Yong Ik Byun

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.

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.

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.

Galex observations of the ultraviolet halos of NGC 253 and M82

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 0.1% 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.

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

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 . Using the SDSS spectra to identify galaxies with even weak z ! 0.2 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 color has a full range of 5 mag. Plotting the color against the metallicity sensitive Lick FUV r FUV r and D4000 indices and against the stellar velocity dispersion, we find no correlation between our measurement Mg2 of UV rising flux and any parameter sensitive to the global metallicity of quiescent early-type galaxies. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: evolution — ultraviolet: galaxies

Ultraviolet Morphology and Star Formation in the Tidal Tails of NGC 4038/39

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.

QUASI-STELLAR OBJECT SELECTION ALGORITHM USING TIME VARIABILITY AND MACHINE LEARNING: SELECTION OF 1620 QUASI-STELLAR OBJECT CANDIDATES FROM MACHO LARGE MAGELLANIC CLOUD DATABASE

We present a new QSO selection algorithm using a Support Vector Machine (SVM), a supervised classification method, on a set of extracted time series features including period, amplitude, color, and autocorrelation value. We train a model that separates QSOs from variable stars, non-variable stars and microlensing events using 58 known QSOs, 1,629 variable stars and 4,288 non-variables using the MAssive Compact Halo Object (MACHO) database as a training set. To estimate the efficiency and the accuracy of the model, we perform a cross-validation test using the training set. The test shows that the model correctly identifies ∼80% of known QSOs with a 25% false positive rate. The majority of the false positives are Be stars. We applied the trained model to the MACHO Large Magellanic Cloud (LMC) dataset, which consists of 40 million lightcurves, and found 1,620 QSO candidates. During the selection none of the 33,242 known MACHO variables were misclassified as QSO candidates. In order to estimate the true false positive rate, we crossmatched the candidates with astronomical catalogs including the Spitzer Surveying the Agents of a Galaxy’s Evolution (SAGE) LMC catalog and a few X-ray catalogs. The results further suggest that the majority of the candidates, more than 70%, are QSOs. Subject headings: Magellanic Clouds methods: data analysis quasars: generalWe present a new quasi-stellar object (QSO) selection algorithm using a Support Vector Machine, a supervised classification method, on a set of extracted time series features including period, amplitude, color, and autocorrelation value. We train a model that separates QSOs from variable stars, non-variable stars, and microlensing events using 58 known QSOs, 1629 variable stars, and 4288 non-variables in the MAssive Compact Halo Object (MACHO) database as a training set. To estimate the efficiency and the accuracy of the model, we perform a cross-validation test using the training set. The test shows that the model correctly identifies ~80% of known QSOs with a 25% false-positive rate. The majority of the false positives are Be stars. We applied the trained model to the MACHO Large Magellanic Cloud (LMC) data set, which consists of 40 million light curves, and found 1620 QSO candidates. During the selection none of the 33,242 known MACHO variables were misclassified as QSO candidates. In order to estimate the true false-positive rate, we crossmatched the candidates with astronomical catalogs including the Spitzer Surveying the Agents of a Galaxys Evolution LMC catalog and a few X-ray catalogs. The results further suggest that the majority of the candidates, more than 70%, are QSOs.

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

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.