Formation of ultra-compact dwarf galaxies: tests of the galaxy threshing scenario in Fornax

TL;DR

This study tests the galaxy threshing scenario for forming ultra-compact dwarf galaxies in the Fornax cluster, finding partial consistency with observations but also highlighting unexplained velocity dispersion peculiarities.

Contribution

It provides a detailed analysis of UCD formation via galaxy threshing in Fornax, including modeling of the population and dynamical properties, and tests a simple threshing model against observations.

Findings

UCDs' magnitude distribution matches that of dwarf nuclei.

The combined UCD and dwarf population fits an NFW profile with 5 kpc radius.

A simple threshing model cannot fully explain the UCD distribution beyond 30 kpc.

Abstract

This paper investigates the possibility that UCD galaxies in the Fornax cluster are formed by the threshing of nucleated, early-type dwarf galaxies (hereafter dwarf galaxies). Similar to the results of Cote et al. (2006) for the Virgo cluster, we show that the Fornax Cluster observations are consistent with a single population in which all dwarfs are nucleated, with a ratio of nuclear to total magnitude that varies slowly with magnitude. Importantly, the magnitude distribution of the UCD population is similar to that of the dwarf nuclei in the Fornax cluster. The joint population of UCDs and the dwarfs from which they may originate is modelled and shown to be consistent with an NFW profile with a characteristic radius of 5 kpc. Furthermore, a steady-state dynamical model reproduces the known mass profile of Fornax. However, there are a number of peculiarities in the velocity dispersion data that remain unexplained. The simplest possible threshing model is tested, in which dwarf galaxies move on orbits in a static cluster potential and are threshed if they pass within a radius at which the tidal force from the cluster exceeds the internal gravity at the core of their dark matter halo. This fails to reproduce the observed fraction of UCDs at radii greater than 30 kpc from the core of Fornax.