Annu. Rev. Astron. Astrophys. 1982. 20: 517-45
Copyright © 1982 by . All rights reserved

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3.2. A Classification Scheme

The data discussed above suggest a categorization of galaxies according to the degree of predominance of the central and disk sources. We define class 1 galaxies as having both strong central sources and well-defined molecular disks, with a monotonic decrease in P(CO) from center to edge. Class 2 galaxies have similarly strong central sources, but with surrounding molecular annuli, rather than complete underlying disks, and a significant and detectable minimum between the two components. Class 3 galaxies have well-defined annuli and absent or very weak central sources [P(CO)center < 0.2 bar P(CO)disk. Class 4 galaxies show emission only from isolated HII regions or dust clouds in their disks. Class 5 galaxies have no detected CO emission at all. Some representative examples of each type are listed in Table 3.

2 characteristics. They may also be correlated with other galactic characteristics. We have already mentioned the tendency of Magellanic irregulars to fall in class 5. Young (private communication) has suggested that the distinction between classes 1 and 2 may lie in the shapes of their rotation curves; most of the class 1 galaxies lack the innermost solid-body rise and subsequent minimum seen tn our Galaxy.

Table 3. Classification of galaxies by CO emission characteristics

Class Characteristics Examples

1 Central source plus disk M51, NGC 6946, IC 342
2 Central source plus annulus Milky Way
3 Annulus without central source M31, M81
4 Emission from isolated regions M33, LMC
5 No detectable emission NGC 6822, SMC

Further subdivision of the proposed molecular classes, based on CO luminosity or on some measure of fractional molecular content, may prove fruitful once large, statistically meaningful surveys have been completed.

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