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

3.1 Unbarred Spiral Galaxies

There are 40 Sc galaxies whose CO radial distributions have been measured along the major axis. Figure 2a shows the CO integrated intensity distributions for a sample of Sc galaxies, with the observed intensities corrected for inclination (as described in Young & Scoville 1982a). Also shown in Figure 2a is the Milky Way CO distribution smoothed to 1 kpc resolution (Sanders et al 1984), illustrating the central peak, the absence of gas between 1 and 4 kpc, and the molecular annulus between 4 and 8 kpc. None of the face-on Sc galaxies has CO radial distributions that resemble that in the Milky Way. The dominant characteristic of the CO distributions for the Sc galaxies is a central peak and monotonic intensity falloff with increasing radius. For a given galaxy, there is typically a factor of 2 variation in the CO integrated intensities at different azimuthal angles, relative to the mean value at each radius (Morris & Rickard 1982, Young & Scoville 1982a, Solomon et al 1983, Scoville & Young 1983, Tacconi & Young 1986, 1990, Kenney & Young 1988a, Lord & Young 1990, Kenney et al 1991).

Figure 2a. Radial distributions of CO integrated intensity, corrected to face-on, for six relatively face-on Sc galaxies (solid lines) and the Milky Way (dashed line, Sanders et al 1984). Galaxy types and references for the CO distributions are found in Table 2 and distances are given in Young et al (1989).

In luminous, face-on, late-type spirals, the azimuthally averaged distributions of H₂ are markedly different from those of H I in the same galaxies (Morris & Rickard 1982, Young & Scoville 1982a, Scoville & Young 1983, Kenney & Young 1989). The central peaks in the H₂ distributions for Sc galaxies contrast markedly with the central depressions seen in H I and the relatively constant H I surface densities [N (HI) 10²¹ cm^-2] across the optical disk, outside the center. In NGC 6946 (see Section 7.1), the ratio of H₂ to H I surface densities decreases from a central value of 30 to unity at a radius of 10 kpc (Tacconi & Young 1986). In most of the luminous Sc galaxies, the molecular gas more than fills in the central hole present in the atomic gas distributions.

Figure 2b. Radial distributions of CO integrated intensity, corrected to face-on, for eight Sb/Sbc galaxies. Galaxy types and references for the CO distributions are found in Table 2. The CO distribution for M31 has been scaled down by a factor of 5. Five of these galaxies exhibit central CO depressions.

The total number of Sb and Sbc galaxies in which the CO distributions have been measured is 25. Among the Sb galaxies that are at distances of less than 25 Mpc (for H₀ = 50 km s^-1 Mpc^-1), fewer than half have been found to exhibit CO depressions in their centers. These galaxies include M31 (Stark 1979), NGC 2841 and 7331 (Young & Scoville 1982b), NGC 1068 (Scoville et al 1983, Myers & Scoville 1987), NGC 891 (Sofue 1988, Young 1987), NGC 4216 (Kenney & Young 1988a), and NGC 488, 2336, 3147, 4725, and IC 356 (Young et al 1991). Figure 2b illustrates the CO radial distributions for a sample of Sb galaxies with and without central CO depressions. There is no obvious correlation of the presence or absence of the central CO depression with other galaxy properties.

Major axis CO distributions in early type spiral galaxies have been published for only a small number of systems, most of which exhibit CO radial distributions that peak in the center and decrease monotonically with radius, like the Sc galaxies and the majority of the Sb galaxies. The central H₂ surface densities in the early type galaxies are as high as those found in the Sc galaxies. The H I distributions for Sa galaxies, on the other hand, have surface densities that peak at 1-4 M pc^-2, as opposed to 5-10 M pc^-2 found for Sc galaxies (Wevers et al 1986, van Driel 1987).

The isophotal diameters for the CO emission in a large sample of spiral galaxies have been analyzed by Kenney & Young (1988a). Overall, the mean ratio of CO diameter to optical diameter (D₂₅) for 68 galaxies has a value of 0.5 ± 0.2, with a tendency for the largest ratios to be found in the late type spirals.