Interstellar properties of a dual nuclear starburst: Far-infrared spectroscopy of M82

We have measured [O I] 63 mu m and [Si II] 35 mu m in the central 44 '' (700 pc) of the starburst galaxy M82. The luminosities in these transitions are each similar to 0.1% of the bolometric luminosity. We model the [O I] in M82 as arising from warm neutral gas photodissociated by far-ultraviolet (FUV) flux from OB stars, while most of the [Si II] emission arises from associated H II regions. The gas-phase Si/H abundance ratio is found to be similar to 1.5 x 10(-5) or about 3 times greater than that seen in galactic nebulae, where silicon is mostly bound up in grains. The enrichment is probably caused by the partial destruction of silicate grains by fast supernovae-driven shocks. The [O I] 63 mu m and the [Si II] 35 mu m spectrum both show an asymmetric line profile indistinguishable in shape from those of the [O III] 52 and 88 mu m and [N III] 57 mu m lines measured previously in M82. We detect two distinct velocity components, which we attribute to emission from two regions associated with the molecular gas concentrations located 190 pc (12 '') on either side of the nucleus. We model separately the two emission lobes and derive the cloud conditions in these two regions. The derived properties suggest that the clouds in these lobes are small, r(cl)similar to 0.4-1.0 pc, have warm, T similar to 230 K, neutral gas surfaces, and are concentrated with volume filling factors of similar to 0.1 and area filling factors of approximately seven to 20. Together, the two lobes, each of diameter similar to 125 pc (similar to 8 ''), are characterized by a large number, similar to 3 x 10(5), of similar to 600 M(.) clouds with surface gas densities of similar to 10(4) cm(-3) and with pressures of P/k similar to 3 x 10(6) cm(-3) K, illuminated by FUV fluxes similar to 10(3) times the average local interstellar value for the Milky Way. These highly pressurized clouds are held in pressure equilibrium by the surrounding H II regions and the hot intercloud medium. The clouds cannot sustain the nuclear starburst long. The intense FUV fluxes have already photodissociated about 10% of the molecular gas, and the combined effects of fragmentation, photoionization, and photodissociation caused by the massive stars will soon destroy the natal molecular environment.