Annu. Rev. Astron. Astrophys. 1978. 16: 103-39
Copyright © 1978 by . All rights reserved

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2.3. Velocity Fields

The general pattern of the velocity fields derived from the observations listed in Table 1 is always dominated by rotation (see Figures 3-7), even for galaxies that appear optically to be very nearly face-on (e.g. NGC 628) or irregular (e.g. NGC 4449, IC10). Roberts (1975b) has stressed that galaxies of all types show rotation, and it is especially noteworthy that this statement applies even to dwarf irregulars (Cottrell 1976b, Tully et al. 1978). Interpretation in terms of rotation curves is reviewed in Section 3 below.

More detailed examination shows that the observed velocity fields frequently show deviations from the symmetry expected from purely circular rotation, and that these deviations can also be large (e.g. NGC 3310, van der Kruit 1976a). Interpretation in terms of bars and oval distortions, warps, density waves, and nuclear explosions is reviewed in Section 4.

It is evident from Table 1 that there are a number of galaxies for which both optical and radio kinematical data exist. For a few systems there is enough resolution in the HI data to make a detailed comparison with the optical results in the regions of overlap. In M31 the agreement is in general very satisfactory (within about 5 km/sec, Emerson 1976), as is the case in M33 (Boulesteix & Monnet 1970, de Jager & Davies 1971, Warner et al. 1973) and the LMC (Feast 1970, Smith & Weedman 1971, Chériguène & Monnet 1972). For M81 the agreement is also good (Rots 1975). The velocities of the HI and HII are also similar to those of planetary nebulae (Smith & Weedman 1972), blue globular clusters (Andrews & Evans 1972), and supergiants (Prevot 1973) in the LMC. The situation is, however, somewhat confused in the SMC (Smith & Weedman 1973).

Differences of 10-20 km/sec for particular objects in M33 may be ascribed to peculiar motions within the HII regions themselves (Warner et al. 1973, Wright 1971) or to the effects of uneven internal obscuration of the light by the dust that must be present there (e.g. van der Kruit & Allen 1976). One must also keep in mind that the radio HI results have inevitably been smoothed by the telescope beam.

The extensive velocity fields of M51 derived from Halpha Fabry-Perot interferograms by Tully (1974a) and from radio-HI synthesis by Shane (1975) are very similar. The orientation parameters and rotation curves obtained from the Halpha data by Tully (1974b) and from the HI data by Segalovitz (1976) (1) agree within the uncertainties when one considers the complexity of the velocity field and the differing angular resolutions of the two observing methods. The optical data for the giant HII complexes in M101 also show no large differences with the radio-HI measurements.

1 The inclination angle quoted by Segalovitz (1976, p. 67) should read 35° and not 15°. Back.

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