Stellar and gaseous abundances in M82

The near-infrared (near-IR) absorption spectra of starburst galaxies show several atomic and molecular lines from red supergiants that can be used to infer reliable stellar abundances. The metals locked in stars give a picture of the galaxy metallicity prior to the last burst of star formation. The enrichment of the new generation of stars born in the last burst can be traced by measuring the hot gas in the X-rays. For the first time, detailed stellar abundances in the nuclear region of the starburst galaxy M82 have been obtained. They are compared with those of the hot gas, as derived from an accurate reanalysis of the XMM-Newton and Chandra nuclear X-ray spectra. The cool stars and the hot gas suggest [Fe/H]=-0.35+/-0.2 dex and an overall [Si/Fe] and [Mg/Fe] enhancement by similar to0.4 and 0.5 dex, respectively. This is consistent with a major chemical enrichment by Type II supernova explosions in recursive bursts on short timescales. Oxygen is more puzzling to interpret, since it is enhanced by similar to0.3 dex in stars and depleted by similar to0.2 dex in the hot gas. None of the standard enrichment scenarios can fully explain such a behavior compared with that of the other alpha-elements.