M82 - THE SAGA CONTINUES

We present new near-infrared observations of the starburst galaxy M82, including high-resolution J, H, and K imagery, 1-5 mum spectrophotome try and photometry, and 1.05-1.35 mum spectroscopy. These measurements are used to examine the stellar population in the galaxy and conditio ns in the interstellar medium. We combine our observations with result s from the literature to derive a new set of constraints for starburst models, which are presented in a companion paper. Our measurements of Paschen line fluxes in combination with the strengths of other recomb ination lines allow study of the reddening in M82. We find that a unif orm foreground screen model for the dust geometry cannot explain the r elative line strengths. A variety of other models can fit the data rea sonably well, such as a nonuniform foreground screen or a homogeneous mix of dust and gas. The derived properties of the galaxy are not stro ngly dependent on which of these models is selected; we derive an ioni zing flux of almost-equal-to 10(54) s-1 and an absolute magnitude of t he starburst region of M(K) < -22.5. Depending on the reddening model, the visual extinction to the nucleus lies between A(V) = 12 and A(V) = 27; we show that the nucleus does not coincide with any feature that is bright at visual wavelengths. The stellar H2O index in the nuclear region is less than 0.06 and the CO index is almost-equal-to 0.21; co ntamination of the CO band by emission by hot dust must be small. We u se several lines in the 1.3 mum region to derive a lower limit to the electron density of n(e) > 10(4.5) cm-3 in the [Fe II] emission region . The [Fe II] line fluxes are consistent with excitation by fast shock s incident on dense gas. We report the tentative detection of several lines of H-2, which, if confirmed, implicate UV fluorescence as the ex citation mechanism.