319 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&
Microwave and radio emission of dusty star-forming galaxies: Implication for the cosmic radio background
We use the most up-to-date cosmological evolution models of star-forming (SF)
galaxies and radio sources to compute the extragalactic number counts and the
cosmic background from 408MHz to 12THz. The model of SF galaxies reproduces the
constraints obtained by Spitzer, Herschel, and ground-based submm/mm
experiments: number counts, redshift distribution of galaxies, cosmic
background intensity and anisotropies. The template SEDs of this model are
extrapolated to the radio adding synchrotron, free-free, and spinning dust
emissions. To fix the synchrotron intensity, we use the IR/radio flux ratio,
q70, and a spectral index beta=-3. For a constant q70, our model added to the
AGN contribution provides a good fit to the number counts from 12THz to 408MHz
and to the CIB. Spinning dust accounts for up to 20% of the cosmic microwave
background produced by SF galaxies, but for less than 10% of the total
background when AGN are included. The SF galaxies account for 77.5% of the
number counts at 1.4GHz for a flux of 1e-4Jy. However, the model does not
explain the CRB measured with the ARCADE2 experiment. Considering the case when
q70 decreases strongly with redshift, this still does not explain the ARCADE2
results. It also yields to an overestimate of the low-flux number counts in the
radio. Thus, we rule out a steep variation of q70 with redshift at least for
z<3.5. Adding a population of faint SF galaxies at high redshift (Lir<1e11Lsun
and 4<z<6), which would reproduce the ARCADE2 results, leads to predictions of
the CIB much higher than what is observed, ruling out this as the explanation
for the ARCADE2 results. Considering our findings and previous studies, we
conclude that if the radio emission measured by ARCADE2 is astrophysical in
origin, it has to originate in the Galaxy or in a new kind of radio sources
(with no mid- to far-IR counterparts) or emission mechanism still to be
discovered.Comment: accepted for publication by A&A, modification of one citatio
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
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&
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
Embodied Artificial Intelligence through Distributed Adaptive Control: An Integrated Framework
In this paper, we argue that the future of Artificial Intelligence research
resides in two keywords: integration and embodiment. We support this claim by
analyzing the recent advances of the field. Regarding integration, we note that
the most impactful recent contributions have been made possible through the
integration of recent Machine Learning methods (based in particular on Deep
Learning and Recurrent Neural Networks) with more traditional ones (e.g.
Monte-Carlo tree search, goal babbling exploration or addressable memory
systems). Regarding embodiment, we note that the traditional benchmark tasks
(e.g. visual classification or board games) are becoming obsolete as
state-of-the-art learning algorithms approach or even surpass human performance
in most of them, having recently encouraged the development of first-person 3D
game platforms embedding realistic physics. Building upon this analysis, we
first propose an embodied cognitive architecture integrating heterogenous
sub-fields of Artificial Intelligence into a unified framework. We demonstrate
the utility of our approach by showing how major contributions of the field can
be expressed within the proposed framework. We then claim that benchmarking
environments need to reproduce ecologically-valid conditions for bootstrapping
the acquisition of increasingly complex cognitive skills through the concept of
a cognitive arms race between embodied agents.Comment: Updated version of the paper accepted to the ICDL-Epirob 2017
conference (Lisbon, Portugal
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
Probing the origin of the microwave anomalous foreground
The galactic anomalous microwave emission detected between 10 and 90 GHz is a
major foreground to CMB fluctuations. Well correlated to dust emission at 100
m, the anomalous emission is interstellar but its origin is still debated.
Some possible explanations relate it to dust: emission of spinning, small
(nanometric) grains carrying a permanent electric dipole or magnetic
fluctuations in larger (submicronic) grains. To probe the origin of the
anomalous emission, we compare microwave data to dust IR emission and search
for specific signatures predicted by models of spinning dust. For the anomalous
emission, we use the 23 GHz all-sky map deduced from WMAP data by
Miville-Deschenes et al. (2008). The dust emission is traced by IRAS data.
Models show that spinning dust emission is little sensitive to the intensity of
the radiation field (Go) for 10<nu<30 GHz while the corresponding mid-IR
emission is proportional to Go. To test this behaviour in our comparison, we
derive Go from the dust temperature maps of Schlegel et al. (1998). From
all-sky maps, we show that the anomalous emission is better correlated to the
emission of small grains (at 12 m) than to that of big grains (at 100
m). In addition we show that the former correlation is significantly
improved when the 12 m flux is divided by Go, as predicted by current
models of spinning dust.Comment: 4 pages, 5 figures, accepted by A&
The cycling of carbon into and out of dust
Observational evidence seems to indicate that the depletion of interstellar
carbon into dust shows rather wide variations and that carbon undergoes rather
rapid recycling in the interstellar medium (ISM). Small hydrocarbon grains are
processed in photo-dissociation regions by UV photons, by ion and electron
collisions in interstellar shock waves and by cosmic rays. A significant
fraction of hydrocarbon dust must therefore be re-formed by accretion in the
dense, molecular ISM. A new dust model (Jones et al., Astron. Astrophys., 2013,
558, A62) shows that variations in the dust observables in the diffuse
interstellar medium (nH = 1000 cm^3), can be explained by systematic and
environmentally-driven changes in the small hydrocarbon grain population. Here
we explore the consequences of gas-phase carbon accretion onto the surfaces of
grains in the transition regions between the diffuse ISM and molecular clouds
(e.g., Jones, Astron. Astrophys., 2013, 555, A39). We find that significant
carbonaceous dust re-processing and/or mantle accretion can occur in the outer
regions of molecular clouds and that this dust will have significantly
different optical properties from the dust in the adjacent diffuse ISM. We
conclude that the (re-)processing and cycling of carbon into and out of dust is
perhaps the key to advancing our understanding of dust evolution in the ISM.Comment: 14 pages, 6 figure
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
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