A NEAR-INFRARED SPECTROSCOPIC STUDY OF THE STARBURST CORE OF M82

Near-infrared spectroscopy of the starburst core of M82 is presented. Complete J, H, and K band spectra with a resolution of 0.0035 μm are shown for the inner 60 pc of the galaxy, and spatial profiles along the starburst ridge are presented for Bry, molecular hydrogen, and [Fe II] line fluxes. The data are used to better define the extinction toward the starburst region, the excitation conditions of the gas, and to characterize the stellar populations in the starburst region. Emission from shocked molecular hydrogen is detected from the core of M82. The distribution of the emission is quite unlike that of other near-infrared lines or continuum emission. Much of the emission can be understood as originating on the inner edge of the molecular "ring" that surrounds the starburst. Large-scale flows from the starburst impacting on the ring can account for this shocked gas. The large-scale flows within the starburst region are probed directly with the Bowen fluorescence transition of Mg II. The difference in the distribution of the molecular hydrogen emission and that of other near-infrared diagnostics suggests that single-beam, aggregate models of starbursts are oversimplified. Hydrogen recombination line measurements between Hα and Brγ indicate that the extinction toward the emitting region is about Av = 5. This is consistent with the far-infrared and submillimeter dust column densities if the extinguishing clouds are well mixed with the stellar population. The near-infrared continuum colors can be reconciled with this if the red starlight from the starburst core is diluted by thermal emission from grains at the long-wavelength end of the K band. If this extinction is applicable to the starburst as a whole, then the need for an unusual initial mass function (IMF) for the starburst is largely obviated. The spatial distribution of [Fe II] is similar to the nonthermal emission that dominates the radio spectrum of M82. This correspondence helps to identify this emission, which is ubiquitous in starburst systems, as being associated with supernova remnants. Using the derived extinction, and allowing for dilution of absorption features by thermal emission from dust at >2.3 μm, the infrared spectrum of M82 appears to be dominated by very late type supergiants, if solar neighborhood stars can be considered to be appropriate analogs. In particular, the strong 1.9 μm H2O band is difficult to account for without an important contribution from red supergiants. If the red starlight from the core of M82 is indeed dominated by supergiants, then the difference between the spatial distribution of near-infrared continuum and that of Brγ can be interpreted as a difference in distribution of stars with an age difference of only a few million years. This would suggest that the morphology of the starburst in the core of M82 has changed in a relatively short period of time, compared with the dynamical time scale of the inner galaxy. This conclusion must be tempered by the possibility that chemical enrichment in the core of M82 has resulted in a population of older stars with spectral properties that resemble supergiants, or that the nucleus of M82 is gas poor.