First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33

Abstract: The 3D velocities of M31 and M33 are important for understanding the evolution and cosmological context of the Local Group. Their most massive stars are detected by Gaia, and we use Data Release 2 (DR2) to determine the galaxy proper motions (PMs). We select galaxy members based on, e.g., parallax, PM, color-magnitude-diagram location, and local stellar density. The PM rotation of both galaxies is confidently detected, consistent with the known line-of-sight rotation curves: $V_{\rm rot} = -206\pm86$ km s$^{-1}$ (counter-clockwise) for M31, and $V_{\rm rot} = 80\pm52$ km s$^{-1}$ (clockwise) for M33. We measure the center-of-mass PM of each galaxy relative to surrounding background quasars in DR2. This yields that $({\mu}{\alpha*},{\mu}{\delta})$ equals $(65 \pm 18 , -57 \pm 15)$ $\mu$as yr$^{-1}$ for M31, and $(31 \pm 19 , -29 \pm 16)$ $\mu$as yr$^{-1}$ for M33. In addition to the listed random errors, each component has an additional residual systematic error of 16 $\mu$as yr$^{-1}$. These results are consistent at 0.8$\sigma$ and 1.0$\sigma$ with the (2 and 3 times higher-accuracy) measurements already available from Hubble Space Telescope (HST) optical imaging and VLBA water maser observations, respectively. This lends confidence that all these measurements are robust. The new results imply that the M31 orbit towards the Milky Way is somewhat less radial than previously inferred, $V_{\rm tan, DR2+HST} = 57^{+35}_{-31}$ km s$^{-1}$, and strengthen arguments that M33 may be on its first infall into M31. The results highlight the future potential of Gaia for PM studies beyond the Milky Way satellite system.

• The paper presents Gaia DR2 measurements that confirm M31's counterclockwise and M33's clockwise rotations with quantified proper motions.
• The study employs rigorous star selection based on parallax, proper motion, CMD location, and local density to isolate genuine galaxy members.
• These findings refine Local Group dynamics by indicating a less radial orbit for M31 and supporting the first infall scenario for M33.

Insights from Gaia DR2 on the Dynamics of the Andromeda System

The paper "First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33" provides a detailed analysis of the proper motions (PMs) of the galaxies M31 (Andromeda) and M33 (Triangulum) using data from the Gaia Data Release 2 (DR2). This research is instrumental in enhancing the understanding of the dynamics and future evolution of the Local Group, which includes the Milky Way (MW), M31, and M33—its three most massive members.

The authors, utilizing the substantial stellar catalogs provided by Gaia DR2, analyze the PMs of both M31 and M33 by selecting stars that are bright enough to be detected at such immense distances. The methodology involved selecting galaxy members based on various criteria including parallax, PM, color-magnitude-diagram location, and local stellar density. This selection allowed the researchers to mitigate contamination and derive more accurate measurements of motion.

Key Findings

The study confirms the PM rotation of M31 (counterclockwise) and M33 (clockwise), consistent with their known line-of-sight rotation curves. For M31, the PM measurements are $$(\mu_{\alpha*},\mu_{\delta}) = (65 \pm 18, -57 \pm 15)$$ with additional systematic errors, while for M33, the values are $(31 \pm 19, -29 \pm 16)$. These measures align with earlier estimates derived from Hubble Space Telescope (HST) optical imaging and Very Long Baseline Array (VLBA) water maser observations, bolstering confidence in their robustness.

Implications

1. M31's Interaction with the Milky Way: The derived tangential velocity of Andromeda ($V_{\rm tan, DR2+HST} = 57^{+35}_{-31}$) suggests that its orbit toward the Milky Way is less radial than previous estimates, implying a delayed collision that will occur with a larger impact parameter. This has significant implications for the future dynamics and eventual merger of the MW and M31.
2. M33's Infall into M31: The outcomes support the hypothesis that M33 is on its first infall into M31, refuting the previous possibility of a long-period orbit with a recent pericentric passage. This insight holds cosmological significance as it aligns with expectations from hierarchical structure formation processes.
3. Methodological Validation: The convergence between the Gaia DR2 measurements and prior robust observations (HST and VLBA) affirms the reliability of current methodologies. This validates the use of data from varied astronomical instruments to elucidate the dynamics of distant galactic systems.

Future Prospects

The study underscores the potential of Gaia for conducting PM studies beyond the Milky Way satellite system. By extending the observation baseline, future Gaia data releases are expected to yield even more precise measurements, decreasing uncertainties by a factor proportional to the 1.5th power of the time-baseline. This will refine models of the Local Group's evolution and offer deeper insights into galaxy dynamics, possibly revealing new complexities about satellite interactions and large-scale structures.

Conclusion

This paper, by providing strong empirical evidence for the motions and interactions of M31 and M33, further integrates observational data with theoretical models, bridging gaps in our understanding of Local Group dynamics. As Gaia continues to advance our ability to measure cosmic kinematics with exceptional precision, it will contribute significantly to the broader field of galactic astronomy, facilitating predictions and models with unprecedented accuracy and reliability. The findings serve to clarify the current and future states of these galaxies and their collective influence on the Local Group's ultimate fate.