We use measurements of the H i content, stellar mass and star formation rates (SFRs) in ∼190 massive galaxies with M★ > 1010 M⊙, obtained from the GALEX (Galaxy Evolution Explorer) Arecibo SDSS ...(Sloan Digital Sky Survey) survey described in Paper I to explore the global scaling relations associated with the bin-averaged ratio of the SFR over the H i mass (i.e. ΣSFR/ΣMHi), which we call the H i-based star formation efficiency (SFE). Unlike the mean specific star formation rate (sSFR), which decreases with stellar mass and stellar mass surface density, the SFE remains relatively constant across the sample with a value close to SFE = 10−9.5 yr−1 (or an equivalent gas consumption time-scale of ∼3 × 109 yr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV −r colour and concentration (R90/R50). We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5 per cent of the sample shows high efficiencies with SFE > 10−9 yr−1, and we suggest that this is very likely due to a deficiency of cold gas rather than an excess SFR. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given sSFR, although these galaxies show both a higher than average gas fraction and lower than average sSFR. Both of these populations are plausible candidates for ‘transition’ galaxies, showing potential for a change (either decrease or increase) in their sSFR in the near future. We also find that 36 ± 5 per cent of the total H i mass density and 47 ± 5 per cent of the total SFR density are found in galaxies with M★ > 1010 M⊙.
We present a clustering analysis of near-ultraviolet (NUV)-optical colour selected luminosity bin samples of green valley galaxies. These galaxy samples are constructed by matching the Sloan Digital ...Sky Survey Data Release 7 with the latest Galaxy Evolution Explorer source catalogue which provides NUV photometry. We present cross-correlation function measurements and determine the halo occupation distribution of green valley galaxies using a new multiple tracer analysis technique.
We extend the halo occupation formalism, which describes the relation between galaxies and halo mass in terms of the probability P(N, M
h) that a halo of given mass M
h contains N galaxies, to model the cross-correlation function between a galaxy sample of interest and multiple tracer populations simultaneously. This method can be applied to commonly used luminosity threshold samples as well as to colour and luminosity bin selected galaxy samples, and improves the accuracy of clustering analyses for sparse galaxy populations.
We confirm the previously observed trend that red galaxies reside in more massive haloes and are more likely to be satellite galaxies than average galaxies of similar luminosity. While the change in central galaxy host mass as a function of colour is only weakly constrained, the satellite fraction and characteristic halo masses of green satellite galaxies are found to be intermediate between those of blue and red satellite galaxies.
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.
For a sample of 43 nearby, late-type galaxies, we have investigated the radial variation of both the current star formation rate and the dust-induced UV light attenuation. To do this we have ...cross-correlated IRAS images and GALEX observations for each of these galaxies and compiled observations of the gas (CO and H I) and metal-abundance gradients found in the literature. We find that attenuation correlates with metallicity. We then use the UV profiles, corrected for attenuation, to study several variants of the Schmidt law and conclude that our results are compatible with a simple law similar to the one of Kennicutt extending smoothly to lower surface densities, but with considerable scatter. We do not detect an abrupt break in the UV light at the threshold radius derived from H alpha data (at which the H alpha profile shows a break and beyond which only a few H II regions are usually found). We interpret the H alpha sudden break not as a change in the star formation regime (as often suggested), but as the vanishingly small number of ionizing stars corresponding to low levels of star formation.
We have used the first matched set of GALEX and SDSS data to investigate the properties of a sample of 74 nearby galaxies with far-ultraviolet luminosities 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 similar to LBGs. The implied star formation rates are SFR ~ 3 to 30 solar mass per year. This overlaps the range of SFRs for LBGs. We find a strong inverse correlation between galaxy mass and far-ultraviolet surface brightness, and on this basis divide the sample into ``large\'\' and ``compact\'\' UVLGs. The compact UVLGs have half-light radii of a few kpc or less (similar to LBGs). They are relatively low mass galaxies (~10 billion solar masses) with typical velocity dispersions of 60 to 150 km/s. They span a range in metallicity from 0.3 to 1 times solar, have blue optical-UV colors, and are forming stars at a rate sufficient to build the present galaxy in ~a Gigayear. 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 analysis of the polycyclic aromatic hydrocarbon (PAH) spectra of a sample of 92 typical star-forming galaxies at 0.03 < z < 0.2 observed with the Spitzer intensified Reticon ...spectrograph (IRS). We compare the relative strengths of PAH emission features with Sloan Digital Sky Survey optical diagnostics to probe the relationship between PAH grain properties and star formation and active galactic nuclei (AGNs) activity. Short-to-long wavelength PAH ratios, and in particular the 7.7 Delta *mm-to-11.3 Delta *mm feature ratio, are strongly correlated with the star formation diagnostics Dn (4000) and H Delta *a equivalent width, increasing with younger stellar populations. This ratio also shows a significant difference between active and non-active galaxies, with the active galaxies exhibiting weaker 7.7 Delta *mm emission. A hard radiation field as measured by and effects PAH ratios differently depending on whether this field results from starburst activity or an AGN. Our results are consistent with a picture in which larger PAH molecules grow more efficiently in richer media and in which smaller PAH molecules are preferentially destroyed by the AGN.
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.
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.
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