|Annu. Rev. Astron. Astrophys. 1978. 16:
Copyright © 1978 by . All rights reserved
4.3. Bars and Oval Distortions
When the large-scale disturbances in the observed velocity fields show a high degree of symmetry it is usual to look for explanations which, although perhaps more prosaic than energetic nuclear explosions, nevertheless may provide some useful information on the kinematics of the mass distributions in the disks. In the case of the barred spiral galaxies the procedure has been to look at specific objects with a view towards correlating noncircular motions with the structure of the bar. On the other hand, routine investigation of a number of disk galaxies in HI has led to the identification of oval distortions as a common feature of many galaxies.
The structure and dynamics of barred spiral galaxies has been reviewed extensively by de Vaucouleurs & Freeman (1973). There is some evidence for gas streaming in the bars (see also Burbidge 1970), although there are conflicting reports as to whether it flows inward or outward. A case for outward streaming in NGC 4027 was made by de Vaucouleurs et al. (1968); Christiansen & Jefferys (1976) have modelled the velocity field of that galaxy with a high-density prolate spheroid representing the bar embedded off-center in a disk.
Chériguène (1975) presented results from optical emission-line slit spectra along the bars of twelve barred spirals. No obvious noncircular motions were found, althoug it should be noted that no observations at other slit position angles were made. The results nevertheless suggest that the inferred rotation curve is a function of morphological type in the sense that the total angular velocities in the bar regions decrease from 100 km/sec/kpc for the SBab, b types to 20-40 km/sec/kpc for the SBcd, m. In the IBm galaxies the rotation is very slow. These results are consistent with the single case of measured stellar rotation of 80 km/sec/kpc in the bar of the SBb galaxy NGC 3351 (Peterson et al. 1976).
The most extensive observations presently available on a barred spiral galaxy are those for NGC 5383 (Figure 3). The velocity field obtained from radio-HI data provide evidence for an underlying massive disk in normal differential rotation, upon which disturbances of the order of 50 km/sec are superposed in the region of the bar (Allen et al. 1973a, Sancisi 1975). Optical measurements of the emission-line velocity field (Peterson et al. 1978, Duval 1977) confirm the presence of velocity perturbations in the bar and can be interpreted as inward-streaming motions of up to 180 km/sec along the bar (Peterson et al. 1978). More complicated streaming patterns associated with shocks in the gas are expected on the basis of hydrodynamical calculations (Huntley et al. 1978, and references therein), and a comparison with the observations should prove rewarding.
Figure 3. The kinematical effects of bars in galaxies are illustrated for the case of NGC 5383. Above is shown an optical picture of the galaxy and at right the velocity field derived from emission lines on optical slit spectra (Peterson et al. 1978).
The presence of an underlying massive disk has been discovered also in NGC 3359 by Siefert et al. (1975), and evidence for noncircular motions near the bar of NGC 660 has been presented by Benvenuti et al. (1976).
We turn now to the evidence for the presence of oval distortions in galaxies which, at least from the appearance of their optical images were not formerly suspected of having them (but see Freeman 1970b). The HI observations of NGC 4151 and NGC 4736 (Bosma et al. 1977a, b) show a large-scale feature in their velocity fields (see Figure 4): The apparent dynamical "major axis" in fact has an S shape, and in the inner regions the position angle is inconsistent with that of the optical isophotes. The interpretation offered is that the potential distribution in the central area is an oval-shaped disk or a "fat bar." A qualitative model for NGC 4736 is able to describe both the velocity perturbations and the distribution of light, including the faint outer ring (Bosma et al. 1977b). The line of nodes found from the HI observations in the far outer parts of NGC 4151 (Davies 1973, Bosma et al. 1977a) indicates that the velocities observed optically near the center are mainly rotational (Anderson 1974, Fricke & Reinhardt 1974), although there is still evidence for noncircular motions of the HII regions near the ends of the "fat bar" (Simkin 1975b).
Figure 4. The kinematical effects of an oval distortion in the inner disk are evident in the radial velocity field of the HI in NGC 4736 (Bosma et al. 1977b). The kinematical "line-of-nodes" has an S-shape, which in the inner region is inconsistent with the major axis of the optical isophotes. Note that HI is detected in the faint outer ring.