117 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
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
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
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
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 effect of temperature mixing on the observable (T,beta)-relation of interstellar dust clouds
Detailed studies of the shape of dust emission spectra are possible thanks to
the current instruments capable of observations in several sub-millimetre bands
(e.g., Herschel and Planck). However, some controversy remains even on the
basic effects resulting from the mixing of temperatures along the
line-of-sight.
Studies have suggested either a positive or a negative correlation between
the colour temperature T_C and the observed spectral index beta_Obs. Our aim is
to show that both cases are possible and to determine the factors leading to
either behaviour. We start by studying the sum of two or three modified black
bodies of different temperature. With radiative transfer modelling, we examine
the probability distributions of the dust mass as a function of the physical
dust temperature. With these results as a guideline, we examine the (T_C,
beta_Obs) relations for different sets of clouds.
Even in the case of modified blackbodies at temperatures T_0 and T_0+ Delta
T_0, the correlation between T_C and beta_Obs can be either positive or
negative. If one compares models where Delta T_0 is varied, the correlation is
negative. If the models differ in their mean temperature T_0 rather than in
Delta T_0, the correlation remains positive. Radiative transfer models show
that externally heated clouds have different mean temperatures but the widths
of their temperature distributions are rather similar. Thus, the correlation
between T_C and beta_Obs is expected to be positive. The same result applies to
clouds illuminated by external radiation fields of different intensity. For
internally heated clouds a negative correlation is the more likely alternative.
If the signal-to-noise ratio is high, the observed negative correlation could
be explained by the temperature dependence of the dust optical properties but
that intrinsic dependence could be even steeper than the observed one.Comment: Accepted to A&
Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation
We examine the recent report by Prochter et al. (2006) that gamma-ray burst
(GRB) sight lines have a much higher incidence of strong MgII absorption than
quasar sight lines. We propose that the discrepancy is due to the different
beam sizes of GRBs and quasars, and that the intervening MgII systems are
clumpy with the dense part of each cloudlet of a similar size as the quasars,
i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational
predictions of our proposed model. Most notably, in some cases the intervening
MgII absorbers in GRB spectra should be seen varying, and quasars with smaller
sizes should show an increased rate of strong MgII absorbers. In fact, our
prediction of variable MgII lines in the GRB spectra has been now confirmed by
Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be
strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation;
accepted for publication in Astrophysics & Space Science as a Lette
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
Frequency of seizures and epilepsy in neurological HIV-infected patients
SummaryBackgroundInfection with the human immunodeficiency virus (HIV) is associated both with infections of the central nervous system and with neurological deficits due to direct effects of the neurotropic virus. Seizures and epilepsy are not rare among HIV-infected patients. We investigated the frequency of acute seizures and epilepsy of patients in different stages of HIV infection. In addition, we compared the characteristics of patients who experienced provoked seizures only with those of patients who developed epilepsy.MethodsThe database of the Department of Neurology, University of MĂĽnster, was searched for patients with HIV infection admitted between 1992 and 2004. Their charts were reviewed regarding all available sociodemographic, clinical, neurophysiological, imaging and laboratory data, therapy and outcome. Stage of infection according to the CDC classification and the epileptogenic zone were determined.ResultsOf 831 HIV-infected patients treated in our department, 51 (6.1%) had seizures or epilepsy. Three of the 51 patients (6%) were diagnosed with epilepsy before the onset of the HIV infection. Fourteen patients (27%) only had single or few provoked seizures in the setting of acute cerebral disorders (eight patients), drug withdrawal or sleep withdrawal (two patients), or of unknown cause (four patients). Thirty-four patients (67%) developed epilepsy in the course of their HIV infection. Toxoplasmosis (seven patients), progressive multifocal leukencephalopathy (seven patients) and other acute or subacute cerebral infections (five patients) were the most frequent causes of seizures. EEG data of 38 patients were available. EEG showed generalized and diffuse slowing only in 9 patients, regional slowing in 14 patients and regional slowing and epileptiform discharges in 1 patient. Only 14 of the patients had normal EEG. At the last contact, the majority of the patients (46 patients=90%) were on highly active antiretroviral therapy (HAART). Twenty-seven patients (53%) were on anticonvulsant therapy (gabapentin: 14 patients, carbamazepine: 9 patients, valproate: 2 patients, phenytoin: 1 patient, lamotrigine: 1 patient). Patients with only provoked seizures had no epilepsy risk factors except HIV infection, and were less likely to be infected via intravenous drug abuse.ConclusionsSeizures are a relevant neurological symptom during the course of HIV infection. Although in some patients seizures only occur provoked by acute disease processes, the majority of patients with new onset seizures eventually develops epilepsy and require anticonvulsant therapy. Intravenous drug abuse and the presence of non-HIV-associated risk factors for epilepsy seem to be associated with the development of chronic seizures in this patient group
Modelling the spectral energy distribution of galaxies. V. The dust and PAH emission SEDs of disk galaxies
We present a self-consistent model of the spectral energy distributions
(SEDs) of spiral galaxies from the ultraviolet (UV) to the mid-infrared
(MIR)/far-infrared (FIR)/submillimeter (submm) based on a full radiative
transfer calculation of the propagation of starlight in galaxy disks. This
model predicts not only the total integrated energy absorbed in the UV/optical
and re-emitted in the infrared/submm, but also the colours of the dust emission
based on an explicit calculation of the strength and colour of the UV/optical
radiation fields heating the dust, and incorporating a full calculation of the
stochastic heating of small dust grains and PAH molecules.
The geometry of the translucent components of the model is empirically
constrained using the results from the radiation transfer analysis of Xilouris
et al. on spirals in the middle range of the Hubble sequence, while the
geometry of the optically thick components is constrained from physical
considerations with a posteriori checks of the model predictions with
observational data.
These geometrical constraints enable the dust emission to be predicted in
terms of a minimum set of free parameters: the central face-on dust opacity in
the B-band tau^f_B, a clumpiness factor F for the star-forming regions, the
star-formation rate SFR, the normalised luminosity of the old stellar
population old and the bulge-to-disk ratio B/D. We show that these parameters
are almost orthogonal in their predicted effect on the colours of the dust/PAH
emission.
The results of the calculations are made available in the form of a large
library of simulated dust emission SEDs spanning the whole parameter space of
our model, together with the corresponding library of dust attenuation
calculated using the same model. (see full abstract in the paper)Comment: 39 pages; accepted for publication in Astronomy & Astrophysics; For a
higher resolution version of Fig.1 and Fig.20 see
http://www.star.uclan.ac.uk/~ccp/index.shtm
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