A Possible Relic Star Cluster in the Sextans Dwarf Galaxy

Abstract

We report a possible discovery of a relic star cluster in the Sextans dwarf spheroidal galaxy. Using the \textit{hk} index ($\equiv$($Ca-b$)$-$($b-y$)) as a photometric metallicity indicator, we discriminate the metal-poor and metal-rich stars in the galaxy and find unexpected number density excess of metal-poor stars located 7'.7 ($\sim$190 pc in projected distance) away from the known galactic center. The $V-I$ color$-$magnitude diagram (CMD) for stars around the density excess reveals that both the main sequence and the giant branch are considerably narrower and redder than the bulk of field stars in Sextans. Our stellar population models show (a) that the narrow CMD is best reproduced by a simple stellar population with an age of $\sim$13 Gyr and [Fe/H] of $\sim$$-$2.3 dex, and (b) that the redder $V-I$ color of the $hk$-weak population is explained $only$ if it is $\sim$2 Gyr older than the field stars. The results lead us to conclude that the off-centered density peak is likely associated with an old, metal-poor globular cluster. The larger spatial extent ($>$ 80 pc in radius) and the smaller number of stars ($\sim$1000) than typical globular clusters point to a star cluster that is in the process of dissolution. The finding serves as the first detection of a surviving star cluster in Sextans, supporting previous suggestions of the presence of star cluster remnants in the galaxy. If confirmed, the survival of a relic star cluster until now implies a $cored$ dark matter halo profile for this dwarf galaxy.

arXiv comment10 pages, 5 figures, published in ApJL

References in corpus (34)

• A Universal Density Profile from Hierarchical Clustering
• The Structure of Cold Dark Matter Halos
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• The USNO-B Catalog
• The Core-Cusp Problem
• Homogeneous Photometry for Star Clusters and Resolved Galaxies. II. Photometric Standard Stars
• Subaru Deep Survey V. A Census of Lyman Break Galaxies at z=4 and 5 in the Subaru Deep Fields: Photometric Properties
• Multi-Element Abundance Measurements from Medium-Resolution Spectra. III. Metallicity Distributions of Milky Way Dwarf Satellite Galaxies
• Stellar Velocities in the Carina, Fornax, Sculptor and Sextans dSph Galaxies: Data from the Magellan/MMFS Survey
• A MegaCam Survey of Outer Halo Satellites. III. Photometric and Structural Parameters
• Multi-Element Abundance Measurements from Medium-Resolution Spectra. II. Catalog of Stars in Milky Way Dwarf Satellite Galaxies
• A Dynamical Fossil in the Ursa Minor Dwarf Spheroidal Galaxy
• Study of the Sextans dwarf spheroidal galaxy from the DART CaII triplet survey
• The Draco and Ursa Minor dwarf spheroidals. A comparative study
• Exploring Halo Substructure with Giant Stars IV: The Extended Structure of the Ursa Minor Dwarf Spheroidal
• Chemical Composition of Extremely Metal-Poor Stars in the Sextans Dwarf Spheroidal Galaxy
• The Presence of Two Distinct Red Giant Branches in the Globular Cluster NGC 1851
• A photometrically and kinematically distinct core in the Sextans dwarf spheroidal galaxy
• Discovery of tidal tails around the distant globular cluster Palomar 14
• Star Formation History and Chemical Evolution of the Sextans Dwarf Spheroidal Galaxy
• Star Formation Histories of Globular Clusters with Multiple Populations. I. ω Cen, M22, and NGC 1851
• On Kinematic Substructure in the Sextans Dwarf Spheroidal Galaxy
• The Complex Distribution of Recently Formed Stars. Bimodal Stellar Clustering in the Star-Forming Region NGC 346
• On the Origin of Bimodal Horizontal-Branches in Massive Globular Clusters: The Case of NGC 6388 and NGC 6441
• The star formation history of the Sextans dwarf spheroidal galaxy: a true fossil of the pre-reionization era
• The Chemical Signature of a Relic Star Cluster in the Sextans Dwarf Spheroidal Galaxy - Implications for Near-Field Cosmology
• Appearances can be deceiving: clear signs of accretion in the seemingly ordinary Sextans dSph
• Structural analysis of the Sextans dwarf spheroidal galaxy
• Open-cluster density profiles derived using a kernel estimator
• Population gradient in Sextans dSph: Comprehensive mapping of a dwarf galaxy by Suprime-Cam
• Sextans' cold substructures as a dynamical judge: Core, Cusp or MOND?
• A Possible Formation Scenario for Dwarf Spheroidal Galaxies I: Fiducial Model
• A Possible Formation Scenario for Dwarf Spheroidal Galaxies - II: A Parameter Study
• A possible formation scenario for dwarf spheroidal galaxies - III. Adding star formation histories to the fiducial model

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