12 research outputs found
Effects of dust scattering albedo and 2175 A bump on ultraviolet colours of normal disc galaxies
We discuss dust properties in the interstellar medium (ISM) of nearby normal
galaxies, by comparing observations in the ultraviolet (UV) with simulations by
a radiative transfer model. The observed UV colours of nearby galaxies show a
reddening relative to their expected intrinsic colours. Some authors argued
that the Milky Way dust cannot reproduce the reddening because of the prominent
2175 \AA absorption bump. Other authors proposed a reduction mechanism of the
bump strength in an {\it attenuation law} derived from the ratio of the
observed intensity to the intrinsic one through an age-selective attenuation
(i.e., young stars are more attenuated selectively). We newly find that the
wavelength dependence of the scattering albedo also has a strong effect on the
UV colour; an albedo decreasing toward shorter wavelengths (except for the
absorption bump range) produces a significant UV reddening. After comparing the
observed UV colours of nearby normal galaxies with those expected from
radiative transfer simulations assumed several dust models, we find two sorts
of dust suitable for these galaxies: (1) dust with a bump and a smaller albedo
for a shorter wavelength (except for the bump range), and (2) dust without any
bump but with an almost constant albedo. If very small carbonaceous grains
responsible for the common unidentified infrared emission band are also the
bump carrier, the former dust is favorable. Finally, we derive mean attenuation
laws of various dust models as a function of the UV attenuation, and derive
some relations between the UV attenuation and observable/theoretical
quantities.Comment: MNRAS in pres
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
UV and FIR selected star-forming galaxies at z=0: differences and overlaps
We study two samples of local galaxies, one is UV (GALEX) selected and the
other FIR (IRAS) selected, to address the question whether UV and FIR surveys
see the two sides ('bright' and 'dark') of the star formation of the same
population of galaxies or two different populations of star forming galaxies.
No significant difference between the L () luminosity
functions of the UV and FIR samples is found. Also, after the correction for
the `Malmquist bias' (bias for flux limited samples), the FIR-to-UV ratio v.s.
L relations of the two samples are consistent with each other. In the
range of 9 \la \log(L_{tot}/L_\sun) \la 12, both can be approximated by a
simple linear relation of \log (L_{60}/L_{FUV})=\log(L_{tot}/L_\sun)-9.66.
These are consistent with the hypothesis that the two samples represent the
same population of star forming galaxies, and their well documented differences
in L and in FIR-to-UV ratio are due only to the selection effect. A
comparison between the UV luminosity functions shows marginal evidence for a
population of faint UV galaxies missing in the FIR selected sample. The
contribution from these 'FIR-quiet' galaxies to the overall UV population is
insignificant, given that the K-band luminosity functions (i.e. the stellar
mass functions) of the two samples do not show any significant difference.Comment: 21 pages, 7 figures. Accepted by Ap
Stellar Population gradients in galaxy discs from the CALIFA survey
While studies of gas-phase metallicity gradients in disc galaxies are common,
very little has been done in the acquisition of stellar abundance gradients in
the same regions. We present here a comparative study of the stellar
metallicity and age distributions in a sample of 62 nearly face-on, spiral
galaxies with and without bars, using data from the CALIFA survey. We measure
the slopes of the gradients and study their relation with other properties of
the galaxies. We find that the mean stellar age and metallicity gradients in
the disc are shallow and negative. Furthermore, when normalized to the
effective radius of the disc, the slope of the stellar population gradients
does not correlate with the mass or with the morphological type of the
galaxies. Contrary to this, the values of both age and metallicity at 2.5
scale-lengths correlate with the central velocity dispersion in a similar
manner to the central values of the bulges, although bulges show, on average,
older ages and higher metallicities than the discs. One of the goals of the
present paper is to test the theoretical prediction that non-linear coupling
between the bar and the spiral arms is an efficient mechanism for producing
radial migrations across significant distances within discs. The process of
radial migration should flatten the stellar metallicity gradient with time and,
therefore, we would expect flatter stellar metallicity gradients in barred
galaxies. However, we do not find any difference in the metallicity or age
gradients in galaxies with without bars. We discuss possible scenarios that can
lead to this absence of difference.Comment: 24 pages, 17 figures, accepted for publication in A&
Number Counts of GALEX Sources in FUV (1530A) and NUV (2310A) Bands
Number Counts of galaxies in two GALEX bands (FUV: 1530A and NUV: 2310A, both
in AB magnitudes) are reported. They provide for the first time in the
literature homogeneously calibrated number counts of UV galaxies covering
continuously a very wide range of UV magnitude (14 -- 23.8). Both the FUV and
NUV counts are inconsistent with a non-evolution model, while they are in good
agreement with evolution models (essentially luminosity evolution) derived from
the high-z UV luminosity functions of Arnouts et al. (2004). It is found that
the contribution from galaxies detected by GALEX to the UV background is
0.68+-0.10 nW m-2 sr-1 at 1530A and 0.99+-0.15 nW m-2 sr-1 at 2310A. These are
66+-9% and 44+-6% of the total contributions of galaxies to the the UV
background at 1530A, respectively, as estimated using the evolution models. ...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