23 research outputs found
On the Classification of UGC1382 as a Giant Low Surface Brightness Galaxy
We provide evidence that UGC1382, long believed to be a passive elliptical
galaxy, is actually a giant low surface brightness (GLSB) galaxy which rivals
the archetypical GLSB Malin 1 in size. Like other GLSB galaxies, it has two
components: a high surface brightness disk galaxy surrounded by an extended low
surface brightness (LSB) disk. For UGC1382, the central component is a
lenticular system with an effective radius of 6 kpc. Beyond this, the LSB disk
has an effective radius of ~38 kpc and an extrapolated central surface
brightness of ~26 mag/arcsec^2. Both components have a combined stellar mass of
~8x10^10 M_sun, and are embedded in a massive (10^10 M_sun) low-density (<3
M_sun/pc^2) HI disk with a radius of 110 kpc, making this one of the largest
isolated disk galaxies known. The system resides in a massive dark matter halo
of at least 2x10^12 M_sun. Although possibly part of a small group, its low
density environment likely plays a role in the formation and retention of the
giant LSB and HI disks. We model the spectral energy distributions and find
that the LSB disk is likely older than the lenticular component. UGC1382 has
UV-optical colors typical of galaxies transitioning through the green valley.
Within the LSB disk are spiral arms forming stars at extremely low
efficiencies. The gas depletion time scale of ~10^11 yr suggests that UGC1382
may be a very long term resident of the green valley. We find that the
formation and evolution of the LSB disk is best explained by the accretion of
gas-rich LSB dwarf galaxies.Comment: 17 pages, 16 figures, 4 tables; accepted to the Astrophysical Journa
The UV-Optical Galaxy Color-Magnitude Diagram. I. Basic Properties
We have analyzed the bivariate distribution of galaxies as a function of ultraviolet-optical colors and absolute magnitudes in the local universe. The sample consists of galaxies with redshifts and optical photometry from the Sloan Digital Sky Survey (SDSS) main galaxy sample matched with detections in the near-ultraviolet (NUV) and far-ultraviolet (FUV) bands in the Medium Imaging Survey being carried out by the Galaxy Evolution Explorer (GALEX) satellite. In the (NUV â r)_(0.1) versus M_(r,0.1) galaxy color-magnitude diagram, the galaxies separate into two well-defined blue and red sequences. The (NUV â r)_(0.1) color distribution at each M_(r,0.1) is not well fit by the sum of two Gaussians due to an excess of galaxies in between the two sequences. The peaks of both sequences become redder with increasing luminosity, with a distinct blue peak visible up to M_(r,0.1) ~ â 23. The r_(0.1)-band luminosity functions vary systematically with color, with the faint-end slope and characteristic luminosity gradually increasing with color. After correcting for attenuation due to dust, we find that approximately one-quarter of the color variation along the blue sequence is due to dust, with the remainder due to star formation history and metallicity. Finally, we present the distribution of galaxies as a function of specific star formation rate and stellar mass. The specific star formation rates imply that galaxies along the blue sequence progress from low-mass galaxies with star formation rates that increase somewhat with time to more massive galaxies with a more or less constant star formation rate. Above a stellar mass of ~10^(10.5) M_â, galaxies with low ratios of current to past averaged star formation rate begin to dominate
The Young and the Dustless: Interpreting Radio Observations of UltraViolet Luminous Galaxies
Ultraviolet Luminous Galaxies (UVLGs) have been identified as intensely
star-forming, nearby galaxies. A subset of these, the supercompact UVLGs, are
believed to be local analogs of high redshift Lyman Break Galaxies. Here we
investigate the radio continuum properties of this important population for the
first time. We have observed 42 supercompact UVLGs with the VLA, all of which
have extensive coverage in the UV/optical by GALEX and SDSS. Our analysis
includes comparison samples of multiwavelength data from the Spitzer First Look
Survey and from the SDSS-Galex matched catalogs. In addition we have Spitzer
MIPS data for 24 of our galaxies and find that they fall on the radio-FIR
correlation of normal star-forming galaxies. We find that our galaxies have
lower radio-to-UV ratios and lower Balmer decrements than other local galaxies
with similar (high) star formation rates. Optical spectra show they have lower
Dn(4000) and HdeltaA indices, higher Hbeta emission-line equivalents widths,
and higher [OIII]5007/Hbeta emission-line ratios than normal star forming
galaxies. Comparing these results to galaxy spectral evolution models we
conclude that supercompact UVLGs are distinguished from normal star forming
galaxies firstly by their high specific star formation rates. Moreover,
compared to other types of galaxies with similar star formation rates, they
have significantly less dust attenuation. In both regards they are similar to
Lyman Break Galaxies. This suggests that the process that causes star formation
in the supercompact UVLGs differs from other local star forming galaxies, but
may be similar to Lyman Break Galaxies.Comment: 37 pages, 13 figures, accepted and scheduled to appear in ApJS
December 2007 (GALEX Special Issue
Extinction Corrected Star Formation Rates Empirically Derived from Ultraviolet-Optical Colors
Using a sample of galaxies from the Sloan Digital Sky Survey spectroscopic
catalog with measured star-formation rates (SFRs) and ultraviolet (UV)
photometry from the GALEX Medium Imaging Survey, we derived empirical linear
correlations between the SFR to UV luminosity ratio and the UV-optical colors
of blue sequence galaxies. The relations provide a simple prescription to
correct UV data for dust attenuation that best reconciles the SFRs derived from
UV and emission line data. The method breaks down for the red sequence
population as well as for very blue galaxies such as the local ``supercompact''
UV luminous galaxies and the majority of high redshift Lyman Break Galaxies
which form a low attenuation sequence of their own.Comment: 20 pages, 11 figures, accepted for publication in the ApJS GALEX
special issu
Clustering Properties of restframe UV selected galaxies I: the correlation length derived from GALEX data in the local Universe
We present the first measurements of the angular correlation function of
galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the
GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions.
The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22,
yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm
0.6 Mpc and no significant trend is seen for this value as a function of the
limiting apparent magnitude or between the GALEX bands. This estimate is close
to that found from samples of blue galaxies in the local universe selected in
the visible, and similar to that derived at z ~ 3 for LBGs with similar rest
frame selection criteria. This result supports models that predict anti-biasing
of star forming galaxies at low redshift, and brings an additional clue to the
downsizing of star formation at z<1.Comment: Accepted for publication in GALEX Special ApJs, December 200
Clustering Properties of restframe UV selected galaxies II: Migration of Star Formation sites with cosmic time from GALEX and CFHTLS
We analyze the clustering properties of ultraviolet selected galaxies by
using GALEX-SDSS data at z<0.6 and CFHTLS deep u' imaging at z=1. These
datasets provide a unique basis at z< 1 which can be directly compared with
high redshift samples built with similar selection criteria. We discuss the
dependence of the correlation function parameters (r0, delta) on the
ultraviolet luminosity as well as the linear bias evolution. We find that the
bias parameter shows a gradual decline from high (b > 2) to low redshift (b ~
0.79^{+0.1}_{-0.08}). When accounting for the fraction of the star formation
activity enclosed in the different samples, our results suggest that the bulk
of star formation migrated from high mass dark matter halos at z>2 (10^12 <
M_min < 10^13 M_sun, located in high density regions), to less massive halos at
low redshift (M_min < 10^12 M_sun, located in low density regions). This result
extends the ``downsizing'' picture (shift of the star formation activity from
high stellar mass systems at high z to low stellar mass at low z) to the dark
matter distribution.Comment: Accepted for Publication in the Special GALEX Ap. J. Supplement,
December 2007 Version with full resolution fig1 available at
http://taltos.pha.jhu.edu/~sebastien/papers/Galex_p2.ps.g
Ultraviolet through Infrared Spectral Energy Distributions from 1000 SDSS Galaxies: Dust Attenuation
The meaningful comparison of models of galaxy evolution to observations is
critically dependent on the accurate treatment of dust attenuation. To
investigate dust absorption and emission in galaxies we have assembled a sample
of ~1000 galaxies with ultraviolet (UV) through infrared (IR) photometry from
GALEX, SDSS, and Spitzer and optical spectroscopy from SDSS. The ratio of IR to
UV emission (IRX) is used to constrain the dust attenuation in galaxies. We use
the 4000A break as a robust and useful, although coarse, indicator of star
formation history (SFH). We examine the relationship between IRX and the UV
spectral slope (a common attenuation indicator at high-redshift) and find
little dependence of the scatter on 4000A break strength. We construct average
UV through far-IR spectral energy distributions (SEDs) for different ranges of
IRX, 4000A break strength, and stellar mass (M_*) to show the variation of the
entire SED with these parameters. When binned simultaneously by IRX, 4000A
break strength, and M_* these SEDs allow us to determine a low resolution
average attenuation curve for different ranges of M_*. The attenuation curves
thus derived are consistent with a lambda^{-0.7} attenuation law, and we find
no significant variations with M_*. Finally, we show the relationship between
IRX and the global stellar mass surface density and gas-phase-metallicity.
Among star forming galaxies we find a strong correlation between IRX and
stellar mass surface density, even at constant metallicity, a result that is
closely linked to the well-known correlation between IRX and star-formation
rate.Comment: 12 pages, 8 figures, 2 tables, appearing in the Dec 2007 GALEX
special issue of ApJ Supp (29 papers
The Extreme Hosts of Extreme Supernovae
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 designation
The UV 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 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.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
The UV-Optical Color Magnitude Diagram II: Physical Properties and Morphological Evolution On and Off of a Star-Forming Sequence
We use the UV-optical color magnitude diagram in combination with
spectroscopic and photometric measurements derived from the SDSS spectroscopic
sample to measure the distribution of galaxies in the local universe (z<0.25)
and their physical properties as a function of specific star formation rate
(SSFR) and stellar mass. Throughout this study our emphasis is on the
properties of galaxies on and off of a local "star-forming sequence." We
discuss how the physical characteristics of galaxies along this sequence are
related to scaling relations typically derived for galaxies of different
morphological types. We find, among other trends that our measure of the star
formation rate surface density is nearly constant along this sequence. We
discuss this result and implications for galaxies at higher redshift. For the
first time, we report on measurements of the local UV luminosity function
versus galaxy structural parameters as well as inclination. We also split our
sample into disk-dominated and bulge-dominated subsamples using the i-band
Sersic index and find that disk-dominated galaxies occupy a very tight locus in
SSFR vs. stellar mass space while bulge-dominated galaxies display a much
larger spread of SSFR at fixed stellar mass. A significant fraction of galaxies
with SSFR and SF surface density above those on the "star-forming sequence" are
bulge-dominated. We can use our derived distribution functions to ask whether a
significant fraction of these galaxies may be experiencing a final episode of
star formation (possibly induced by a merger or other burst), soon to be
quenched, by determining whether this population can explain the growth rate of
the non-star-forming galaxies on the "red sequence." (Abridged)Comment: 30 pages, 28 figures, scheduled to appear as part of the GALEX
Special Ap.J.Suppl., December, 2007 (29 papers