5,363 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
HST imaging of hyperluminous infrared galaxies
We present HST WFPC2 I band imaging for a sample of 9 Hyperluminous Infrared
Galaxies spanning a redshift range 0.45 < z < 1.34. Three of the sample have
morphologies showing evidence for interactions, six are QSOs. Host galaxies in
the QSOs are reliably detected out to z ~ 0.8. The detected QSO host galaxies
have an elliptical morphology with scalelengths spanning 6.5 < r_{e}(Kpc) < 88
and absolute k corrected magnitudes spanning -24.5 < M_{I} < -25.2. There is no
clear correlation between the IR power source and the optical morphology. None
of the sources in the sample, including F15307+3252, show any evidence for
gravitational lensing. We infer that the IR luminosities are thus real. Based
on these results, and previous studies of HLIRGs, we conclude that this class
of object is broadly consistent with being a simple extrapolation of the ULIRG
population to higher luminosities; ULIRGs being mainly violently interacting
systems powered by starbursts and/or AGN. Only a small number of sources whose
infrared luminosities exceed 10^{13}Lsun are intrinsically less luminous
objects which have been boosted by gravitational lensing.Comment: 16 Pages. Accepted for publication in MNRA
Sub-millimetre observations of hyperluminous infrared galaxies
We present sub-mm photometry for 11 Hyperluminous Infrared Galaxies (HLIRGs)
and use radiative transfer models for starbursts and AGN to investigate the IR
emission. In all sources both a starburst and AGN are required to explain the
IR emission. The mean starburst fraction is 35%, with a range spanning 80%
starburst dominated to 80% AGN dominated. In all cases the starburst dominates
at rest-frame wavelengths >50 microns, with star formation rates >500 solar
masses per year. The trend of increasing AGN fraction with increasing IR
luminosity seen in IRAS galaxies peaks in HLIRGs, and is not higher than the
fraction seen in bright ULIRGs. The AGN and starburst luminosities correlate,
suggesting that a common physical factor, plausibly the dust masses, governs
their luminosities. Our results suggest that the HLIRG population is comprised
both of ULIRG-like galaxy mergers, and of young galaxies going through their
maximal star formation periods whilst harbouring an AGN. The coeval AGN and
starburst activity in our sources implies that starburst and AGN activity, and
the peak starburst and AGN luminosities, can be coeval in active galaxies
generally. When extrapolated to high-z our sources have comparable sub-mm
fluxes to sub-mm survey sources. At least some sub-mm survey sources are
therefore likely to be comprised of similar galaxy populations to those found
in the HLIRG population. It is also plausible from these results that high-z
sub-mm sources harbour heavily obscured AGN. The differences in X-ray and
sub-mm properties between HLIRGs at z~1 and sub-mm sources at z~3 implies
evolution between the two epochs. Either the mean AGN obscuration level is
greater at z~3 than at z~1, or the fraction of IR-luminous sources at z~3 that
contain AGN is smaller than that at z~1.Comment: 15 pages. Accepted for publication in MNRA
The European Large Area ISO Survey - ISOPHOT results using the MPIA-pipeline
The European Large Area ISO Survey (ELAIS) will provide Infrared observations
of 4 regions in the sky with ISO. Around 2000 Infrared sources have been
detected at 7 and 15 microns (with ISOCAM), 90 and 175 microns (with ISOPHOT))
over 13 square degrees of the sky. We present the source extraction pipeline of
the 90 microns ISOPHOT observations, describe and discuss the results obtained
and derive the limits of the ELAIS observational strategy.Comment: 4 pages, 5 figures, to appear in the ISO conference "The Universe as
seen by ISO", 1998, UNESCO, Pari
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
Survival after postoperative morbidity: a longitudinal observational cohort study
Prolonged morbidity after surgery is associated with a risk of premature death for a longer duration than perhaps is commonly thought; however, this risk falls with time. We suggest that prolonged postoperative morbidity measured in this way may be a valid indicator of the quality of surgical healthcare. Our findings reinforce the importance of research and quality improvement initiatives aimed at reducing the duration and severity of postoperative complication
PSCz Superclusters: Detection, Shapes & Cosmological Implications
We study the possibility of correctly identifying superclusters, from the
smooth galaxy density field of the PSCz redshift catalogue, and of recovering
their true shapes in the presence of a bias introduced by the coupling between
the selection function and the constant radius smoothing. Using simulations we
quantify such systematic biases in the smoothed PSCz density field and after
applying the necessary corrections we study the PSCz supercluster multiplicity
and morphologies using a differential geometry definition of shape. Our results
strongly suggest that filamentariness is the dominant morphological feature of
PSCz superclusters. Finally, we compare our results with those expected in
three different cosmological models and find that the LCDM model performs
better than Omega_m=1 CDM models.Comment: 9 pages, 9 figures, submitted to MNRA
A possible dearth of hot gas in galaxy groups at intermediate redshift
We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at
redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing
signal of the CNOC2 groups has shown that they are likely to be genuine,
gravitationally bound objects. Of the 21 groups in the field of view of the
EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of
observation, even though three of the them have velocity dispersions high
enough that they would easily be visible if their luminosities scaled with
their velocity dispersions in the same way as nearby groups' luminosities
scale. We consider the possibility that this is due to the reported velocity
dispersions being erroneously high, and conclude that this is unlikely. We
therefore find tentative evidence that groups at intermediate redshift are
underluminous relative to their local cousins.Comment: 16 pages, 5 figures, reference added in section 1, typos corrected,
published in Ap
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