Near-infrared integral field spectroscopy and mid-infrared spectroscopy ofthe starburst galaxy M82

We present new infrared observations of the central regions of the starburs t galaxy M82. The observations consist of near-infrared integral field spec troscopy in the H and K bands obtained with the MPE 3D instrument and of la mbda = 2.4-45 mum spectroscopy from the Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory. These measurements are used, toget her with data from the literature, to (1) reexamine the controversial issue of extinction, (2) determine the physical conditions of the interstellar m edium (ISM) within the star-forming regions, and (3) characterize the compo sition of the stellar populations. Our results provide a set of constraints for detailed starburst modeling, which we present in a companion paper. We find that purely foreground extinction cannot reproduce the global relativ e intensities of H recombination lines from optical to radio wavelengths. A good fit is provided by a homogeneous mixture of dust and sources, and wit h a visual extinction of A(V) = 52 mag. The SWS data provide evidence for d eviations from commonly assumed extinction laws between 3 and 10 mum. The f ine-structure lines of Ne, Ar, and S detected with SWS imply an electron de nsity of approximate to 300 cm(-3), and abundance ratios Ne/H and Ar/H near ly solar and S/H about one-fourth solar. The excitation of the ionized gas indicates an average effective temperature for the OB stars of 37,400 K, wi th little spatial variation across the starburst regions. We find that a ra ndom distribution of closely packed gas clouds and ionizing clusters and an ionization parameter of approximate to 10(-2.3) represent well the star-fo rming regions on spatial scales ranging from a few tens to a few hundreds o f parsecs. From detailed population synthesis and the mass-to-a-light ratio , we conclude that the near-infrared continuum emission across the starburs t regions is dominated by red supergiants with average effective temperatur es ranging from 3600 to 4500 K and roughly solar metallicity. Our data rule out significant contributions from older, metal-rich giants in the central few tens of parsecs of M82.