1,335 research outputs found
Age and Dust Degeneracy for Starburst Galaxies Solved?
A spectral evolution model of galaxies that includes both stellar and dust
effects is newly built. xApplying the model to 22 nearby starburst galaxies, we
have shown that far infrared luminosity of galaxies helps to break the
age-dustiness degeneracy. We have derived a unique solution of age and the
dustiness for each starburst galaxy. The resulting starburst ages and optical
depths are in the range and , respectively. The result is robust and is almost independent of model
assumptions such as dust distributions, extinction curves, and burst strengths.
With the rapidly growing sensitivity of submillimeter detectors, it should
become possible in the near future to determine the age and of
star-forming galaxies at redshifts and beyond. Accurate estimates
of for Lyman-break galaxies and high-z galaxies might require a
substantial revision of the previously claimed picture of star formation
history over the Hubble time.Comment: Latex (aas2pp4) 15 pages, 1 table, 6 figures. Accepted for Ap
A Robust Age Indicator for Old Stellar Populations
We derive new spectral H_gamma index definitions which are robust age
indicators for old and relatively old stellar populations and thus have great
potential for solving the age-metallicity degeneracy of galaxy spectra. To
study H_gamma as a function of age, metallicity and resolution, we used a new
spectral synthesis model which predicts SEDs of single-age, single-metallicity
stellar populations at resolution FWHM=1.8A (which can be smoothed to different
resolutions), allowing direct measurements of the equivalent widths of
particular absorption features. We find that the H_gamma strong age
disentangling power strongly depends strongly on the adopted resolution and
galaxy velocity dispersion. We propose a system of indices which are completely
insensitive to metallicity and stable against resolution, allowing the study of
galaxies up to ~300 km/s. Observational spectra of very high S/N and relatively
high dispersion, are required to gain this unprecedented age discriminating
power. Once such spectra are obtained, accurate and reliable estimates for the
luminosity-weighted average stellar ages of these galaxies will become possible
for the first time, without assessing their metallicities. We measured this
index for two globular clusters, a number of low-luminosity elliptical galaxies
and a standard S0 galaxy. We find a large spread in the average stellar ages of
a sample of low-luminosity ellipticals. In particular these indices yield 4 Gyr
for M32, in agreement with the age provided by an extraordinary fit to the full
spectrum of this galaxy that we achieve here.Comment: 22 pages, 4 figures. ApJ, in press. Models and details can be found
at http://www.ioa.s.u-tokyo.ac.jp/~vazdekis
Diffuse Extragalactic Background Light versus Deep Galaxy Counts in the Subaru Deep Field: Missing Light in the Universe?
Deep optical and near-infrared galaxy counts are utilized to estimate the
extragalactic background light (EBL) coming from normal galactic light in the
universe. Although the slope of number-magnitude relation of the faintest
counts is flat enough for the count integration to converge, considerable
fraction of EBL from galaxies could still have been missed in deep galaxy
surveys because of various selection effects including the cosmological dimming
of surface brightness of galaxies. Here we give an estimate of EBL from galaxy
counts, in which these selection effects are quantitatively taken into account
for the first time, based on reasonable models of galaxy evolution which are
consistent with all available data of galaxy counts, size, and redshift
distributions. We show that the EBL from galaxies is best resolved into
discrete galaxies in the near-infrared bands (J, K) by using the latest data of
the Subaru Deep Field; more than 80-90% of EBL from galaxies has been resolved
in these bands. Our result indicates that the contribution by missing galaxies
cannot account for the discrepancy between the count integration and recent
tentative detections of diffuse EBL in the K-band (2.2 micron), and there may
be a very diffuse component of EBL which has left no imprints in known galaxy
populations.Comment: ApJ Letters in press. Two new reports on the diffuse EBL at 1.25 and
2.2 microns are added to the reference list and Table
The Ages of Dwarf Ellipticals
We present narrow band photometry of 91 dwarf ellipticals in the Coma and
Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the
sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs
reveals two distinct populations of early-type systems based on integrated
colors. The class of dEN galaxies are redder in their continuum colors as
compared to bright cluster ellipticals and dE type dwarfs, and their position
in multi-color diagrams can only be explained by an older mean age for their
underlying stellar populations. By comparison with the narrow band photometry
of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are
a higher metallicity continuation of the old, metal-poor color sequence of
galactic globulars and the blue population of M87 globulars. Bright ellipticals
and dE dwarfs, on the other hand, follow the color sequence of the metal-rich,
red population of M87 globulars. A comparison to SED models, convolved to a
simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs
older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The
implication is that globulars and dEN galaxies are primordial and have
metallicities set by external constraints such as the enrichment of their
formation clouds. Bright ellipticals and dE galaxies have metallicities and
ages that suggest an extended phase of initial star formation to produce a
younger mean age, even if their formation epoch is similar to that of dENs and
blue globulars, and an internally driven chemical evolutionary history.Comment: 13 pages AAS LaTeX, 6 figures, accepted for publication in A
Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant
We show that evolution of the luminosity density of galaxies in the universe
provides a powerful test for the geometry of the universe. Using reasonable
galaxy evolution models of population synthesis which reproduce the colors of
local galaxies of various morphological types, we have calculated the
luminosity density of galaxies as a function of redshift . Comparison of the
result with recent measurements by the Canada-France Redshift Survey in three
wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the
\Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the
lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if
\Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0,
\lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however
ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels
quoted apply unless the standard assumptions on galaxy evolution are
drastically violated. We have also calculated a global star formation rate in
the universe to be compared with the observed rate beyond z \sim 2. We find
from this comparison that spiral galaxies are formed from material accretion
over an extended period of a few Gyrs, while elliptical galaxies are formed
from initial star burst at z >~ 5 supplying enough amount of metals and
ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ
Letter
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