Tom A. Barlow

California Institute of Technology, Pasadena, California, United States

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Publications (157)889 Total impact

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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that might exist in all quasars and could play a major role in feedback to galaxy evolution. The viability of BAL outflows as a feedback mechanism depends on their kinetic energies, as derived from the outflow velocities, column densities, and distances from the central quasar. We estimate these quantities for the quasar, Q1413+1143 (redshift $z_e = 2.56$), aided by the first detection of PV $\lambda\lambda$1118,1128 BAL variability in a quasar. In particular, PV absorption at velocities where the CIV trough does not reach zero intensity implies that the CIV BAL is saturated and the absorber only partially covers the background continuum source (with characteristic size <0.01 pc). With the assumption of solar abundances, we estimate that the total column density in the BAL outflow is log N_H > 22.3 (cm^-2). Variability in the PV and saturated CIV BALs strongly disfavors changes in the ionization as the cause of the BAL variability, but supports models with high-column density BAL clouds moving across our lines of sight. The observed variability time of 1.6 yr in the quasar rest frame indicates crossing speeds >750 km/s and a radial distance from the central black hole of <3.5 pc, if the crossing speeds are Keplerian. The total outflow mass is ~4100 M_solar, the kinetic energy ~4x10^54 erg, and the ratio of the outflow kinetic energy luminosity to the quasar bolometric luminosity is ~0.02 (at the minimum column density and maximum distance), which might be sufficient for important feedback to the quasar's host galaxy.
    Monthly Notices of the Royal Astronomical Society 07/2014; 444(2). DOI:10.1093/mnras/stu1502 · 5.23 Impact Factor
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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra are prominent signatures of high-velocity outflows, which might be present in all quasars and could be a major contributor to feedback to galaxy evolution. Studying the variability in these BALs allows us to further our understanding of the structure, evolution, and basic physical properties of the outflows. This is the third paper in a series on a monitoring programme of 24 luminous BAL quasars at redshifts 1.2 < z < 2.9. We focus here on the time-scales of variability in CIV 1549A BALs in our full multi-epoch sample, which covers time-scales from 0.02-8.7 yr in the quasar rest-frame. Our sample contains up to 13 epochs of data per quasar, with an average of 7 epochs per quasar. We find that both the incidence and the amplitude of variability are greater across longer time-scales. Part of our monitoring programme specifically targeted half of these BAL quasars at rest-frame time-scales <2 months. This revealed variability down to the shortest time-scales we probe (8-10 days). Observed variations in only portions of BAL troughs or in lines that are optically thick suggest that at least some of these changes are caused by clouds (or some type of outflow substructures) moving across our lines of sight. In this crossing cloud scenario, the variability times constrain both the crossing speeds and the absorber locations. Typical variability times of order ~1 year indicate crossing speeds of a few thousand km/s and radial distances near ~1 pc from the central black hole. However, the most rapid BAL changes occurring in 8-10 days require crossing speeds of 17 000 - 84 000 km/s and radial distances of only 0.001-0.02 pc. These speeds are similar to or greater than the observed radial outflow speeds, and the inferred locations are within the nominal radius of the broad emission line region.
    Monthly Notices of the Royal Astronomical Society 11/2012; 429(3). DOI:10.1093/mnras/sts427 · 5.23 Impact Factor
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    ABSTRACT: The PTF (ATEL #1964, #3253; http://www.astro.caltech.edu/ptf/ ; Law et al. 2009, PASP, 121, 1395; Rau et al. 2009, PASP, 121, 1334) reports the discovery of 27 new supernovae. PTF discoveries are made by autonomous PTF software (Bloom et al. 2011, http://adsabs.harvard.edu/abs/2011arXiv1106.5491B ), as well as by the Galaxy Zoo Supernova Project (Smith et al. 2011, MNRAS, 412, 1309; http://supernova.galaxyzoo.org ).
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    ABSTRACT: The PTF (ATEL #1964, #3253; http://www.astro.caltech.edu/ptf/ ; Law et al. 2009, PASP, 121, 1395; Rau et al. 2009, PASP, 121, 1334) reports the discovery of 20 new supernovae. PTF discoveries are made by autonomous PTF software (Bloom et al. 2011, http://adsabs.harvard.edu/abs/2011arXiv1106.5491B ), as well as by the Galaxy Zoo Supernova Project (Smith et al. 2011, MNRAS, 412, 1309; http://supernova.galaxyzoo.org ).
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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. Studying the variability in these BALs can help us understand the structure, evolution, and basic physical properties of these outflows. We are conducting a BAL monitoring program, which so far includes 163 spectra of 24 luminous quasars, covering time-scales from \sim 1 week to 8 years in the quasar rest-frame. We investigate changes in both the CIV {\lambda}1550 and SiIV {\lambda}1400 BALs, and we report here on some of the results from this program.
    Proceedings of the International Astronomical Union 04/2012; DOI:10.1017/S1743921310006782
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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra indicate high-velocity outflows that may be present in all quasars and could be an important contributor to feedback to their host galaxies. Variability studies of BALs help illuminate the structure, evolution, and basic physical properties of the outflows. Here we present further results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2 < z < 2.9. We directly compare the variabilities in the CIV 1549 and SiIV 1400 absorption to try to ascertain the cause(s) of the variability. We find that SiIV BALs are more likely to vary than CIV BALs. When looking at flow speeds >-20 000 km/s, 47 per cent of quasars exhibited SiIV variability while 31 per cent exhibited CIV variability. Furthermore, ~50 per cent of the variable SiIV regions did not have corresponding CIV variability at the same velocities. When both CIV and SiIV varied, those changes always occurred in the same sense (either getting weaker or stronger). We also include our full data set so far in this paper, which includes up to 10 epochs of data per quasar. The multi-epoch data show that the BAL changes were not generally monotonic across the full ~5 to ~8 yr time span of our observations, suggesting that the characteristic time-scale for significant line variations, and (perhaps) for structural changes in the outflows, is less than a few years. Coordinated variabilities between absorption regions at different velocities in individual quasars seems to favor changing ionization of the outflowing gas as the cause of the observed BAL variability. However, variability in limited portions of broad troughs fits naturally in a scenario where movements of individual clouds, or substructures in the flow, across our lines-of-sight cause the absorption to vary. The actual situation may be a complex mixture of changing ionization and cloud movements.
    Monthly Notices of the Royal Astronomical Society 03/2012; 422(4). DOI:10.1111/j.1365-2966.2012.20846.x · 5.23 Impact Factor
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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. Studying the variability in these BALs can help us understand the structure, evolution, and basic physical properties of these outflows. We are conducting a BAL monitoring program, which so far includes 163 spectra of 24 luminous quasars at z=1.2-2.9, covering time-scales from 1 week to 8 years in the quasar rest-frame. We investigate changes in both the CIV BALs and the SiIV BALs, and we see a variety of phenomena, including some BALs with dramatic variability over a wide range in outflow velocities and other BALs that did not change at all over the entire observation period. Variability generally occurred in only portions of BAL troughs. The fraction of quasars with CIV BAL variability increased as we added more observing epochs to our sample, with 88% of the quasars showing CIV BAL variability. When comparing CIV to SiIV BAL variability, we found that SiIV BALs are more likely to vary than CIV BALs. When both the CIV and SiIV BALs vary within the same quasar, they always vary in the same sense (with both lines either getting stronger or weaker). We have new data that probe the shortest time-scales (<1 month rest-frame), which provide constraints on the sizes and location of the outflowing gas. We also present some preliminary results on the study of key diagnostic lines, such as PV λ1118, 1128. Studying these lines provides crucial constraints on the total column densities in the flows, which we can then use to estimate their mass outflow rates and kinetic energy yields. These quantities will help determine the viability of these outflows as a feedback mechanism.
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    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. DOI:10.1088/0004-637X/730/2/88 · 6.28 Impact Factor
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    ABSTRACT: Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. The variability of BALs can help us understand the structure, evolution, and basic physical properties of the outflows. Here we report on our first results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2<z<2.9, focusing on C IV 1549 BAL variability in two different time intervals: 4 to 9 months (short-term) and 3.8 to 7.7 years (long-term) in the quasar rest-frame. We find that 39% (7/18) of the quasars varied in the short-term, whereas 65% (15/23) varied in the long-term, with a larger typical change in strength in the long-term data. The variability occurs typically in only portions of the BAL troughs. The components at higher outflow velocities are more likely to vary than those at lower velocities, and weaker BALs are more likely to vary than stronger BALs. The fractional change in BAL strength correlates inversely with the strength of the BAL feature, but does not correlate with the outflow velocity. Both the short-term and long-term data indicate the same trends. The observed behavior is most readily understood as a result of the movement of clouds across the continuum source. If the crossing speeds do not exceed the local Keplerian velocity, then the observed short-term variations imply that the absorbers are <6 pc from the central quasar. Comment: 14 pages, 7 figures, accepted for publication in MNRAS
    Monthly Notices of the Royal Astronomical Society 12/2010; 413(2). DOI:10.1111/j.1365-2966.2010.18185.x · 5.23 Impact Factor
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    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; 727(1). DOI:10.1088/0004-637X/727/1/15 · 6.28 Impact Factor
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    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; DOI:10.1111/j.1365-2966.2010.16908.x · 5.23 Impact Factor
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    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. DOI:10.1088/2041-8205/714/1/L171 · 5.60 Impact Factor
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    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. DOI:10.1088/0004-637X/707/2/1449 · 6.28 Impact Factor
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    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. DOI:10.1088/0004-6256/138/6/1911 · 4.05 Impact Factor
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    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. DOI:10.1088/0004-637X/699/2/1307 · 6.28 Impact Factor
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    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; 700(1). DOI:10.1088/0004-637X/700/1/L11 · 6.28 Impact Factor
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    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. DOI:10.1088/0004-637X/698/2/1838 · 6.28 Impact Factor
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    ABSTRACT: We present a detailed study of the Galaxy Evolution Explorer's photometric catalogs with special focus on the statistical properties of the All-sky and Medium Imaging Surveys. We introduce the concept of primaries to resolve the issue of multiple detections and follow a geometric approach to define clean catalogs with well-understood selection functions. We cross-identify the GALEX sources (GR2+3) with Sloan Digital Sky Survey (DR6) observations, which indirectly provides an invaluable insight about the astrometric model of the UV sources and allows us to revise the band merging strategy. We derive the formal description of the GALEX footprints as well as their intersections with the SDSS coverage along with analytic calculations of their areal coverage. The crossmatch catalogs are made available for the public. We conclude by illustrating the implementation of typical selection criteria in SQL for catalog subsets geared toward statistical analyses, e.g., correlation and luminosity function studies.
    The Astrophysical Journal 05/2009; 694(2). DOI:10.1088/0004-637X/694/2/1281 · 6.28 Impact Factor
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    ABSTRACT: Our initial search for variability within the time-tagged source photon lists was performed with data generated from 1802 individual sky-field images contained in the GALEX data release IR1.1+GR1. (2 data files).

Publication Stats

5k Citations
889.00 Total Impact Points

Institutions

  • 2000–2014
    • California Institute of Technology
      • • Infrared Processing and Analysis Center
      • • Division of Physics, Mathematics, and Astronomy
      • • Spitzer Science Center
      Pasadena, California, United States
  • 2012
    • Columbia University
      New York, New York, United States
  • 2004–2009
    • Yonsei University
      • Center for Space Astrophysics
      Sŏul, Seoul, South Korea
  • 2008
    • Tohoku University
      • Astronomical Institute
      Sendai, Kagoshima-ken, Japan
  • 2007
    • University of Hawaiʻi at Hilo
      Hilo, Hawaii, United States
    • Carnegie Institution for Science
      Washington, West Virginia, United States
  • 2006
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States
  • 1989
    • University of California, San Diego
      • Center for Astrophysics and Space Sciences (CASS)
      San Diego, CA, United States