Leo IV and V – A possible dwarf galaxy pair?

Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
e-mail: mblana@udec.cl

Received: 11 November 2011
Accepted: 30 March 2012

Abstract

The last few years have seen the discovery of many faint and ultra-faint dwarf spheroidal galaxies around the Milky Way. Among these is a pair of satellites called Leo IV and Leo V. This pair is found at large distances from the Milky Way (154 and 175 kpc, respectively). The fairly small difference in radial distance, and the fact that they also show a close projected distance on the sky, has led to the idea that we might be seeing a new pair of bound galaxies – like the Magellanic Clouds. In this paper we investigate this speculation by means of a simple integration code (confirming the results with full N-body simulations). Because the luminous mass of the two faint dwarfs is far too low to allow them to be bound, we simulate the pair assuming extended dark matter haloes. Our results show that the minimum dark matter mass required for the pair to be bound is quite high – ranging from 1.6 × 10¹⁰ M_⊙ to 5.4 × 10¹⁰ M_⊙ (within the virial radii). Computing the mass of dark matter within a commonly adopted radius of 300 pc shows that our models are well within the predicted range of dark matter content for satellites so faint. We therefore conclude that it could be possible that the two galaxies constitute a bound pair.