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Astron. Astrophys. 350, 777-790 (1999)

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Diffuse thermal emission from very hot gas in starburst galaxies

M. Cappi 1, M. Persic 2, L. Bassani 1, A. Franceschini 3, L.K. Hunt 4, S. Molendi 5, E. Palazzi 1, G.G.C. Palumbo 6,1, Y. Rephaeli 7 and P. Salucci 8

1 ITeSRE/CNR, via Gobetti 101, 40129 Bologna, Italy
2 Trieste Astronomical Observatory, via G.B.Tiepolo 11, 34131 Trieste, Italy
3 Astronomy Department, University of Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
4 CAISMI/CNR, Largo E.Fermi 5, 50125 Firenze, Italy
5 IFCTR/CNR, via Bassini 15, 20133 Milano, Italy
6 Astronomy Department, University of Bologna, via Zamboni 33, 40126 Bologna, Italy
7 School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
8 SISSA/ISAS, via Beirut 2-4, 34013 Trieste, Italy

Received 24 June 1999 / Accepted 30 August 1999


BeppoSAX observations of the nearby archetypical starburst galaxies NGC253 and M82 are presented. Spectral analysis shows that the 2-10 keV spectra of both galaxies, extracted from the central [FORMULA] regions, are best fitted by a thermal emission model with kT [FORMULA] 6-9 keV and metal abundances [FORMULA] 0.1-0.3 solar. The spatial analysis yields clear evidence that this emission is extended in NGC 253, and possibly also in M82. These results clearly rule out a LLAGN as the main origin of the X-ray emission in NGC 253. For M82, the presence of an Fe-K line at [FORMULA] 6.7 keV, and the convex profile of its 2-10 keV continuum, indicate a significant thermal component. Contributions from point sources (e.g. X-ray binaries, supernova remnants, and/or a LLAGN) and Compton emission are also likely. Altogether, BeppoSAX results provide compelling evidence for the existence of a hot interstellar plasma in both galaxies, possibly in the form of superwind outflows from the disks of these galaxies. Order-of-magnitude estimates and some implications, such as the expelled mass and the energetics of the outflowing gas of this superwind scenario, are discussed. These new results also suggest some similarity between the X-ray emission from these galaxies and that from the Galactic Ridge.

Key words: galaxies: individual: M 82 – galaxies: individual: NGC 253 – galaxies: starburst – X-rays: galaxies

Send offprint requests to: M. Cappi (mcappi@tesre.bo.cnr.it)

© European Southern Observatory (ESO) 1999

Online publication: October 14, 1999