274 research outputs found
UV-optical colors as probes of early-type galaxy evolution
We have studied âŒ2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range O <z <0.1.1. Combining GALEXUV photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least âŒ30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical ACDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 <z <0.11 have ⌠1 % -3 % of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is âŒ300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r <5.5) seen in a significant fraction (âŒ30%) of our observed sample.Peer reviewe
The GALEX Time Domain Survey. I. Selection and Classification of Over a Thousand Ultraviolet Variable Sources
We present the selection and classification of over a thousand ultraviolet (UV) variable sources discovered in
âŒ40 deg^2 of GALEX Time Domain Survey (TDS) NUV images observed with a cadence of 2 days and a baseline
of observations of âŒ3 years. The GALEX TDS fields were designed to be in spatial and temporal coordination with
the Pan-STARRS1 Medium Deep Survey, which provides deep optical imaging and simultaneous optical transient
detections via image differencing.We characterize the GALEX photometric errors empirically as a function of mean
magnitude, and select sources that vary at the 5Ï level in at least one epoch. We measure the statistical properties
of the UV variability, including the structure function on timescales of days and years. We report classifications for
the GALEX TDS sample using a combination of optical host colors and morphology, UV light curve characteristics,
and matches to archival X-ray, and spectroscopy catalogs. We classify 62% of the sources as active galaxies (358
quasars and 305 active galactic nuclei), and 10% as variable stars (including 37 RR Lyrae, 53 M dwarf flare stars, and 2 cataclysmic variables). We detect a large-amplitude tail in the UV variability distribution for M-dwarf flare stars and RR Lyrae, reaching up to |Îm| = 4.6 mag and 2.9 mag, respectively. The mean amplitude of the structure function for quasars on year timescales is five times larger than observed at optical wavelengths. The remaining unclassified sources include UV-bright extragalactic transients, two of which have been spectroscopically confirmed to be a young core-collapse supernova and a flare from the tidal disruption of a star by dormant supermassive black hole. We calculate a surface density for variable sources in the UV with NUV 0.2 mag of âŒ8.0, 7.7, and 1.8 deg^(â2) for quasars, active galactic nuclei, and RR Lyrae stars, respectively. We also calculate a surface density rate in the UV for transient sources, using the effective survey time at the cadence appropriate to each class, of âŒ15 and 52 deg^(â2) yr^(â1) for M dwarfs and extragalactic transients, respectively
The infrared emission of ultraviolet selected galaxies from z = 0 to z=1
We select galaxies in UV rest-frame at z=0, z~0.7 and z~1 together with a
sample of LBGs at z~1, the samples are built in order to sample the same range
of luminosity at any redshift. The evolution of the IR and UV luminosities with
z is analysed for individual galaxies as well as in terms of luminosity
functions. The L_IR/L_UV ratio is used to measure dust attenuation. This ratio
does not exhibit a strong evolution with z for the bulk of our sample galaxies
but some trends are found for galaxies with a strong dust attenuation and for
UV luminous sources: galaxies with L_IR/L_UV>10 are more frequent at z>0 than
at z=0 and the largest values of L_IR/L_UV are found for UV faint objects;
conversely the most luminous galaxies of our samples (L_UV> 2 10^{10} L_sun$),
detected at z=1, exhibit a lower dust attenuation than the fainter ones.
L_IR/L_UV increases with the K rest-frame luminosity of the galaxies at all the
redshifts considered and shows a residual anti-correlation with L_UV. The most
massive and UV luminous galaxies exhibit quite large specific star formation
rates. LBGs exhibit systematically lower dust attenuation than UV selected
galaxies of same luminosity but similar specific star formation rates. The
analysis of the UV+IR luminosity functions leads to the conclusion that up to z
= 1 most of the star formation activity of UV selected galaxies is emitted in
IR. Whereas we are able to retrieve all the star formation from our UV
selection at z=0.7, at z = 1 we miss a large fraction of galaxies more luminous
than ~ 10^{11} L_sun. The effect is found larger for Lyman Break Galaxies.Comment: 13 pages. accepted for publication (Astronomy and Astrophysics
UV to IR SEDs of UV selected galaxies in the ELAIS fields: evolution of dust attenuation and star formation activity from z=0.7 to z=0.2
We study the ultraviolet to far-infrared (hereafter UV-to-IR) SEDs of a
sample of intermediate redshift (0.2 < z < 0.7) UV-selected galaxies from the
ELAIS-N1 and ELAIS-N2 fields by fitting a multi-wavelength dataset to a library
of GRASIL templates. Star formation related properties of the galaxies are
derived from the library of models by using the Bayesian statistics. We find a
decreasing presence of galaxies with low attenuation and low total luminosity
as redshift decreases, which does not hold for high total luminosity galaxies.
In addition the dust attenuation of low mass galaxies increases as redshift
decreases, and this trend seems to disappear for galaxies with M* > 10^11
M_sun. This result is consistent with a mass dependent evolution of the dust to
gas ratio, which could be driven by a mass dependent efficiency of star
formation in star forming galaxies. The specific star formation rates (SSFR)
decrease with increasing stellar mass at all redshifts, and for a given stellar
mass the SSFR decreases with decreasing redshift. The differences in the slope
of the M*--SSFR relation found between this work and others at similar redshift
could be explained by the adopted selection criteria of the samples which, for
a UV selected sample, favours blue, star forming galaxies.Comment: 21 figures, accepted for publication in Ap
UV Star Formation Rates in the Local Universe
We measure star formation rates of ~50,000 optically-selected galaxies in the
local universe (z~0.1), spanning a range from gas-rich dwarfs to massive
ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS
(optical) photometry to a library of population synthesis models that include
dust attenuation. For star-forming galaxies, our UV-based SFRs compare
remarkably well with those derived from SDSS H alpha. Deviations from perfect
agreement between these two methods are due to differences in the dust
attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for
galaxies with weak or no H alpha emission, and where H alpha is contaminated
with an emission from an AGN. We use full-SED SFRs to calibrate a simple
prescription that uses GALEX UV magnitudes to produce good SFRs for normal
star-forming galaxies. The specific SFR is considered as a function of stellar
mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and
for (3) galaxies without H alpha emission. We find that the three have distinct
star formation histories, with AGN lying intermediate between the star-forming
and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie
on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong
AGN appear to represent the massive continuation of this sequence. Weak AGN,
while also massive, have lower SFR, sometimes extending to the realm of
quiescent galaxies. We propose an evolutionary sequence for massive galaxies
that smoothly connects normal star-forming galaxies to quiescent (red sequence)
galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha
emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at
z=0.1 with smaller total error than previous measurements (abridged).Comment: Accepted for publication in ApJ (Special GALEX Supplement issue - Dec
2007). v2: Typo in Eq. 2 correcte
Star formation and dust extinction properties of local galaxies from AKARI-GALEX All-Sky Surveys: First results from most secure multiband sample from FUV to FIR
The AKARI All-Sky Survey provided the first bright point source catalog
detected at 90um. Starting from this catalog, we selected galaxies by matching
AKARI sources with those in the IRAS PSCz. Next, we have measured total GALEX
FUV and NUV flux densities. Then, we have matched this sample with SDSS and
2MASS galaxies. By this procedure, we obtained the final sample which consists
of 607 galaxies. If we sort the sample with respect to 90um, their average SED
shows a coherent trend: the more luminous at 90um, the redder the global SED
becomes. The M_r--NUV-r color-magnitude relation of our sample does not show
bimodality, and the distribution is centered on the green valley between the
blue cloud and red sequence seen in optical surveys. We have established
formulae to convert FIR luminosity from AKARI bands to the total infrared (IR)
luminosity L_TIR. With these formulae, we calculated the star formation
directly visible with FUV and hidden by dust. The luminosity related to star
formation activity (L_SF) is dominated by L_TIR even if we take into account
the far-infrared (FIR) emission from dust heated by old stars. At high star
formation rate (SFR) (> 20 Msun yr^-1), the fraction of directly visible SFR,
SFR_FUV, decreases. We also estimated the FUV attenuation A_FUV from
FUV-to-total IR (TIR) luminosity ratio. We also examined the L_TIR/L_FUV-UV
slope (FUV- NUV) relation. The majority of the sample has L_TIR/L_FUV ratios 5
to 10 times lower than expected from the local starburst relation, while some
LIRGs and all the ULIRGs of this sample have higher L_TIR/L_FUV ratios. We
found that the attenuation indicator L_TIR/L_FUV is correlated to the stellar
mass of galaxies, M*, but there is no correlation with specific SFR (SSFR),
SFR/M*, and dust attenuation L_TIR/L_FUV. (abridged)Comment: 13 pages, 18 figures, accepted for publication in A&
Deep GALEX Imaging of the HST/COSMOS Field: A First Look at the Morphology of z~0.7 Star-forming Galaxies
We present a study of the morphological nature of redshift z~0.7 star-forming
galaxies using a combination of HST/ACS, GALEX and ground-based images of the
COSMOS field. Our sample consists of 8,146 galaxies, 5,777 of which are
detected in the GALEX near-ultraviolet band down to a limiting magnitude of
25.5 (AB). We make use of the UV to estimate star formation rates, correcting
for the effect of dust using the UV-slope, and compute, from the ACS F814W
images, the C,A,S,G,M20 morphological parameters for all objects in our sample.
We observe a morphological bimodality in the galaxy population and show that it
has a strong correspondence with the FUV - g color bimodality. We conclude that
UV-optical color predominantly evolves concurrently with morphology. We observe
many of the most star-forming galaxies to have morphologies approaching that of
early-type galaxies, and interpret this as evidence that strong starburst
events are linked to bulge growth and constitute a process through which
galaxies can be brought from the blue to the red sequence while simultaneously
modifying their morphology accordingly. We conclude that the red sequence has
continued growing at z~<0.7. We also observe z~0.7 galaxies to have physical
properties similar to that of local galaxies, except for higher star formation
rates. Whence we infer that the dimming of star-forming galaxies is responsible
for most of the evolution in the star formation rate density of the Universe
since that redshift, although our data are also consistent with a mild number
evolution. [abridged]Comment: 29 pages including 22 figures. Accepted for publication in ApJS
COSMOS Special Issue. A copy of the paper with high resolution figures is
available at http://www.astro.columbia.edu/~michel/galex_cosmos_paper.pd
IR and UV Galaxies at z=0.6 -- Evolution of Dust Attenuation and Stellar Mass as Revealed by SWIRE and GALEX
We study dust attenuation and stellar mass of star-forming
galaxies using new SWIRE observations in IR and GALEX observations in UV. Two
samples are selected from the SWIRE and GALEX source catalogs in the
SWIRE/GALEX field ELAIS-N1-00 ( deg). The UV selected sample
has 600 galaxies with photometric redshift (hereafter photo-z) and NUV (corresponding to \rm L_{FUV} \geq 10^{9.6} L_\sun).
The IR selected sample contains 430 galaxies with mJy
(\rm L_{dust} \geq 10^{10.8} L_\sun) in the same photo-z range. It is found
that the mean ratios of the z=0.6 UV galaxies are
consistent with that of their z=0 counterparts of the same . For
IR galaxies, the mean ratios of the z=0.6 LIRGs (\rm
L_{dust} \sim 10^{11} L_\sun) are about a factor of 2 lower than local LIRGs,
whereas z=0.6 ULIRGs (\rm L_{dust} \sim 10^{12} L_\sun) have the same mean
ratios as their local counterparts. This is consistent
with the hypothesis that the dominant component of LIRG population has changed
from large, gas rich spirals at z to major-mergers at z=0. The stellar
mass of z=0.6 UV galaxies of \rm L_{FUV} \leq 10^{10.2} L_\sun is about a
factor 2 less than their local counterparts of the same luminosity, indicating
growth of these galaxies. The mass of z=0.6 UV lunmous galaxies (UVLGs: \rm
L_{FUV} > 10^{10.2} L_\sun) and IR selected galaxies, which are nearly
exclusively LIRGs and ULIRGs, is the same as their local counterparts.Comment: 27 pages, 8 figures, to be published in the Astrophysical Journal
Supplement series dedicated to GALEX result
Star formation in the nearby universe: the ultraviolet and infrared points of view
This work presents the main ultraviolet (UV) and far-infrared (FIR)
properties of two samples of nearby galaxies selected from the GALEX (\AA, hereafter NUV) and IRAS (m) surveys respectively.
They are built in order to get detection at both wavelengths for most of the
galaxies. Star formation rate (SFR) estimators based on the UV and FIR
emissions are compared. Systematic differences are found between the SFR
estimators for individual galaxies based on the NUV fluxes corrected for dust
attenuation and on the total IR luminosity. A combined estimator based on NUV
and IR luminosities seems to be the best proxy over the whole range of values
of SFR. Although both samples present similar average values of the birthrate
parameter b, their star-formation-related properties are substantially
different: NUV-selected galaxies tend to show larger values of for lower
masses, SFRs and dust attenuations, supporting previous scenarios for the star
formation history (SFH). Conversely, about 20% of the FIR-selected galaxies
show high values of , SFR and NUV attenuation. These galaxies, most of them
being LIRGs and ULIRGs, break down the downsizing picture for the SFH, however
their relative contribution per unit volume is small in the local Universe.
Finally, the cosmic SFR density of the local Universe is estimated in a
consistent way from the NUV and IR luminosities.Comment: 43 pages, 13 figures, accepted for publication in Astrophysical
Journal Supplement Serie
Far-Ultraviolet and Far-Infrared Bivariate Luminosity Function of Galaxies: Complex Relation between Stellar and Dust Emission
Far-ultraviolet (FUV) and far-infrared (FIR) luminosity functions (LFs) of
galaxies show a strong evolution from to , but the FIR LF
evolves much stronger than the FUV one. The FUV is dominantly radiated from
newly formed short-lived OB stars, while the FIR is emitted by dust grains
heated by the FUV radiation field. It is known that dust is always associated
with star formation activity. Thus, both FUV and FIR are tightly related to the
star formation in galaxies, but in a very complicated manner. In order to
disentangle the relation between FUV and FIR emissions, we estimate the UV-IR
bivariate LF (BLF) of galaxies with {\sl GALEX} and {\sl AKARI} All-Sky Survey
datasets. Recently we invented a new mathematical method to construct the BLF
with given marginals and prescribed correlation coefficient. This method makes
use of a tool from mathematical statistics, so called "copula". The copula
enables us to construct a bivariate distribution function from given marginal
distributions with prescribed correlation and/or dependence structure. With
this new formulation and FUV and FIR univariate LFs, we analyze various FUV and
FIR data with {\sl GALEX}, {\sl Spitzer}, and {\sl AKARI} to estimate the UV-IR
BLF. The obtained BLFs naturally explain the nonlinear complicated relation
between FUV and FIR emission from star-forming galaxies. Though the faint-end
of the BLF was not well constrained for high- samples, the estimated linear
correlation coefficient was found to be very high, and is remarkably
stable with redshifts (from 0.95 at to 0.85 at ). This implies
the evolution of the UV-IR BLF is mainly due to the different evolution of the
univariate LFs, and may not be controlled by the dependence structure.Comment: 10 pages, 7 figures, Earth, Planets and Space, in pres
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