The Discovery of an Ultra-Faint Star Cluster in the Constellation of Ursa Minor

TL;DR

This paper reports the discovery of Munoz 1, an ultra-faint stellar cluster near Ursa Minor, characterized by its small size, low luminosity, and distinct velocity, expanding knowledge of faint star clusters in the Milky Way's halo.

Contribution

The paper presents the first identification and detailed characterization of Munoz 1, an ultra-faint star cluster, using photometry and spectroscopy, highlighting its unique properties and independence from nearby dwarf galaxies.

Findings

Munoz 1 is located at 45 kpc from Earth.

It has a half-light radius of 7 pc and an absolute magnitude of about -0.4.

Its velocity suggests it is not associated with Ursa Minor.

Abstract

We report the discovery of a new ultra-faint globular cluster in the constellation of Ursa Minor, based on stellar photometry from the MegaCam imager at the Canada-France-Hawaii Telescope (CFHT). We find that this cluster, Munoz 1, is located at a distance of 45 +/- 5 kpc and at a projected distance of only 45 arcmin from the center of the Ursa Minor dSph galaxy. Using a Maximum Likelihood technique we measure a half-light radius of 0.5 arcmin, or equivalently 7 pc and an ellipticity consistent with being zero. We estimate its absolute magnitude to be M_V=-0.4 +/- 0.9, which corresponds to L_V=120 (+160, -65) L_sun and we measure a heliocentric radial velocity of -137 +/- 4 km/s based on Keck/DEIMOS spectroscopy. This new satellite is separate from Ursa Minor by ~30 kpc and 110 km/s suggesting the cluster is not obviously associated with the dSph, despite the very close angular separation. Based on its photometric properties and structural parameters we conclude that Munoz 1 is a new ultra-faint stellar cluster. Along with Segue 3 this is one of the faintest stellar clusters known to date.