Decoding the Galactic Twirl: The Downfall of Milky Way-mass Galaxies Rotation Curves in the FIRE Simulations

Abstract: Recent measurements of the Milky Way rotation curve found a sharp decline at around $15$-$20$ kpc from the center of the Galaxy, suggesting that the Galactic dark matter halo is much less massive than predicted by other dynamical tracers. To address this tension, we study the validity of the assumptions made in calculating the Milky Way's rotation curve. To do so, we apply Jeans' equation, the current standard approach of measuring rotation curves, to three cosmological zoom-in simulations of Milky Way-like galaxies from the FIRE-2 Latte suite. Using synthetic Gaia surveys, we replicate the sample selection process and calculation employed in measuring the Milky Way rotation curve. We examine four failure modes of this calculation and find that the measured curves deviate from the true curve by $5$-$20\%$ rather than below $5\%$, as estimated by previous works. Interestingly, there is a large galaxy-to-galaxy variance, and different systematics dominate different galaxies. We rederive the Milky Way's dark matter density profile with the rotation curve while incorporating systematics from the simulations. The posterior distribution of the density profiles is consistent with a fiducial NFW profile when assuming a gNFW profile for dark matter. We find that the virial mass, $7.32^{+1.98}{-1.53}\times10^{11}~M{\odot}$, consistent with other probes of the Milky Way's mass. However, we recommend that the field moves away from relying solely on the rotation curve when studying the dark matter profile, and adopts methods that incorporate additional probes and/or do not heavily depend on assumptions described in this study.

Overview of "Decoding the Galactic Twirl: The Downfall of Milky Way-mass Galaxies Rotation Curves in the FIRE Simulations"

This paper undertakes an evaluation of the uncertainties inherent in deriving the rotation curves of Milky Way-like galaxies, particularly as informed by the Feedback in Realistic Environments (FIRE-2) simulations. Rotation curves are crucial for understanding a galaxy’s mass distribution and estimating the mass of its dark matter halo. However, recent observations have shown inconsistencies in the outer regions of the Milky Way's rotation curve that suggest a less massive dark matter halo than predicted. This study probes these discrepancies by dissecting the methodology and assumptions involved in deriving such curves.

Key Findings

1. Systematic Uncertainties: The authors identify several failure modes in the Jeans equation, which is the standard method used to calculate rotation curves from stellar kinematics. These include biases due to survey selection functions, inaccuracies in asymmetric drift correction, potential dynamical disequilibrium from recent mergers, and non-axisymmetry in the galaxy's gravitational potential.

2. Simulation Insights: Using the Latte suite of the FIRE-2 simulations, the study assesses the biases and uncertainties introduced by these factors. It finds that deviations from the true rotation curve due to systematic uncertainties can range between 5-20%.

3. Galaxy-to-Galaxy Variance: Significant variability in the dominant contributors to systematic uncertainties is noted across different galaxies. For instance, asymmetric drift corrections dominate in some cases, while axis asymmetry is more significant in others.

4. Implication of Results for the Milky Way: When applying these findings to the Milky Way, the paper suggests that the previously reported sharp decline in its rotation curve and subsequent interpretation of a lighter dark matter halo should be reconsidered. The paper instead recommends integrating more systematics-driven uncertainties, which align the Milky Way's mass estimates with various other observational dark matter constraints.

Methodology

The study employs cosmological zoom-in simulations of Milky Way-like galaxies and simulations coupled with synthetic surveys replicating the selection process of real surveys. Detailed numerical analyses exploit the FIRE-2 simulations to derive the rotation curves, applying the Jeans equation while simulating typical observational biases and deviations from the assumed classical distributions.

Implications

1. Rotation Curves in Astronomical Analysis: The quantification of systematic uncertainties in this paper urges caution in relying solely on stellar-derived rotation curves for inferring the dark matter distribution of a galaxy.

2. Future Methodology Recommendations: The authors advocate for methods that go beyond rotation curves alone and employ multiple lines of evidence to better constrain dark matter profiles. This involves incorporating other probes such as globular clusters, satellite galaxies, and stellar streams into such analyses.

3. Galactic Modeling: The findings highlight the need for improved models and methodologies that anticipate these systematic biases, potentially leading to adjustments in the theoretical models of galaxy formation and evolution.

Conclusion

This investigation provides critical insights into the methodological limitations of current rotation curve measurements while offering pathways for refining such analyses. Emphasizing the need for integrated approaches that consider a wider range of observational data, the paper sets a significant precedent for how future studies should handle dark matter distribution assessments in galaxy disks. As astrophysical research progresses, the tools and methods refined in this paper will be invaluable in achieving more accurate models of the Milky Way and other galaxies.