ARlogo Annu. Rev. Astron. Astrophys. 1991. 29: 581-625
Copyright © 1991 by Annual Reviews. All rights reserved

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9.1 M82

M82 is one of the first galaxies in which CO was detected (Rickard et al 1975, Solomon & de Zafra 1975), and the CO emission in this nearly edge-on irregular galaxy has been mapped extensively (see Table 2 for references). The total H2 mass within the inner 6 kpc could be as high as 2 x 109 Msun (assuming the standard galactic CO to H2 conversion), in which case, it is an order of magnitude greater than the HI mass within the same region. It is one of the few galaxies in which the molecular emission is sufficiently bright to enable studies of molecules other than CO. Rickard & Palmer (1977) first detected HCN and CS here. In addition, 13CO J = 1-0 is observed with an intensity approximately 1/20 that of CO (Stark & Carlson 1984, Young & Scoville 1984), and the CO J = 2-1 emission is significantly brighter than the J = 1-0 emission at the central position (Knapp et al 1980, Sutton et al 1983). Both of these characteristics have led a number of authors to suggest that the CO emission in the center of M82 is optically thin, an excitation regime seldom seen in galactic GMCs.

Carlstrom (1989) has published high resolution aperture synthesis maps of the CO, HCO+, HCN, and 3 mm continuum emission, as shown in Figure 13 together with the 6 cm and 2 µm continuum. The 2 µm continuum arises mostly from late-type giant stars; the 6 cm continuum is mainly synchrotron emission from high energy particles injected into the ISM from supernovae; and the 3 mm continuum is predominately free-free emission from H II regions, ionized by young stars. All of the distributions show an elongation of the central galactic disk, and within the disk all of the molecular line maps show a clear double-peaked structure. The latter feature was first discovered by Nakai et al (1987) and is generally interpreted as an edge-on torus at 200 pc radius. The total mass of gas in this structure is 0.6-1 x 108 Msun, which is 10% of the dynamical mass within the same region. The central peak in the 2 µm continuum probably represents the nuclear star cluster with a significant enrichment by super-giant stars produced from the starburst. It is noteworthy that the distribution of nonthermal radiation shown in the 6 cm continuum shows roughly the same spatial extent as the molecular torus, but no evidence of the two peaks seen in the molecular emission. Comparison of the 3 mm continuum map with the distribution of gas seen in the CO, HCO+, and HCN suggests that at present, the formation of high mass stars as probed by the distribution of free-free emission is occurring predominately to the west of the nucleus.

Figure 13

Figure 13. The central disk of M82 with CO (5.5" resolution) on an R-band gray scale image; the 2 µm continuum (Telesco et al 1991); HCO+ (10" resolution); HCN (10" resolution); 6-cm radio continuum (0.35" resolution, Kronberg et a1 1985); and 3 mm continuum (6" resolution). The diamond denotes the position of the 2 µm ``stellar'' peak. The molecular images and the 3 mm continuum are from Carlstrom (1989).

Based upon an excitation analysis of the HCN and HCO+ emission, Carlstrom finds that the density of the gas in the emitting regions probably exceeds 5 x 104 cm-3, that is, a factor of 200 higher than the mean volume densities of galactic GMCs. This dense gas has a filling factor ltapprox 0.1%. The Lymann continuum production rate required to explain the 3 mm free-free emission is 6 x 1053 sec-1 (Carlstrom 1989), exceeding by a factor of four the Lyman continuum production rate required for the entire Milky Way (Mezger 1978). The implied rate of star formation within the molecular torus is Mdot appeq 6-10 Msun yr-1 assuming a Miller-Scalo initial mass function with an upper mass cutoff of 45 Msun and lower mass cutoffs of 0.1-1 Msun (see Scoville & Soifer 1991). At this rate, the gas in the torus at 200 pc radius would be entirely converted into stars in 107 years (if low mass stars do indeed form) and in order to maintain the nuclear starburst, gas must flow in from further out in the galaxy. If low mass stars do not form, the starburst can be sustained for a considerably longer time.

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