Evidence That Hydra I is a Tidally Disrupting Milky Way Dwarf Galaxy

Abstract

The Eastern Banded Structure (EBS) and Hydra~I halo overdensity are very nearby (d $\sim$ 10 kpc) objects discovered in SDSS data. Previous studies of the region have shown that EBS and Hydra I are spatially coincident, cold structures at the same distance, suggesting that Hydra I may be the EBS's progenitor. We combine new wide-field DECam imaging and MMT/Hectochelle spectroscopic observations of Hydra I with SDSS archival spectroscopic observations to quantify Hydra I's present-day chemodynamical properties, and to infer whether it originated as a star cluster or dwarf galaxy. While previous work using shallow SDSS imaging assumed a standard old, metal-poor stellar population, our deeper DECam imaging reveals that Hydra~I has a thin, well-defined main sequence turnoff of intermediate age ($\sim 5-6$ Gyr) and metallicity ([Fe/H] = $-0.9$ dex). We measure statistically significant spreads in both the iron and alpha-element abundances of ${\sigma }_{[Fe/H]}=0.13\pm 0.02$ dex and ${\sigma }_{[\alpha /\mathrm{Fe}]}=0.09\pm 0.03$ dex, respectively, and place upper limits on both the rotation and its proper motion. Hydra~I's intermediate age and [Fe/H] -- as well as its low [$\alpha$/Fe], apparent [Fe/H] spread, and present-day low luminosity -- suggest that its progenitor was a dwarf galaxy, which subsequently lost more than $99.99\%$ of its stellar mass.

arXiv comment17 pages, 14 figures, submitted to ApJ

References in corpus (78)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• MultiNest: an efficient and robust Bayesian inference tool for cosmology and particle physics
• The observed properties of dwarf galaxies in and around the Local Group
• The Dartmouth Stellar Evolution Database
• Multimodal nested sampling: an efficient and robust alternative to MCMC methods for astronomical data analysis
• The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III
• SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20
• The Dark Energy Camera
• Tracing Galaxy Formation with Stellar Halos I: Methods
• The Field of Streams: Sagittarius and its Siblings
• The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies
• Stellar Chemical Signatures And Hierarchical Galaxy Formation
• Accurate masses for dispersion-supported galaxies
• A Universal Mass Profile for Dwarf Spheroidal Galaxies?
• Empirical Color Transformations Between SDSS Photometry and Other Photometric Systems
• Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data
• Galactic stellar haloes in the CDM model
• Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds
• Eight Ultra-faint Galaxy Candidates Discovered in Year Two of the Dark Energy Survey
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• The Star Formation Histories of Local Group Dwarf Galaxies I. Hubble Space Telescope / Wide Field Planetary Camera 2 Observations
• Detection of massive tidal tails around the globular cluster Pal 5 with SDSS commissioning data
• The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon
• The Star Formation History of the Large Magellanic Cloud
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• The Quenching of the Ultra-Faint Dwarf Galaxies in the Reionization Era
• Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
• Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• The DART imaging and CaT survey of the Fornax Dwarf Spheroidal Galaxy
• A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with H.E.S.S
• Rings and Radial Waves in the Disk of the Milky Way
• Chemical Evolution of the Magellanic Clouds: analytical models
• "Galaxy," Defined
• Accreted versus In Situ Milky Way Globular Clusters
• A comprehensive model for the Monoceros tidal stream
• Globular clusters in the outer Galactic halo: new HST/ACS imaging of 6 globular clusters and the Galactic globular cluster age-metallicity relation
• Star Clusters in M31: II. Old Cluster Metallicities and Ages from Hectospec Data
• Exploring Halo Substructure with Giant Stars: Spectroscopy of Stars in the Galactic Anticenter Stellar Structure
• The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
• Substructure in Tidal Streams; Tributaries in the Anticenter Ring
• Clean Kinematic Samples in Dwarf Spheroidals: An Algorithm for Evaluating Membership and Estimating Distribution Parameters When Contamination is Present
• An Age Difference of 2 Gyr between a Metal-Rich and a Metal-Poor Globular Cluster
• Halo globular clusters observed with AAOmega: dark matter content, metallicity and tidal heating
• A re-interpretation of the Triangulum-Andromeda stellar clouds: a population of halo stars kicked out of the Galactic disk
• Na-O anticorrelation and HB. IX. Kinematics of the program clusters. A link between systemic rotation and HB morphology?
• Measuring Consistent Masses for 25 Milky Way Globular Clusters
• Insight Into the Formation of the Milky Way Through Cold Halo Substructure. I. The ECHOS of Milky Way Formation
• Bayesian analysis of resolved stellar spectra: application to MMT/Hectochelle Observations of the Draco dwarf spheroidal
• The PAndAS Field of Streams: stellar structures in the Milky Way halo toward Andromeda and Triangulum
• Discovery of a cold stellar stream in the ATLAS DR1 data
• The Complex Structure of Stars in the Outer Galactic Disk as revealed by Pan-STARRS1
• Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters
• Serendipitous Discovery of a Thin Stellar Stream near the Galactic Bulge in the Pan-STARRS1 3Pi Survey
• Pal 1: Another Young Galactic Halo Globular Cluster?
• Testing Newtonian Gravity with AAOmega: Mass-To-Light Profiles and Metallicity Calibrations From 47 Tuc and M55
• Exploring Halo Substructure with Giant Stars: The Nature of the Triangulum-Andromeda Stellar Features
• On Rings and Streams in the Galactic Anti-Center
• Photometry of Magellanic Cloud clusters with the Advanced Camera for Surveys - II. The unique LMC cluster ESO 121-SC03
• Age and metallicity of star clusters in the Small Magellanic Cloud from integrated spectroscopy
• A Search for RR Lyrae Stars in Segue 2 and Segue 3
• Another Look at the EBS: A Stellar Debris Stream and a Possible Progenitor
• On the Relative Ages of the $\alpha$-Rich and $\alpha$-Poor Stellar Populations in the Galactic Halo
• Tidal radii of globular clusters and the mass of the Milky Way
• Fishing in Tidal Streams: New Radial Velocity and Proper Motion Constraints on the Orbit of the Anticenter Stream
• Insight Into the Formation of the Milky Way Through Cold Halo Substructure. II. The Elemental Abundances of ECHOS
• Slicing The Monoceros Overdensity with Suprime-Cam
• The shell game: a panoramic view of Fornax
• Ursa Major II - Reproducing the observed properties through tidal disruption
• A radial velocity survey of low Galactic latitude structures: III. The Monoceros Ring in front of the Carina and Andromeda galaxies
• The Metallicity of the Monoceros Stream
• The metallicity of Palomar 1
• Properties of the Young Milky Way Globular Cluster Whiting 1 from Near-Infrared Photometry
• Kinematics in Kapteyn's Selected Area 76: Orbital Motions Within the Highly Substructured Anticenter Stream
• A Megacam Survey of Outer Halo Satellites. IV. Two foreground populations possibly associated with the Monoceros substructure in the direction of NGC2419 and Koposov2

Cited by in corpus (8)

• Revised and new proper motions for confirmed and candidate Milky Way dwarf galaxies
• Gemini/GRACES Spectroscopy of Stars in Triangulum II
• The velocity anisotropy of the Milky Way satellite system
• Search for globular clusters associated with the Milky Way dwarf galaxies using Gaia DR2
• The properties of dwarf spheroidal galaxies in the Cen A group: Stellar populations, internal dynamics, and a heart-shaped Halpha ring
• Cresting the wave: Proper motions of the Eastern Banded Structure
• Tails from the Orphanage
• Line-of-sight mass estimator and the masses of the Milky Way and Andromeda Galaxy