4,114 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
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
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
Dust Emission from Active Galactic Nuclei
Unified schemes of active galactic nuclei (AGN) require an obscuring dusty
torus around the central source, giving rise to Seyfert 1 line spectrum for
pole-on viewing and Seyfert 2 characteristics in edge-on sources. Although the
observed IR is in broad agreement with this scheme, the behavior of the 10
micron silicate feature and the width of the far-IR emission peak remained
serious problems in all previous modeling efforts. We show that these problems
find a natural explanation if the dust is contained in about 5-10 clouds along
radial rays through the torus. The spectral energy distributions (SED) of both
type 1 and type 2 sources are properly reproduced from different viewpoints of
the same object if the visual optical depth of each cloud is larger than about
60 and the clouds' mean free path increases roughly in proportion to radial
distance.Comment: 11 pages, submitted to ApJ Letter
Mechanical loss of a hydroxide catalysis bond between sapphire substrates and its effect on the sensitivity of future gravitational wave detectors
Hydroxide catalysis bonds are low mechanical loss joints which are used in the fused silica mirror suspensions of current room temperature interferometric gravitational wave detectors, one of the techniques which was essential to allow the recent detection of gravitational radiation by LIGO. More sensitive detectors may require cryogenic techniques with sapphire as a candidate mirror and suspension material, and thus hydroxide catalysis bonds are under consideration for jointing sapphire. This paper presents the first measurements of the mechanical loss of such a bond created between sapphire substrates and measured down to cryogenic temperatures. The mechanical loss is found to be 0.03±0.01 at room temperature, decreasing to (3±1)Ă10â4 at 20 K. The resulting thermal noise of the bonds on several possible mirror suspensions is presented
The Power Spectrum of the PSC Redshift Survey
We measure the redshift-space power spectrum P(k) for the recently completed
IRAS Point Source Catalogue (PSC) redshift survey, which contains 14500
galaxies over 84% of the sky with 60 micron flux >= 0.6 Jansky. Comparison with
simulations shows that our estimated errors on P(k) are realistic, and that
systematic errors due to the finite survey volume are small for wavenumbers k
>~ 0.03 h Mpc^-1. At large scales our power spectrum is intermediate between
those of the earlier QDOT and 1.2 Jansky surveys, but with considerably smaller
error bars; it falls slightly more steeply to smaller scales. We have fitted
families of CDM-like models using the Peacock-Dodds formula for non-linear
evolution; the results are somewhat sensitive to the assumed small-scale
velocity dispersion \sigma_V. Assuming a realistic \sigma_V \approx 300 km/s
yields a shape parameter \Gamma ~ 0.25 and normalisation b \sigma_8 ~ 0.75; if
\sigma_V is as high as 600 km/s then \Gamma = 0.5 is only marginally excluded.
There is little evidence for any `preferred scale' in the power spectrum or
non-Gaussian behaviour in the distribution of large-scale power.Comment: Latex, uses mn.sty, 14 pages including 11 Postscript figures.
Accepted by MNRA
Silicon mirror suspensions for gravitational wave detectors
One of the most significant limits to the sensitivity of current, and future, long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test masses and their suspensions. This paper reports results of analytical and experimental studies of the limits to thermal noise performance of cryogenic silicon test mass suspensions set by two constraints on suspension fibre dimensions: the minimum dimensions required to allow conductive cooling for extracting incident laser beam heat deposited in the mirrors; and the minimum dimensions of fibres (set by their tensile strength) which can support test masses of the size envisaged for use in future detectors. We report experimental studies of breaking strength of silicon ribbons, and resulting design implications for the feasibility of suspension designs for future gravitational wave detectors using silicon suspension fibres. We analyse the implication of this study for thermal noise performance of cryogenically cooled silicon suspensions
Apparatus for dimensional characterization of fused silica fibers for the suspensions of advanced gravitational wave detectors
Detection of gravitational waves from astrophysical sources remains one of the most challenging problems faced by experimental physicists. A significant limit to the sensitivity of future long-baseline interferometric gravitational wave detectors is thermal displacement noise of the test mass mirrors and their suspensions. Suspension thermal noise results from mechanical dissipation in the fused silica suspension fibers suspending the test mass mirrors and is therefore an important noise source at operating frequencies between âŒ10 and 30 Hz. This dissipation occurs due to a combination of thermoelastic damping, surface and bulk losses. Its effects can be reduced by optimizing the thermoelastic and surface loss, and these parameters are a function of the cross sectional dimensions of the fiber along its length. This paper presents a new apparatus capable of high resolution measurements of the cross sectional dimensions of suspension fibers of both rectangular and circular cross section, suitable for use in advanced detector mirror suspensions
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