A DECam View of the Diffuse Dwarf Galaxy Crater II: The Colour-Magnitude Diagram

Abstract

We present a deep Blanco/DECam colour-magnitude diagram (CMD) for the large but very diffuse Milky Way satellite dwarf galaxy Crater II. The CMD shows only old stars with a clearly bifurcated subgiant branch (SGB) that feeds a narrow red giant branch. The horizontal branch (HB) shows many RR Lyrae and red HB stars. Comparing the CMD with [Fe/H] = -2.0 and [$\alpha$/Fe] = +0.3 alpha-enhanced BaSTI isochrones indicates a mean age of 12.5 Gyr for the main event and a mean age of 10.5 Gyr for the brighter SGB. With such multiple star formation events Crater II shows similarity to more massive dwarfs that have intermediate age populations, however for Crater II there was early quenching of the star formation and no intermediate age or younger stars are present. The spatial distribution of Crater II stars overall is elliptical in the plane of the sky, the detailed distribution shows a lack of strong central concentration, and some inhomogeneities. The 10.5 Gyr subgiant and upper main sequence stars show a slightly higher central concentration when compared to the 12.5 Gyr population. Matching to Gaia DR2 we find the proper motion of Crater II: ${\mu }_{\alpha }\cos \delta$=-0.14 $\pm$ 0.07 , ${\mu }_{\delta }$=-0.10 $\pm$ 0.04 mas yr${}^{-1}$, approximately perpendicular to the semi-major axis of Crater II. Our results provide constraints on the star formation and chemical enrichment history of Crater II, but cannot definitively determine whether or not substantial mass has been lost over its lifetime.

arXiv comment13 pages, 9 figures, 3 tables. Accepted for publication in MNRAS

References in corpus (38)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Gaia Data Release 2. Summary of the contents and survey properties
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III
• Overview of the DESI Legacy Imaging Surveys
• The Faintest Dwarf Galaxies
• Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART
• The Updated BaSTI Stellar Evolution Models and Isochrones: I. Solar Scaled Calculations
• Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall
• Population Gradients in Local Group Dwarf Spheroidals
• The feeble giant. Discovery of a large and diffuse Milky Way dwarf galaxy in the constellation of Crater
• The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy
• The ACS Survey of Galactic Globular Clusters: M54 and Young Populations in the Sagittarius Dwarf Spheroidal Galaxy
• The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2
• Multi-Element Abundance Measurements from Medium-Resolution Spectra. IV. Alpha Element Distributions in Milky Way Dwarf Satellite Galaxies
• The SUMO project I. A survey of multiple populations in globular clusters
• The ACS LCID Project: On the origin of dwarf galaxy types: a manifestation of the halo assembly bias?
• SMASH - Survey of the MAgellanic Stellar History
• Mass loss on the red giant branch: the value and metallicity dependence of Reimers' η in globular clusters
• Tidal stripping and the structure of dwarf galaxies in the Local Group
• Global and non-global parameters of horizontal branch morphology of globular clusters
• The Distribution of Alpha Elements in Ultra-Faint Dwarf Galaxies
• Tidal disruption of dwarf spheroidal galaxies: the strange case of Crater II
• Anomalous Cepheids in the Large Magellanic Cloud: Insight into their origin and connection with the star formation history
• Instrumental response model and detrending for the Dark Energy Camera
• Satellite dwarf galaxies: Stripped but not quenched
• Dynamical Histories of the Crater II and Hercules Dwarf Galaxies
• The ISLANDS project I: Andromeda XVI, An Extremely Low Mass Galaxy not Quenched by Reionization
• Ruprecht 106: A riddle, wrapped in a mystery, inside an enigma
• Population gradient in Sextans dSph: Comprehensive mapping of a dwarf galaxy by Suprime-Cam
• The not so simple globular cluster $\omega$ Cen. I. Spatial distribution of the multiple stellar populations
• CCD PHOTOMETRY OF VARIABLE STARS IN THE GLOBULAR CLUSTER RU 106
• On the absolute age of the metal-rich globular M71 (NGC 6838): I. optical photometry
• On Binary Channels to Anomalous Cepheids
• RR Lyrae Variable Stars in the Crater II Dwarf Galaxy
• The post-infall evolution of a satellite galaxy
• Variable Stars in Local Group Galaxies. IV. RR Lyrae stars in the central regions of the low-density galaxy Crater II
• Color-Magnitude Diagram Constraints on the Metallicities, Ages, and Star Formation History of the Stellar Populations in the Carina Dwarf Spheroidal Galaxy

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