5,417 research outputs found
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 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
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
Line Profiles of Cores within Clusters. III. What is the most reliable tracer of core collapse in dense clusters?
Recent observational and theoretical investigations have emphasised the
importance of filamentary networks within molecular clouds as sites of star
formation. Since such environments are more complex than those of isolated
cores, it is essential to understand how the observed line profiles from
collapsing cores with non-spherical geometry are affected by filaments. In this
study, we investigate line profile asymmetries by performing radiative transfer
calculations on hydrodynamic models of three collapsing cores that are embedded
in filaments. We compare the results to those that are expected for isolated
cores. We model the five lowest rotational transition line (J = 1-0, 2-1, 3-2,
4-3, and 5-4) of both optically thick (HCN, HCO) as well as optically thin
(NH, HCO) molecules using constant abundance laws. We find
that less than 50% of simulated (1-0) transition lines show blue infall
asymmetries due to obscuration by the surrounding filament. However, the
fraction of collapsing cores that have a blue asymmetric emission line profile
rises to 90% when observed in the (4-3) transition. Since the densest gas
towards the collapsing core can excite higher rotational states, upper level
transitions are more likely to produce blue asymmetric emission profiles. We
conclude that even in irregular, embedded cores one can trace infalling gas
motions with blue asymmetric line profiles of optically thick lines by
observing higher transitions. The best tracer of collapse motions of our sample
is the (4-3) transition of HCN, but the (3-2) and (5-4) transitions of both HCN
and HCO are also good tracers.Comment: accepted by MNRAS; 13 pages, 16 figures, 6 table
CO-dark gas and molecular filaments in Milky Way type galaxies
We use the moving mesh code AREPO coupled to a time-dependent chemical
network to investigate the formation and destruction of molecular gas in
simulated spiral galaxies. This allows us to determine the characteristics of
the gas that is not traced by CO emission. Our extremely high resolution AREPO
simulations allow us to capture the chemical evolution of the disc, without
recourse to a parameterised `clumping factor'. We calculate H2 and CO column
densities through our simulated disc galaxies, and estimate the CO emission and
CO-H2 conversion factor. We find that in conditions akin to those in the local
interstellar medium, around 42% of the total molecular mass should be in
CO-dark regions, in reasonable agreement with observational estimates. This
fraction is almost insensitive to the CO integrated intensity threshold used to
discriminate between CO-bright and CO-dark gas, as long as this threshold is
less than 10 K km/s. The CO-dark molecular gas primarily resides in extremely
long (>100 pc) filaments that are stretched between spiral arms by galactic
shear. Only the centres of these filaments are bright in CO, suggesting that
filamentary molecular clouds observed in the Milky Way may only be small parts
of much larger structures. The CO-dark molecular gas mainly exists in a
partially molecular phase which accounts for a significant fraction of the
total disc mass budget. The dark gas fraction is higher in simulations with
higher ambient UV fields or lower surface densities, implying that external
galaxies with these conditions might have a greater proportion of dark gas.Comment: Accepted by MNRA
The effect of crystal orientation on the cryogenic strength of hydroxide catalysis bonded sapphire
Hydroxide catalysis bonding has been used in gravitational wave detectors to precisely and securely join components of quasi-monolithic silica suspensions. Plans to operate future detectors at cryogenic temperatures has created the
need for a change in the test mass and suspension material. Mono-crystalline sapphire is one candidate material for use at cryogenic temperatures and is being investigated for use in the KAGRA detector. The crystalline structure of sapphire may influence the properties of the hydroxide catalysis bond formed. Here, results are presented of studies of the potential influence of the crystal orientation of sapphire on the shear strength of the hydroxide catalysis bonds formed between sapphire samples. The strength was tested at approximately 8 K; this is the first measurement of the strength of such bonds between
sapphire at such reduced temperatures. Our results suggest that all orientation combinations investigated produce bonds of sufficient strength for use in typical mirror suspension designs, with average strengths >23 MPa
Submillimeter Observations of the Ultraluminous BAL Quasar APM 08279+5255
With an inferred bolometric luminosity of 5\times10^{15}{\rm \lsun}, the
recently identified z=3.87, broad absorption line quasar APM 08279+5255 is
apparently the most luminous object currently known. As half of its prodigious
emission occurs in the infrared, APM 08279+5255 also represents the most
extreme example of an Ultraluminous Infrared Galaxy. Here, we present new
submillimeter observations of this phenomenal object; while indicating that a
vast quantity of dust is present, these data prove to be incompatible with
current models of emission mechanisms and reprocessing in ultraluminous
systems. The influence of gravitational lensing upon these models is considered
and we find that while the emission from the central continuum emitting region
may be significantly enhanced, lensing induced magnification cannot easily
reconcile the models with observations. We conclude that further modeling,
including the effects of any differential magnification is required to explain
the observed emission from APM 08279+5255.Comment: 12 Pages with Two figures. Accepted for publication in the
Astrophysical Journal Letter
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