Time evolution of the radio continuum of young starbursts: the importance of synchrotron emission

Free access article

Issue		A&A Volume 460, Number 1, December II 2006


Page(s)		67 - 81
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20065629

A&A 460, 67-81 (2006)
DOI: 10.1051/0004-6361:20065629

Time evolution of the radio continuum of young starbursts: the importance of synchrotron emission

H. Hirashita¹ and L. K. Hunt²

¹ Center for Computational Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
e-mail: hirasita@ccs.tsukuba.ac.jp
² INAF - Istituto di Radioastronomia-Sezione Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: hunt@arcetri.astro.it

(Received 18 May 2006 / Accepted 30 August 2006)

Abstract
We investigate the radio spectral energy distributions (SEDs) of young star-forming galaxies and how they evolve with time. The duration and luminosity of the nonthermal radio emission from supernova remnants (SNRs) are constrained by using the observational radio SEDs of SBS 0335-052 and I Zw 18, which are the two lowest-metallicity blue compact dwarf galaxies in the nearby universe. The typical radio "fluence" for SNRs in SBS 0335-052, that is the radio energy emitted per SNR over its radiative lifetime, is estimated to be ~6- $22\times 10^{22}~{\rm W~Hz^{-1}~yr}$ at 5 GHz. On the other hand, the radio fluence in I Zw 18 is ~1- $3\times 10^{22}~{\rm W~Hz^{-1}~yr}$ at 5 GHz. We discuss the origin of this variation and propose scaling relations between synchrotron luminosity and gas density. We have also predicted the time dependence of the radio spectral index and of the spectrum itself, for both the "active" (SBS 0335-052) and "passive" (I Zw 18) cases. These models enable us to roughly age date and classify radio spectra of star-forming galaxies into active/passive classes. Implications for high-z galaxy evolution are also discussed.

Key words: galaxies: dwarf -- galaxies: evolution -- galaxies: ISM -- ISM: supernova remnants -- radio continuum: galaxies

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