examine in different projections. We also determine simple fits to this relation.

A supermassive black hole in the nucleus of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces and a flare of radiation is emitted by the stream of stellar debris that plunges into the black hole. Since common active galactic nuclei have accreting black holes that can also produce flares, a convincing demonstration that a stellar tidal disruption has occurred generally begins with a “normal” galaxy that has no evidence of prior nuclear activity. Here we report a luminous UV flare from an elliptical galaxy at z = 0.37 in the Groth field of the GALEX Deep Imaging Survey that has no evidence of a Seyfert nucleus from optical spectroscopy and X-ray imaging obtained during the flare. Multiwavelength data collected at the time of the event, and for 2 years following, allow us to constrain, for the first time, the spectral energy distribution of a candidate tidal disruption flare from optical through X-rays. The luminosity and temperature of the radiation and the decay curve of the flare are in excellent agreement with theoretical predictions for the tidal disruption of a star, and provide the strongest empirical evidence for a stellar disruption event to date.

We present Version 1.0 of the NASA Galaxy Evolution Explorer (GALEX) ultraviolet variability catalog (GUVV) that contains information on 84 time-variable and transient sources gained with simultaneous near and far ultraviolet photometric observations. These time-variable sources were serendipitously revealed in the various 1.2 degree star fields currently being surveyed by the GALEX satellite in two ultraviolet bands (NUV 1750-2750A, FUV 1350-1750A) with limiting AB magnitudes of 23-25. The largest-amplitude variable objects presently detected by GALEX are M-dwarf flare stars, which can brighten by 5-10 mag in both the NUV and FUV bands during short duration (< 500s) outbursts. Other types of large-amplitude ultraviolet variable objects include ab-type RR Lyrae stars, which can vary periodically by 2-5mag in the GALEX FUV band. This first GUVV catalog lists galactic positions and possible source identifications in order to provide the astronomical community with a list of time-variable objects that can now be repeatedly observed at other wavelengths. We expect the total number of time-variable source detections to increase as the GALEX mission progresses, such that later version numbers of the GUVV catalog will contain substantially more variable sources.

We present the GALEX NUV (2310 A) and FUV (1530 A) galaxy luminosity functions of the nearby cluster of galaxies A1367 in the magnitude range -20.3< M_AB < -13.3. The luminosity functions are consistent with previous ( 2 mag shallower) estimates based on the FOCA and FAUST experiments, but display a steeper faint-end slope than the GALEX luminosity function for local field galaxies. Using spectro-photometric optical data we select out star-forming systems from quiescent galaxies and study their separate contributions to the cluster luminosity function. We find that the UV luminosity function of cluster star-forming galaxies is consistent with the field. The difference between the cluster and field LF is entirely due to the contribution at low luminosities (M_AB >-16 mag) of non star-forming, early-type galaxies that are significantly over dense in clusters.

Number counts of galaxies in two Galaxy Evolution Explorer (GALEX) bands [far-UV (FUV: 1530 Angstrom) and near-UV (NUV: 2310 Angstrom); both in AB magnitudes] are reported. They provide for the first time in the literature homogeneously calibrated number counts of UV galaxies continuously covering a very wide range in UV magnitude (14 - 23.8). Both the FUV and NUV counts are inconsistent with a nonevolution model, whereas they are in good agreement with evolution models (essentially luminosity evolution) derived from the high-z UV luminosity functions of Arnouts et al. We find that the contribution from GALEX-detected galaxies to the UV background is 0.68 +/- 0.10 nW m(-2) sr(-1) at 1530 Angstrom and 0.99 +/- 0.15 nW m(-2) sr(-1) at 2310 Angstrom. These are 66% +/- 9% and 44% +/- 6% of the total contributions of galaxies to the UV background at 1530 Angstrom (1.03 +/- 0.15 nW m(-2) sr(-1)) and at 2310 Angstrom (2.25 +/- 0.32 nW m(-2) sr(-1)), respectively, as estimated using the evolution models. Galaxy counts and star counts in seven regions, each containing a few square degrees of GALEX coverage in an area of similar to4 - 13 deg(2), are compared with each other to study the region-by-region variance. This shows that for the galaxy counts, the cosmic variance is comparable to the net error due to other uncertainties. The star counts increase with decreasing absolute Galactic latitude. \b\.

We compare the dust attenuation properties of two samples of galaxies purely selected in the Galaxy Evolution Explorer (GALEX) near-ultraviolet band (NUV; 1750-2750 Å, λmean = 2310 Å) and in the far-infrared (FIR) at 60 μm. These samples are built using the GALEX and IRAS sky surveys over ~600 deg2. The NUV-selected sample contains 95 galaxies detected down to NUV = 16 mag (AB system). Eighty-three galaxies in this sample are spiral or irregular, and only two of them are not detected at 60 μm. The FIR-selected sample is built from the IRAS PSCz survey, which is complete down to 0.6 Jy. Among the 163 sources, we select 118 star-forming galaxies that are well measured by IRAS; all but one are detected in NUV, and 14 galaxies are not detected in the far-ultraviolet band (FUV; 1350-1750 Å, λmean = 1530 Å). The dust-to-ultraviolet (NUV and FUV) flux ratio is calibrated to estimate the dust attenuation at both wavelengths. The median value of the attenuation in NUV is found to be ~1 mag for the NUV-selected sample, versus ~2 mag for the FIR-selected one. Within both samples, the dust attenuation is found to correlate with the luminosity of the galaxies. Almost all the NUV-selected galaxies and two-thirds of the FIR-selected sample exhibit a lower dust attenuation than expected from the tight relation found previously for starburst galaxies between dust attenuation and the slope of the ultraviolet continuum. The situation is reversed for the remaining third of the FIR-selected galaxies: their extinction is higher than that deduced from their FUV - NUV color and the relation for starbursts.

The Galaxy Evolution Explorer (GALEX) satellite has obtained high time resolution ultraviolet photometry during a large flare on the M4 dwarf star GJ 3685A. Simultaneous Near-ultraviolet (NUV, 1750-2800 Å) and Far-ultraviolet (FUV, 1350-1750 Å) time-tagged photometry with time resolution better than 0.1 s shows that the overall brightness in the FUV band increased by a factor of 1000 in 200 s. Under the assumption that the NUV emission is mostly due to a stellar continuum, and that the FUV flux is shared equally between emission lines and continuum, there is evidence for two distinct flare components for this event. The first flare type is characterized by an exponential increase in flux with little or no increase in temperature. The other involves rapid increases in both temperature and flux. While the decay time for the first flare component may be several hours, the second flare event decayed over less than 1 minute, suggesting that there was little or no confinement of the heated plasma.

We present the results of a determination of the galaxy luminosity function at ultraviolet wavelengths at redshifts of z = 0.0-0.1 from Galaxy Evolution Explorer (GALEX) data. We determined the luminosity function in the GALEX far-UV and near-UV bands from a sample of galaxies with UV magnitudes between 17 and 20 that are drawn from a total of 56.73 deg2 of GALEX fields overlapping the bJ-selected Two-Degree Field Galaxy Redshift Survey. The resulting luminosity functions are fainter than previous UV estimates and result in total UV luminosity densities of 1025.55±0.12 and 1025.72±0.12 ergs s-1 Hz-1 Mpc-3 at 1530 and 2310 Å, respectively. This corresponds to a local star formation rate density in agreement with previous estimates made with Hα-selected data for reasonable assumptions about the UV extinction.

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 Sloan Digital Sky Survey Data Release 1 (SDSS DR1) main spectroscopic sample. Parameters are estimated in a statistical way by comparing each observed broadband spectral energy distribution (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 attain sensitivity to SFRs as low as 10-3 M☉ yr-1, 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 ~108 M☉ to ~5% for ~1011 M☉ galaxies.

We present Galaxy Evolution Explorer (GALEX) images of the prototypical edge-on starburst galaxies M82 and NGC253. Our initial analysis is restricted to the complex of ultraviolet (UV) filaments in the starburst-driven outflows in the galaxy halos. The UV luminosities in the halo are too high to be provided by shock-heated or photoionized gas except perhaps in the brightest filaments in M82, suggesting that most of the UV light is the stellar continuum of the starburst scattered into our line of sight by dust in the outflow. This interpretation agrees with previous results from optical imaging polarimetry in M82. The morphology of the UV filaments in both galaxies shows a high degree of spatial correlation with H-alpha and X-ray emission. This indicates that these outflows contain cold gas and dust, some of which may be vented into the intergalactic medium (IGM). UV light is seen in the “H-alpha cap” 11 kpc North of M82. If this cap is a result of the wind fluid running into a pre-existing gas cloud, the gas cloud contains dust and is not primordial in nature but was probably stripped from M82 or M81. If starburst winds efficiently expel dust into the IGM, this could have significant consequences for the observation of cosmologically distant objects.

In a companion Letter (Arnouts et al.) we present new measurements of the galaxy luminosity function at 1500 Å out to z ~ 1 using Galaxy Evolution Explorer VIMOS-VLT Deep Survey observations (1039 galaxies with NUV ≤ 24.5 and z > 0.2) and at higher z using existing data sets. In this Letter we use the same sample to study evolution of the FUV luminosity density ρ1500. We detect evolution consistent with a (1 + z)2.5±0.7 rise to z ~ 1 and (1 + z)0.5±0.4 for z > 1. The luminosity density from the most UV-luminous galaxies (UVLGs) is undergoing dramatic evolution (30 times) between 0 < z < 1. UVLGs are responsible for a significant fraction (>25%) of the total far-ultraviolet luminosity density at z ~ 1. We measure dust attenuation and star formation rates of our sample galaxies and determine the star formation rate density (⋆) as a function of redshift, both uncorrected and corrected for dust. We find good agreement with other measures of ⋆ in the rest ultraviolet and Hα given the still significant uncertainties in the attenuation correction.

We have used the first matched set of Galaxy Evolution Explorer (GALEX) and Sloan Digital Sky Survey (SDSS) data to investigate the properties of a sample of 74 nearby (z < 0.3) galaxies with far-ultraviolet luminosities greater than 2 × 1010 L☉, chosen to overlap the luminosity range of typical high-z Lyman break galaxies (LBGs). GALEX deep surveys have shown that ultraviolet-luminous galaxies (UVLGs) similar to these are the fastest evolving component of the UV galaxy population. Model fits to the combined GALEX and SDSS photometry yield typical FUV extinctions in UVLGs of 0.5-2 mag (similar to LBGs and less luminous GALEX-selected galaxies). The implied star formation rates are SFR ~ 3-30 M☉ yr-1. This overlaps the range of SFRs for LBGs. We find a strong inverse correlation between galaxy mass and FUV surface brightness, and on this basis we divide the sample into "large" and "compact" UVLGs. The large UVLGs are relatively massive (M* ~ 1011 M☉) late-type disk galaxies forming stars at a rate similar to their past average (M*/SFR ~ tHubble). They are metal rich (approximately solar), have intermediate optical-UV colors (FUV - r ~ 2-3), and about a third host a type 2 (obscured) active galactic nucleus. In contrast, the compact UVLGs have half-light radii of a few kpc or less (similar to LBGs ). They are relatively low-mass galaxies (M* ~ 1010 M☉) with typical velocity dispersions of 60-150 km s-1. They span a range in metallicity from ~0.3 to 1 times solar, have blue optical-UV colors (FUV - r ~ 0.5-2), and are forming stars at a rate sufficient to build the present galaxy in ~1-2 Gyr. In all these respects they appear similar to the LBG population. These "living fossils" may therefore provide an opportunity for detailed investigation of the physical processes occurring in typical star-forming galaxies in the early universe.

We present the first measurement of the galaxy luminosity function (LF) at 1500 Angstrom in the range 0.2 less than or equal to z less than or equal to 1.2 based on Galaxy Evolution Explorer VIMOS-VLT Deep Survey observations (similar to1000 spectroscopic redshifts for galaxies with NUV less than or equal to 24.5) and at higher z using existing data sets. Our main results are summarized as follows: (1) Luminosity evolution is observed with DeltaM(*) similar to -2.0 mag between z = 0 and z = 1 and DeltaM(*) similar to -1.0 mag between z = 1 and z = 3. This confirms that the star formation activity was significantly higher in the past. (2) The LF slopes vary in the range -1.2 greater than or equal to alpha greater than or equal to -1.65, with a marginally significant hint of increase at higher z. (3) We split the sample in three rest-frame (B-I) intervals, providing an approximate spectral type classification: Sb - Sd, Sd - Irr, and unobscured starbursts. We find that the bluest class evolves less strongly in luminosity than the two other classes. On the other hand, their number density increases sharply with z (similar to 15% in the local universe to similar to55% at z similar to1), while that of the reddest classes decreases.