Revealing the Structure and Internal Rotation of the Sagittarius Dwarf Spheroidal Galaxy with Gaia and Machine Learning

Abstract

We present a detailed study of the internal structure and kinematics of the core of the Sagittarius dwarf spheroidal galaxy (Sgr). Using machine-learning techniques, we have combined the information provided by 3300 RR Lyrae stars, more than 2000 spectroscopically observed stars, and the Gaia second data release to derive the full phase space, i.e. 3D positions and kinematics, of more than $1.2\times 10^5$ member stars in the core of the galaxy. Our results show that Sgr has a bar structure $\sim 2.5$ kpc long, and that tidal tails emerge from its tips to form what it is known as the Sgr stream. The main body of the galaxy, strongly sheared by tidal forces, is a triaxial (almost prolate) ellipsoid with its longest principal axis of inertia inclined $43^{\circ }\pm 6^{\circ }$ with respect to the plane of the sky and axis ratios of 1:0.67:0.60. Its external regions are expanding mainly along its longest principal axis, yet the galaxy conserves an inner core of about $500\times 330\times 300$ pc that shows no net expansion and is rotating at $v_{\mathrm{rot}}=4.13\pm 0.16$ $\mathrm{km}{\mathrm{s}}^{-1}$. The internal angular momentum of Sgr forms an angle $\theta =18^{\circ }\pm 6^{\circ }$ with respect to its orbital angular momentum, meaning that the galaxy is in an inclined prograde orbit around the Milky Way. We compared our results with predictions from $N$-body models with spherical, pressure-supported progenitors and a model whose progenitor is a flattened rotating disk. Only the rotating model, based on preexisting simulations aimed at reproducing the line-of-sight velocity gradients observed in Sgr, was able to reproduce the observed properties in the core of the galaxy.

37 pages, 26 figures, 2 tables. Accepted for publication in the Astrophysical Journal

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• GaiaHub: A method for combining data from the Gaia and Hubble space telescopes to derive improved proper motions for faint stars
• Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars
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• Kinematic Structure of the Large Magellanic Cloud Globular Cluster System from Gaia eDR3 and Hubble Space Telescope Proper Motions
• The metallicity distribution in the core of the Sagittarus dwarf spheroidal: minimising the metallicity biases
• Tidally induced velocity gradients in the Milky Way dwarf spheroidal satellites
• The Pristine Inner Galaxy Survey (PIGS) IV: A photometric metallicity analysis of the Sagittarius dwarf spheroidal galaxy
• The Dark Matter Halo of M54
• The Pristine Inner Galaxy Survey (PIGS) X. Probing the early chemical evolution of the Sagittarius dwarf galaxy with carbon abundances
• The Dark Matter Tidal Stripping History of the Sagittarius Core with N-body simulations
• HubPUG: Proper Motions for Local Group Dwarfs observed with HST utilizing Gaia as a Reference Frame
• Forward and Back: Kinematics of the Palomar 5 Tidal Tails
• RR Lyrae stars as probes of the outer Galactic halo: Chemical and kinematic analysis of a pilot sample
• Spiral arms and the angular momentum gap in Milky Way Cepheids
• A Sagittarius-like simulated dwarf spheroidal galaxy from TNG50
• Stellar angular momentum of disk galaxies at z = 0.7 in the MAGIC survey I. Impact of the environment
• Assessment of Dark Matter Models Using Dark Matter Correlations across Dwarf Spheroidal Galaxies
• Kinematics of the Sagittarius Dwarf Spheroidal core: A 5D Analysis for a 6D Methodology with Gaia DR3
• Timing Mass of the Local Group
• Identifications of RR Lyrae stars and Quasars from the simulated data of Mephisto-W Survey
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