3,354 research outputs found
Temperature inversion on the surface of externally heated optically thick multigrain dust clouds
It was recently discovered that the temperature in the surface layer of
externally heated optically thick gray dust clouds increases with the optical
depth for some distance from the surface, as opposed to the normal decrease in
temperature with distance in the rest of the cloud. This temperature inversion
is a result of efficient absorption of diffuse flux from the cloud interior by
the surface dust exposed to the external radiation. A micron or bigger size
grains experience this effect when the external flux is of stellar spectrum. We
explore what happens to the effect when dust is a mixture of grain sizes
(multigrain). Two possible boundary conditions are considered: i) a constant
external flux without constrains on the dust temperature, and ii) the maximum
dust temperature set to the sublimation temperature. We find that the first
condition allows small grains to completely suppress the temperature inversion
of big grains if the overall opacity is dominated by small grains. The second
condition enables big grains to maintain the inversion even when they are a
minor contributor to the opacity. In reality, the choice of boundary condition
depends on the dust dynamics. When applied to the physics of protoplanetary
disks, the temperature inversion leads to a previously unrecognized disk
structure where optically thin dust can exist inside the dust destruction
radius of an optically thick disk. We conclude that the transition between the
dusty disk and the gaseous inner clearing is not a sharp edge, but rather a
large optically thin region.Comment: 8 pages, 10 figures, Accepted for publication in the Astrophysical
Journa
The Correlation Function of Rich Clusters of Galaxies in CDM-like Models
We use ensembles of high-resolution CDM simulations to investigate the shape
and amplitude of the two point correlation function of rich clusters. The
standard scale-invariant CDM model with provides a poor description
of the clustering measured from the APM rich cluster redshift survey, which is
better fitted by models with more power at large scales. The amplitudes of the
rich cluster correlation functions measured from our models depend weakly on
cluster richness. Analytic calculations of the clustering of peaks in a
Gaussian density field overestimate the amplitude of the N-body cluster
correlation functions, but reproduce qualitatively the weak trend with cluster
richness. Our results suggest that the high amplitude measured for the
correlation function of richness class Abell clusters is either an
artefact arising from incompleteness in the Abell catalogue, or an indication
that the density perturbations in the early universe were very non-Gaussian.Comment: uuencoded compressed postscript ,MNRAS, in press, OUAST-93-1
Anthropic versus cosmological solutions to the coincidence problem
In this paper we investigate possible solutions to the coincidence problem in
flat phantom dark energy models with a constant dark energy equation of state
and quintessence models with a linear scalar field potential. These models are
representative of a broader class of cosmological scenarios in which the
universe has a finite lifetime. We show that, in the absence of anthropic
constraints, including a prior probability for the models inversely
proportional to the total lifetime of the universe excludes models very close
to the model. This relates a cosmological solution to the
coincidence problem with a dynamical dark energy component having an equation
of state parameter not too close to -1 at the present time. We further show,
that anthropic constraints, if they are sufficiently stringent, may solve the
coincidence problem without the need for dynamical dark energy.Comment: 7 pages, 7 figure
Vacuum Energy: If Not Now, Then When?
We review the cosmological evidence for a low matter density universe and a
cosmological constant or dynamical vacuum energy and address the cosmolog$
coincidence problem: why is the matter density about one-half the vacuum energy
{\em now}. This is reasonble, following the anthropic argument of Efstathiou
and of Martel, Schapiro & Weinberg.Comment: 4 pages (latex
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
Combining Physical galaxy models with radio observations to constrain the SFRs of high-z dusty star forming galaxies
We complement our previous analysis of a sample of z~1-2 luminous and
ultra-luminous infrared galaxies ((U)LIRGs), by adding deep VLA radio
observations at 1.4 GHz to a large data-set from the far-UV to the sub-mm,
including Spitzer and Herschel data. Given the relatively small number of
(U)LIRGs in our sample with high S/N radio data, and to extend our study to a
different family of galaxies, we also include 6 well sampled near IR-selected
BzK galaxies at z~1.5. From our analysis based on the radiative transfer
spectral synthesis code GRASIL, we find that, while the IR luminosity may be a
biased tracer of the star formation rate (SFR) depending on the age of stars
dominating the dust heating, the inclusion of the radio flux offers
significantly tighter constraints on SFR. Our predicted SFRs are in good
agreement with the estimates based on rest-frame radio luminosity and the Bell
(2003) calibration. The extensive spectro-photometric coverage of our sample
allows us to set important constraints on the SF history of individual objects.
For essentially all galaxies we find evidence for a rather continuous SFR and a
peak epoch of SF preceding that of the observation by a few Gyrs. This seems to
correspond to a formation redshift of z~5-6. We finally show that our physical
analysis may affect the interpretation of the SFR-M* diagram, by possibly
shifting, with respect to previous works, the position of the most dust
obscured objects to higher M* and lower SFRs.Comment: 26 pages, 15 figures, 3 tables, accepted for publication in MNRAS on
Dec. 4th, 201
Forecast B-modes detection at large scales in presence of noise and foregrounds
We investigate the detectability of the primordial CMB polarization B-mode
power spectrum on large scales in the presence of instrumental noise and
realistic foreground contamination. We have worked out a method to estimate the
errors on component separation and to propagate them up to the power spectrum
estimation. The performances of our method are illustrated by applying it to
the instrumental specifications of the Planck satellite and to the proposed
configuration for the next generation CMB polarization experiment COrE. We
demonstrate that a proper component separation step is required in order
achieve the detection of B-modes on large scales and that the final sensitivity
to B-modes of a given experiment is determined by a delicate balance between
noise level and residual foregrounds, which depend on the set of frequencies
exploited in the CMB reconstruction, on the signal-to-noise of each frequency
map, and on our ability to correctly model the spectral behavior of the
foreground components. We have produced a flexible software tool that allows
the comparison of performances on B-mode detection of different instrumental
specifications (choice of frequencies, noise level at each frequency, etc.) as
well as of different proposed approaches to component separation.Comment: 7 pages, 2 tables, 1 figure, accepted by MNRA
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