Oswald H. W. Siegmund

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.

THE COSMIC ORIGINS SPECTROGRAPH

The Cosmic Origins Spectrograph (COS) is a moderate-resolution spectrograph with unprecedented sensitivity that was installed into the Hubble Space Telescope (HST) in 2009 May, during HST Servicing Mission 4 (STS-125). We present the design philosophy and summarize the key characteristics of the instrument that will be of interest to potential observers. For faint targets, with flux F ? 1.0 ? 10?14?erg?cm?2?s?1 ??1, COS can achieve comparable signal to noise (when compared to Space Telescope Imaging Spectrograph echelle modes) in 1%-2% of the observing time. This has led to a significant increase in the total data volume and data quality available to the community. For example, in the first 20 months of science operation (2009 September-2011 June) the cumulative redshift pathlength of extragalactic sight lines sampled by COS is nine times than sampled at moderate resolution in 19 previous years of Hubble observations. COS programs have observed 214 distinct lines of sight suitable for study of the intergalactic medium as of 2011 June. COS has measured, for the first time with high reliability, broad Ly? absorbers and Ne VIII in the intergalactic medium, and observed the He II reionization epoch along multiple sightlines. COS has detected the first CO emission and absorption in the UV spectra of low-mass circumstellar disks at the epoch of giant planet formation, and detected multiple ionization states of metals in extra-solar planetary atmospheres. In the coming years, COS will continue its census of intergalactic gas, probe galactic and cosmic structure, and explore physics in our solar system and Galaxy.

THE ON-ORBIT PERFORMANCE OF THE GALAXY EVOLUTION EXPLORER

We report the first years on-orbit performance results for the Galaxy Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter modified Ritchey-Chretien telescope with a 125 field of view, selectable imaging and objective-grism spectroscopic modes, and an innovative optical system with a thin-film multilayer dichroic beam splitter that enables simultaneous imaging by a pair of photon-counting, microchannel-plate, delay-line readout detectors. Initial measurements demonstrate that GALEX is performing well, meeting its requirements for resolution, efficiency, astrometry, bandpass definition, and survey sensitivity.

Recent Star Formation in the Extreme Outer Disk of M83

Ultraviolet imaging with the Galaxy Evolution Explorer (GALEX) has revealed an extensive sample of UV-bright stellar complexes in the extreme outer disk of M83, extending to about 4 times the radius at which the majority of H II regions are detected (R = 51, or 6.6 kpc). These sources are typically associated with large-scale filamentary H I structures in the warped outer disk of M83 and are distributed beyond the galactocentric radii at which molecular interstellar medium has yet been detected. We present measured properties of these stellar complexes, including far-UV and near-UV magnitudes and local gas surface density. Only a subset of the outer-disk UV sources have corresponding H II regions detected in Hα imaging, consistent with a sample of mixed age in which some sources are a few megayears old and others are much more evolved (~108 yr).

The Ultraviolet Galaxy Luminosity Function in the Local Universe from GALEX Data

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 A, 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.

New Constraints on the Star Formation Histories and Dust Attenuation of Galaxies in the Local Universe from GALEX

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.

The Properties of Ultraviolet-luminous Galaxies at the Current Epoch

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.

Panoramic Galex far- and near-ultraviolet imaging of M31 and M33

We present Galaxy Evolution Explorer (GALEX) far-UV (FUV) and near-UV (NUV) mosaic observations covering the entirety of M31 and M33. For both targets, we measure the decline of surface brightness (in FUV and NUV) and changes in FUV - NUV color as a function of galactocentric radius. These UV radial profiles are compared to the distribution of ionized gas traced by Hα emission. We find that the extent of the UV emission, in both targets, is greater than the extent of the observed H II regions and diffuse ionized gas. We determine the ultraviolet diffuse fraction in M33 using our FUV observations and compare it to the Hα diffuse fraction obtained from wide-field narrowband imaging. The FUV diffuse fraction appears to be remarkably constant near 0.65 over a large range in galactocentric radius, with departures to higher values in circumnuclear regions and, most notably, at the limit of the Hα disk. We suggest that the increase in the FUV diffuse fraction at large galactocentric radii could indicate that a substantial portion of the diffuse emission beyond this point is not generated in situ but rather scattered from dust, after originating in the vicinity of the disks outermost H II regions. The radial variation of the Hα diffuse fraction was also measured. We found the Hα diffuse fraction generally near 0.4 but rising toward the galaxy center, up to 0.6. We made no attempt to correct our diffuse fraction measurements for position-dependent extinction, so the quoted values are best interpreted as upper limits given the plausibly higher extinction for stellar clusters relative to their surroundings.

Microchannel Plate EUV Detectors for the Extreme Ultraviolet Explorer

The design and operating characteristics of the prototype imaging microchannel plate (MCP) detector for the Extreme Ultraviolet Explorer (EUVE) Satellite [1] are discussed. We show that this detector has achieved high position resolution performance (> 512×512 pixels) and has low (< 1%) image distortion. In addition, the channel plate scheme used has tight pulse height distributions (< 40% FWHM) for UV radiation and displays low (<.2 cnt cm-2 sec-1) dark background counting rates. We also describe work that has been done on EUV filters in relation to the envisaged filter and photocathode complement.

High-resolution cross delay line detectors for the GALEX mission

The GALEX instrument consists of a 50cm normal incidence mirror telescope in combination with a grism, and a dichroic beamsplitter system projecting images onto two detectors simultaneously. The objective of this instrument is to provide sensitive high resolution imaging of galaxies in two bandpasses, with the option of the modest resolution spectroscopy. We are currently developing the microchannel plate, delay line, sealed tube detectors for the Galaxy Evolution Explorer mission to be launched in 2001.