Dwarf elliptical galaxies with kinematically decoupled cores

¹ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281, S9, 9000 Gent, Belgium e-mail: [sven.derijcke;herwig.dejonghe]@UGent.be
² Institut für Astronomie, Universität Wien, Türkenschanzstraße 17, 1180 Wien, Austria e-mail: zeilinger@astro.univie.ac.at
³ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: ghau@eso.org

Received: 30 April 2004
Accepted: 7 July 2004

Abstract

We present, for the first time, photometric and kinematical evidence, obtained with FORS2 on the VLT, for the existence of kinematically decoupled cores (KDCs) in two dwarf elliptical galaxies; FS76 in the NGC 5044 group and FS373 in the NGC 3258 group. Both kinematically peculiar subcomponents rotate in the same sense as the main body of their host galaxy but betray their presence by a pronounced bump in the rotation velocity profiles at a radius of about 1''. The KDC in FS76 rotates at  km s^-1, with the host galaxy rotating at  km s^-1; the KDC in FS373 has a rotation velocity of  km s^-1 while the galaxy itself rotates at  km s^-1. FS373 has a very complex rotation velocity profile with the velocity changing sign at 1.5 R_e. The velocity and velocity dispersion profiles of FS76 are asymmetric at larger radii. This could be caused by a past gravitational interaction with the giant elliptical NGC 5044, which is at a projected distance of 50 kpc. We argue that these decoupled cores are most likely not produced by mergers in a group or cluster environment because of the prohibitively large relative velocities. A plausible alternative is offered by flyby interactions between a dwarf elliptical or its disky progenitor and a massive galaxy. The tidal forces during an interaction at the relative velocities and impact parameters typical for a group environment exert a torque on the dwarf galaxy that, according to analytical estimates, transfers enough angular momentum to its stellar envelope to explain the observed peculiar kinematics.