194 research outputs found
The degeneracy between the dust colour temperature and the spectral index. The problem of multiple chi^2 minima
Because of the Herschel and Planck satellite missions, there is strong
interest in the interpretation the sub-millimetre dust spectra from
interstellar clouds. Much work has been done to understand the dependence
between the spectral index beta_Obs and the colour temperature T_C that is
partly caused by the noise. The (T_C, beta_Obs) confidence regions are
elongated, banana-shaped structures. We studied under which conditions these
exhibit anomalous, strongly non-Gaussian behaviour that could affect the
interpretation of the observed (T_C, beta_Obs) relations. We examined modified
black body spectra and spectra calculated from radiative transfer models of
filamentary clouds at wavelengths 100um-850um. We performed modified black body
fits and examined the structure of the chi^2(T_, beta_Obs) function. We show
cases where, when the signal-to-noise ratio is low, the chi^2 has multiple
local minima in the (T_C, beta_Obs) plane. A small change in the weighting of
the data points can cause the solution to jump to completely different values.
In particular, noise can lead to the appearance of a separate population of
solutions with low colour temperatures and high spectral indices. The anomalies
are caused by the noise but the tendency to show multiple chi^2 minima depends
on the model and the wavelengths analysed. Deviations from the assumed single
modified black body spectrum are not important. The presence of local minima
implies that the results obtained from the chi^2 minimisation depend on the
starting point of the optimisation and may correspond to non-global minima. The
(T_C,beta_Obs) distributions may be contaminated by a few solutions with
unrealistically low colour temperatures and high spectral indices. Proper
weighting must be applied to avoid the determination of the beta_Obs(T_C)
relation to be unduly affected by these measurements.Comment: 11 pages, accepted to A&
The degeneracy between dust colour temperature and spectral index. Comparison of methods for estimating the beta(T) relation
Sub-millimetre dust emission provides information on the physics of
interstellar clouds and dust. Noise can produce anticorrelation between the
colour temperature T_C and the spectral index beta. This must be separated from
the intrinsic beta(T) relation of dust. We compare methods for the analysis of
the beta(T) relation. We examine sub-millimetre observations simulated as
simple modified black body emission or using 3D radiative transfer modelling.
In addition to chi^2 fitting, we examine the results of the SIMEX method, basic
Bayesian model, hierarchical models, and one method that explicitly assumes a
functional form for beta(T). All methods exhibit some bias. Bayesian method
shows significantly lower bias than direct chi^2 fits. The same is true for
hierarchical models that also result in a smaller scatter in the temperature
and spectral index values. However, significant bias was observed in cases with
high noise levels. Beta and T estimates of the hierarchical model are biased
towards the relation determined by the data with the highest S/N ratio. This
can alter the recovered beta(T) function. With the method where we explicitly
assume a functional form for the beta(T) relation, the bias is similar to the
Bayesian method. In the case of an actual Herschel field, all methods agree,
showing some degree of anticorrelation between T and beta.
The Bayesian method and the hierarchical models can both reduce the
noise-induced parameter correlations. However, all methods can exhibit
non-negligible bias. This is particularly true for hierarchical models and
observations of varying signal-to-noise ratios and must be taken into account
when interpreting the results.Comment: Submitted to A&A, 18 page
Mantle formation, coagulation and the origin of cloud/core-shine: II. Comparison with observations
Many dense interstellar clouds are observable in emission in the near-IR,
commonly referred to as "Cloudshine", and in the mid-IR, the so-called
"Coreshine". These C-shine observations have usually been explained with grain
growth but no model has yet been able to self-consistently explain the dust
spectral energy distribution from the near-IR to the submm. We want to
demonstrate the ability of our new core/mantle evolutionary dust model THEMIS
(The Heterogeneous dust Evolution Model at the IaS), which has been shown to be
valid in the far-IR and submm, to reproduce the C-shine observations. Our
starting point is a physically motivated core/mantle dust model. It consists of
3 dust populations: small aromatic-rich carbon grains; bigger core/mantle
grains with mantles of aromatic-rich carbon and cores either made of amorphous
aliphatic-rich carbon or amorphous silicate. We assume an evolutionary path
where these grains, when entering denser regions, may first form a second
aliphatic-rich carbon mantle (coagulation of small grains, accretion of carbon
from the gas phase), second coagulate together to form large aggregates, and
third accrete gas phase molecules coating them with an ice mantle. To compute
the corresponding dust emission and scattering, we use a 3D Monte-Carlo
radiative transfer code. We show that our global evolutionary dust modelling
approach THEMIS allows us to reproduce C-shine observations towards dense
starless clouds. Dust scattering and emission is most sensitive to the cloud
central density and to the steepness of the cloud density profile. Varying
these two parameters leads to changes, which are stronger in the near-IR, in
both the C-shine intensity and profile. With a combination of aliphatic-rich
mantle formation and low-level coagulation into aggregates, we can
self-consistently explain the observed C-shine and far-IR/submm emission
towards dense starless clouds.Comment: Paper accepted for publication in A&A with companion paper "Mantle
formation, coagulation and the origin of cloud/core-shine: I. Dust scattering
and absorption in the IR", A.P Jones, M. Koehler, N. Ysard, E. Dartois, M.
Godard, L. Gavila
Modelling the spinning dust emission from dense interstellar clouds
Electric dipole emission arising from PAHs is often invoked to explain the
anomalous microwave emission (AME). This assignation is based on an observed
tight correlation between the mid-IR emission of PAHs and the AME; and a good
agreement between models of spinning dust and the broadband AME spectrum. So
far often detected at large scale in the diffuse interstellar medium, the AME
has recently been studied in detail in well-known dense molecular clouds with
the help of Planck data. While much attention has been given to the physics of
spinning dust emission, the impact of varying local physical conditions has not
yet been considered in detail. Our aim is to study the emerging spinning dust
emission from interstellar clouds with realistic physical conditions and
radiative transfer. We use the DustEM code from Compiegne et al. to describe
the extinction and IR emission of all dust populations. The spinning dust
emission is obtained with SpDust, as described by Silsbee et al., that we have
coupled to DustEM. We carry out full radiative transfer simulations and
carefully estimate the local gas state as a function of position within
interstellar clouds. We show that the spinning dust emission is sensitive to
the abundances of the major ions and we propose a simple scheme to estimate
these abundances. We also investigate the effect of changing the cosmic-ray
rate. In dense media, where radiative transfer is mandatory, we show that the
relationship between the spinning and mid-IR emissivities of PAHs is no longer
linear and that the spinning dust emission may actually be strong at the centre
of clouds where the mid-IR PAH emission is weak. These results provide new ways
to trace grain growth from diffuse to dense medium and will be useful for the
analysis of AME at the scale of interstellar clouds.Comment: 7 pages, 10 figures, accepted by A&
The dust masses of powerful radio galaxies: clues to the triggering of their activity
We use deep Herschel Space Observatory observations of a 90% complete sample
of 32 intermediate-redshift 2Jy radio galaxies (0.05 < z < 0.7) to estimate the
dust masses of their host galaxies and thereby investigate the triggering
mechanisms for their quasar-like AGN. The dust masses derived for the radio
galaxies (7.2x10^5 < M_d < 2.6x10^8 M_sun) are intermediate between those of
quiescent elliptical galaxies on the one hand, and ultra luminous infrared
galaxies (ULIRGs) on the other. Consistent with simple models for the
co-evolution of supermassive black holes and their host galaxies, these results
suggest that most of the radio galaxies represent the late time re-triggering
of AGN activity via mergers between the host giant elliptical galaxies and
companion galaxies with relatively low gas masses. However, a minority of the
radio galaxies in our sample (~20%) have high, ULIRG-like dust masses, along
with evidence for prodigious star formation activity. The latter objects are
more likely to have been triggered in major, gas-rich mergers that represent a
rapid growth phase for both their host galaxies and their supermassive black
holes.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
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
Mantle formation, coagulation and the origin of cloud/core shine: I. Modelling dust scattering and absorption in the infra-red
Context. The observed cloudshine and coreshine (C-shine) have been explained
in terms of grain growth leading to enhanced scatter- ing from clouds in the J,
H and K photometric bands and the Spitzer IRAC 3.6 and 4.5 {\mu}m bands. Aims.
Using our global dust modelling approach THEMIS (The Heterogeneous dust
Evolution Model at the IaS) we explore the effects of dust evolution in dense
clouds, through aliphatic-rich carbonaceous mantle formation and grain-grain
coagulation. Methods. We model the effects of wide band gap a-C:H mantle
formation and the low-level aggregation of diffuse interstellar medium dust in
the moderately-extinguished outer regions of molecular clouds. Results. The
formation of wide band gap a-C:H mantles on amorphous silicate and amorphous
carbon (a-C) grains leads to a decrease in their absorption cross-sections but
no change in their scattering cross-sections at near-IR wavelengths, resulting
in higher albedos. Conclusions. The evolution of dust, with increasing density
and extinction in the diffuse to dense molecular cloud transition, through
mantle formation and grain aggregation, appears to be a likely explanation for
the observed C-shine.Comment: 12 pages, 15 figures, accepted for publication in A&A along with the
companion paper entitled, Mantle formation, coagulation and the origin of
cloud/core shine: II Comparison with observations, by Ysard et al. (also
accepted for publication in A&A
New radio observations of anomalous microwave emission in the HII region RCW175
We have observed the HII region RCW175 with the 64m Parkes telescope at
8.4GHz and 13.5GHz in total intensity, and at 21.5GHz in both total intensity
and polarization. High angular resolution, high sensitivity, and polarization
capability enable us to perform a detailed study of the different constituents
of the HII region. For the first time, we resolve three distinct regions at
microwave frequencies, two of which are part of the same annular diffuse
structure. Our observations enable us to confirm the presence of anomalous
microwave emission (AME) from RCW175. Fitting the integrated flux density
across the entire region with the currently available spinning dust models,
using physically motivated assumptions, indicates the presence of at least two
spinning dust components: a warm component with a relatively large hydrogen
number density n_H=26.3/cm^3 and a cold component with a hydrogen number
density of n_H=150/cm^3. The present study is an example highlighting the
potential of using high angular-resolution microwave data to break model
parameter degeneracies. Thanks to our spectral coverage and angular resolution,
we have been able to derive one of the first AME maps, at 13.5GHz, showing
clear evidence that the bulk of the AME arises in particular from one of the
source components, with some additional contribution from the diffuse
structure. A cross-correlation analysis with thermal dust emission has shown a
high degree of correlation with one of the regions within RCW175. In the center
of RCW175, we find an average polarized emission at 21.5GHz of
2.2\pm0.2(rand.)\pm0.3(sys.)% of the total emission, where we have included
both systematic and statistical uncertainties at 68% CL. This polarized
emission could be due to sub-dominant synchrotron emission from the region and
is thus consistent with very faint or non-polarized emission associated with
AME.Comment: Accepted for publication in the Astrophysical Journa
Nano-grain depletion in photon-dominated regions
Context. Carbonaceous nano-grains play a fundamental role in the
physico-chemistry of the interstellar medium (ISM) and especially of
photon-dominated regions (PDRs). Their properties vary with the local physical
conditions and affect the local chemistry and dynamics. Aims. We aim to
highlight the evolution of carbonaceous nano-grains in three different PDRs and
propose a scenario of dust evolution as a response to the physical conditions.
Methods. We used Spitzer/IRAC (3.6, 4.5, 5.8, and 8 m) and Spitzer/MIPS
(24 m) together with Herschel/PACS (70 m) to map dust emission in
IC63 and the Orion Bar. To assess the dust properties, we modelled the dust
emission in these regions using the radiative transfer code SOC together with
the THEMIS dust model. Results. Regardless of the PDR, we find that nano-grains
are depleted and that their minimum size is larger than in the diffuse ISM
(DISM), which suggests that the mechanisms that lead nano-grains to be
photo-destroyed are very efficient below a given critical size limit. The
evolution of the nano-grain dust-to-gas mass ratio with both G0 and the
effective temperature of the illuminating star indicates a competition between
the nano-grain formation through the fragmentation of larger grains and
nano-grain photo-destruction. We modelled dust collisions driven by radiative
pressure with a classical 1D approach to show that this is a viable scenario
for explaining nano-grain formation through fragmentation and, thus, the
variations observed in nano-grain dust-to-gas mass ratios from one PDR to
another. Conclusions. We find a broad variation in the nano-grain dust
properties from one PDR to another, along with a general trend of nano-grain
depletion in these regions. We propose a viable scenario of nano-grain
formation through fragmentation of large grains due to radiative
pressure-induced collisions
Dust evolution across the Horsehead nebula
Context. Micro-physical processes on interstellar dust surfaces are tightly connected to dust properties (i.e. dust composition, size, and shape) and play a key role in numerous phenomena in the interstellar medium (ISM). The large disparity in physical conditions (i.e. density and gas temperature) in the ISM triggers an evolution of dust properties. The analysis of how dust evolves with the physical conditions is a stepping stone towards a more thorough understanding of interstellar dust.Aims. We highlight dust evolution in the Horsehead nebula photon-dominated region.Methods. We used Spitzer/IRAC (3.6, 4.5, 5.8 and 8 mu m) and Spitzer/MIPS (24 mu m) together with Herschel/PACS (70 and 160 mu m) and Herschel/SPIRE (250, 350 and 500 mu m) to map the spatial distribution of dust in the Horsehead nebula over the entire emission spectral range. We modelled dust emission and scattering using the THEMIS interstellar dust model together with the 3D radiative transfer code SOC.Results. We find that the nano-grain dust-to-gas ratio in the irradiated outer part of the Horsehead is 6-10 times lower than in the diffuse ISM. The minimum size of these grains is 2-2.25 times larger than in the diffuse ISM, and the power-law exponent of their size distribution is 1.1-1.4 times lower than in the diffuse ISM. In the denser part of the Horsehead nebula, it is necessary to use evolved grains (i.e. aggregates, with or without an ice mantle).Conclusions. It is not possible to explain the observations using grains from the diffuse medium. We therefore propose the following scenario to explain our results. In the outer part of the Horsehead nebula, all the nano-grain have not yet had time to re-form completely through photo-fragmentation of aggregates and the smallest of the nano-grain that are sensitive to the radiation field are photo-destroyed. In the inner part of the Horsehead nebula, grains most likely consist of multi-compositional mantled aggregates.Peer reviewe
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