The total mass of the Large Magellanic Cloud from its perturbation on the Orphan stream (1812.08192v2)

Abstract: In a companion paper by Koposov et al., RR Lyrae from \textit{Gaia} Data Release 2 are used to demonstrate that stars in the Orphan stream have velocity vectors significantly misaligned with the stream track, suggesting that it has received a large gravitational perturbation from a satellite of the Milky Way. We argue that such a mismatch cannot arise due to any realistic static Milky Way potential and then explore the perturbative effects of the Large Magellanic Cloud (LMC). We find that the LMC can produce precisely the observed motion-track mismatch and we therefore use the Orphan stream to measure the mass of the Cloud. We simultaneously fit the Milky Way and LMC potentials and infer that a total LMC mass of $1.38^{+0.27}_{-0.24} \times10^{11}\,\rm{M_\odot}$ is required to bend the Orphan Stream, showing for the first time that the LMC has a large and measurable effect on structures orbiting the Milky Way. This has far-reaching consequences for any technique which assumes that tracers are orbiting a static Milky Way. Furthermore, we measure the Milky Way mass within 50 kpc to be $3.80^{+0.14}_{-0.11}\times10^{11} M_\odot$. Finally, we use these results to predict that, due to the reflex motion of the Milky Way in response to the LMC, the outskirts of the Milky Way's stellar halo should exhibit a bulk, upwards motion.

• The paper demonstrates that the LMC’s gravitational pull causes a marked misalignment in the Orphan Stream’s velocity vectors.
• Using Gaia DR2 data, the study refines the LMC mass to ~1.38×10¹¹ M☉, highlighting the need to incorporate external perturbations in Milky Way models.
• The findings stress that dynamic interactions must be considered to accurately model galactic structures and satellite motions.

Analysis of the Perturbative Influence of the Large Magellanic Cloud on the Orphan Stream

The paper by Erkal et al. comprehensively examines the gravitational impact of the Large Magellanic Cloud (LMC) on the Orphan Stellar Stream, a structure orbiting the Milky Way. This paper explores the implications of using the Orphan Stream as a novel diagnostic tool to weigh the mass of the LMC, contributing to a nuanced understanding of its influence on surrounding stellar formations.

Key Observations and Findings

The researchers identified a significant misalignment between the velocity vectors of stars in the Orphan Stream and the stream's track itself, a deviation initially signaled by Gaia Data Release 2. The analysis demonstrates that such a mismatch is unlikely to be caused by the gravitational potential of the Milky Way alone, suggesting a substantial external influence, specifically from the LMC. By incorporating the effects of the LMC into the gravitational modeling, the paper finds that this dwarf galaxy's mass could produce exactly the observed stream misalignment.

Numerical Insights and Implications

The paper estimates the total mass of the LMC to be approximately $1.38^{+0.27}_{-0.24} \times 10^{11} M_\odot$, emphasizing its significant gravitational perturbation on the Orphan Stream. This provides a fresh perspective, showing for the first time that the LMC exerts a measurable gravitational effect on celestial bodies orbiting the Milky Way. The paper reveals that this interaction should influence any analyses assuming that stellar structures are moving under a static Milky Way potential—challenging previous assumptions about galactic dynamics.

Additionally, the paper refines the mass estimate of the Milky Way within 50 kpc to be $3.80^{+0.14}_{-0.11} \times 10^{11} M_\odot$, enhancing the precision of our understanding of the galactic mass profile.

Theoretical and Practical Implications

The findings underscore the necessity of accounting for dynamic interactions between galactic structures, particularly the LMC, when analyzing or modeling the Milky Way's gravitational potential. These insights hold considerable implications for various astrophysical techniques relying on the assumption of a stable galactic environment.

Looking ahead, the paper posits that the reflexive motion of the Milky Way, induced by the LMC, could trigger a discernible upward motion in the Milky Way's halo periphery. This forecasting not only reshapes our current models but also opens avenues for future research into the mechanics of nearby galactic interactions, potentially informing studies of galaxy formation and evolution.

Future Directions

The paper suggests that future studies could leverage the Orphan Stream and other similar structures to further refine mass estimates of satellite galaxies. Continued exploration of the dynamic interplay between the Milky Way and its satellites could yield valuable insights into the past and future trajectories of these complex interactions, offering greater clarity in our understanding of the large-scale structure of the Universe.