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Astron. Astrophys. 335, 431-442 (1998)

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Counterrotating galaxies formed by cosmological collapses

M. Harsoula and N. Voglis

Department of Astronomy, University of Athens, GR-157 84 Athens, Greece

Received 21 January 1998 / Accepted 16 March 1998


We investigate numerically the possibility that dissipationless cosmological collapses can lead to the formation of counterrotating galaxies. We consider systems formed from initially small density excesses embedded in the environment of other density perturbations in an otherwise homogeneous and isotropic expanding early Universe.

We find that a central bar-like density excess, that can make bound a mass of the order of galactic mass can work as a seed able to initiate counterrotating galaxies. Such objects are formed rather naturally as a result of the partial mixing between the material in which positive angular momentum dominates with the material in which negative angular momentum dominates. The initial axial ratio of the bar-like perturbation is a control parameter in this process. Counterrotation is favored for an axial ratio [FORMULA]. The rotational velocity profile in the relaxed state of these configurations matches to a great extent the curves given from observations, for some elliptical galaxies, presenting kinematically decoupled cores.

Performing parallel simulations of the same system by N-body and by conservative technique codes we show that the rotational velocity curve, established after the relaxation, maintains its basic features even for a Hubble time. Therefore counterrotating galaxies observed today could have been formed directly from cosmological initial conditions.

Key words: galaxies: formation – galaxies: evolution – galaxies: kinematics and dynamics

Send offprint requests to: M. Harsoula

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998