134 research outputs found
On the missing 2175 Angstroem-bump in the Calzetti extinction curve
The aim of the paper is to give a physical explanation of the absence of the
feature in the Calzetti extinction curve. We analyze the dust attenuation of a
homogeneous source seen through a distant inhomogeneous distant screen. The
inhomogeneities are described through an idealized isothermal turbulent medium
where the probability distribution function (PDF) of the column density is
log-normal. In addition it is assumed that below a certain critical column
density the carriers of the extinction bump at 2175 Angstroem are being
destroyed by the ambient UV radiation field. Turbulence is found to be a
natural explanation not only of the flatter curvature of the Calzetti
extinction curve but also of the missing bump provided the critical column
density is N_H >= 10^21 cm^-2. The density contrast needed to explain both
characteristics is well consistent with the Mach number of the cold neutral
medium of our own Galaxy which suggests a density contrast sigma_(rho/) 6.Comment: 6 pages, 6 figures accepted for publication in A&A, section
On the thermal behaviour of small iron grains
The optical properties of small spherical iron grains are derived using a
Kramers-Kronig-consistent model of the dielectric function including its
dependence on temperature and size. Especially discussed is the effect of the
size dependence, which results from the limitation of the free path of the free
electrons in the metal by the size of the grain, on the absorption behaviour of
small iron spheres and spheroids. The estimated absorption properties are
applied to study the temperature behaviour of spherical and spheroidal grains
which are heated by the interstellar radiation field.Comment: 12 pages, 16 figure
Modelling the Pan-Spectral Energy Distributions of Starburst & Active Galaxies
We present results of a self-consistent model of the spectral energy
distribution (SED) of starburst galaxies. Two parameters control the IR SED,
the mean pressure in the ISM and the destruction timescale of molecular clouds.
Adding a simplified AGN spectrum provides mixing lines on IRAS color : color
diagrams. This reproduces the observed colors of both AGNs and starbursts.Comment: Poster Paper for IAU 222: The Interplay among Black Holes, Stars and
ISM in Galactic Nucle
Physical properties of interstellar filaments
We analyze the physical parameters of interstellar filaments that we describe
by an idealized model of isothermal self-gravitating infinite cylinder in
pressure equilibrium with the ambient medium. Their gravitational state is
characterized by the ratio f_cyl of their mass line density to the maximum
possible value for a cylinder in a vacuum. Equilibrium solutions exist only for
f_cyl < 1. This ratio is used in providing analytical expressions for the
central density, the radius, the profile of the column density, the column
density through the cloud centre, and the fwhm. The dependence of the physical
properties on external pressure and temperature is discussed and directly
compared to the case of pressure-confined isothermal self-gravitating spheres.
Comparison with recent observations of the fwhm and the central column
density N_H(0) show good agreement and suggest a filament temperature of ~10 K
and an external pressure p_ext/k in the range 1.5x10^4 K/cm^3 to 5x10^4 K/cm^3.
Stability considerations indicate that interstellar filaments become
increasingly gravitationally unstable with mass line ratio f_cyl approaching
unity. For intermediate f_cyl>0.5 the instabilities should promote core
formation through compression, with a separation of about five times the fwhm.
We discuss the nature of filaments with high mass line densities and their
relevance to gravitational fragmentation and star formation.Comment: 18 pages, 12 figures accepted for publication (13/4/2012
Modelling the spectral energy distribution of galaxies. I. Radiation fields and grain heating in the edge-on spiral NGC891
We describe a new tool for the analysis of the UV to the sub-millimeter (sub-mm) spectral energy distribution (SED) of spiral galaxies. We use a consistent treatment of grain heating and emission, solve the radiation transfer problem for a finite disk and bulge, and self-consistently calculate the stochastic heating of grains placed in the resulting radiation field.
We use this tool to analyse the well-studied nearby edge- on spiral galaxy NGC 891. First we investigate whether the old stellar population in NGC 891, along with a reasonable assumption about the young stellar population, can account for the heating of the dust and the observed far-infrared and sub- mm emission. The dust distribution is taken from the model of Xilouris et al. (1999), who used only optical and near-infrared observations to determine it. We have found that such a simple model cannot reproduce the SED of NGC 891, especially in the sub-mm range. It underestimates by a factor of 2–4 the observed sub-mm flux.
A number of possible explanations exist for the missing sub-mm flux. We investigate a few of them and demonstrate that one can reproduce the observed SED in the far-infrared and the sub-mm quite well, as well as the observed radial profile at 850 µm.
For the models calculated we give the relative proportion of the dust radiation powered by the old and young stellar popula- tions as a function of FIR/sub-mm wavelength. In all models we find that the dust is predominantly heated by the young stellar population
The dust emission SED of X-ray emitting regions in Stephan's Quintet
We analysed the Spitzer maps of Stephan's Quintet in order to investigate the
nature of the dust emission associated with the X-ray emitting regions of the
large scale intergalactic shock and of the group halo. This emission can in
principle be powered by dust-gas particle collisions, thus providing efficient
cooling of the hot gas. However the results of our analysis suggest that the
dust emission from those regions is mostly powered by photons. Nonetheless dust
collisional heating could be important in determining the cooling of the IGM
gas and the large scale star formation morphology observed in SQ.Comment: Conference proceedings IAU Symposium 284 "The Spectral energy
distribution of galaxies", 5-9 September 2011, Preston, U
Dust in dwarf galaxies: The case of NGC 4214
We have carried out a detailed modelling of the dust heating and emission in
the nearby, starbursting dwarf galaxy NGC 4214. Due to its proximity and the
great wealth of data from the UV to the millimeter range (from GALEX, HST, {\it
Spitzer}, Herschel, Planck and IRAM) it is possible to separately model the
emission from HII regions and their associated photodissociation regions (PDRs)
and the emission from diffuse dust. Furthermore, most model parameters can be
directly determined from the data leaving very few free parameters. We can fit
both the emission from HII+PDR regions and the diffuse emission in NGC 4214
with these models with "normal" dust properties and realistic parameters.Comment: 4pages, 3 figures. To appear in 'The Spectral Energy Distribution of
Galaxies' Proceedings IAU Symposium No 284, 201
The star formation efficiency in Stephan's Quintet intragroup regions
We investigated the star formation efficiency for all the dust emitting
sources in Stephan's Quintet (SQ). We inferred star formation rates using
Spitzer MIR/FIR and GALEX FUV data and combined them with gas column density
measurements by various authors, in order to position each source in a
Kennicutt-Schmidt diagram. Our results show that the bright IGM star formation
regions in SQ present star formation efficiencies consistent with those
observed within local galaxies. On the other hand, star formation in the
intergalactic shock region seems to be rather inhibited.Comment: Conference proceedings "Galaxy Mergers in an evolving Universe",
23-28 October 2011, Hualien, Taiwa
High Resolution mid-Infrared Imaging of SN 1987A
Using the Thermal-Region Camera and Spectrograph (T-ReCS) attached to the
Gemini South 8m telescope, we have detected and resolved 10 micron emission at
the position of the inner equatorial ring (ER) of supernova SN 1987A at day
6067. ``Hot spots'' similar to those found in the optical and near-IR are
clearly present. The morphology of the 10 micron emission is globally similar
to the morphology at other wavelengths from X-rays to radio. The observed
mid-IR flux in the region of SN1987A is probably dominated by emission from
dust in the ER. We have also detected the ER at 20 micron at a 4 sigma level.
Assuming that thermal dust radiation is the origin of the mid-IR emission, we
derive a dust temperature of 180^{+20}_{-10} K, and a dust mass of 1.- 8.
10^{-5} Mo for the ER. Our observations also show a weak detection of the
central ejecta at 10 micron. We show that previous bolometric flux estimates
(through day 2100) were not significantly contaminated by this newly discovered
emission from the ER. If we assume that the energy input comes from radioactive
decays only, our measurements together with the current theoretical models set
a temperature of 90 leq T leq 100 K and a mass range of 10^{-4} - 2. 10^{-3} Mo
for the dust in the ejecta. With such dust temperatures the estimated thermal
emission is 9(+/-3) 10^{35} erg s^{-1} from the inner ring, and 1.5 (+/-0.5)
10^{36} erg s^{-1} from the ejecta. Finally, using SN 1987A as a template, we
discuss the possible role of supernovae as major sources of dust in the
Universe.Comment: aastex502, 14 pages, 4 figures; Accepted for publication in ApJ
Content changed: new observations, Referee's comments and suggestion
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