The Magellanic Stream System: I. Ram-pressure tails and the relics of the collision between the Magellanic Clouds (1510.00096v3)

Abstract: We have analyzed the Magellanic Stream (MS) using the deepest and the most resolved H I survey of the Southern Hemisphere (the Galactic All-Sky Survey). The overall Stream is structured into two filaments, suggesting two ram-pressure tails lagging behind the Magellanic Clouds (MCs), and resembling two close, transonic, von Karman vortex streets. The past motions of the Clouds appear imprinted in them, implying almost parallel initial orbits, and then a radical change after their passage near the N(H I) peak of the MS. This is consistent with a recent collision between the MCs, $200-300$ Myr ago, which has stripped their gas further into small clouds, spreading them out along a gigantic bow shock, perpendicular to the MS. The Stream is formed by the interplay between stellar feedback and the ram pressure exerted by hot gas in the Milky Way (MW) halo with $\rho_{hot}$= $10^{-4}$ $cm^{-3}$ at 50-70 kpc, a value necessary to explain the MS multiphase high-velocity clouds. The corresponding hydrodynamic modeling provides the currently most accurate reproduction of the whole H I Stream morphology, of its velocity, and column density profiles along $L_{MS}$. The 'ram pressure plus collision' scenario requires tidal dwarf galaxies, which are assumed to be the Cloud and dSph progenitors, to have left imprints in the MS and the Leading Arm, respectively. The simulated LMC and SMC have baryonic mass, kinematics and proper motions consistent with observations. This supports a novel paradigm for the MS System, which could have its origin in material expelled toward the MW by the ancient gas-rich merger that formed M31.

• The paper re-interprets the Magellanic Stream as primarily two ram-pressure tails shaped by the Milky Way's hot halo, resembling von Karman vortex streets.
• Utilizing GASS data and hydrodynamical modeling, the study suggests a close encounter between the LMC and SMC 200-300 Myr ago dramatically altered their orbits and the Stream's structure.
• This research highlights the critical role of ram-pressure in satellite galaxy evolution and has implications for understanding galactic halo properties and hierarchical galaxy formation.

An Analysis of the Magellanic Stream System and Implications for Galactic Dynamics

The paper by Hammer et al. presents a sophisticated examination of the Magellanic Stream (MS) system resulting from interactions between the Magellanic Clouds (MCs) and the Milky Way's (MW) halo. Offering a novel interpretation, the paper combines high-resolution Galactic All-Sky Survey (GASS) data with hydrodynamical modeling to elucidate the structural properties and origins of the MS, the Magellanic Bridge, and the Leading Arm, expanding upon traditional models of galactic dynamics.

Key Findings:

The MS is identified as comprising two dominant filamentary structures attributed to ram-pressure effects exerted by the Milky Way's hot halo gas on the LMC and SMC. Utilizing data from GASS, the authors provide strong evidence that these filaments represent two ram-pressure tails trailing the Magellanic Clouds, demonstrating a striking paralleling and intertwining pattern akin to von Karman vortex streets, indicative of their transonic motion through the halo. This patterning suggests significant influences from both stellar feedback processes and the hydrodynamic environment, reframing our understanding of how dwarf galaxies lose their gaseous content upon interaction with larger galactic systems.

Ram-Pressure and Collision Dynamics:

Among the more compelling claims, Hammer et al. assert that a close encounter between the LMC and SMC approximately 200-300 Myr ago induced significant changes in their orbits. This interaction, they argue, led to a reconfiguration of the MS architecture, validating their hypothesis with evidence of filamentary overlaps and shifts observable in the neutral hydrogen (H I) distribution maps. The paper also posits that the present orbital dynamics of these galaxies can be reconciled with recent proper motion measurements, suggesting greater complexity in the Clouds' historical trajectory than previously considered.

The team’s models integrate these dynamics with the presence of massive ionized gas regions within the MS, suggesting that tidal and ram-pressure stripping processes may coexist, contributing to the observed multiphase high-velocity clouds. Interestingly, their modeling efforts imply that previous assumptions—such as continuous gas dominance from tidal interactions—are inadequate without considering the significant role of ram-pressure stripping.

Theoretical and Practical Implications:

This paper underscores the necessity of incorporating hydrodynamic interactions when considering the evolution of satellite galaxies, specifically in understanding their structural and kinematic signatures. The implications span adjustments in the evaluations of the MW’s halo density profiles and their influence on nearby dwarf galaxies.

Additionally, the proposed analytical framework has implications for broader cosmological models: the hypothesis that tidal dwarf galaxies (TDGs) may have been expelled from a merger event associated with M31 provides a novel context for interpreting current observations of MW satellite formations. Such a model suggests coordinated infalls of dwarfs and draws attention to the potential linkage between distant galaxy interactions and local satellite dynamics.

Future Directions:

Hammer et al.'s paper invites further exploration into the interplay between ram-pressure dynamics, stellar feedback, and tidal interactions. It sets a foundation for utilizing more sophisticated SPH simulation techniques and the next generation of astronomical surveys to test the robustness of such models across varied galactic environments.

In embracing a comprehensive mechanistic understanding of the Magellanic System’s evolution, this research enhances our grasp of hierarchical galaxy formation processes and the interdependencies of small-scale galactic structures within their larger cosmic environment.