[show abstract][hide abstract] ABSTRACT: We characterize the star formation in the low-metallicity galaxy NGC 6822 over the past few hundred million years, using GALEX far-UV (FUV, 1344-1786 Å) and near-UV (NUV, 1771-2831 Å) imaging, and ground-based Hα imaging. From the GALEX FUV image, we define 77 star-forming (SF) regions with area >860 pc2, and surface brightness 26.8 mag (AB) arcsec–2, within 02 (1.7 kpc) of the center of the galaxy. We estimate the extinction by interstellar dust in each SF region from resolved photometry of the hot stars it contains: E(B – V) ranges from the minimum foreground value of 0.22 mag up to 0.66 ± 0.21 mag. The integrated FUV and NUV photometry, compared with stellar population models, yields ages of the SF complexes up to a few hundred Myr, and masses from 2 × 102 to 1.5 × 106 . The derived ages and masses strongly depend on the assumed type of interstellar selective extinction, which we find to vary across the galaxy. The total mass of the FUV-defined SF regions translates into an average star formation rate (SFR) of 1.4 × 10–2 yr–1 over the past 100 Myr, and SFR = 1.0 × 10–2 yr–1 in the most recent 10 Myr. The latter is in agreement with the value that we derive from the Hα luminosity, SFR = 0.008 yr–1. The SFR in the most recent epoch becomes higher if we add the SFR = 0.02 yr–1 inferred from far-IR measurements, which trace star formation still embedded in dust (age a few Myr).
The Astrophysical Journal 03/2011; 730(2):88. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use GALEX ultraviolet (UV) and optical integrated photometry of the hosts of seventeen luminous supernovae (LSNe, having peak M_V < -21) and compare them to a sample of 26,000 galaxies from a cross-match between the SDSS DR4 spectral catalog and GALEX interim release 1.1. We place the LSNe hosts on the galaxy NUV-r versus M_r color magnitude diagram (CMD) with the larger sample to illustrate how extreme they are. The LSN hosts appear to favor low-density regions of the galaxy CMD falling on the blue edge of the blue cloud toward the low luminosity end. From the UV-optical photometry, we estimate the star formation history of the LSN hosts. The hosts have moderately low star formation rates (SFRs) and low stellar masses (M_*) resulting in high specific star formation rates (sSFR). Compared with the larger sample, the LSN hosts occupy low-density regions of a diagram plotting sSFR versus M_* in the area having higher sSFR and lower M_*. This preference for low M_*, high sSFR hosts implies the LSNe are produced by an effect having to do with their local environment. The correlation of mass with metallicity suggests that perhaps wind-driven mass loss is the factor that prevents LSNe from arising in higher-mass, higher-metallicity hosts. The massive progenitors of the LSNe (>100 M_sun), by appearing in low-SFR hosts, are potential tests for theories of the initial mass function that limit the maximum mass of a star based on the SFR. Comment: 8 pages, 3 figures, 2 tables, accepted to ApJ, amended references and updated SN designations
The Astrophysical Journal 11/2010; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We measure the UV-optical color dependence of galaxy clustering in the local universe. Using the clean separation of the red and blue sequences made possible by the NUV - r color-magnitude diagram, we segregate the galaxies into red, blue and intermediate "green" classes. We explore the clustering as a function of this segregation by removing the dependence on luminosity and by excluding edge-on galaxies as a means of a non-model dependent veto of highly extincted galaxies. We find that \xi (r_p, \pi) for both red and green galaxies shows strong redshift space distortion on small scales -- the "finger-of-God" effect, with green galaxies having a lower amplitude than is seen for the red sequence, and the blue sequence showing almost no distortion. On large scales, \xi (r_p, \pi) for all three samples show the effect of large-scale streaming from coherent infall. On scales 1 Mpc/h < r_p < 10 Mpc/h, the projected auto-correlation function w_p(r_p) for red and green galaxies fits a power-law with slope \gamma ~ 1.93 and amplitude r_0 ~ 7.5 and 5.3, compared with \gamma ~ 1.75 and r_0 ~ 3.9 Mpc/h for blue sequence galaxies. Compared to the clustering of a fiducial L* galaxy, the red, green, and blue have a relative bias of 1.5, 1.1, and 0.9 respectively. The w_p(r_p) for blue galaxies display an increase in convexity at ~ 1 Mpc/h, with an excess of large scale clustering. Our results suggest that the majority of blue galaxies are likely central galaxies in less massive halos, while red and green galaxies have larger satellite fractions, and preferentially reside in virialized structures. If blue sequence galaxies migrate to the red sequence via processes like mergers or quenching that take them through the green valley, such a transformation may be accompanied by a change in environment in addition to any change in luminosity and color. Comment: accepted by MNRAS
Monthly Notices of the Royal Astronomical Society 04/2010; · 5.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have discovered recent star formation in the outermost portion ((1-4) × R 25) of the nearby lenticular (S0) galaxy NGC 404 using Galaxy Evolution Explorer UV imaging. FUV-bright sources are strongly concentrated within the galaxy's H I ring (formed by a merger event according to del Río et al.), even though the average gas density is dynamically subcritical. Archival Hubble Space Telescope imaging reveals resolved upper main-sequence stars and conclusively demonstrates that the UV light originates from recent star formation activity. We present FUV, NUV radial surface brightness profiles, and integrated magnitudes for NGC 404. Within the ring, the average star formation rate (SFR) surface density (ΣSFR) is ~2.2 × 10–5 M ☉ yr–1 kpc–2. Of the total FUV flux, 70% comes from the H I ring which is forming stars at a rate of 2.5 × 10–3 M ☉ yr–1. The gas consumption timescale, assuming a constant SFR and no gas recycling, is several times the age of the universe. In the context of the UV-optical galaxy color-magnitude diagram, the presence of the star-forming H I ring places NGC 404 in the green valley separating the red and blue sequences. The rejuvenated lenticular galaxy has experienced a merger-induced, disk-building excursion away from the red sequence toward bluer colors, where it may evolve quiescently or (if appropriately triggered) experience a burst capable of placing it on the blue/star-forming sequence for up to ~1 Gyr. The green valley galaxy population is heterogeneous, with most systems transitioning from blue to red but others evolving in the opposite sense due to acquisition of fresh gas through various channels.
The Astrophysical Journal Letters 04/2010; 714(1):L171. · 6.35 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use multi-wavelength, matched aperture, integrated photometry from the Galaxy Evolution Explorer (GALEX), the Sloan Digital Sky Survey, and the RC3 to estimate the physical properties of 166 nearby galaxies hosting 168 well-observed Type Ia supernovae (SNe Ia). The ultraviolet (UV) imaging of local SN Ia hosts from GALEX allows a direct comparison with higher-redshift hosts measured at optical wavelengths that correspond to the rest-frame UV. Our data corroborate well-known features that have been seen in other SN Ia samples. Specifically, hosts with active star formation produce brighter and slower SNe Ia on average, and hosts with luminosity-weighted ages older than 1 Gyr produce on average more faint, fast, and fewer bright, slow SNe Ia than younger hosts. New results include that in our sample, the faintest and fastest SNe Ia occur only in galaxies exceeding a stellar mass threshold of ~1010 M ☉, leading us to conclude that their progenitors must arise in populations that are older and/or more metal rich than the general SN Ia population. A low host extinction subsample hints at a residual trend in peak luminosity with host age, after correcting for light-curve shape, giving the appearance that older hosts produce less-extincted SNe Ia on average. This has implications for cosmological fitting of SNe Ia, and suggests that host age could be useful as a parameter in the fitting. Converting host mass to metallicity and computing 56Ni mass from the supernova light curves, we find that our local sample is consistent with a model that predicts a shallow trend between stellar metallicity and the 56Ni mass that powers the explosion, but we cannot rule out the absence of a trend. We measure a correlation between 56Ni mass and host age in the local universe that is shallower and not as significant as that seen at higher redshifts. The details of the age-56Ni mass correlations at low and higher redshift imply a luminosity-weighted age threshold of ~3 Gyr for SN Ia hosts, above which they are less likely to produce SNe Ia with 56Ni masses above ~0.5 M ☉.
The Astrophysical Journal 12/2009; 707(2):1449. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: One possible channel for the formation of dwarf galaxies involves birth in the tidal tails of interacting galaxies. We report the detection of a bright UV tidal tail and several young tidal dwarf galaxy (TDG) candidates in the post-merger galaxy NGC 4922 in the Coma cluster. Based on a two-component population model (combining young and old stellar populations), we find that the light of tidal tail predominantly comes from young stars (a few Myr old). The Galaxy Evolution Explorer ultraviolet data played a critical role in the parameter (age and mass) estimation. Our stellar mass estimates of the TDG candidates are ~106–7 M ☉, typical for dwarf galaxies.
The Astronomical Journal 11/2009; 138(6):1911. · 4.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Studying the environments of 0.4 < z < 1.2 ultraviolet (UV)-selected galaxies, as examples of extreme star-forming galaxies (with star formation rates (SFRs) in the range of 3-30 M ☉ yr–1), we explore the relationship between high rates of star formation, host halo mass, and pair fractions. We study the large- and small-scale environments of local ultraviolet luminous galaxies (UVLGs) by measuring angular correlation functions. We cross-correlate these systems with other galaxy samples: a volume-limited sample (ALL), a blue luminous galaxy sample, and a luminous red galaxy (LRG) sample. We determine the UVLG comoving correlation length to be r 0 = 4.8+11.6 –2.4 h –1 Mpc at z = 1.0, which is unable to constrain the halo mass for this sample. However, we find that UVLGs form close (separation <30 kpc) pairs with the ALL sample, but do not frequently form pairs with LRGs. A rare subset of UVLGs, those with the highest FUV surface brightnesses, are believed to be local analogs of high-redshift Lyman break galaxies (LBGs) and are called Lyman break analogs (LBAs). LBGs and LBAs share similar characteristics (i.e., color, size, surface brightness, specific SFRs, metallicities, and dust content). Recent Hubble Space Telescope images of z ~ 0.2 LBAs show disturbed morphologies, signs of mergers and interactions. UVLGs may be influenced by interactions with other galaxies and we discuss this result in terms of other high star-forming, merging systems.
The Astrophysical Journal 06/2009; 699(2):1307. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We explore the age distribution of the globular cluster (GC) system of the nearby elliptical galaxy NGC 5128 using ultraviolet (UV) photometry from Galaxy Evolution Explorer (GALEX) observations, with UV - optical colors used as the age indicator. Most GCs in NGC 5128 follow the general trends of GCs in M31 and Milky Way in UV - optical color-color diagram, which indicates that the majority of GCs in NGC 5128 are old similar to the age range of old GCs in M31 and Milky Way. A large fraction of spectroscopically identified intermediate-age GC (IAGC) candidates with ~ 3-8 Gyr are not detected in the FUV passband. Considering the nature of intermediate-age populations being faint in the far-UV (FUV) passband, we suggest that many of the spectroscopically identified IAGCs may be truly intermediate in age. This is in contrast to the case of M31 where a large fraction of spectroscopically suggested IAGCs are detected in FUV and therefore may not be genuine IAGCs but rather older GCs with developed blue horizontal branch stars. Our UV photometry strengthens the results previously suggesting the presence of GC and stellar subpopulation with intermediate age in NGC 5128. The existence of IAGCs strongly indicates the occurrence of at least one more major star formation episode after a starburst at high redshift. Comment: 8 pages, 3 figures, accepted for ApJ Letter
The Astrophysical Journal 06/2009; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We measure the projected spatial correlation function wp (rp ) from a large sample combining Galaxy Evolution Explorer ultraviolet imaging with the Sloan Digital Sky Survey spectroscopic sample. We study the dependence of the clustering strength for samples selected on (NUV – r)abs color, specific star formation rate (SSFR), and stellar mass. We find that there is a smooth transition in the clustering of galaxies as a function of this color from weak clustering among blue galaxies to stronger clustering for red galaxies. The clustering of galaxies within the "green valley" has an intermediate strength, and is consistent with that expected from galaxy groups. The results are robust to the correction for dust extinction. The comparison with simple analytical modeling suggests that the halo occupation number increases with older star formation epochs. When splitting according to SSFR, we find that the SSFR is a more sensitive tracer of environment than stellar mass.
The Astrophysical Journal 06/2009; 698(2):1838. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: A dormant supermassive black hole lurking in the center of a galaxy will be revealed when a star passes close enough to be torn apart by tidal forces, and a flare of electromagnetic radiation is emitted when the bound fraction of the stellar debris falls back onto the black hole and is accreted. Although the tidal disruption of a star is a rare event in a galaxy, 10–4 yr–1, observational candidates have emerged in all-sky X-ray and deep ultraviolet (UV) surveys in the form of luminous UV/X-ray flares from otherwise quiescent galaxies. Here we present the third candidate tidal disruption event discovered in the Galaxy Evolution Explorer (GALEX) Deep Imaging Survey: a 1.6 × 1043 erg s–1 UV/optical flare from a star-forming galaxy at z = 0.1855. The UV/optical spectral energy distribution (SED) during the peak of the flare measured by GALEX and Palomar Large Field Camera imaging can be modeled as a single temperature blackbody with T bb = 1.7 × 105 K and a bolometric luminosity of 3 × 1045 erg s–1, assuming an internal extinction with E(B – V)gas = 0.3. The Chandra upper limit on the X-ray luminosity during the peak of the flare, LX (2 – 10 keV)<1041 erg s–1, is 2 orders of magnitude fainter than expected from the ratios of UV to X-ray flux density observed in active galaxies. We compare the light curves and broadband properties of all three tidal disruption candidates discovered by GALEX, and find that (1) the light curves are well fitted by the power-law decline expected for the fallback of debris from a tidally disrupted solar-type star and (2) the UV/optical SEDs can be attributed to thermal emission from an envelope of debris located at roughly 10 times the tidal disruption radius of a 107 M ☉ central black hole. We use the observed peak absolute optical magnitudes of the flares (–17.5>Mg > – 18.9) to predict the detection capabilities of upcoming optical synoptic surveys.
The Astrophysical Journal 05/2009; 698(2):1367. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We investigate the nature of the star formation law at low gas surface densities using a sample of 19 low surface brightness (LSB) galaxies with existing H I maps in the literature, UV imaging from the Galaxy Evolution Explorer satellite, and optical images from the Sloan Digital Sky Survey. All of the LSB galaxies have (NUV – r) colors similar to those for higher surface brightness star-forming galaxies of similar luminosity indicating that their average star formation histories are not very different. Based upon four LSB galaxies with both UV and far-infrared (FIR) data, we find FIR/UV ratios significantly less than 1, implying low amounts of internal UV extinction in LSB galaxies. We use the UV images and H I maps to measure the star formation rate (SFR) and hydrogen gas surface density within the same region for all the galaxies. The LSB galaxy star formation rate surface densities lie below the extrapolation of the power law fit to the SFR surface density as a function of the total gas density for higher surface brightness galaxies. Although there is more scatter, the LSB galaxies also lie below a second version of the star formation law in which the SFR surface density is correlated with the gas density divided by the orbital time in the disk. The downturn seen in both star formation laws is consistent with theoretical models that predict lower star formation efficiencies in LSB galaxies due to the declining molecular fraction with decreasing density.
The Astrophysical Journal 04/2009; 696(2):1834. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the first far-ultraviolet (FUV; λ ~ 1500 Å) image of the nearby peculiar SAB(r)bc galaxy NGC 3310. The small 15'' (945 pc) diameter circumnuclear starburst ring is the most luminous structure, producing 30% of the total observed FUV luminosity. Diffuse emission from the inner disk (20'' < R < 40'') contributes another ~20% of the observed FUV flux. A linear feature (the "arrow") appears to be a star-forming tidal feature. A diffuse arc observed at optical wavelengths (the "bow") is not visible in the FUV and is probably a tidally induced shell composed of older stars. Mean star formation rates range from 0.031 M☉ yr-1 kpc-2 in the arrow to 2.1 M☉ yr-1 kpc-2 at the brightest FUV source. The striking similarity between the R1/4 law behavior of the FUV and B-band surface brightness profiles, combined with the very blue colors of NGC 3310, strongly argues that the present morphology is the result of a global starburst triggered by a merger with a dwarf companion.
The Astrophysical Journal 01/2009; 473(1):L21. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: Images of the globular cluster M13 were obtained with the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission. A total of 488 stars are detected in the UIT B5 filter centered at 1620 Å. Of an uncrowded sample of 247 stars, 232 are well correlated with ground-based Johnson V photometry, which was provided by K. Montgomery (1995, private communication). A color-magnitude diagram, constructed from the (m1620 − V) color index and the m1620 magnitude, shows a well-populated, blue horizontal branch extending to a temperature in excess of 35,000 K. A King plot of radial density distribution is satisfied by an exponential of index α=3.0, which is consistent with Laget et al. Recent evolutionary models by Sweigart that include the effects of helium mixing into the hydrogen envelope of horizontal-branch stars, in addition to the latest canonical models, are considered when discussing the extreme blue horizontal-branch morphology. An offset toward a higher luminosity of stars near the zero-age horizontal branch in the 4.1 < logTeff < 4.3 region is also discussed.
The Astrophysical Journal 01/2009; 501(1):L67. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have used the GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of ~8 Å in the far-ultraviolet (FUV; 1350-1750 Å) and ~20 Å in the near-ultraviolet (NUV; 1950-2750 Å) for a systematic search of Lyα-emitting galaxies at low redshift. Our aim is to fill a gap between high-redshift surveys and a small set of objects studied in detail in the nearby universe. A blind search of 7018 spectra extracted in five deep exposures (5.65 deg2) has resulted in 96 Lyα-emitting galaxy candidates in the FUV domain after accounting for broad-line AGNs. The Lyα equivalent widths (EWs) are consistent with stellar population model predictions and show no trends as a function of UV color or UV luminosity, with the exception of a possible decrease in the most luminous objects that may be due to small-number statistics. The objects' distribution in EW is similar to that at z ~ 3, but their fraction among star-forming galaxies is smaller. Avoiding uncertain candidates, a subsample of 66 objects in the range 0.2 < z < 0.35 has been used to build a Lyα luminosity function (LF). The incompleteness due to objects with significant Lyα emission but a UV continuum too low for spectral extraction has been evaluated. A comparison with Hα LFs in the same redshift domain is consistent with an average Lyα/Hα of ~1 in about 15% of the star-forming galaxies. A comparison with high-redshift Lyα LFs implies an increase of the Lyα luminosity density by a factor of about 16 from z ~ 0.3 to z ~ 3. By comparison with the factor of 5 increase in the UV luminosity density in the same redshift range, this suggests an increase of the average Lyα escape fraction with redshift.
The Astrophysical Journal 12/2008; 680(2):1072. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present ultraviolet (UV) photometry of M31 globular clusters (GCs) found in 23 Galaxy Evolution Explorer (GALEX) images covering the entirety of M31. We detect 485 and 273 GCs (and GC candidates) in the near-ultraviolet (NUV) and far-ultraviolet (FUV), respectively. The UV properties of GCs have been analyzed using various combinations of UV–optical and optical–optical colors. Comparing M31 data with those of Galactic GCs in the UV with the aid of population models, we find that the age ranges of old GCs in M31 and the Galactic halo are similar. Three metal-rich ([Fe/H] > − 1) GCs in M31 produce significant FUV flux making their FUV–V colors unusually blue for their metallicities. These are thought to be analogs of the two peculiar Galactic GCs, NGC 6388 and NGC 6441, with extended blue HB stars. Based on the models incorporating helium enriched subpopulations in addition to the majority of the population that have a normal helium abundance, we suggest that even small fraction of super-helium-rich subpopulations in GCs can reproduce the observed UV bright metal-rich GCs. Young clusters in M31 show distinct UV and optical properties from GCs in Milky Way. Population models indicate that their typical age is less than ~2 Gyr and is consistent with the age derived from the most recent high-quality spectroscopic observations. A large fraction of young GCs have the kinematics of the thin, rapidly rotating disk component. Most GCs with bulge kinematics show old ages. The existence of young GCs on the outskirts of M31 disk suggests the occurrence of a significant recent star formation in the thin-disk of M31. We detect 12 (10) intermediate-age GC candidates in NUV (FUV) identified by previous spectroscopic observations. On the basis of comparing our UV photometry to population models, we suggest that some of spectroscopically identified intermediate-age GCs may not be truly intermediate in age, but rather older GCs that possess developed HB stars which contribute to enhanced UV flux as well as Balmer lines.
The Astrophysical Journal Supplement Series 12/2008; 173(2):643. · 16.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present neutral hydrogen, ultraviolet, optical, and near-infrared imaging, and optical spectroscopy, of Minkowski's Object (MO), a star-forming peculiar galaxy near NGC 541. The observations strengthen evidence that star formation in MO was triggered by the radio jet from NGC 541. Key new results are the discovery of a 4.9 × 108 M☉ double H I cloud straddling the radio jet downstream from MO, where the jet changes direction and decollimates; strong detections of MO, also showing double structure, in UV and Hα; and numerous H II regions and associated clusters in MO. In UV, MO resembles the radio-aligned, rest-frame UV morphologies in many high-redshift radio galaxies (HzRGs), also thought to be caused by jet-induced star formation. MO's stellar population is dominated by a 7.5 Myr old, 1.9 × 107 M☉ instantaneous burst, with a current star formation rate of 0.52 M☉ yr-1 (concentrated upstream from where the H I column density is high). This is unlike the jet-induced star formation in Centaurus A, where the jet interacts with preexisting cold gas; in MO, the H I may have cooled out of a warmer, clumpy intergalactic or interstellar medium as a result of jet interaction, followed by the collapse of the cooling clouds and subsequent star formation (consistent with numerical simulations). Since the radio source that triggered star formation in MO is much less luminous, and therefore more common than powerful HzRGs, and because the environment around MO is not particularly special in terms of abundant dense, cold gas, jet-induced star formation in the early universe might be even more prevalent than previously thought.
The Astrophysical Journal 12/2008; 647(2):1040. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the results from the analysis of optical spectra of 31 Hα-selected regions in the extended UV (XUV) disks of M83 (NGC 5236) and NGC 4625 recently discovered by GALEX. The spectra were obtained using IMACS at the Las Campanas Observatory 6.5 m Magellan I telescope and COSMIC at the Palomar 200 inch (5 m) telescope, respectively, for M83 and NGC 4625. The line ratios measured indicate nebular oxygen abundances (derived from the R23 parameter) of the order of Z☉/5-Z☉/10. For most emission-line regions analyzed the line fluxes and ratios measured are best reproduced by models of photoionization by single stars with masses in the range 20-40 M☉ and oxygen abundances comparable to those derived from the R23 parameter. We find indications for a relatively high N/O abundance ratio in the XUV disk of M83. Although the metallicities derived imply that these are not the first stars formed in the XUV disks, such a level of enrichment could be reached in young spiral disks only 1 Gyr after these first stars would have formed. The amount of gas in the XUV disks allows maintaining the current level of star formation for at least a few Gyr.
The Astrophysical Journal 12/2008; 661(1):115. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We use the Galaxy Evolution Explorer (GALEX) Medium and All-Sky Imaging Survey (MIS and AIS) data from the first public data release (GR1), matched to the Sloan Digital Sky Survey (SDSS) DR3 catalog, to perform source classification. The GALEX surveys provide photometry in far- and near-UV bands and the SDSS in five optical bands (u, g, r, i, z). The GR1/DR3 overlapping areas are 363 (86) deg2 for the GALEX AIS (MIS), for sources within the 0.5° central area of the GALEX fields. Our sample covers mostly |b| > 30° Galactic latitudes. We present statistical properties of the GALEX-SDSS matched sources catalog, containing >2 × 106 objects detected in at least one UV band. We classify the matched sources by comparing the seven-band photometry to model colors constructed for different classes of astrophysical objects. For sources with photometric errors <0.3 mag, the corresponding typical AB-magnitude limits are mFUV ~ 21.5, mNUV ~ 22.5 for AIS, and mFUV ~ 24, mNUV ~ 24.5 for MIS. At AIS depth, the number of Galactic and extragalactic objects are comparable, but the latter predominate in the MIS. On the basis of our stellar models, we estimate the GALEX surveys detect hot white dwarfs throughout the Milky Way halo (down to a radius of 0.04 R☉ at MIS depth), providing an unprecedented improvement in the Galactic WD census. Their observed surface density is consistent with Milky Way model predictions. We also select low-redshift QSO candidates, extending the known QSO samples to lower magnitudes, and providing z ≈ 1 candidates for detailed z ≈ 1 follow-up investigations. SDSS optical spectra available for a large subsample confirm the classification for the photometrically selected candidates with 97% purity for single hot stars, 45% (AIS) or 31% (MIS) for binaries containing a hot star and a cooler companion, and about 85% for QSOs.
The Astrophysical Journal Supplement Series 12/2008; 173(2):659. · 16.24 Impact Factor
[show abstract][hide abstract] ABSTRACT: We test the empirical relation between ultraviolet color and attenuation as derived for starburst galaxies with a wide assortment of galaxy types detected by the Galaxy Evolution Explorer and find that it systematically overestimates the far-ultraviolet attenuation of our sample by ~0.5 mag. Our efforts to find an additional parameter that could improve the starburst reddening relation were unsuccessful. In particular, UV - Ks colors (in nonmatching apertures) show no correlation with the offset from the starburst reddening relation, suggesting either that UV - Ks colors are a poor tracer of present to past average star formation history (the "b" parameter) or that the intrinsic dust distribution/geometry may be responsible for moving galaxies off the correlation. It is possible to reduce the systematic overestimate of AFUV by using the linear correlation derived from our sample, which simply lowers the starburst predicted values of AFUV by 0.58 mag. The scatter, however, remains large at 0.89 mag.
The Astrophysical Journal 12/2008; 619(1):L55. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We select far-infrared (FIR: 60 μm) and far-ultraviolet (FUV: 530 Å) samples of nearby galaxies in order to discuss the biases encountered by monochromatic surveys (FIR or FUV). Very different volumes are sampled by each selection, and much care is taken to apply volume corrections to all the analyses. The distributions of the bolometric luminosity of young stars are compared for both samples: they are found to be consistent with each other for galaxies of intermediate luminosities, but some differences are found for high (>5 × 1010 L☉) luminosities. The shallowness of the IRAS survey prevents us from securing a comparison at low luminosities (<2 × 109 L☉). The ratio of the total infrared (TIR) luminosity to the FUV luminosity is found to increase with the bolometric luminosity in a similar way for both samples up to 5 × 1010 L☉. Brighter galaxies are found to have a different behavior according to their selection: the LTIR/LFUV ratio of the FUV-selected galaxies brighter than 5 × 1010 L☉ reaches a plateau, whereas LTIR/LFUV continues to increase with the luminosity of bright galaxies selected in FIR. The volume-averaged specific star formation rate (SFR per unit galaxy stellar mass, SSFR) is found to decrease toward massive galaxies within each selection. The mean values of the SSFR are found to be larger than those measured for optical and NIR-selected samples over the whole mass range for the FIR selection, and for masses larger than 1010 M☉ for the FUV selection. Luminous and massive galaxies selected in FIR appear as active as galaxies with similar characteristics detected at z ~ 0.7.
The Astrophysical Journal Supplement Series 12/2008; 173(2):404. · 16.24 Impact Factor