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

9. GALACTIC NUCLEI

In the center of our Galaxy, the predominant gas phase is molecular and a total H₂ mass of approximately 1-5 x 10⁸ M is estimated for the inner 800 pc radius. The dominant large-scale morphological feature is an expanding ring or arms at approximately 200-300 pc radius, made up of extremely massive clouds ( 10⁶ M). The mean gas densities in these clouds are at least a factor of 10 higher than those of typical Galactic GMCs.

In all but the nearest external galaxies, high resolution observations are required to detect the existence of such structures, if they exist. Three of the nearby spiral galaxies (IC 342, NGC 6946, and NGC 253) for which aperture synthesis observations have been obtained, exhibit elongated barlike distributions for the molecular gas in the central kpc (Section 3.2). In three other nearby galaxies studied at high resolution (Maffei 2, NGC 2146, and NGC 3079), elongated distributions are also seen in the nuclei, but these galaxies are observed at high inclination, and the observed structures may simply be inclined nuclear disks (Nakai et al 1989, Young et al 1988a, b, Jackson & Ho 1988). In the center of M51 (see Figure 3), there is an apparent deficiency of molecular gas within the central 600 pc (Lo et al 1987b, Rand & Kulkarni 1990). Two galaxies analyzed in considerably greater detail are the nearby irregular galaxy M82 and the Seyfert II galaxy NGC 1068.