5,231 research outputs found
The Global Star Formation Rate from the 1.4 GHz Luminosity Function
The decimetric luminosity of many galaxies appears to be dominated by
synchrotron emission excited by supernova explosions. Simple models suggest
that the luminosity is directly proportional to the rate of supernova
explosions of massive stars averaged over the past 30 Myr. The proportionality
may be used together with models of the evolving 1.4 GHz luminosity function to
estimate the global star formation rate density in the era z < 1. The local
value is estimated to be 0.026 solar masses per year per cubic megaparsec, some
50% larger than the value inferred from the Halpha luminosity density. The
value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec.
The 10-fold increase in star formation rate density is consistent with the
increase inferred from mm-wave, far-infrared, ultra-violet and Halpha
observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS
version has improved figure placemen
The star-formation history of the universe - an infrared perspective
A simple and versatile parameterized approach to the star formation history
allows a quantitative investigation of the constraints from far infrared and
submillimetre counts and background intensity measurements.
The models include four spectral components: infrared cirrus (emission from
interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN
dust torus. The 60 m luminosity function is determined for each chosen
rate of evolution using the PSCz redshift data for 15000 galaxies. The
proportions of each spectral type as a function of 60 m luminosity are
chosen for consistency with IRAS and SCUBA colour-luminosity relations, and
with the fraction of AGN as a function of luminosity found in 12 m
samples. The luminosity function for each component at any wavelength can then
be calculated from the assumed spectral energy distributions. With assumptions
about the optical seds corresponding to each component and, for the AGN
component, the optical and near infrared counts can be accurately modelled.
A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850
m can be found with pure luminosity evolution in all 3 cosmological models
investigated: = 1, = 0.3 ( = 0), and
= 0.3, = 0.7.
All 3 models also give an acceptable fit to the integrated background
spectrum. Selected predictions of the models, for example redshift
distributions for each component at selected wavelengths and fluxes, are shown.
The total mass-density of stars generated is consistent with that observed,
in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details
of models can be found at http://astro.ic.ac.uk/~mrr/countmodel
The Redshift of GRB 970508
GRB 970508 is the second gamma-ray burst (GRB) for which an optical afterglow
has been detected. It is the first GRB for which a distance scale has been
determined: absorption and emission features in spectra of the optical
afterglow place GRB 970508 at a redshift of z >= 0.835 (Metzger et al. 1997a,
1997b). The lack of a Lyman-alpha forest in these spectra further constrains
this redshift to be less than approximately 2.3. I show that the spectrum of
the optical afterglow of GRB 970508, once corrected for Galactic absorption, is
inconsistent with the relativistic blast-wave model unless a second, redshifted
source of extinction is introduced. This second source of extinction may be the
yet unobserved host galaxy. I determine its redshift to be z =
1.09^{+0.14}_{-0.41}, which is consistent with the observed redshift of z =
0.835. Redshifts greater than z = 1.40 are ruled out at the 3 sigma confidence
level.Comment: Accepted to The Astrophysical Journal (Letters), 10 pages, LaTe
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
Low-metallicity star formation: Relative impact of metals and magnetic fields
Low-metallicity star formation poses a central problem of cosmology, as it
determines the characteristic mass scale and distribution for the first and
second generations of stars forming in our Universe. Here, we present a
comprehensive investigation assessing the relative impact of metals and
magnetic fields, which may both be present during low-metallicity star
formation. We show that the presence of magnetic fields generated via the
small-scale dynamo stabilises the protostellar disc and provides some degree of
support against fragmentation. In the absence of magnetic fields, the
fragmentation timescale in our model decreases by a factor of ~10 at the
transition from Z=0 to Z>0, with subsequently only a weak dependence on
metallicity. Similarly, the accretion timescale of the cluster is set by the
large-scale dynamics rather than the local thermodynamics. In the presence of
magnetic fields, the primordial disc can become completely stable, therefore
forming only one central fragment. At Z>0, the number of fragments is somewhat
reduced in the presence of magnetic fields, though the shape of the mass
spectrum is not strongly affected in the limits of the statistical
uncertainties. The fragmentation timescale, however, increases by roughly a
factor of 3 in the presence of magnetic fields. Indeed, our results indicate
comparable fragmentation timescales in primordial runs without magnetic fields
and Z>0 runs with magnetic fields.Comment: MNRAS in pres
Faint Radio Sources and Star Formation History
Faint extragalactic radio sources provide important information about the
global history of star formation. Sensitive radio observations of the Hubble
Deep Field and other fields have found that sub-mJy radio sources are
predominantly associated with star formation activity rather than AGN. Radio
observations of star forming galaxies have the advantage of being independent
of extinction by dust. We use the FIR-radio correlation to compare the radio
and FIR backgrounds, and make several conclusions about the star forming
galaxies producing the FIR background. We then use the redshift distribution of
faint radio sources to determine the evolution of the radio luminosity
function, and thus estimate the star formation density as a function of
redshift.Comment: 12 pages, 9 figures, latex using texas.sty, to appear in the CD-ROM
Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology, held in Paris, France, Dec. 14-18, 1998. Eds.: J. Paul, T.
Montmerle, and E. Aubourg (CEA Saclay). No changes to paper, just updated
publication info in this commen
Impact and collisional processes in the solar system
As impact cratered terrains have been successively recognized on certain planets and planetary satellites, it has become clear that impact processes are important to the understanding of the accretion and evolution of all solid planets. The noble gases in the normalized atmospheric inventories of the planets and the normalized gas content of meteorites are grossly similar, but demonstrate differences from each other which are not understood. In order to study shock devolatilization of the candidate carrier phases which are principally thought to be carbonaceous or hydrocarbons in planetesimals, experiments were conducted on noble gase implantation in various carbons: carbon black, activated charcoal, graphite, and carbon glass. These were candidate starting materials for impact devolatilization experiments. Initial experiments were conducted on vitreous amorphous carbon samples which were synthesized under vapor saturated conditions using argon as the pressurizing medium. An amino acid and surface analysis by laser ionization analyses were performed on three samples of shocked Murchison meteorite. A first study was completed in which a series of shock loading experiments on a porous limestone and on a non-porous gabbro in one and three dimensions were performed. Also a series of recovery experiments were conducted in which shocked molten basalt a 1700 C is encapsulated in molybdenum containers and shock recovered from up to 6 GPa pressures
Detection and mapping of hydrothermally altered rocks in the vicinity of the comstock lode, Virginia Range, Nevada, using enhanced LANDSAT images
The author has identified the following significant results. LANDSAT images enhanced by the band-ratioing method can be used for reconnaissance alteration mapping in moderately heavily vegetated semiarid terrain as well as in sparsely vegetated to semiarid terrain where the technique was originally developed. Significant vegetation cover in a scene, however, requires the use of MSS ratios 4/5, 4/6, and 6/7 rather than 4/5, 5/6, and 6/7, and requires careful interpretation of the results. Supplemental information suitable to vegetation identification and cover estimates, such as standard LANDSAT false-color composites and low altitude aerial photographs of selected areas is desirable
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