A&A 399, 39-50 (2003)
DOI: 10.1051/0004-6361:20021600

The 2-10 keV luminosity as a Star Formation Rate indicator

P. Ranalli¹, A. Comastri² and G. Setti<sup>1

1</sup>  Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
²  INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy

(Received 30 July 2002 / Accepted 4 November 2002)

Abstract
Radio and far infrared luminosities of star-forming galaxies follow a tight linear relation. Making use of ASCA and BeppoSAX observations of a well-defined sample of nearby star-forming galaxies, we argue that tight linear relations hold between the X-ray, radio and far infrared luminosities. The effect of intrinsic absorption is investigated taking NGC 3256 as a test case. It is suggested that the hard X-ray emission is directly related to the Star Formation Rate. Star formation processes may also account for most of the 2-10 keV emission from LLAGNs of lower X-ray luminosities (for the same FIR and radio luminosity). Deep Chandra observations of a sample of radio-selected star-forming galaxies in the Hubble Deep Field North show that the same relation holds also at high ( ) redshift. The X-ray/radio relations also allow a derivation of X-ray number counts up to very faint fluxes from the radio Log  N-Log  S, which is consistent with current limits and models. Thus the contribution of star-forming galaxies to the X-ray background can be estimated.

Key words: X-rays: galaxies -- radio continuum: galaxies -- galaxies: high-redshift -- infrared: galaxies -- galaxies: fundamental parameters -- galaxies: starburst

Offprint request: P. Ranalli, ranalli@bo.astro.it

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