Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...271..391c&link_type=abstract
Astronomy and Astrophysics, Vol.271, NO. 2/APR, P. 391, 1993
Astronomy and Astrophysics
Astrophysics
105
Scientific paper
N-body simulations are used to study bar formation and pattern speeds in galaxies with various bulge-to-disk mass ratios. The stellar bars are composed of trapped, elongated, prograde orbits that precess at a rate close to {OMEGA} - K/2, so the bar pattern speed depends critically on the relative bulge mass and the disk scale-length. For a given ratio of bulge-to-disk scale length, galaxies with a large bulge- to-disk mass ratio, such as those with early Hubble types, have large maxima in the distribution of {OMEGA} - k/2 and therefore form bars with large pattern speeds, placing corotation at a small radius and near the ends of the bars. The density profiles in such bars are found to be flat, and a ring in both gas and stars forms at the inner Lindblad resonance, which is inside the bar. Galaxies with a low bulge-to-disk mass ratio, such as late-type galaxies, have low values of {OMEGA} - k/2 everywhere and consequently low pattern speeds for the bars that form. This places corotation far out in the disks, beyond the ends of the bars, and lets the disk scale length determine the bar length. It also makes the density profile along the bar exponential and nearly indistinguishable from the outer disk profile. In all cases, the length of the bar is limited approximately by either the disk scale length or corotation, whichever is smaller. The early type bars in the models continuously grow and slow down with time as angular momentum gets transferred to the corotation and outer resonance regions, which both occur within the stellar disk. Late type bars stop growing at an early stage, however, because corotation is so far out in the disk that there are few stars to absorb the bar's angular momentum. We propose that these numerical simulations explain the observed differences in the properties of bars along the Hubble sequence; i.e., bars in late-type galaxies are in fact shorter with respect to either the overall galaxy size or the length of the rising part of the velocity curve, compared to early-type galaxy bars, and the intensity profiles of late-type galaxy bars have the same exponential shape as the outlying disks whereas in early-type galaxies the bars have a nearly uniform intensity. We also simulate self-consistently the gas behavior in barred galaxies. In the late types, where the central part is not dominated by a large bulge, the gas becomes self-gravitating and a short gaseous bar forms. This result may explain the common observation of CO bars in the nuclei of late type galaxies. The gas between the inner Lindblad resonance and corotation in these galaxies also accretes into the nucleus because of spiral torques. We suggest that this accretion can fuel a star burst after a companion galaxy first moves the outer disk gas to the accreting region inside corotation. Early type galaxies accrete matter in a similar fashion, but much earlier in the evolution, and the result is a pronounced ring at the inner Lindblad resonance.
Combes François
Elmegreen Bruce G.
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