Ted K. Wyder

The Galaxy Evolution Explorer: A Space ultraviolet survey mission

We give an overview of the Galaxy Evolution Explorer (GALEX), a NASA Explorer Mission launched on 2003 April 28. GALEX is performing the first space UV sky survey, including imaging and grism surveys in two bands (1350-1750 and 1750-2750 ?). The surveys include an all-sky imaging survey (mAB 20.5), a medium imaging survey of 1000 deg2 (mAB 23), a deep imaging survey of 100 deg2 (mAB 25), and a nearby galaxy survey. Spectroscopic (slitless) grism surveys (R = 100-200) are underway with various depths and sky coverage. Many targets overlap existing or planned surveys in other bands. We will use the measured UV properties of local galaxies, along with corollary observations, to calibrate the relationship of the UV and global star formation rate in local galaxies. We will apply this calibration to distant galaxies discovered in the deep imaging and spectroscopic surveys to map the history of star formation in the universe over the redshift range 0 < z < 2 and probe the physical drivers of star formation in galaxies. The GALEX mission includes a guest investigator program, supporting the wide variety of programs made possible by the first UV sky survey.

UV STAR FORMATION RATES IN THE LOCAL UNIVERSE

We measure star formation rates (SFRs) of ≈50,000 optically selected galaxies in the local universe (z ≈ 0.1)—from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (ultraviolet) and SDSS photometry to a library of dust-attenuated population synthesis models. For star-forming galaxies, our UV-based SFRs compare remarkably well with those from SDSS-measured emission lines (Hα). Deviations from perfect agreement are shown to be due to differences in the dust attenuation estimates. In contrast to Hα measurements, UV provides reliable SFRs for galaxies with weak Hα, and where Hα is contaminated with AGN emission (1/2 of the sample). Using full-SED SFRs, we calibrate a simple prescription that uses GALEX far- and near-UV magnitudes to produce dust-corrected SFRs for normal star-forming galaxies. The specific SFR is considered as a function of stellar mass for (1) star-forming galaxies with no AGNs, (2) those hosting an AGN, and (3) galaxies without Hα emission. We find that the three have distinct star formation histories, with AGNs lying intermediate between the star-forming and the quiescent galaxies. Star-forming galaxies without an AGN lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. On the other hand, weak AGNs, while also massive, have lower SFRs, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent galaxies via strong and weak AGNs. We confirm that some galaxies with no Hα show signs of star formation in the UV. We derive a cosmic star formation density at z = 0.1 with significantly smaller total error than previous measurements.

The WiggleZ Dark Energy Survey: mapping the distance–redshift relation with baryon acoustic oscillations

We present measurements of the baryon acoustic peak at redshifts z = 0.44, 0.6 and 0.73 in the galaxy correlation function of the final dataset of the WiggleZ Dark Energy Survey. We combine our correlation function with lower-redshift measurements from the 6-degree Field Galaxy Survey and Sloan Digital Sky Survey, producing a stacked survey correlation function in which the statistical significance of the detection of the baryon acoustic peak is 4.9-σ relative to a zero-baryon model with no peak. We fit cosmological models to this combined baryon acoustic oscillation (BAO) dataset comprising six distance-redshift data points, and compare the results to similar fits to the latest compilation of supernovae (SNe) and Cosmic Microwave Background (CMB) data. The BAO and SNe datasets produce consistent measurements of the equation-ofstate w of dark energy, when separately combined with the CMB, providing a powerful check for systematic errors in either of these distance probes. Combining all datasets we determine w = 1.03 ± 0.08 for a flat Universe, consistent with a cosmological constant model. Assuming dark energy is a cosmological constant and varying the spatial curvature, we find k = 0.004± 0.006.

The WiggleZ Dark Energy Survey: Joint measurements of the expansion and growth history at z < 1

We perform a joint determination of the distance–redshift relation and cosmic expansion rate at redshifts z = 0.44, 0.6 and 0.73 by combining measurements of the baryon acoustic peak and Alcock–Paczynski distortion from galaxy clustering in the WiggleZ Dark Energy Survey, using a large ensemble of mock catalogues to calculate the covariance between the measurements. We find that D_A(z) = (1205 ± 114, 1380 ± 95, 1534 ± 107) Mpc and H(z) = (82.6 ± 7.8, 87.9 ± 6.1, 97.3 ± 7.0) km s^(−1) Mpc^(−1) at these three redshifts. Further combining our results with other baryon acoustic oscillation and distant supernovae data sets, we use a Monte Carlo Markov Chain technique to determine the evolution of the Hubble parameter H(z) as a stepwise function in nine redshift bins of width Δz = 0.1, also marginalizing over the spatial curvature. Our measurements of H(z), which have precision better than 7 per cent in most redshift bins, are consistent with the expansion history predicted by a cosmological constant dark energy model, in which the expansion rate accelerates at redshift z < 0.7.

The WiggleZ Dark Energy Survey: survey design and first data release

The WiggleZ Dark Energy Survey is a survey of 240 000 emission-line galaxies in the distant Universe, measured with the AAOmega spectrograph on the 3.9-m Anglo-Australian Telescope (AAT). The primary aim of the survey is to precisely measure the scale of baryon acoustic oscillations (BAO) imprinted on the spatial distribution of these galaxies at look-back times of 4–8 Gyr. The target galaxies are selected using ultraviolet (UV) photometry from the Galaxy Evolution Explorer satellite, with a flux limit of NUV < 22.8 mag . We also require that the targets are detected at optical wavelengths, specifically in the range 20.0 < r < 22.5 mag . We use the Lyman break method applied to the UV colours, with additional optical colour limits, to select high-redshift galaxies. The galaxies generally have strong emission lines, permitting reliable redshift measurements in relatively short exposure times on the AAT. The median redshift of the galaxies is z_(med)= 0.6 . The redshift range containing 90 per cent of the galaxies is 0.2 < z < 1.0 . The survey will sample a volume of ~1 Gpc^3 over a projected area on the sky of 1000 deg^2, with an average target density of 350 deg^(−2). Detailed forecasts indicate that the survey will measure the BAO scale to better than 2 per cent and the tangential and radial acoustic wave scales to approximately 3 and 5 per cent, respectively. Combining the WiggleZ constraints with existing cosmic microwave background measurements and the latest supernova data, the marginalized uncertainties in the cosmological model are expected to be σ(Ω_m) = 0.02 and σ(w) = 0.07 (for a constant w model). The WiggleZ measurement of w will constitute a robust, precise and independent test of dark energy models. This paper provides a detailed description of the survey and its design, as well as the spectroscopic observations, data reduction and redshift measurement techniques employed. It also presents an analysis of the properties of the target galaxies, including emission-line diagnostics which show that they are mostly extreme starburst galaxies, and Hubble Space Telescope images, which show that they contain a high fraction of interacting or distorted systems. In conjunction with this paper, we make a public data release of data for the first 100 000 galaxies measured for the project.

The WiggleZ Dark Energy Survey: the growth rate of cosmic structure since redshift z=0.9

We present precise measurements of the growth rate of cosmic structure for the redshift range 0.1 < z < 0.9, using redshift-space distortions in the galaxy power spectrum of the WiggleZ Dark Energy Survey. Our results, which have a precision of around 10 per cent in four independent redshift bins, are well fitted by a flat Λ cold dark matter (ΛCDM) cosmological model with matter density parameter Ω_m = 0.27. Our analysis hence indicates that this model provides a self-consistent description of the growth of cosmic structure through large-scale perturbations and the homogeneous cosmic expansion mapped by supernovae and baryon acoustic oscillations. We achieve robust results by systematically comparing our data with several different models of the quasi-linear growth of structure including empirical models, fitting formulae calibrated to N-body simulations, and perturbation theory techniques. We extract the first measurements of the power spectrum of the velocity divergence field, P_(θθ) (k), as a function of redshift (under the assumption that P_(gθ) (k) = − √P_(gg)(k)P_(θθ) (k), where g is the galaxy overdensity field), and demonstrate that the WiggleZ galaxy–mass cross-correlation is consistent with a deterministic (rather than stochastic) scale-independent bias model for WiggleZ galaxies for scales k < 0.3 h Mpc^(−1). Measurements of the cosmic growth rate from the WiggleZ Survey and other current and future observations offer a powerful test of the physical nature of dark energy that is complementary to distance–redshift measures such as supernovae and baryon acoustic oscillations.

The GALEX Arecibo SDSS Survey - I. Gas fraction scaling relations of massive galaxies and first data release

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 10^(10) 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 10^(11) M_⊙, the detected fraction does not fall below ~40 per cent. We find that the atomic gas fraction M_(HI)/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 10^(8.5) 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 M_(HI)/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.

The UV-Optical Galaxy Color-Magnitude Diagram. I. Basic Properties

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)_(0.1) versus M_(r,0.1) galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV − r)_(0.1) color distribution at each M_(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_(r,0.1) ~ − 23. The r_(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 ~10^(10.5) M_☉, galaxies with low ratios of current to past averaged star formation rate begin to dominate.

Galaxy evolution explorer ultraviolet color-magnitude relations and evidence of recent star formation in early-type galaxies

We have used the Galaxy Evolution Explorer UV photometric data to construct a first near-UV (NUV) color-magnitude relation (CMR) for the galaxies preclassified as early-type by Sloan Digital Sky Survey studies. The NUV CMR is a powerful tool for tracking the recent star formation history in early-type galaxies, owing to its high sensitivity to the presence of young stellar populations. Our NUV CMR for UV-weak galaxies shows a well-defined slope and thus will be useful for interpreting the rest-frame NUV data of distant galaxies and studying their star formation history. Compared to optical CMRs, the NUV CMR shows a substantially larger scatter, which we interpret as evidence of recent star formation activities. Roughly 15% of the recent epoch (z < 0.13) bright [M(r) < -22] early-type galaxies show a sign of recent (1 Gyr) star formation at the 1%-2% level (lower limit) in mass compared to the total stellar mass. This implies that low-level residual star formation was common during the last few billion years even in bright early-type galaxies.

The GALEX-VVDS measurement of the evolution of the far-ultraviolet luminosity density and the cosmic star formation rate

In a companion paper (Arnouts et al. 2004) we presented new measurements of the galaxy luminosity function at 1500 Angstroms out to z~1 using GALEX-VVDS observations (1039 galaxies with NUV 0.2) and at higher z using existing data sets. In this paper we use the same sample to study evolution of the FUV luminosity density. 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 (UVLG) is undergoing dramatic evolution (x30) between 025%) of the total FUV 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 the SFR density in the rest ultraviolet and Halpha given the still significant uncertainties in the attenuation correction.