On stability of galactic disks in the modified dynamics and the distribution of their mean surface brightness
1989; IOP Publishing; Volume: 338; Linguagem: Inglês
10.1086/167184
ISSN1538-4357
Autores Tópico(s)Geophysics and Gravity Measurements
Resumoview Abstract Citations (64) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On Stability of Galactic Disks in the Modified Dynamics and the Distribution of Their Mean Surface-Brightness Milgrom, Mordehai Abstract I begin an examination of the stability of the disk component of galaxies in the modified dynamics (MOND). A disk in a galaxy with a mean acceleration, a, of the order of, or smaller than the acceleration constant of the theory, a_0_, is, as a rule, more stable according to MOND than it is in Newtonian dynamics. This comes about because in MOND-indeed in any like theory-the acceleration increases more slowly with the attracting mass; a smaller relative change in the gravitational acceleration results from a given perturbation of the mass distribution. An exact result is that the degree of stability does not increase indefinitely in the deep MOND limit; the stability criteria, for an arbitrary equilibrium state of a stellar or gaseous system, become independent of a, when this is much smaller than a_0_. For example, the maximal added stability of a galactic disk is, loosely speaking, similar to that achieved by a Newtonian disk that is furnished with a nonresponsive halo of a mass comparable to its own. A crucial prediction emerges: the fictitious halo that would be required, in a Newtonian framework, to account for the equilibrium properties (mainly the rotation curves) of very low surface density disks, is not the same as the "halo" one would invoke for stabilizing them. The former becomes increasingly "massive" with decreasing surface density; the latter does not. Accounting for such a state of things, in Newtonian dynamics with dark halos, would entail added contrivance. I study the stability to tightly wound perturbations in the WKB approximation and derive the relevant Toomre condition for local stability in MOND; it is satisfied more easily when a < a_0_ than when a > a_0_. Disks with a >> a_0_-or equivalently, with surface density {SIGMA} >> {SIGMA}_0_ = a_0_G^-1^-obey Newtonian dynamics and, since they possess no halo to speak of, they are subject to the much-discussed instabilities of bare Newtonian disks. It is proposed that the observed paucity of galactic disks with mean surface-densities above a certain limit-the updated version of the Freeman law-is related to this fact (the cutoff value being {SIGMA}_0_). Publication: The Astrophysical Journal Pub Date: March 1989 DOI: 10.1086/167184 Bibcode: 1989ApJ...338..121M Keywords: Brightness Distribution; Galactic Structure; Stability; Mass Distribution; Poisson Equation; Vlasov Equations; Wentzel-Kramer-Brillouin Method; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: STRUCTURE full text sources ADS |
Referência(s)