Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...465..703l&link_type=abstract
Astrophysical Journal v.465, p.703
Astronomy and Astrophysics
Astronomy
56
Ism: Dust, Extinction, Galaxies: Abundances, Galaxies: Individual Messier Number: M82, Galaxies: Ism, Galaxies: Starburst, Infrared: Galaxies
Scientific paper
We have measured [O I] 63 micron and [Si II] 35 micron in the central 44" (700 pc) of the starburst galaxy M82. The luminosities in these transitions are each ~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 ~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 micron and the [Si II] 35 micron spectrum both show an asymmetric line profile indistinguishable in shape from those of the [O III] 52 and 88 micron and [N III] 57 micron 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_ ~ 0.4-1.0 pc, have warm, T ~ 230 K, neutral gas surfaces, and are concentrated with volume filling factors of ~0.1 and area filling factors of approximately seven to 20. Together, the two lobes, each of diameter ~125 pc (~8"), are characterized by a large number, ~3 x 10^5^, of ~600 M_sun_ clouds with surface gas densities of ~1^4^ cm^-3^ and with pressures of P/k ~3 x 10^6^ cm^-3^ K, illuminated by FUV fluxes ~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.
Colgan Sean J. W.
Erickson Edwin F.
Haas Michael R.
Hollenbach David J.
Lord Steve D.
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