484 research outputs found
Physical properties of a very diffuse HI structure at high Galactic latitude
The main goal of this analysis is to present a new method to estimate the
physical properties of diffuse cloud of atomic hydrogen observed at high
Galactic latitude. This method, based on a comparison of the observations with
fractional Brownian motion simulations, uses the statistical properties of the
integrated emission, centroid velocity and line width to constrain the physical
properties of the 3D density and velocity fields, as well as the average
temperature of HI. We applied this method to interpret 21 cm observations
obtained with the Green Bank Telescope of a very diffuse HI cloud at high
Galactic latitude located in Firback North 1. We first show that the
observations cannot be reproduced solely by highly-turbulent CNM type gas and
that there is a significant contribution of thermal broadening to the line
width observed. To reproduce the profiles one needs to invoke two components
with different average temperature and filling factor. We established that, in
this very diffuse part of the ISM, 2/3 of the column density is made of WNM and
1/3 of thermally unstable gas (T ~2600 K). The WNM gas is mildly supersonic
(~1) and the unstable phase is definitely sub-sonic (~0.3). The density
contrast (i.e., the standard deviation relative to the mean of density
distribution) of both components is close to 0.8. The filling factor of the WNM
is 10 times higher that of the unstable gas, which has a density structure
closer to what would be expected for CNM gas. This field contains a signature
of CNM type gas at a very low level (N_H ~ 3 x 10^19) which could have been
formed by a convergent flow of WNM gas.Comment: 13 pages, 12 figures, accepted for publication in A&
Dust models post-Planck: constraining the far-infrared opacity of dust in the diffuse interstellar medium
We compare the performance of several dust models in reproducing the dust
spectral energy distribution (SED) per unit extinction in the diffuse
interstellar medium (ISM). We use our results to constrain the variability of
the optical properties of big grains in the diffuse ISM, as published by the
Planck collaboration.
We use two different techniques to compare the predictions of dust models to
data from the Planck HFI, IRAS and SDSS surveys. First, we fit the far-infrared
emission spectrum to recover the dust extinction and the intensity of the
interstellar radiation field (ISRF). Second, we infer the ISRF intensity from
the total power emitted by dust per unit extinction, and then predict the
emission spectrum. In both cases, we test the ability of the models to
reproduce dust emission and extinction at the same time.
We identify two issues. Not all models can reproduce the average dust
emission per unit extinction: there are differences of up to a factor
between models, and the best accord between model and observation is obtained
with the more emissive grains derived from recent laboratory data on silicates
and amorphous carbons. All models fail to reproduce the variations in the
emission per unit extinction if the only variable parameter is the ISRF
intensity: this confirms that the optical properties of dust are indeed
variable in the diffuse ISM.
Diffuse ISM observations are consistent with a scenario where both ISRF
intensity and dust optical properties vary. The ratio of the far-infrared
opacity to the band extinction cross-section presents variations of the
order of ( in extreme cases), while ISRF intensity varies
by ( in extreme cases). This must be accounted for in
future modelling.Comment: A&A, in pres
FIRBACK: II. Data Reduction and Calibration of the 170 micron ISO Deep Cosmological Survey
We present the final reduction and calibration of the FIRBACK ISOPHOT data.
FIRBACK is a deep cosmological survey performed at 170 microns. This paper
deals with the ISOPHOT C200 camera with the C160 filter. We review the whole
data reduction process and compare our final calibration with DIRBE (for the
extended emission) and IRAS (for point sources). The FIRBACK source extraction
and galaxy counts is discussed in a companion paper (Dole et al., 2001).Comment: Accepted for publication in A&A. 9 pages, includes new aa.cls. Also
available (with better quality figures) at http://wwwfirback.ias.u-psud.fr
and http://mips.as.arizona.edu/~hdole/firback (new aa.cls is here
Direct Estimate of Cirrus Noise in Herschel Hi-GAL Images
In Herschel images of the Galactic plane and many star forming regions, a
major factor limiting our ability to extract faint compact sources is cirrus
confusion noise, operationally defined as the "statistical error to be expected
in photometric measurements due to confusion in a background of fluctuating
surface brightness". The histogram of the flux densities of extracted sources
shows a distinctive faint-end cutoff below which the catalog suffers from
incompleteness and the flux densities become unreliable. This empirical cutoff
should be closely related to the estimated cirrus noise and we show that this
is the case. We compute the cirrus noise directly, both on Herschel images from
which the bright sources have been removed and on simulated images of cirrus
with statistically similar fluctuations. We connect these direct estimates with
those from power spectrum analysis, which has been used extensively to predict
the cirrus noise and provides insight into how it depends on various
statistical properties and photometric operational parameters. We report
multi-wavelength power spectra of diffuse Galactic dust emission from Hi-GAL
observations at 70 to 500 microns within Galactic plane fields at l= 30 degrees
and l= 59 degrees. We find that the exponent of the power spectrum is about -3.
At 250 microns, the amplitude of the power spectrum increases roughly as the
square of the median brightness of the map and so the expected cirrus noise
scales linearly with the median brightness. Generally, the confusion noise will
be a worse problem at longer wavelengths, because of the combination of lower
angular resolution and the rising power spectrum of cirrus toward lower spatial
frequencies, but the photometric signal to noise will also depend on the
relative spectral energy distribution of the source compared to the cirrus.Comment: 4 pages (in journal), 3 figures, Astronomy and Astrophysics, accepted
for publication 13 May 201
Statistical properties of dust far-infrared emission
The description of the statistical properties of dust emission gives
important constraints on the physics of the interstellar medium but it is also
a useful way to estimate the contamination of diffuse interstellar emission in
the cases where it is considered a nuisance. The main goals of this analysis of
the power spectrum and non-Gaussian properties of 100 micron dust emission are
1) to estimate the power spectrum of interstellar matter density in three
dimensions, 2) to review and extend previous estimates of the cirrus noise due
to dust emission and 3) to produce simulated dust emission maps that reproduce
the observed statistical properties. The main results are the following. 1) The
cirrus noise level as a function of brightness has been previously
overestimated. It is found to be proportional to instead of ^1.5, where
is the local average brightness at 100 micron. This scaling is in
accordance with the fact that the brightness fluctuation level observed at a
given angular scale on the sky is the sum of fluctuations of increasing
amplitude with distance on the line of sight. 2) The spectral index of dust
emission at scales between 5 arcmin and 12.5 degrees is =-2.9 on average
but shows significant variations over the sky. Bright regions have
systematically steeper power spectra than diffuse regions. 3) The skewness and
kurtosis of brightness fluctuations is high, indicative of strong
non-Gaussianity. 4) Based on our characterization of the 100 micron power
spectrum we provide a prescription of the cirrus confusion noise as a function
of wavelength and scale. 5) Finally we present a method based on a modification
of Gaussian random fields to produce simulations of dust maps which reproduce
the power spectrum and non-Gaussian properties of interstellar dust emission.Comment: 13 pages, 13 figures. Accepted for publication in A&
An Imprint of Molecular Cloud Magnetization in the Morphology of the Dust Polarized Emission
We describe a morphological imprint of magnetization found when considering
the relative orientation of the magnetic field direction with respect to the
density structures in simulated turbulent molecular clouds. This imprint was
found using the Histogram of Relative Orientations (HRO): a new technique that
utilizes the gradient to characterize the directionality of density and column
density structures on multiple scales. We present results of the HRO analysis
in three models of molecular clouds in which the initial magnetic field
strength is varied, but an identical initial turbulent velocity field is
introduced, which subsequently decays. The HRO analysis was applied to the
simulated data cubes and mock-observations of the simulations produced by
integrating the data cube along particular lines of sight. In the 3D analysis
we describe the relative orientation of the magnetic field with
respect to the density structures, showing that: 1.The magnetic field shows a
preferential orientation parallel to most of the density structures in the
three simulated cubes. 2.The relative orientation changes from parallel to
perpendicular in regions with density over a critical density in the
highest magnetization case. 3.The change of relative orientation is largest for
the highest magnetization and decreases in lower magnetization cases. This
change in the relative orientation is also present in the projected maps. In
conjunction with simulations HROs can be used to establish a link between the
observed morphology in polarization maps and the physics included in
simulations of molecular clouds.Comment: (16 pages, 11 figures, submitted to ApJ 05MAR2013, accepted
07JUL2013
Low-Velocity Halo Clouds
Models that reproduce the observed high-velocity clouds (HVCs) also predict
clouds at lower radial velocities that may easily be confused with Galactic
disk (|z| < 1 kpc) gas. We describe the first search for these low-velocity
halo clouds (LVHCs) using IRAS data and the initial data from the Galactic
Arecibo L-band Feed Array survey in HI (GALFA-HI). The technique is based upon
the expectation that such clouds should, like HVCs, have very limited infrared
thermal dust emission as compared to their HI column density. We describe our
'displacement-map' technique for robustly determining the dust-to-gas ratio of
clouds and the associated errors that takes into account the significant
scatter in the infrared flux from the Galactic disk gas. We find that there
exist lower-velocity clouds that have extremely low dust-to-gas ratios,
consistent with being in the Galactic halo - candidate LVHCs. We also confirm
the lack of dust in many HVCs with the notable exception of complex M, which we
consider to be the first detection of warm dust in HVCs. We do not confirm the
previously reported detection of dust in complex C. In addition, we find that
most Intermediate- and Low-Velocity clouds that are part of the Galactic disk
have a higher 60 micron/100 micron flux ratio than is typically seen in
Galactic HI, which is consistent with a previously proposed picture in which
fast-moving Galactic clouds have smaller, hotter dust grains.Comment: 30 pages, 7 figures. Accepted to the Ap
Foreground removal from WMAP 5yr temperature maps using an MLP neural network
One of the main obstacles for extracting the cosmic microwave background
(CMB) signal from observations in the mm/sub-mm range is the foreground
contamination by emission from Galactic component: mainly synchrotron,
free-free, and thermal dust emission. The statistical nature of the intrinsic
CMB signal makes it essential to minimize the systematic errors in the CMB
temperature determinations. The feasibility of using simple neural networks to
extract the CMB signal from detailed simulated data has already been
demonstrated. Here, simple neural networks are applied to the WMAP 5yr
temperature data without using any auxiliary data. A simple \emph{multilayer
perceptron} neural network with two hidden layers provides temperature
estimates over more than 75 per cent of the sky with random errors
significantly below those previously extracted from these data. Also, the
systematic errors, i.e.\ errors correlated with the Galactic foregrounds, are
very small. With these results the neural network method is well prepared for
dealing with the high - quality CMB data from the ESA Planck Surveyor
satellite.Comment: 6 pages, 13 figure
Velocity centroids and the structure of interstellar turbulence: I. Analytical study
We present an analytical study of the statistical properties of integrated
emission and velocity centroids for a slightly compressible turbulent slab
model, to retrieve the underlying statistics of three-dimensional density and
velocity fluctuations. Under the assumptions that the density and velocity
fields are homogeneous and isotropic, we derive the expressions of the antenna
temperature for an optically thin spectral line observation, and of its
successive moments with respect to the line of sight velocity component,
focusing on the zeroth (intensity or integrated emission I) and first
(non-normalized velocity centroid C) moments. The ratio of the latter to the
former is the normalized centroid C_0, whose expression can be linearized for
small density fluctuations. To describe the statistics of I, C and C_0, we
derive expansions of their autocorrelation functions in powers of density
fluctuations and perform a lowest-order real-space calculation of their scaling
behaviour, assuming that the density and velocity fields are fractional
Brownian motions. We hence confirm, within the scope of this study, the
property recently found numerically by Miville-Deschenes, Levrier and Falgarone
(2003, ApJ, 593, 831) that the spectral index of the normalized centroid is
equal to that of the full velocity field. However, it is also argued that, in
order to retrieve the velocity statistics, normalization of centroids may
actually not be the best way to remove the influence of density fluctuations.
In this respect, we discuss the modified velocity centroids introduced by
Lazarian and Esquivel (2003, ApJL, 592, 37) as a possible alternative. In a
following paper, we shall present numerical studies aimed at assessing the
validity domain of these results.Comment: 23 pages, 2 figures, to be published in Astronomy & Astrophysic
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