The radio emission of normal galaxies may become opaque at low radio frequencies due to thermal ionized gas. We performed modelling of the free-free absorption to reproduce the ocal spectrum of SgrA Complex and the global spectrum of the starburst galaxy M82. We show the importance of resolution of radio observations and the value of filling factor of the absorbing gas for correct modelling of the absorption.
We describe radio observations at 244 and 610 MHz of a sample of 20 luminous and ultra-luminous IRAS galaxies. These are a sub-set of a sample of 31 objects that have well-measured radio spectra up to at least 23 GHz. The radio spectra of these objec
ts below 1.4 GHz show a great variety of forms and are rarely a simple power-law extrapolation of the synchrotron spectra at higher frequencies. Most objects of this class have spectral turn-overs or bends in their radio spectra. We interpret these spectra in terms of free-free absorption in the starburst environment. Several objects show radio spectra with two components having free-free turn-overs at different frequencies (including Arp 220 and Arp 299), indicating that synchrotron emission originates from regions with very different emission measures. In these sources, using a simple model for the supernova rate, we estimate the time for which synchrotron emission is subject to strong free-free absorption by ionized gas, and compare this to expected HII region lifetimes. We find that the ionized gas lifetimes are an order of magnitude larger than plausible lifetimes for individual HII regions. We discuss the implications of this result and argue that those sources which have a significant radio component with strong free-free absorption are those in which the star formation rate is still increasing with time. We note that if ionization losses modify the intrinsic synchrotron spectrum so that it steepens toward higher frequencies, the often observed deficit in fluxes higher than ~10 GHz would be much reduced.
In this paper we test 8 models of the free electron distribution in the Milky Way that have been published previously, and we introduce 4 additional models that explore the parameter space of possible models further. These new models consist of a simple exponential thick disk model, and updat
We have observed the canonical gigahertz-peaked spectrum source 0108+388 with the VLBA at a range of frequencies above and below the spectral peak. The activity that dominates the radio emission from 0108+388, which is also classified as a Compact Sy
mmetric Object, is thought to be less than 1000 years old. We present strong evidence that the spectral turnover in 0108+388 results from free-free absorption by non-uniform gas, possibly in the form of a disk in the central tens of parsecs.
We searched for X-ray supernova remnants (SNRs) in the starburst region of M82, using archival data from the Chandra X-ray Observatory with a total effective exposure time of 620 ks with an X-ray spectroscopic selection. Strong line-emission from Fe
xxv at 6.7 keV is a characteristic spectral feature of hot, shocked gas of young SNRs and distinctive among the discrete sources in the region populated by X-ray binaries. We selected candidates using narrow-band imaging aimed at the line excess and identified six (and possibly a seventh) X-ray SNRs. Two previously known examples were recovered by our selection. Five of them have radio counterparts, including the radio supernova SN2008iz, which was discovered as a radio transient in 2008. It shows a hard X-ray spectrum with a blueshifted Fe K feature with v ~ -2700 km/s, both of which suggest its youth. The 4-8 keV luminosities of the selected SNRs are in the range of (0.3-3)e38 erg/s. We made a crude estimate of the supernova rate, assuming that more luminous SNRs are younger, and found 0.06 (0.03-0.13) /yr, in agreement with the supernova rates estimated by radio observations and the generally believed star formation rate of M82, although the validity of the assumption is questionable. A sum of the Fe xxv luminosity originating from the selected X-ray SNRs consists of half of the total Fe xxv luminosity observed in the central region of M82. We briefly discuss its implications for starburst winds and the Fe xxv emission in more luminous starburst galaxies.
We report on the discovery of SN2014J in the nearby galaxy M82. Given its proximity, it offers the best opportunity to date to study a thermonuclear supernova over a wide range of the electromagnetic spectrum. The first set of optical, near-IR and mi
d-IR observations of SN2014J, orchestrated by the intermediate Palomar Transient Factory (iPTF), show that SN2014J is a spectroscopically normal Type Ia supernova, albeit exhibiting high-velocity features in its spectrum and heavily reddened by dust in the host galaxy. Our earliest detections start just hours after the fitted time of explosion. We use high-resolution optical spectroscopy to analyze the dense intervening material and do not detect any evolution in the resolved absorption features during the lightcurve rise. Similarly to other highly reddened Type Ia supernovae, a low value of total-to-selective extinction, Rv < 2, provides the best match to our observations. We also study pre-explosion optical and near-IR images from HST with special emphasis on the sources nearest to the SN location.
Wojciech S. Jurusik
,Krzysztof T. Chyzy
,Robert T. Drzazga
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(2013)
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"Modelling of the free-free absorption in the starburst galaxy M82 and the Sgr A Complex"
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Wojciech Jurusik
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