280 research outputs found
Large and small-scale structures and the dust energy balance problem in spiral galaxies
The interstellar dust content in galaxies can be traced in extinction at
optical wavelengths, or in emission in the far-infrared. Several studies have
found that radiative transfer models that successfully explain the optical
extinction in edge-on spiral galaxies generally underestimate the observed
FIR/submm fluxes by a factor of about three. In order to investigate this
so-called dust energy balance problem, we use two Milky Way-like galaxies
produced by high-resolution hydrodynamical simulations. We create mock optical
edge-on views of these simulated galaxies (using the radiative transfer code
SKIRT), and we then fit the parameters of a basic spiral galaxy model to these
images (using the fitting code FitSKIRT). The basic model includes smooth
axisymmetric distributions along a S\'ersic bulge and exponential disc for the
stars, and a second exponential disc for the dust. We find that the dust mass
recovered by the fitted models is about three times smaller than the known dust
mass of the hydrodynamical input models. This factor is in agreement with
previous energy balance studies of real edge-on spiral galaxies. On the other
hand, fitting the same basic model to less complex input models (e.g. a smooth
exponential disc with a spiral perturbation or with random clumps), does
recover the dust mass of the input model almost perfectly. Thus it seems that
the complex asymmetries and the inhomogeneous structure of real and
hydrodynamically simulated galaxies are a lot more efficient at hiding dust
than the rather contrived geometries in typical quasi-analytical models. This
effect may help explain the discrepancy between the dust emission predicted by
radiative transfer models and the observed emission in energy balance studies
for edge-on spiral galaxies.Comment: 9 pages, 5 figures, accepted for publication in A&
The mass, location and heating of the dust in the Cassiopeia A supernova remnant
We model the thermal dust emission from dust grains heated by synchrotron
radiation and by particle collisions, under conditions appropriate for four
different shocked and unshocked gas components of the Cassiopeia A (Cas A)
supernova remnant (SNR). By fitting the resulting spectral energy distributions
(SEDs) to the observed SNR dust fluxes, we determine the required mass of dust
in each component. We find the observed SED can be reproduced by of silicate grains, the majority of which is in the unshocked
ejecta and heated by the synchrotron radiation field. Warmer dust, located in
the X-ray emitting reverse shock and blastwave regions, contribute to the
shorter wavelength infrared emission but make only a small fraction of the
total dust mass. Carbon grains can at most make up of the total
dust mass. Combined with estimates for the gas masses, we obtain dust-to-gas
mass ratios for each component, which suggest that the condensation efficiency
in the ejecta is high, and that dust in the shocked ejecta clumps is well
protected from destruction by sputtering in the reverse shock.Comment: Accepted by MNRAS, 14 pages, 6 figures. Author accepted manuscript.
Accepted on 05/02/2019. Deposited on 05/02/201
HERschel Observations of Edge-on Spirals (HEROES). II: Tilted-ring modelling of the atomic gas disks
Context. Edge-on galaxies can offer important insights in galaxy evolution as
they are the only systems where the distribution of the different components
can be studied both radially and vertically. The HEROES project was designed to
investigate the interplay between the gas, dust, stars and dark matter (DM) in
a sample of 7 massive edge-on spiral galaxies.
Aims. In this second HEROES paper we present an analysis of the atomic gas
content of 6 out of 7 galaxies in our sample. The remaining galaxy was recently
analysed according to the same strategy. The primary aim of this work is to
constrain the surface density distribution, the rotation curve and the geometry
of the gas disks in a homogeneous way. In addition we identify peculiar
features and signs of recent interactions.
Methods. We construct detailed tilted-ring models of the atomic gas disks
based on new GMRT 21-cm observations of NGC 973 and UGC 4277 and re-reduced
archival HI data of NGC 5907, NGC 5529, IC 2531 and NGC 4217. Potential
degeneracies between different models are resolved by requiring a good
agreement with the data in various representations of the data cubes.
Results. From our modelling we find that all but one galaxy are warped along
the major axis. In addition, we identify warps along the line of sight in three
galaxies. A flaring gas layer is required to reproduce the data only for one
galaxy, but (moderate) flares cannot be ruled for the other galaxies either. A
coplanar ring-like structure is detected outside the main disk of NGC 4217,
which we suggest could be the remnant of a recent minor merger event. We also
find evidence for a radial inflow of 15 +- 5 km/s in the disk of NGC 5529,
which might be related to the ongoing interaction with two nearby companions.
(Abridged)Comment: 39 pages, 38 figures, Accepted for publication in Astronomy &
Astrophysic
Constraining early-time dust formation in core-collapse supernovae
There is currently a severe discrepancy between theoretical models of dust
formation in core-collapse supernovae (CCSNe), which predict
M of ejecta dust forming within days, and observations at
these epochs, which infer much lower masses. We demonstrate that, in the
optically thin case, these low dust masses are robust despite significant
observational and model uncertainties. For a sample of 11 well-observed CCSNe,
no plausible model reaches carbon dust masses above M, or
silicate masses above M. Optically thick models can
accommodate larger dust masses, but the dust must be clumped and have a low
() covering fraction to avoid conflict with data at optical wavelengths.
These values are insufficient to reproduce the observed infrared fluxes, and
the required covering fraction varies not only between SNe but between epochs
for the same object. The difficulty in reconciling large dust masses with
early-time observations of CCSNe, combined with well-established detections of
comparably large dust masses in supernova remnants, suggests that a mechanism
for late-time dust formation is necessary.Comment: 14 pages, 13 figures. MNRAS accepted 10/07/2
Properties of shocked dust grains in supernova remnants
Shockwaves driven by supernovae both destroy dust and reprocess the surviving
grains, greatly affecting the resulting dust properties of the interstellar
medium (ISM). While these processes have been extensively studied
theoretically, observational constraints are limited. We use
physically-motivated models of dust emission to fit the infrared (IR) spectral
energy distributions of seven Galactic supernova remnants, allowing us to
determine the distribution of dust mass between diffuse and dense gas phases,
and between large and small grain sizes. We find that the dense (), relatively cool () gas phase contains
of the dust mass, making the warm dust located in the X-ray emitting
plasma (/) a negligible fraction of
the total, despite dominating the mid-IR emission. The ratio of small
() to large () grains in
the cold component is consistent with that in the ISM, and possibly even
higher, whereas the hot phase is almost entirely devoid of small grains. This
suggests that grain shattering, which processes large grains into smaller ones,
is ineffective in the low-density gas, contrary to model predictions.
Single-phase models of dust destruction in the ISM, which do not account for
the existence of the cold swept-up material containing most of the dust mass,
are likely to greatly overestimate the rate of dust destruction by supernovae.Comment: 13 pages, 12 figures. MNRAS accepte
Revisiting the dust destruction efficiency of supernovae
Dust destruction by supernovae is one of the main processes removing dust from the interstellar medium (ISM). Estimates of the efficiency of this process, both theoretical and observational, typically assume a shock propagating into a homogeneous medium, whereas the ISM possesses significant substructure in reality. We self-consistently model the dust and gas properties of the shocked ISM in three supernova remnants (SNRs), using X-ray and infrared (IR) data combined with corresponding emission models. Collisional heating by gas with properties derived from X-ray observations produces dust temperatures too high to fit the far-IR fluxes from each SNR. An additional colder dust component is required, which has a minimum mass several orders of magnitude larger than that of the warm dust heated by the X-ray emitting gas. Dust-to-gas mass ratios indicate that the majority of the dust in the X-ray emitting material has been destroyed, while the fraction of surviving dust in the cold component is plausibly close to unity. As the cold component makes up virtually all the total dust mass, destruction timescales based on homogeneous models, which cannot account for multiple phases of shocked gas and dust, may be significantly overestimating actual dust destruction efficiencies, and subsequently underestimating grain lifetimes
HERschel Observations of Edge-on Spirals (HEROES). I: Far-infrared morphology and dust mass determination
Context. Edge-on spiral galaxies with prominent dust lanes provide us with an
excellent opportunity to study the distribution and properties of the dust
within them. The HEROES project was set up to observe a sample of seven large
edge-on galaxies across various wavelengths for this investigation.
Aims. Within this first paper, we present the Herschel observations and
perform a qualitative and quantitative analysis on them, and we derive some
global properties of the far infrared and submillimetre emission.
Methods. We determine horizontal and vertical profiles from the Herschel
observations of the galaxies in the sample and describe the morphology.
Modified black-body fits to the global fluxes, measured using aperture
photometry, result in dust temperatures and dust masses. The latter values are
compared to those that are derived from radiative transfer models taken from
the literature.
Results. On the whole, our Herschel flux measurements agree well with
archival values. We find that the exponential horizontal dust distribution
model often used in the literature generally provides a good description of the
observed horizontal profiles. Three out of the seven galaxies show signatures
of extended vertical emission at 100 and 160 {\mu}m at the 5{\sigma} level, but
in two of these it is probably due to deviations from an exactly edge-on
orientation. Only for NGC 4013, a galaxy in which vertically extended dust has
already been detected in optical images, we can detect vertically extended
dust, and the derived scaleheight agrees with the value estimated through
radiative transfer modelling. Our analysis hints at a correlation between the
dust scaleheight and its degree of clumpiness, which we infer from the
difference between the dust masses as calculated from modelling of optical data
and from fitting the spectral energy distribution of Herschel datapoints.Comment: 21 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Cognitive and Structural Correlates of Conversational Speech Timing in Mild Cognitive Impairment and Mild-to-Moderate Alzheimer’s Disease : Relevance for Early Detection Approaches.
FUNDING This study was supported by a Centre for Ageing Research and Development in Ireland (CARDI) Leadership Fellowship (grant number 13533).Peer reviewedPublisher PD
The first maps of κd - the dust mass absorption coefficient - in nearby galaxies, with DustPedia
The dust mass absorption coefficient, κd is the conversion function used to infer physical dust masses from observations of dust emission. However, it is notoriously poorly constrained, and it is highly uncertain how it varies, either between or within galaxies. Here we present the results of a proof-of-concept study, using the DustPedia data for two nearby face-on spiral galaxies M 74 (NGC 628) and M 83 (NGC 5236), to create the first ever maps of κd in galaxies. We determine κd using an empirical method that exploits the fact that the dust-to-metals ratio of the interstellar medium is constrained by direct measurements of the depletion of gas-phase metals. We apply this method pixel-by-pixel within M 74 and M 83, to create maps of κd. We also demonstrate a novel method of producing metallicity maps for galaxies with irregularly sampled measurements, using the machine learning technique of Gaussian process regression. We find strong evidence for significant variation in κd. We find values of κd at 500 μm spanning the range 0.11-0.25 m^{2 kg^{-1}} in M 74, and 0.15-0.80 m^{2 kg^{-1}} in M 83. Surprisingly, we find that κd shows a distinct inverse correlation with the local density of the interstellar medium. This inverse correlation is the opposite of what is predicted by standard dust models. However, we find this relationship to be robust against a large range of changes to our method - only the adoption of unphysical or highly unusual assumptions would be able to suppress it
The dust content of the Crab Nebula
We have modelled the near-infrared to radio images of the Crab Nebula with a Bayesian SED
model to simultaneously fit its synchrotron, interstellar (IS), and supernova dust emission. We
infer an IS dust extinction map with an average AV = 1.08 ± 0.38 mag, consistent with a small
contribution (22 per cent) to the Crab’s overall infrared emission. The Crab’s supernova dust
mass is estimated to be between 0.032 and 0.049 M (for amorphous carbon grains) with an
average dust temperature Tdust = 41 ± 3 K, corresponding to a dust condensation efficiency
of 8–12 per cent. This revised dust mass is up to an order of magnitude lower than some
previous estimates, which can be attributed to our different IS dust corrections, lower SPIRE
flux densities, and higher dust temperatures than were used in previous studies. The dust
within the Crab is predominantly found in dense filaments south of the pulsar, with an average
V-band dust extinction of AV = 0.20–0.39 mag, consistent with recent optical dust extinction
studies. The modelled synchrotron power-law spectrum is consistent with a radio spectral
index αradio = 0.297 ± 0.009 and an infrared spectral index αIR = 0.429 ± 0.021. We have
identified a millimetre excess emission in the Crab’s central regions, and argue that it most
likely results from two distinct populations of synchrotron emitting particles. We conclude
that the Crab’s efficient dust condensation (8–12 per cent) provides further evidence for a
scenario where supernovae can provide substantial contributions to the IS dust budgets in
galaxies
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