Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius (1611.00089v2)

Abstract: The extensive span of the Sagittarius (Sgr) stream makes it a promising tool for studying the Milky Way gravitational potential. Characterizing its stellar kinematics can constrain halo properties and provide a benchmark for the Cold Dark Matter galaxy formation paradigm. Accurate models of the disruption dynamics of the Sgr progenitor are necessary to employ this tool. Using a combination of analytic modeling and N-body simulations, we build a new model of the Sgr orbit and resulting stellar stream. In contrast to previous models, we simulate the full infall trajectory of the Sgr progenitor from the time it first crossed the Milky Way virial radius 8 Gyr ago. An exploration of the parameter space of initial phase-space conditions yields tight constraints on the angular momentum of the Sgr progenitor. Our best-fit model is the first to reproduce accurately existing data on the 3D positions and radial velocities of the debris detected 100 kpc away in the MW halo. In addition to replicating the mapped stream, the simulation also predicts the existence of several arms of the Sgr stream extending to hundreds of kiloparsecs. The two most distant stars known in the Milky Way halo coincide with the predicted structure. Additional stars in the newly predicted arms can be found with future data from the Large Synoptic Survey Telescope. Detecting a statistical sample of stars in the most distant Sgr arms would provide an opportunity to constrain the Milky Way potential out to unprecedented Galactocentric radii.

Overview of the Predicted Extension of the Sagittarius Stream to the Milky Way Virial Radius

The paper conducted by Marion I. P. Dierickx and Abraham Loeb presents a comprehensive investigation into the Sagittarius (Sgr) stream, a massive tidal structure that originates from the disruption of the Sgr dwarf spheroidal galaxy as it interacts with the Milky Way (MW). This work combines analytic modeling with N-body simulations to create a novel model of the Sgr orbit, extending our understanding of the stream's behavior and its potential as a probe of the MW's gravitational potential across vast scales.

Simulation Approach and Results

Contrary to previous methodologies that focused primarily on integrating the current orbital coordinates backwards, this research probes the forward evolution of the Sgr dwarf, starting from its initial infall into the MW halo approximately 8 Gyr ago. By exploring a wide parameter space for initial conditions such as velocity and angular momentum, the authors provide a model that successfully reproduces the current observed 3D positions and radial velocities of Sgr stream stars located roughly 100 kpc from the MW center.

Key aspects of the paper include:

• Initial Conditions and Modeling: The Sgr progenitor is modeled using a combination of analytic framework and various initial phase-space conditions, constraining its initial angular momentum. The best-fit model initially positions Sgr at the MW's virial radius with a specific set of dynamic parameters, simulating its orbital evolution and mass loss through tidal stripping.
• N-body Simulation: Utilizing detailed N-body simulations, the paper refines the analytic predictions. These simulations use a live MW halo to account for dynamical interaction effects and play a crucial role in generating a realistic stellar stream that is compared against observed data.
• Stream Features: The generated model excels in mirroring the spatial distribution and velocity characteristics of prominent Sgr stream segments, particularly the leading and trailing arms. A significant achievement of this paper is predicting the presence of distant arms of the stream that extend to hundreds of kiloparsecs, surpassing previously simulated extents.

Implications and Future Directions

The implications of a more extended Sgr stream are multifaceted. The predicted arms offer an unprecedented opportunity to utilize the Sgr debris as a galactic probe, providing constraints on the MW's potential out to its virial radius and beyond. This setup can be pivotal in improving our understanding of the MW's mass distribution and the dynamics of halo substructures, thus offering empirical benchmarks for models of dark matter and galaxy formation.

Moreover, these predictions hold significant implications for current and forthcoming observational campaigns. Existing UKIDSS and Pan-STARRS datasets, as well as future missions like LSST and WFIRST, are poised to probe these distant arms, potentially confirming the extended structures suggested by the model. Should these structures be verified, it would not only validate aspects of the Cold Dark Matter paradigm but also necessitate refinements in our understanding of MW dynamics and its past accretion history.

In summary, this paper proposes a refined model of the Sgr stream, emphasizing its significant extension and its applicability in studying the MW's gravitational characteristics. It paves the way for future theoretical and observational efforts needed to fully understand the interplay between the MW and its satellite galaxies and sheds light on the galaxy's formation and evolution within the local cosmic web.