A New Channel for the Formation of Kinematically Decoupled Cores in Early-type galaxies

TL;DR

This paper demonstrates a novel formation mechanism for kinematically decoupled cores in elliptical galaxies through prograde mergers that temporarily reverse orbital spin, resulting in large-scale counter-rotating cores.

Contribution

It introduces a new formation channel for KDCs in early-type galaxies via prograde mergers causing temporary retrograde orbits, expanding current understanding.

Findings

Formation of large (~2 kpc) counter-rotating cores in merger remnants.

KDCs are mainly composed of old stars and persist for over 2 Gyr.

Remnant velocity dispersion shows two symmetrical off-centered peaks.

Abstract

We present the formation of a Kinematically Decoupled Core (KDC) in an elliptical galaxy, resulting from a major merger simulation of two disk galaxies. We show that although the two progenitor galaxies are initially following a prograde orbit, strong reactive forces during the merger can cause a short-lived change of their orbital spin; the two progenitors follow a retrograde orbit right before their final coalescence. This results in a central kinematic decoupling and the formation of a large-scale (~2 kpc radius) counter-rotating core (CRC) at the center of the final elliptical-like merger remnant (M*=1.3x10^11 Msun), while its outer parts keep the rotation direction of the initial orbital spin. The stellar velocity dispersion distribution of the merger remnant galaxy exhibits two symmetrical off-centered peaks, comparable to the observed "2-sigma galaxies". The KDC/CRC consists mainly of old, pre-merger population stars (older than 5 Gyr), remaining prominent in the center of the galaxy for more than 2 Gyr after the coalescence of its progenitors. Its properties are consistent with KDCs observed in massive elliptical galaxies. This new channel for the formation of KDCs from prograde mergers is in addition to previously known formation scenarios from retrograde mergers and can help towards explaining the substantial fraction of KDCs observed in early-type galaxies.