1,495 research outputs found
Dust Emission from Evolved and Unevolved HII Regions in the Large Magellanic Cloud
We present a study of the dust properties of 12 classical and superbubble HII
regions in the Large Magellanic Cloud. We use infrared photometry from Spitzer
(8, 24, 70, and 160 \mum bands), obtained as part of the Surveying the Agents
of a Galaxy's Evolution (SAGE) program, along with archival spectroscopic
classifications of the ionizing stars to examine the role of stellar sources on
dust heating and processing. Our infrared observations show surprisingly little
correlation between the emission properties of the dust and the effective
temperatures or bolometric magnitudes of stars in the HII regions, suggesting
that the HII region evolutionary timescale is not on the order of the dust
processing timescale. We find that the infrared emission of superbubbles and
classical HII regions shows little differentiation between the two classes,
despite the significant differences in age and morphology. We do detect a
correlation of the 24 \mum emission from hot dust with the ratio of 70 to 160
\mum flux. This correlation can be modeled as a trend in the temperature of a
minority hot dust component, while a majority of the dust remains significantly
cooler.Comment: 15 pages, 5 figures. Accepted to Ap
NGC 300: an extremely faint, outer stellar disk observed to 10 scale lengths
We have used the Gemini Multi-object Spectrograph (GMOS) on the Gemini South
8m telescope in exceptional conditions (0.6" FWHM seeing) to observe the outer
stellar disk of the Sculptor group galaxy NGC 300 at two locations. At our
point source detection threshold of r' = 27.0 (3-sigma) mag, we trace the
stellar disk out to a radius of 24', or 2.2 R_25 where R_25 is the 25
mag/arcsec**2 isophotal radius. This corresponds to about 10 scale lengths in
this low-luminosity spiral (M_B = -18.6), or about 14.4 kpc at a cepheid
distance of 2.0 +/- 0.07 Mpc. The background galaxy counts are derived in the
outermost field, and these are within 10% of the mean survey counts from both
Hubble Deep Fields. The luminosity profile is well described by a nucleus plus
a simple exponential profile out to 10 optical scale lengths. We reach an
effective surface brightness of 30.5 mag/arcsec**2 (2-sigma) at 55%
completeness which doubles the known radial extent of the optical disk. These
levels are exceedingly faint in the sense that the equivalent surface
brightness in B or V is about 32 mag/arcsec**2. We find no evidence for
truncation of the stellar disk. Only star counts can be used to reliably trace
the disk to such faint levels, since surface photometry is ultimately limited
by nonstellar sources of radiation. In the Appendix, we derive the expected
surface brightness of one such source: dust scattering of starlight in the
outer disk.Comment: ApJ accepted -- 30 pages, 13 figures -- see
ftp://www.aao.gov.au/pub/local/jbh/astro-ph/N300 for full resolution figures
and preprin
Generalization of the coupled dipole method to periodic structures
We present a generalization of the coupled dipole method to the scattering of
light by arbitrary periodic structures. This new formulation of the coupled
dipole method relies on the same direct-space discretization scheme that is
widely used to study the scattering of light by finite objects. Therefore, all
the knowledge acquired previously for finite systems can be transposed to the
study of periodic structures.Comment: 5 pages, 2 figures, and 1 tabl
Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Bayesian Estimation of CMB Polarization Maps
We describe a sampling method to estimate the polarized CMB signal from
observed maps of the sky. We use a Metropolis-within-Gibbs algorithm to
estimate the polarized CMB map, containing Q and U Stokes parameters at each
pixel, and its covariance matrix. These can be used as inputs for cosmological
analyses. The polarized sky signal is parameterized as the sum of three
components: CMB, synchrotron emission, and thermal dust emission. The polarized
Galactic components are modeled with spatially varying power law spectral
indices for the synchrotron, and a fixed power law for the dust, and their
component maps are estimated as by-products. We apply the method to simulated
low resolution maps with pixels of side 7.2 degrees, using diagonal and full
noise realizations drawn from the WMAP noise matrices. The CMB maps are
recovered with goodness of fit consistent with errors. Computing the likelihood
of the E-mode power in the maps as a function of optical depth to reionization,
tau, for fixed temperature anisotropy power, we recover tau=0.091+-0.019 for a
simulation with input tau=0.1, and mean tau=0.098 averaged over 10 simulations.
A `null' simulation with no polarized CMB signal has maximum likelihood
consistent with tau=0. The method is applied to the five-year WMAP data, using
the K, Ka, Q and V channels. We find tau=0.090+-0.019, compared to
tau=0.086+-0.016 from the template-cleaned maps used in the primary WMAP
analysis. The synchrotron spectral index, beta, averaged over high
signal-to-noise pixels with standard deviation sigma(beta)<0.25, but excluding
~6% of the sky masked in the Galactic plane, is -3.03+-0.04. This estimate does
not vary significantly with Galactic latitude, although includes an informative
prior.Comment: 11 pages, 9 figures, matches version accepted by Ap
The circumstellar disc in the Bok globule CB 26: Multi-wavelength observations and modelling of the dust disc and envelope
Circumstellar discs are expected to be the nursery of planets. Grain growth
within such discs is the first step in the planet formation process. The Bok
globule CB 26 harbours such a young disc. We present a detailed model of the
edge-on circumstellar disc and its envelope in the Bok globule CB 26. The model
is based on HST near-infrared maps in the I, J, H, and K bands, OVRO and SMA
radio maps at 1.1mm, 1.3mm and 2.7mm, and the spectral energy distribution
(SED) from 0.9 microns to 3mm. New photometric and spectroscopic data from the
Spitzer Space Telescope and the Caltech Submilimeter Observatory have been
obtained and are part of our analysis. Using the self-consistent radiative
transfer code MC3D, the model we construct is able to discriminate parameter
sets and dust properties of both its parts, namely envelope and disc. We find
that the disc has an inner hole with a radius of 45 +/- 5 AU. Based on a dust
model including silicate and graphite the maximum grain size needed to
reproduce the spectral millimetre index is 2.5 microns. Features seen in the
near-infrared images, dominated by scattered light, can be described as a
result of a rotating envelope. Successful employment of ISM dust in both the
disc and envelope hint that grain growth may not yet play a significant role
for the appearance of this system. A larger inner hole gives rise to the
assumption that CB 26 is a circumbinary disc.Comment: 18 pages, 15 figures, Accepted for publication in A&
Determining the Parameters of Massive Protostellar Clouds via Radiative Transfer Modeling
A one-dimensional method for reconstructing the structure of prestellar and
protostellar clouds is presented. The method is based on radiative transfer
computations and a comparison of theoretical and observed intensity
distributions at both millimeter and infrared wavelengths. The radiative
transfer of dust emission is modeled for specified parameters of the density
distribution, central star, and external background, and the theoretical
distribution of the dust temperature inside the cloud is determined. The
intensity distributions at millimeter and IR wavelengths are computed and
quantitatively compared with observational data. The best-fit model parameters
are determined using a genetic minimization algorithm, which makes it possible
to reveal the ranges of parameter degeneracy as well. The method is illustrated
by modeling the structure of the two infrared dark clouds IRDC-320.27+029 (P2)
and IRDC-321.73+005 (P2). The derived density and temperature distributions can
be used to model the chemical structure and spectral maps in molecular lines.Comment: Accepted for publication in Astronomy Report
Quantifying the anisotropy in the infrared emission of powerful AGN
We use restframe near- and mid-IR data of an isotropically selected sample of
quasars and radio galaxies at 1.0 \leq z \leq 1.4, which have been published
previously, to study the wavelength-dependent anisotropy of the IR emission.
For that we build average SEDs of the quasar subsample (= type 1 AGN) and radio
galaxies (= type 2 AGN) from ~1-17 {\mu}m and plot the ratio of both average
samples. From 2 to 8 {\mu}m restframe wavelength the ratio gradually decreases
from 20 to 2 with values around 3 in the 10{\mu}m silicate feature. Longward of
12{\mu}m the ratio decreases further and shows some high degree of isotropy at
15 {\mu}m (ratio ~1.4). The results are consistent with upper limits derived
from the X-ray/mid-IR correlation of local Seyfert galaxies. We find that the
anisotropy in our high-luminosity radio-loud sample is smaller than in
radio-quiet lower-luminosity AGN which may be interpreted in the framework of a
receding torus model with luminosity-dependent obscuration properties. It is
also shown that the relatively small degree of anisotropy is consistent with
clumpy torus models.Comment: 6 pages, 2 figures; accepted by Ap
IR-correlated 31 GHz radio emission from Orion East
Lynds dark cloud LDN1622 represents one of the best examples of anomalous
dust emission, possibly originating from small spinning dust grains. We present
Cosmic Background Imager (CBI) 31 GHz data of LDN1621, a diffuse dark cloud to
the north of LDN1622 in a region known as Orion East. A broken ring with
diameter g\approx 20 arcmin of diffuse emission is detected at 31 GHz, at
\approx 20-30 mJy beam with an angular resolution of \approx 5 arcmin.
The ring-like structure is highly correlated with Far Infra-Red emission at
m with correlation coefficients of r \approx 0.7-0.8, significant
at . Multi-frequency data are used to place constraints on other
components of emission that could be contributing to the 31 GHz flux. An
analysis of the GB6 survey maps at 4.85 GHz yields a upper limit on
free-free emission of 7.2 mJy beam (\la 30 per cent of the observed
flux) at the CBI resolution. The bulk of the 31 GHz flux therefore appears to
be mostly due to dust radiation. Aperture photometry, at an angular resolution
of 13 arcmin and with an aperture of diameter 30 arcmin, allowed the use of
IRAS maps and the {\it WMAP} 5-year W-band map at 93.5 GHz. A single modified
blackbody model was fitted to the data to estimate the contribution from
thermal dust, which amounts to \sim\sim100 \mu18.1\pm4.4 \mu^{-1}$, consistent with the values found for
LDN1622.Comment: 8 pages, 3 figures, 3 tables, submitted to MNRA
Cool gas and dust in M33: Results from the Herschel M33 extended survey (HERM33ES)
We present an analysis of the first space-based far-IR-submm observations of
M 33, which measure the emission from the cool dust and resolve the giant
molecular cloud complexes. With roughly half-solar abundances, M33 is a first
step towards young low-metallicity galaxies where the submm may be able to
provide an alternative to CO mapping to measure their H content. In this
Letter, we measure the dust emission cross-section using SPIRE and
recent CO and \HI\ observations; a variation in is present from a
near-solar neighborhood cross-section to about half-solar with the maximum
being south of the nucleus. Calculating the total H column density from the
measured dust temperature and cross-section, and then subtracting the \HI\
column, yields a morphology similar to that observed in CO. The H/\HI\ mass
ratio decreases from about unity to well below 10% and is about 15% averaged
over the optical disk. The single most important observation to reduce the
potentially large systematic errors is to complete the CO mapping of M 33.Comment: 5 pages, 5 figures Accepted for publication in Astronomy and
Astrophysic
Structure Formation, Melting, and the Optical Properties of Gold/DNA Nanocomposites: Effects of Relaxation Time
We present a model for structure formation, melting, and optical properties
of gold/DNA nanocomposites. These composites consist of a collection of gold
nanoparticles (of radius 50 nm or less) which are bound together by links made
up of DNA strands. In our structural model, the nanocomposite forms from a
series of Monte Carlo steps, each involving reaction-limited cluster-cluster
aggregation (RLCA) followed by dehybridization of the DNA links. These links
form with a probability which depends on temperature and particle
radius . The final structure depends on the number of monomers (i. e. gold
nanoparticles) , , and the relaxation time. At low temperature, the
model results in an RLCA cluster. But after a long enough relaxation time, the
nanocomposite reduces to a compact, non-fractal cluster. We calculate the
optical properties of the resulting aggregates using the Discrete Dipole
Approximation. Despite the restructuring, the melting transition (as seen in
the extinction coefficient at wavelength 520 nm) remains sharp, and the melting
temperature increases with increasing as found in our previous
percolation model. However, restructuring increases the corresponding link
fraction at melting to a value well above the percolation threshold. Our
calculated extinction cross section agrees qualitatively with experiments on
gold/DNA composites. It also shows a characteristic ``rebound effect,''
resulting from incomplete relaxation, which has also been seen in some
experiments. We discuss briefly how our results relate to a possible sol-gel
transition in these aggregates.Comment: 12 pages, 10 figure
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