The discovery of two extremely low luminosity Milky Way globular clusters

TL;DR

This paper reports the discovery of two extremely low luminosity globular clusters in the Milky Way halo, expanding the known diversity and suggesting a larger original population of such clusters.

Contribution

It presents the identification and confirmation of two new ultra-faint globular clusters, Koposov 1 and 2, with detailed analysis of their properties and implications for globular cluster populations.

Findings

Koposov 1 and 2 are the faintest known globular clusters in the Milky Way.

These clusters have very short relaxation and evaporation timescales.

Their discovery indicates a larger, previously unrecognized population of globular clusters.

Abstract

We report the discovery of two extremely low luminosity globular clusters in the Milky Way Halo. These objects were detected in the Sloan Digital Sky Survey Data Release 5 and confirmed with deeper imaging at the Calar Alto Observatory. The clusters, Koposov 1 and Koposov 2, are located at $\sim 40-50$ kpc and appear to have old stellar populations and luminosities of only $M_V \sim -1$ mag. Their observed sizes of $\sim 3$ pc are well within the expected tidal limit of $\sim$10 pc at that distance. Together with Palomar 1, AM 4 and Whiting 1, these new clusters are the lowest luminosity globulars orbiting the Milky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close to distant branch of the Sagittarius stream. The half-mass relaxation times of Koposov 1 and 2 are only $\sim 70$ and $\sim 55$ Myr respectively (2 orders of magnitude shorter than the age of the stellar populations), so it would seem that they have undergone drastic mass segregation. Since they do not appear to be very concentrated, their evaporation timescales may be as low as $\sim 0.1 t_{\rm Hubble}$. These discoveries show that the structural parameter space of globular clusters in the Milky Way halo is not yet fully explored. They also add, through their short remaining survival times, significant direct evidence for a once much larger population of globular clusters.