The formation of counterrotating cores in elliptical galaxies

Abstract

Counterrotating cores, one of the various kinematic peculiarities recently discovered in elliptical galaxies, are likely to be the signature of a past merger event. These structures consist of velocity anomalies in the innermost zones of some ellipticals with amplitudes ranging from 10 to 150 km s -1 , sometimes accompanied by a core-within-a-core structure in the surface brightness profile. In a merger between a high- and a low-luminosity elliptical galaxy, the denser core of the smaller galaxy might survive the tidal field of the primary and sink to the remnant center by dynamical friction. This may provide a mechanism for formation of counterrotating cores. An alternative possibility for the counterrotation is a nuclear stellar disk formed out of the gas furnished by a captured gas-rich satellite. We study the first of these mechanisms with self-consistent numerical simulations of mergers between unequal elliptical galaxies. The simulations show that counter-rotation results only when the merging orbits are retrograde, due to the large change in the secondary spin during the merger. It can appear along the photometric major, minor, or intermediate axes. Both primary and secondary particles are involved in the counterrotation. For the mass ratios explored, the secondary does survive the tidal field and settles at the center of the remnant. The secondary light dominates the surface brightness in a small central region, but a core-within-a-core structure is seen only if the ratio of sizes is sufficiently large. The absorption by the primary particles of the orbital energy and angular momentum causes the isophotes within 1 R e to align with those of the central, secondary-dominated region, therefore strong isophote twists are not expected at the edge of that region. Isophote twists appear, though, farther out. All of these properties generally match those observed in ellipticals with counterrotating cores.