A Unified Theory of the Formation of Galactic Bars

Details A Unified Theory of the Formation of Galactic Bars A Unified Theory of the Formation of Galactic Bars

Jul 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003astl...29..447p&link_type=abstract

Astronomy Letters, vol. 29, p. 447-462 (2003)

Astronomy and Astrophysics

Astronomy

3

Scientific paper

We give arguments for a basically unified formation mechanism of slow (Lynden-Bell) and fast (common) galactic bars. This mechanism is based on an instability that is akin to the well-known instability of radial orbits and is produced by the mutual attraction and alignment of precessing stellar orbits (so far, only the formation of slow bars has been explained in this way). We present a general theory of the low-frequency modes in a disk that consists of orbits precessing at different angular velocities. The problem of determining these modes is reduced to integral equations of moderately complex structure. The characteristic pattern angular velocities Omega_p of the low-frequency modes are of the order of the mean orbital precession angular velocity . Bar modes are also among the low-frequency modes; while Omega_p is approximately equal to for slow bars, Omega_p for fast bars can appreciably exceed even the maximum orbital precession angular velocity in the disk Omega_pr(max) (however, it remains of the order of these precession angular velocities). The possibility of such an excess of Omega_p over Omega_pr(max) is associated with the effect of "repelling" orbits. The latter tend to move in a direction opposite to the direction in which they are pushed. We analyze the pattern of orbital precession in potentials typical of galactic disks. We note that the maximum radius of an "attracting" circular orbit r_c can serve as a reasonable estimate of the bar length l_b. Such an estimate is in good agreement with the available results of N-body simulations.

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