441 research outputs found
The Herschel Virgo Cluster Survey. VII. Dust in cluster dwarf elliptical galaxies
We use the science demonstration phase data of the Herschel Virgo Cluster Survey to search for dust emission of early-type dwarf galaxies in the central regions of the Virgo cluster as an alternative way of identifying the interstellar medium. We present the first possible far-infrared detection of cluster early-type dwarf galaxies: VCC781 and VCC951 are detected at the 10σ level in the SPIRE 250 μm image. Both detected galaxies have
dust masses of the order of 10^5 M_⊙ and average dust temperatures ≈20 K. The detection rate (less than 1%) is quite high compared to the 1.7% detection rate for Hi emission, considering that dwarfs in the central regions are more Hi deficient. We conclude that the removal of interstellar dust from dwarf galaxies resulting from ram pressure stripping, harassment, or tidal effects must be as efficient as the removal of interstellar gas
The efficiency of grain growth in the diffuse interstellar medium
Grain growth by accretion of gas-phase metals is a common assumption in models of dust evolution, but in dense gas, where the timescale is short enough for accretion to be effective, material is accreted in the form of ice mantles rather than adding to the refractory grain mass. It has been suggested that negatively-charged small grains in the diffuse interstellar medium (ISM) can accrete efficiently due to the Coulomb attraction of positively-charged ions, avoiding this issue. We show that this inevitably results in the growth of the small-grain radii until they become positively charged, at which point further growth is effectively halted. The resulting gas-phase depletions under diffuse ISM conditions are significantly overestimated when a constant grain size distribution is assumed. While observed depletions can be reproduced by changing the initial size distribution or assuming highly efficient grain shattering, both options result in unrealistic levels of far-ultraviolet extinction. We suggest that the observed elemental depletions in the diffuse ISM are better explained by higher initial depletions, combined with inefficient dust destruction by supernovae at moderate (nH ∼ 30 cm−3) densities, rather than by higher accretion efficiences
The impact of metallicity-dependent dust destruction on the dust-to-metals ratio in galaxies
The ratio of the mass of interstellar dust to the total mass of metals (the dust-to-metals/DTM ratio) tends to increase with metallicity. This can be explained by the increasing efficiency of grain growth in the interstellar medium (ISM) at higher metallicities, with a corollary being that the low DTM ratios seen at low metallicities are due to inefficient stellar dust production. This interpretation assumes that the efficiency of dust destruction in the ISM is constant, whereas it might be expected to increase at low metallicity; the decreased cooling efficiency of low-metallicity gas should result in more post-shock dust destruction via thermal sputtering. We show that incorporating a sufficiently strong metallicity dependence into models of galaxy evolution removes the need for low stellar dust yields. The contribution of stellar sources to the overall dust budget may be significantly underestimated, and that of grain growth overestimated, by models assuming a constant destruction efficiency
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
Сократівсько-платонівські версії мужності як спроба знайдення засобу синхронізації індивідуальної та колективної суверенності
У статті аналізується соціально-політична цінність мужності в її сократівсько-платонівській версії, що
забезпечує синхронізацію індивідуальної та колективної суверенності та є ключовою чеснотою ефективного
існування публічного простору.В статье анализируется социально-политическая ценность мужества в ее сократовско-платоновской версии,
которая обеспечивает синхронизацию индивидуальной и коллективной суверенности и является ключевой
добродетелью существования публичного пространства.In the article the socio-political value of courage is analysed in its sokrat-plato version which provides synchronization
of individual and collective sovereignty and is the key virtue of existence of public space
Dust masses and grain size distributions of a sample of Galactic pulsar wind nebulae
We calculate dust spectral energy distributions (SEDs) for a range of grain sizes and compositions, using physical properties appropriate for five pulsar wind nebulae (PWNe) from which dust emission associated with the ejecta has been detected. By fitting the observed dust SED with our models, with the number of grains of different sizes as the free parameters, we are able to determine the grain size distribution and total dust mass in each PWN. We find that all five PWNe require large (≥0.1μm) grains to make up the majority of the dust mass, with strong evidence for the presence of micron-sized or larger grains. Only two PWNe contain non-negligible quantities of small (<0.01μm) grains. The size distributions are generally well-represented by broken power laws, although our uncertainties are too large to rule out alternative shapes. We find a total dust mass of 0.02−0.28M⊙ for the Crab Nebula, depending on the composition and distance from the synchrotron source, in agreement with recent estimates. For three objects in our sample, the PWN synchrotron luminosity is insufficient to power the observed dust emission, and additional collisional heating is required, either from warm, dense gas as found in the Crab Nebula, or higher temperature shocked material. For G54.1+0.3, the dust is heated by nearby OB stars rather than the PWN. Inferred dust masses vary significantly depending on the details of the assumed heating mechanism, but in all cases large mass fractions of micron-sized grains are required
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
A decade of ejecta dust formation in the Type IIn SN 2005ip
In order to understand the contribution of core-collapse supernovae to the
dust budget of the early universe, it is important to understand not only the
mass of dust that can form in core-collapse supernovae but also the location
and rate of dust formation. SN 2005ip is of particular interest since dust has
been inferred to have formed in both the ejecta and the post-shock region
behind the radiative reverse shock. We have collated eight optical archival
spectra that span the lifetime of SN 2005ip and we additionally present a new
X-shooter optical-near-IR spectrum of SN 2005ip at 4075d post-discovery. Using
the Monte Carlo line transfer code DAMOCLES, we have modelled the blueshifted
broad and intermediate width H, H and He I lines from 48d to
4075d post-discovery using an ejecta dust model. We find that dust in the
ejecta can account for the asymmetries observed in the broad and intermediate
width H, H and He I line profiles at all epochs and that it is
not necessary to invoke post-shock dust formation to explain the blueshifting
observed in the intermediate width post-shock lines. Using a Bayesian approach,
we have determined the evolution of the ejecta dust mass in SN 2005ip over 10
years presuming an ejecta dust model, with an increasing dust mass from
~10 M at 48d to a current dust mass of 0.1 M.Comment: Accepted by MNRAS, 17 pages, 11 figures. Author accepted manuscript.
Accepted on 04/03/19. Deposited on 07/03/1
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