We introduce the GALEX Arecibo SDSS Survey (GASS), an on-going large programme that is gathering high quality H i-line spectra using the Arecibo radio telescope for an unbiased sample of ∼1000 ...galaxies with stellar masses greater than 1010 M⊙ and redshifts 0.025 < z < 0.05, selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5–5 per cent) is reached. This paper presents the first Data Release, consisting of ∼20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H i gas fraction with galaxy structure and NUV−r colour. A large fraction (∼60 per cent) of the galaxies in our sample are detected in H i. Even at stellar masses above 1011 M⊙, the detected fraction does not fall below ∼40 per cent. We find that the atomic gas fraction MH i/M★ decreases strongly with stellar mass, stellar surface mass density and NUV−r colour, but is only weakly correlated with the galaxy bulge-to-disc ratio (as measured by the concentration index of the r-band light). We also find that the fraction of galaxies with significant (more than a few per cent) H i decreases sharply above a characteristic stellar surface mass density of 108.5 M⊙ kpc−2. The fraction of gas-rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are transitioning between the blue, star-forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between MH i/M★ and other galaxy properties. We have fitted a plane to the two-dimensional relation between the H i mass fraction, stellar surface mass density and NUV−r colour. Interesting outliers from this plane include gas-rich red sequence galaxies that may be in the process of regrowing their discs, as well as blue, but gas-poor spirals.
We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ...63, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGNs)than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA, and ROSAT data, where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as 50% for individual quasars when inferring a bolometric luminosity from an optical luminosity. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio.
The Calibration and Data Products of GALEX Morrissey, Patrick; Conrow, Tim; Barlow, Tom A ...
The Astrophysical journal. Supplement series,
12/2007, Letnik:
173, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We describe the calibration status and data products pertaining to the GR2 and GR3 data releases of the Galaxy Evolution Explorer (GALEX). These releases have identical pipeline calibrations that are ...significantly improved over the GR1 data release. GALEX continues to survey the sky in the far-ultraviolet (FUV, similar to 154 nm) and near-ultraviolet (NUV, similar to 232 nm) bands, providing simultaneous imaging with a pair of photon-counting, microchannel plate, delay line readout detectors. These 1.25 degree field of view detectors are well suited to ultraviolet observations because of their excellent red rejection and negligible background. A dithered mode of observing and photon list output pose complex requirements on the data processing pipeline, entangling detector calibrations, and aspect reconstruction algorithms. Recent improvements have achieved photometric repeatability of 0.05 and 0.03 m sub(AB) in the FUV and NUV, respectively. We have detected a long-term drift of order 1% FUV and 6% NUV over the mission. Astrometric precision is of order 0.5" rms in both bands. In this paper we provide the GALEX user with a broad overview of the calibration issues likely to be confronted in the current release. Improvements are likely as the GALEX mission continues into an extended phase with a healthy instrument, no consumables, and increased opportunities for guest investigations.
We use the UV-optical color magnitude diagram in combination with spectroscopic and photometric measurements derived from the SDSS spectroscopic sample to measure the distribution of galaxies in the ...local universe (z < 0.25) and their physical properties as a function of specific star formation rate (SFR/M unk) and stellar mass (M unk). Throughout this study our emphasis is on the properties of galaxies on and off of a local "star-forming sequence." We discuss how the physical characteristics of galaxies along this sequence are related to scaling relations typically derived for galaxies of different morphological types. We find, among other trends, that our measure of the star formation rate surface density, capital sigma sub(SFR), is nearly constant along this sequence. We discuss this result and implications for galaxies at higher redshift. For the first time, we report on measurements of the local UV luminosity function versus galaxy structural parameters, as well as inclination. We also split our sample Into disk-dominated and bulge-dominated subsamples using the l-band Sersic index and find that disk-dominated galaxies occupy a very tight locus in SFR/M unk vs. M unk space, while bulge-dominated galaxies display a much larger spread of SFR/M unk at fixed stellar mass. A significant fraction of galaxies with SFR/M unk and capital sigma sub(SFR) above those on the "star-forming sequence" are bulge-dominated. We can use our derived distribution functions to ask whether a significant fraction of these galaxies may be experiencing a final episode of star formation (possibly induced by a merger or other burst), soon to be quenched, by determining whether this population can explain the growth rate of the non-star-forming galaxies on the "red sequence." We find that this is a plausible scenario for bulge-dominated galaxies near the characteristic transition mass under reasonable assumptions regarding quenching timescales. Similarly, we use this technique to estimate the rate of mergers/starbursts that take galaxies off of the star-forming sequence and show that the implied merger rates are consistent with local measurements.
We have analyzed the bivariate distribution of galaxies as a function of ultraviolet-optical colors and absolute magnitudes in the local universe. The sample consists of galaxies with redshifts and ...optical photometry from the Sloan Digital Sky Survey (SDSS) main galaxy sample matched with detections in the near-ultraviolet (NUV) and far-ultraviolet (FUV) bands in the Medium Imaging Survey being carried out by the Galaxy Evolution Explorer (GALEX) satellite. In the (NUV - r) sub(0.1) versus M sub(r.01) galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV - r) sub(0.1) color distribution at each M sub(r.0.1) is not well fit by the sum of two Gaussians due to an excess of galaxies in between the two sequences. The peaks of both sequences become redder with increasing luminosity, with a distinct blue peak visible up to M sub(r.0.1) similar to -23. The r sub(0.1) -band luminosity functions vary systematically with color, with the faint-end slope and characteristic luminosity gradually increasing with color. After correcting for attenuation due to dust, we find that approximately one-quarter of the color variation along the blue sequence is due to dust, with the remainder due to star formation history and metallicity. Finally, we present the distribution of galaxies as a function of specific star formation rate and stellar mass. The specific star formation rates imply that galaxies along the blue sequence progress from low-mass galaxies with star formation rates that increase somewhat with time to more massive galaxies with a more or less constant star formation rate. Above a stellar mass of similar to 10 super(10.5) M unk, galaxies with low ratios of current to past averaged star formation rate begin to dominate.
Abstract
Building on previous Bayesian approaches, we introduce a novel formulation of probabilistic cross-identification, where detections are directly associated to (hypothesized) astronomical ...objects in a globally optimal way. We show that this new method scales better for processing multiple catalogs than enumerating all possible candidates, especially in the limit of crowded fields, which is the most challenging observational regime for new-generation astronomy experiments such as the Rubin Observatory Legacy Survey of Space and Time. Here we study simulated catalogs where the ground truth is known and report on the statistical and computational performance of the method. The paper is accompanied by a public software tool to perform globally optimal catalog matching based on directional data.
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