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The integrated radio continuum spectrum of M33 - Evidence for free-free absorption by cool ionized gasWe present measurements of the integrated radio continuum flux density of M33 at frequencies between 22 and 610 MHz and discuss the radio continuum spectrum of M33 between 22 MHz and 10 GHz. This spectrum has a turnover between 500 and 900 MHz, depending on the steepness of the high frequency radio spectrum of M33. Below 500 MHz the spectrum is relatively flat. We discuss possible mechanisms to explain this spectral shape and consider efficient free-free absorption of nonthermal emission by a cool (not greater than 1000 K) ionized gas to be a very likely possibility. The surface filling factor of both the nonthermal and the thermal material appears to be small (of order 0.001), which could be explained by magnetic field/density fluctuations in the M 33 interstellar medium. We briefly speculate on the possible presence of a nuclear radio source with a steep spectrum.
Document ID
Document Type
Reprint (Version printed in journal)
Israel, F. P.
(Leiden Rijksuniv., Sterrewacht, Netherlands)
Mahoney, M. J.
(Clark Lake Radio Observatory, Borrego Springs, MD; JPL, Pasadena CA, United States)
Howarth, N.
(Mullard Radio Astronomy Observatory Cambridge, United Kingdom)
Date Acquired
August 16, 2013
Publication Date
July 1, 1992
Publication Information
Publication: Astronomy and Astrophysics
Volume: 261
Issue: 1
ISSN: 0004-6361
Subject Category
Accession Number
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