A Hero's Little Horse: Discovery of a Dissolving Star Cluster in Pegasus

Abstract

We report the discovery of an ultra-faint stellar system in the constellation of Pegasus. This concentration of stars was detected by applying our overdensity detection algorithm to the SDSS-DR10 and confirmed with deeper photometry from the Dark Energy Camera at the 4-m Blanco telescope. The best-fitting model isochrone indicates that this stellar system, Kim 1, features an old (12 Gyr) and metal-poor ([Fe/H]$\sim -1.7$) stellar population at a heliocentric distance of $19.8\pm 0.9$ kpc. We measure a half-light radius of $6.9\pm 0.6$ pc using a Plummer profile. The small physical size and the extremely low luminosity are comparable to the faintest known star clusters Segue 3, Koposov 1 & 2, and Muñoz 1. However, Kim 1 exhibits a lower star concentration and is lacking a well defined center. It also has an unusually high ellipticity and irregular outer isophotes, which suggests that we are seeing an intermediate mass star cluster being stripped by the Galactic tidal field. An extended search for evidence of an associated stellar stream within the 3 sqr deg DECam field remains inconclusive. The finding of Kim 1 is consistent with current overdensity detection limits and supports the hypothesis that there are still a substantial number of extreme low luminosity star clusters undetected in the wider Milky Way halo.

9 pages, 8 figures, and 1 table. Accepted for publication in ApJ

References in corpus (16)

• The Dartmouth Stellar Evolution Database
• Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
• The Luminosity Function of the Milky Way Satellites
• Introduction to astroML: Machine Learning for Astrophysics
• The spatial distribution of the Milky Way and Andromeda satellite galaxies
• The Invisibles: A Detection Algorithm to Trace the Faintest Milky Way Satellites
• The Hercules-Aquila Cloud
• A new distant Milky Way globular cluster in the Pan-STARRS1 3π survey
• ATLAS lifts the Cup: Discovery of a New Milky Way satellite in Crater
• Discs of Satellites: the new dwarf spheroidals
• Whiting 1: the youngest globular cluster associated with the Sgr dSph
• Bootes II ReBooted: An MMT/MegaCam Study of An Ultra-Faint Milky Way Satellite
• A chemical confirmation of the faint Bootes II dwarf Spheroidal Galaxy
• The globular cluster AM 4: yet another young globular associated with the Sgr Dwarf Spheroidal galaxy?
• Two New Halo Debris Streams in the Sloan Digital Sky Survey
• Are the Ultra-Faint Dwarf Galaxies Just Cusps?

Cited by in corpus (25)

• Milky Way Satellite Census. I. The Observational Selection Function for Milky Way Satellites in DES Y3 and Pan-STARRS DR1
• A Hero's Dark Horse: Discovery of an Ultra-Faint Milky Way Satellite in Pegasus
• Two Ultra-Faint Milky Way Stellar Systems Discovered in Early Data from the DECam Local Volume Exploration Survey
• Discovery of a Faint Outer Halo Milky Way Star Cluster in the Southern Sky
• Six More Ultra-Faint Milky Way Companions Discovered in the DECam Local Volume Exploration Survey
• On the nature of ultra-faint dwarf galaxy candidates I: DES1, Eridanus III and Tucana V
• Portrait of a Dark Horse: a Photometric and Spectroscopic Study of the Ultra-faint Milky Way Satellite Pegasus III
• Discovery of an Ultra-Faint Stellar System near the Magellanic Clouds with the DECam Local Volume Exploration (DELVE) Survey
• Eppur si muove: Positional and kinematic correlations of satellite pairs in the low Z universe
• The Pristine Dwarf-Galaxy survey -- III. Revealing the nature of the Milky Way globular cluster Sagittarius II
• Stellar mass segregation as separating classifier between globular clusters and ultra-faint dwarf galaxies
• Extended stellar substructure surrounding the Boötes I dwarf spheroidal galaxy
• Detailed study of the Milky Way globular cluster Laevens 3
• A panoramic view of the Local Group dwarf galaxy NGC 6822
• Halo intruders in the Galactic bulge revealed by HST and Gaia: the globular clusters Terzan 10 and Djorgovski 1
• The contribution of dissolving star clusters to the population of ultra-faint objects in the outer halo of the Milky Way
• Gamma-ray and Synchrotron Radiation from Dark Matter annihilations in Ultra-faint Dwarf Galaxies
• A modern view of galaxies and their stellar populations
• The Likelihood of Undiscovered Globular Clusters in the Outskirts of the Milky Way
• First evidence of a stripped star cluster from the Small Magellanic Cloud
• The emergence of globular clusters and globular-cluster-like dwarfs
• A Multi-Wavelength Study of the Segue 3 Cluster
• Ultra-Faint Milky Way Satellites Discovered in Carina, Phoenix, and Telescopium with DELVE Data Release 3
• Quantifying the Detectability of Milky Way Satellites with Image Simulations: a Case Study with KiDS
• Forward-modelling the Luminosity, Distance, and Size distributions of the Milky Way Satellites