|Annu. Rev. Astron. Astrophys. 1978. 16:
Copyright © 1978 by . All rights reserved
In all of the velocity fields presently available, one can easily find evidence for deviations from the textbook style of regular, planar circular motion on all length scales ranging right down to the resolution limit of the observations. On the scale of a few hundred parsecs, local erratic motions of 5-10 km /sec are found everywhere over the disks of spirals and in the irregular galaxies; the viability of the obvious explanations in terms of supernovae explosions or the outflow of matter and radiation from young stars has yet to be examined in detail. In this section we concentrate mainly on deviations at scales of the order of 1 kpc and larger that seem to form systematic patterns over an appreciable part of the disk.
4.1. The Central Regions
The necessary angular resolution in the central regions is available only from the optical observations. Motions in or very near to the nuclei of many galaxies have been reviewed by Burbidge (1972) and Ulrich (1974); for a detailed discussion of the central regions of our Galaxy, see Oort (1977).
The gas motions within about 0.4kpc of the nucleus of M31 show a complex velocity field superposed on rapid rotation (Rubin & Ford 1971), with evidence for expansion at velocities up to 100 km/sec. Observations of the interstellar H and K absorption lines provide further indications of gas ejection from the nucleus with velocities up to 450 km /sec (Morton & Andereck 1976). The gas motions in the inner 1.5 kpc of M81 are equally complex (Goad 1974, 1976); similar to the situation in M31, the rotation curve rises rapidly to a sharp peak of ~ 300 km/sec at about 1 kpc, followed by a deep minimum and a second maximum further out in the main part of the disk. (4) Within 300 pc there is an outflow of gas, but just outside this radius the velocity field shows a general inflow towards the center. The nucleus of M81 contains a compact ( 6 × 10-3 pc = 1300au) radio continuum source that varies in time (de Bruyn et al. 1976, Kellermann et al. 1976), althoug it is not particularly intense. The evidence in M31 and M81 seems to indicate a mild case of nuclear activity and gas ejection on a relatively small scale; however, the stars in the disk and the bulge of M31 do not seem to follow the pattern of noncircular motions seen in the gas (Pellet 1976).
Although strong noncircular motions in the central regions of M51 were originally reported by Burbidge & Burbidge (1964), these were not confirmed in the extensive H Fabry-Perot observations by Tully (1974a, c), who instead found only small (~ 20 km/sec) deviations that he suggested were associated with an inner Lindbland resonance. The situation remains ambiguous, however, since de Veny et al. (1976) have recently reconfirmed the earlier reports with new evidence for noncircular motions of up to 75 km/sec.
The kinematics of the inner rings of bright HII regions found in many galaxies has been investigated in several cases. The ring at ~ 340 pc from the nucleus of the SBb galaxy NGC 3351 is apparently contracting (Rubin et al. 1975). Noncircular motions of the inner ring of HII regions have been observed in NGC 5364 by Goad et al. (1975); a preliminary analysis of the general rotation in that galaxy shows that the HII ring may coincide with the position of the inner Lindblad resonance. A detailed study of the bright inner ring of HII regions in NGC 4736 has been made by van der Kruit (1974a, 1976c). Besides the H II regions themselves, the ring contains diffuse H emission with a sharp outer boundary at a radius of about 1 kpc, where a strong decrease in the optical continuum surface brightness and an increase in (B - V) color is also found. In the ring there are two concentrations of nonthermal radio continuum emission (van der Kruit 1971, de Bruyn 1977a) located on roughly opposite sides of the nucleus (itself also a weak radio source) near the places where stubby armlike features seem to emerge from the ring. The HII kinematics studied by van der Kruit shows that expansion motions exist in the ring and that the expansion is strongest at the positions of the radio continuum peaks (but see also Bosma et al. 1977b). Although an explanation in terms of the inner Lindblad resonance would seem attractive (Schommer & Sullivan 1976), the direction of the noncircular motions does not appear to agree with that expected in the linear approximation (van der Kruit 1976c). The possibilities for large-scale organized effects of nuclear activity (see for example Sanders & Bania 1976) or gravitational disturbances remain to be investigated in detail.
Contrasting results are available for the emission-line "hot spots" found near the nuclei of some galaxies. Noncircular motions are possible for such features in NGC 2903 (Simkin 1975b), however, the "hot spots" in the central 2.5 kpc of the "active" galaxy NGC 2782 show a pattern of normal rotation with no noncircular motions in excess of 15 km/sec along the line of sight (van der Kruit 1977b).
Finally, we mention evidence for substantial noncircular motions in two other cases: Walker's (1968) study of the central region of the Seyfert galaxy NGC 1068 provides detailed evidence for expansion velocities up to 150 km/sec in the entire area within a radius of 1.5 kpc; Demoulin & Burbidge (1970) and Ulrich (1978) present observations indicating outflow of gas from the nucleus of NGC 253 into a cone-shaped region with velocities reaching 300 km/sec. The apparent outflow of matter is confirmed by radio measurements of HI absorption against the radio source in the nucleus (Gottesman et al. 1976).
In summary there is good evidence in the velocity patterns of the central regions for noncircular motions and even ejection of the gas on the 1 kpc scale. The explanations are, however, diverse and not always consistent with all the data; in particular the (admittedly sparse) evidence that the stars are not appreciably affected by all the goings-on leaves the suspicion that the peculiar motions in the central regions may not reflect any real asymmetries in the distribution of mass there. More observational material on this latter topic would be very desirable.
4 The value of 300 km/sec was derived from spectra near the major axis only. Examination of all Goad's spectra suggests that a significantly lower value of 250-270 km/sec is a better estimate, illustrating once again that spectra at many position angles are required for an accurate picture. Back.