Showing papers by "Oswald H. W. Siegmund published in 2005"

The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission

Abstract: 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 On-Orbit Performance of the Galaxy Evolution Explorer

Abstract: We report the first year's 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

Abstract: 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_H II = 5´.1, 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 (~10^8 yr).

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

Abstract: We present the results of a determination of the galaxy luminosity function at ultraviolet wavelengths at redshifts of $z=0.0-0.1$ from GALEX data. We determined the luminosity function in the GALEX FUV and NUV bands from a sample of galaxies with UV magnitudes between 17 and 20 that are drawn from a total of 56.73 deg^2 of GALEX fields overlapping the b_j-selected 2dF Galaxy Redshift Survey. The resulting luminosity functions are fainter than previous UV estimates and result in total UV luminosity densities of 10^(25.55+/-0.12) ergs s^-1 Hz^-1 Mpc^-3 and 10^(25.72+/-0.12) ergs s^-1 Hz^-1 Mpc^-3 at 1530 Ang. and 2310 Ang., respectively. This corresponds to a local star formation rate density in agreement with previous estimates made with H-alpha-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

Abstract: 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

Abstract: 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

Abstract: 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 disk's 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.

Number Counts of GALEX Sources in Far-Ultraviolet (1530 Å) and Near-Ultraviolet (2310 Å) Bands

Abstract: Number Counts of galaxies in two GALEX bands (FUV: 1530A and NUV: 2310A, both in AB magnitudes) are reported. They provide for the first time in the literature homogeneously calibrated number counts of UV galaxies covering continuously a very wide range of UV magnitude (14 -- 23.8). Both the FUV and NUV counts are inconsistent with a non-evolution model, while they are in good agreement with evolution models (essentially luminosity evolution) derived from the high-z UV luminosity functions of Arnouts et al. (2004). It is found that the contribution from galaxies detected by GALEX to the UV background is 0.68+-0.10 nW m-2 sr-1 at 1530A and 0.99+-0.15 nW m-2 sr-1 at 2310A. These are 66+-9% and 44+-6% of the total contributions of galaxies to the the UV background at 1530A, respectively, as estimated using the evolution models. ...

Galex observations of the ultraviolet halos of NGC 253 and M82

Abstract: We present Galaxy Evolution Explorer (GALEX) images of the prototypical edge-on starburst galaxies M82 and NGC 253. 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 continuum and line emission from 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 observed luminosity of the halo UV light is lesssim0.1% of the bolometric luminosity of the starburst. The morphology of the UV filaments in both galaxies shows a high degree of spatial correlation with Hα 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 "Hα cap" 11 kpc north of M82. If this cap is a result of the wind fluid running into a preexisting 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.

The star formation rate function of the local universe

Abstract: We have derived the bivariate luminosity function for the far-ultraviolet (1530 A) and far-infrared (60 μm). We use matched GALEX and IRAS data, and redshifts from NED and the PSCz survey. We have derived a total star formation luminosity function (Ltot), with Ltot = LFUV + LFIR. Using these, we determined the cosmic "star formation rate" (SFR) function and density for the local universe. The total SFR function (Ltot) is fitted very well by a lognormal distribution over 5 decades of luminosity. We find that the bivariate luminosity function (LFUV, LFIR) shows a bimodal behavior, with LFIR tracking LFUV for Ltot < 1010 L☉ and LFUV saturating at ~1010 L☉, while Ltot ~ LFIR for higher luminosities. We also calculate the SFR density and compare it with other measurements.

Systematics of the ultraviolet rising flux in a Galex/SDSS sample of early-type galaxies

Abstract: We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey (SDSS) First Data Release with the Galaxy Evolution Explorer (GALEX) Medium and All-sky Imaging Surveys We obtain a working sample of 1032 early-type galaxies with GALEX far-UV (FUV) detections, SDSS spectroscopy, and z < 02 Using the SDSS spectra to identify galaxies with even weak star formation or evidence of active galactic nuclei, and further removing galaxies with any evidence of non-early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies We find that the FUV - r color has a full range of 5 mag Plotting the FUV - r color against the metallicity sensitive Lick Mg2 and D4000 indices and against the stellar velocity dispersion, we find no correlation between our measurement of UV rising flux and any parameter sensitive to the global metallicity of quiescent early-type galaxies

Ultraviolet morphology and star formation in the tidal tails of ngc 4038/39

Abstract: We present Galaxy Evolution Explorer far-ultraviolet (1530 A) and near-ultraviolet (2310 A) observations of the archetypal merging system NGC 4038/39, "the Antennae." Both tails are relatively bright in the UV, especially in the vicinity of the tidal dwarf galaxy candidates at the end of the southern tail. The UV light generally falls within the optically delineated tails, although the UV light is considerably more structured, with a remarkably similar morphology to the tidal H I. The UV colors suggest that there has been continuing star formation within the tidal tails, even outside the previously studied tidal dwarf regions. Within the inner disk regions, there are interesting UV features that appear to be related to the extended soft X-ray loops and halo recently discovered by Chandra.

Ultraviolet emission and star formation in Stephan's Quintet

Abstract: We present the first Galaxy Evolution Explorer (GALEX) UV images of the well-known interacting group of galaxies, Stephan's Quintet (SQ). We detect widespread UV emission throughout the group. However, there is no consistent coincidence between UV structure and emission in the optical, Hα, or H I. Excluding the foreground galaxy NGC 7320 (Sd), most of the UV emission is found in regions associated with the two spiral members of the group, NGC 7319 and NGC 7318b, and the intragroup medium starburst SQ-A. The extinction-corrected UV data are analyzed to investigate the overall star formation activity in SQ. We find that the total star formation rate (SFR) of SQ is 6.69 ± 0.65 M☉ yr-1. Of this, 1.34 ± 0.16 M☉ yr-1 is due to SQ-A. This is in excellent agreement with that derived from the extinction-corrected Hα luminosity of SQ-A. The SFR in regions related to NGC 7319 is 1.98 ± 0.58 M☉ yr-1, most of which (68%) is contributed by the disk. The contribution from the young tail is only 15%. In the UV, the young tail is more extended (~100 kpc) and shows a looplike structure, including the optical tail, the extragalactic H II regions recently discovered in Hα, and other UV emission regions discovered for the first time. The UV and optical colors of the old tail are consistent with a single stellar population of age t 108.5±0.4 yr. The UV emission associated with NGC 7318b is found in a very large (~80 kpc) disk, with a net SFR of 3.37 ± 0.25 M☉ yr-1. Several large UV emission regions are 30-40 kpc away from the nucleus of NGC 7318b. Although both NGC 7319 and NGC 7318b show peculiar UV morphology, their SFR is consistent with that of normal Sbc galaxies, indicating that the strength of star formation activity is not enhanced by interactions.

The Ultraviolet Luminosity Function of GALEX Galaxies at Photometric Redshifts between 0.07 and 0.25

Abstract: We present measurements of the UV galaxy luminosity function and the evolution of luminosity density from Galaxy Evolution Explorer (GALEX) observations matched to the Sloan Digital Sky Survey (SDSS). We analyze galaxies in the Medium Imaging Survey overlapping the SDSS First Data Release, with a total coverage of 44 deg2. Using the combined GALEX + SDSS photometry, we compute photometric redshifts and study the luminosity function in three redshift shells between z = 0.07 and 0.25. The Schechter function fits indicate that the faint-end slope α is consistent with -1.1 at all redshifts, but the characteristic UV luminosity M* brightens by 0.2 mag from z = 0.07 to 0.25. In the lowest redshift bin, early- and late-type galaxies are studied separately, and we confirm that red galaxies tend to be brighter and have a shallower slope α than blue ones. The derived luminosity densities are consistent with other GALEX results based on a local spectroscopic sample from the Two-Degree Field, and the evolution follows the trend reported by deeper studies.

Recent Star Formation in Nearby Galaxies from Galaxy Evolution Explorer Imaging: M101 and M51

Abstract: The Galaxy Evolution Explorer (GALEX) Nearby Galaxies Survey is providing deep far-UV and near-UV imaging for a representative sample of galaxies in the local universe. We present early results for M51 and M101, from GALEX UV imaging and Sloan Digital Sky Survey optical data in five bands. The multiband photometry of compact stellar complexes in M101 is compared to population synthesis models, to derive ages, reddening, reddening-corrected luminosities, and current/initial masses. The GALEX UV photometry provides a complete census of young compact complexes on a ≈160 pc scale. A galactocentric gradient of the far-UV/near-UV color indicates younger stellar populations toward the outer parts of the galaxy disks, the effect being more pronounced in M101 than in M51.

A comparative study of the spatial distribution of ultraviolet and far-infrared fluxes from M101

Abstract: The total ultraviolet (UV) flux (from 1412 to 2718 A) of M101 is compared on a pixel-to-pixel basis with the total far-infrared (FIR) flux (from 60 to 170 μm) using the maps of the galaxy taken by the Galaxy Evolution Explorer (GALEX) in the near-UV and far-UV and by the Infrared Space Observatory (ISO) at 60, 100, and 170 μm. The main result of this investigation is the discovery of a tight dependence of the FIR/UV ratio on radius, with values monotonically decreasing from ~4 in the nuclear region to nearly zero toward the edge of the optical disk. Although the tightness of this dependence is in part attributable to resolution effects, the result is consistent with the presence of a large-scale distribution of diffuse dust having a face-on optical depth that decreases with radius and that dominates over the more localized variations in opacity between the arm and interarm regions. We also find a trend for the FIR/UV ratio of taking on higher values in the regions of diffuse interarm emission than in the spiral-arm regions, at a given radius. This is interpreted quantitatively in terms of the escape probability of UV photons from spiral arms and their subsequent scattering in the interarm regions, and in terms of the larger relative contribution of optical photons to the heating of the dust in the interarm regions.

Classification and characterization of objects from the galaxy evolution explorer survey and the sloan digital sky survey

Abstract: We use the Galaxy Evolution Explorer (GALEX) Medium Imaging Survey (MIS) and All-sky Imaging Survey (AIS) data available in the first internal release, matched to the Sloan Digital Sky Survey (SDSS) catalogs in the overlapping regions, to classify objects by comparing the multiband photometry to model colors. We show an example of the advantage of such broad wavelength coverage (GALEX far-UV and near-UV, SDSS ugriz) in classifying objects and augmenting the existing samples and catalogs. From the MIS (AIS) sample over an area of 75 (92) deg2, we select a total of 1736 (222) QSO candidates at redshifts less than 2, significantly extending the number of fainter candidates and moderately increasing the number of bright objects in the SDSS list of spectroscopically confirmed QSOs. Numerous hot stellar objects are also revealed by the UV colors, as expected.

The Ultraviolet Galaxy Luminosity Function from GALEX Data: Color-Dependent Evolution at Low Redshift

Abstract: We present measurements of the FUV (1530A) and NUV (2310A) galaxy luminosity functions (LF) at low redshift (z<0.2) from GALEX observations matched to the 2dF Galaxy Redshift Survey. We split our FUV and NUV samples into two UV-bj color bins and two redshift bins. As observed at optical wavelengths, the local LF of the bluest galaxies tend to have steeper faint end slopes and fainter characteristic magnitudes M* than the reddest subsamples. We find evidence for color dependent evolution at very low redshift in both bands, with bright blue galaxies becoming dominant in the highest redshift bin. The evolution of the total LF is consistent with an 0.3 magnitude brightening between z=0 and 0.13, in agreement with the first analysis of deeper GALEX fields probing adjacent and higher redshifts.

The Look-back Time Evolution of Far-Ultraviolet Flux from Elliptical Galaxies: The Fornax Cluster and A2670

Abstract: In order to investigate the origin of the far-UV (FUV) flux from the early-type galaxies, the Galaxy Evolution Explorer is collecting the UV data for the elliptical-rich clusters at moderate redshifts (z < 0.2) where the dominant FUV source is predicted to be hot horizontal-branch (HB) stars and their post-HB progeny. Here we present our first result for the early-type galaxies in A2670 at z = 0.076. Compared to NGC 1399, a nearby giant elliptical galaxy in the Fornax Cluster, it appears that the rest-frame FUV - V color of the giant elliptical galaxies gets redder by ~0.7 mag at the distance of A2670 (z = 0.076; look-back time ≈ 1.0 Gyr). Although a detailed comparison with the models is postponed until more cluster data are accumulated, it is interesting to note that this value is consistent with the variation predicted by the population synthesis models where the mean temperature of HB stars declines rapidly with increasing look-back time.

Optically sensitive Medipix2 detector for adaptive optics wavefront sensing

Abstract: A new hybrid optical detector is described that has many of the attributes desired for the next generation adaptive optics (AO) wavefront sensors. The detector consists of a proximity focused microchannel plate (MCP) read out by multi-pixel application specific integrated circuit (ASIC) chips developed at CERN (“Medipix2”) with individual pixels that amplify, discriminate and count input events. The detector has 256×256 pixels, zero readout noise (photon counting), can be read out at 1 kHz frame rates and is abutable on 3 sides. The Medipix2 readout chips can be electronically shuttered down to a temporal window of a few microseconds with an accuracy of 10 ns. When used in a Shack–Hartmann style wavefront sensor, a detector with 4 Medipix chips should be able to centroid approximately 5000 spots using 7×7 pixel sub-apertures resulting in very linear, off-null error correction terms. The quantum efficiency depends on the optical photocathode chosen for the bandpass of interest.

The GALEX Ultraviolet Variability Catalog

Abstract: We present version 1.0 of the NASA Galaxy Evolution Explorer (GALEX) ultraviolet variability (GUVV) catalog, which contains information on 84 time-variable and transient sources gained with simultaneous near-ultraviolet (NUV) and far-ultraviolet (FUV) photometric observations. These time-variable sources were serendipitously revealed in the various 12 diameter star fields currently being surveyed by the GALEX satellite in two ultraviolet bands (NUV 1750–2750 A, FUV 1350–1750 A) with limiting AB magnitudes of 23–25. The largest amplitude variable objects currently 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 (<500 s) outbursts. Other types of large-amplitude ultraviolet variable objects include ab-type RR Lyrae stars, which can vary periodically by 2–5 mag 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.

The Galex Ultraviolet Variability (GUVV) Catalog

Abstract: 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.

Characterizations of microchannel plate quantum efficiency

Abstract: Microchannel plates have been used over many years in astronomical applications for X-ray, UV and visible sensing. Their adaptability to various size and configuration formats have allowed a wide range of devices to be realized, employing many different forms of readout techniques and photocathode types. One problem that has arisen for a number of these programs is the issue of obtaining high quantum efficiency on a consistent basis. Several missions have suffered from problems when microchannel plates have not achieved the quantum efficiency values that are the generally accepted normal values. We have compiled an extensive set of quantum efficiency measurements which cover missions and devices produced over the past 20 years. These show that the deficiencies in MCP QE of bare MCP's also result in a corresponding decrease in QE for same MCP's coated with a photocathode. This may be interpreted as a deficiency of the MCP detection efficiency for low energy photoelectrons produced by the MCP/cathode. Recent measurements of MCP's produced by alteration of the processing procedures shows that this problem is avoidable, and gives excellent results for current generation MCP's with 5-6 micron pore sizes.

The quantum efficiency and stability of UV and soft x-ray photocathodes

Abstract: The sensitivity of many detection devices is established by the use photocathodes for the conversion of incoming photons into photoelectrons. The choice of photocathode material is determined by the spectral range where the sensitivity of the device is most important. Alkali halides are very efficient photocathodes in the ultraviolet and soft X-ray wavelength ranges and are widely used in many scientific applications. Although they are relatively stable under short exposure to atmosphere, which substantially simplifies production and handling of detection devices, it was found that their sensitivity can be substantially reduced by intense UV or X-ray irradiation (photocathode's ageing). A detailed study of alkali halide photocathodes efficiency and their ageing under intense UV and X-ray irradiation as well as some methods of increasing the stability are presented. The quantum efficiency of amorphous diamond films were shown to be slightly lower than the efficiency of some alkali halide films, but the chemical and mechanical stability and yet to be confirmed radiation hardness of diamond photocathodes make them very attractive for many UV and soft X-ray applications. Multialkalis and new materials such as GaN, AlGaN, GaAs could be used to extend the sensitivity to longer wavelengths, but require in situ processing in very high vacuum.

Galaxy Evolution Explorer ultraviolet spectroscopy and deep imaging of luminous infrared galaxies in the European Large-Area ISO Survey S1 field

Abstract: The European Large-Area ISO Survey (ELAIS) S1 field was observed by the Galaxy Evolution Explorer (GALEX) in both its Wide Spectroscopic and Deep Imaging Survey modes. This field was previously observed by the Infrared Space Observatory, and we made use of the catalog of multiwavelength data published by the ELAIS consortium to select galaxies common to the two samples. Among the 959 objects with GALEX spectroscopy, 88 are present in the ELAIS catalog and 19 are galaxies with an optical spectroscopic redshift. The distribution of redshifts covers the range 0 < z < 1.6. The selected galaxies have bolometric IR luminosities 10 < log L_IR < 13 (deduced from the 15 μm flux using ISOCAM), which means that we cover a wide range of galaxies from normal to ultraluminous IR galaxies. The mean (σ) UV luminosity (not corrected for extinction) amounts to log λL_1530 = 9.8(0.6) L_☉ for the low-z (z ≤ 0.35) sample. The UV slope β (assuming f_λ ∝ λ^β) correlates with the GALEX FUV - NUV color if the sample is restricted to galaxies below z < 0.1. Taking advantage of the UV and IR data, we estimate the dust attenuation from the IR/UV ratio and compare it to the UV slope β. We find that it is not possible to uniquely estimate the dust attenuation from β for our sample of galaxies. These galaxies are highly extinguished with a median value A_FUV = 2.7 ± 0.8. Once the dust correction is applied, the UV- and IR-based star formation rates correlate. For the closest galaxy with the best quality spectrum, we see a feature consistent with being produced by a bump near 220 nm in the attenuation curve.

Galaxy Evolution Explorer Ultraviolet Photometry of Globular Clusters in M31

Abstract: We present ultraviolet photometry for globular clusters (GCs) in M31 from 15 square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect 200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750 Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses, respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV, to an approximate limiting V magnitude of 19. Out of six clusters with [Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore, we find no candidate metal-rich clusters with significant FUV flux, because of the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC 6441, which are metal-rich Galactic GCs with hot HB stars. We show that our GALEX photometry follows the general color trends established in previous UV studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in the UV and with population synthesis models, we suggest that the age range of M31 and Galactic halo GCs are similar.

Large-Amplitude Ultraviolet Variations in the RR Lyrae Star ROTSE-I J143753.84+345924.8

Abstract: The NASA Galaxy Evolution Explorer (GALEX) satellite has obtained simultaneous near-ultraviolet (NUV) and far-ultraviolet (FUV) light curves of the ROTSE-I Catalog RR Lyrae type ab variable star J143753.84+345924.8. A series of 38 GALEX Deep Imaging Survey observations well distributed in phase within the star's 0.56432 day period shows an AB = 4.9 mag variation in the FUV (1350-1750 A) band and an AB = 1.8 mag variation in the NUV (1750-2750 A) band, compared with only a 0.8 mag variation in the broad, unfiltered ROTSE-I (≈4500-10,000 A) band. These GALEX UV observations are the first to reveal a large RR Lyrae amplitude variation at wavelengths below 1800 A. We compare the GALEX and ROTSE-I observations to predictions made by recent Kurucz stellar atmosphere models. We use published physical parameters for the comparable period (0.57433 days), well-observed RR Lyrae star WY Antliae to compute predicted FUV, NUV, and ROTSE-I light curves for J143753.84+345924.8. The observed light curves agree with the Kurucz predictions for [Fe/H] = -1.25 to within AB = 0.2 mag in the GALEX NUV and ROTSE-I bands and to within 0.5 mag in the FUV. At all metallicities between solar and 1/100 solar, the Kurucz models predict 6-8 mag of variation at wavelengths between 1000 and 1700 A. Other variable stars with similar temperature variations, such as Cepheids, should also have large-amplitude FUV light curves, observable during the ongoing GALEX imaging surveys.

Noiseless kilohertz-frame-rate imaging detector based on microchannel plates readout with the Medipix2 CMOS pixel chip

Abstract: A new hybrid imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors. The detector consists of proximity focused microchannel plates (MCPs) read out by pixelated CMOS application specific integrated circuit (ASIC) chips developed at CERN ("Medipix2"). Each Medipix2 pixel has an amplifier, lower and upper charge discriminators, and a 14-bit counter. The 256×256 array can be read out noiselessly (photon counting) in 286 μs. The Medipix2 is abutable on 3 sides to produce 512× (n*256) pixel devices. The readout can be electronically shuttered down to a temporal window of a few microseconds with an accuracy of 10 ns. Good quantum efficiencies can be achieved from the x-ray (open faced with opaque photocathodes) to the optical (sealed tube with semi-transparent GaAs photocathode).

Galaxy Evolution Explorer observations of the ultraviolet surface brightness and color profiles of the local group elliptical galaxy M32 (NGC 221)

Abstract: M32, the compact elliptical galaxy companion to the Andromeda spiral galaxy, has been imaged by the Galaxy Evolution Explorer (GALEX) in two UV bands, centered at ~1500 (far-ultraviolet [FUV]) and 2300 A (near-ultraviolet). The imaging data have been carefully decomposed so as to properly account for the complicated background contamination from the disk of M31. We have derived the surface brightness and color profiles finding a slightly positive color gradient of Δ(FUV-B)/Δ log r = +0.15 ± 0.03 within one effective radius. Earlier data from the Ultraviolet Imaging Telescope suggested that M32 had an extremely large (negative) FUV-optical color gradient [Δ(FUV-B)/Δ log r < -2], inverted with respect to the majority of gradients seen in giant elliptical galaxies. Our new results show that despite its very low UV upturn, M32 has similar UV properties to those observed in luminous elliptical galaxies.