Segue 2: A Prototype of the Population of Satellites of Satellites

TL;DR

Segue 2 is a newly discovered faint satellite of the Milky Way, potentially representing a population of 'satellites of satellites' that are relics from early cosmic epochs, providing insights into galaxy formation.

Contribution

This paper reports the discovery and detailed characterization of Segue 2, a new ultra-faint satellite, and proposes it as part of a novel population of satellites of satellites.

Findings

Segue 2 has a luminosity of M_v = -2.5 and a half-light radius of 34 pc.

Velocity dispersion of Segue 2 is measured at 3.4 km/s.

Evidence of a surrounding stream suggests complex dynamical history.

Abstract

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope. From this, we derive a luminosity of M_v = -2.5, a half-light radius of 34 pc and a systemic velocity of -40$ km/s. Our MMT data also provides evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighboring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km/s and the possible stream as about 7 km/s. This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites -- survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts z > 10 and are good candidates for fossils of the reionization epoch.