[Paper Review] An analysis of satellite planar configurations around the MW and M31: singling out new high quality planes

Isabel Santos-Santos, R. Domı́nguez-Tenreiro+1 more|arXiv (Cornell University)|Aug 6, 2019

Stellar, planetary, and galactic studies1 references4 citations

TL;DR

This study introduces an enhanced 4-galaxy-normal density plot method to identify high-quality planar configurations of satellites around the Milky Way (MW) and M31, using tensor of inertia fitting to quantify plane quality via population (N_sat) and flattening (c/a). It identifies a previously unrecognized, high-quality plane in M31 with 18 satellites—comparable to the Great Plane of Andromeda (GPoA) despite greater distance uncertainties—while showing that satellite mass does not influence plane membership.

ABSTRACT

We present a detailed characterization of planes of satellites in the Milky Way (MW) and M31 systems. To this end we introduce an extension to the '4-galaxy-normal density plot' method \citep{Pawlowski13}, by which plot over-densities signal the normal direction to predominant planes of satellites within a given sample. For a given over-density, the extension provides a extit{collection} of planes, each including a different number of objects $N_{ m sat}$. We apply this method to the position data of confirmed MW and M31 satellites and quantify the quality of planes through the outputs of a Tensor of Inertia plane-fitting technique. Plane quality is quantified in terms of population ($N_{ m sat}$) and flattening (the short-to-long axis ratio $c/a$ or the rms thickness normal to the plane). Therefore, planes with the same population or flattening can be compared with each other allowing us to single-out best-quality planes. For the first time, we study the second-most predominant planar configuration of satellites in M31, singling out a plane with 18 satellite members that shows a quality comparable to the Great Plane of Andromeda (GPoA, with $N_{ m sat}=19$) despite it being more affected by distance uncertainties. This structure is viewed nearly face-on from the MW and is approximately normal to the GPoA. Overall, we find planes of satellites around the MW and M31 with higher qualities than those previously reported with a given $N_{ m sat}$. We also show that mass plays no role in determining a satellite's membership or not to the respective best-quality planes.

Motivation & Objective

• To improve the identification of high-quality planar configurations of satellites in the MW and M31 beyond previously known structures like the VPOS and GPoA.
• To quantify plane quality using both the number of satellites (N_sat) and flattening (c/a or RMS thickness), enabling direct comparison across different-sized planes.
• To detect and characterize secondary planar configurations in M31, particularly the second-most predominant one, which had not been previously identified.
• To test whether satellite mass influences membership in the best-quality planes, given that globular clusters and streams also align with these planes.
• To provide a robust, statistically grounded method for identifying planar structures that accounts for distance uncertainties and sample size variations.

Proposed method

• Extends the 4-galaxy-normal density plot method (Pawlowski et al., 2013) by generating multiple planes for each over-density, each with a different number of satellites (N_sat), to assess plane quality across varying populations.
• Applies tensor of inertia (ToI) fitting to 3D position data of confirmed MW and M31 satellites within 300 kpc, incorporating distance uncertainties via Monte Carlo sampling.
• Quantifies plane quality using two key metrics: the short-to-long axis ratio (c/a) for flattening and the root-mean-square (RMS) thickness normal to the plane.
• Identifies peaks in the 4-galaxy-normal density plot to locate dominant normal directions to planar structures, then evaluates stability of plane normals as N_sat increases.
• Uses bootstrapped distance sampling (1000 iterations) to propagate distance uncertainties into plane fitting and assesses normal vector stability up to N_sat = 24.
• Performs correlation tests to determine whether satellite mass correlates with contribution to the main over-density regions (i.e., plane membership).

Experimental results

Research questions

• RQ1 What are the highest-quality planar configurations of satellites in the MW and M31, defined by both N_sat and c/a, beyond the previously known VPOS and GPoA?
• RQ2 Does a secondary planar structure in M31 exist with comparable quality to the GPoA, and what is its N_sat and c/a?
• RQ3 How does the stability of the plane normal vector change as the number of satellites (N_sat) increases, and at what N_sat does quality peak?
• RQ4 Is satellite mass a determining factor for membership in the best-quality planes, given that globular clusters and streams also align with these planes?
• RQ5 How do distance uncertainties affect the identification and characterization of high-quality planar structures, especially in M31?

Key findings

• A new, high-quality planar configuration in M31 with 18 satellite members was identified, showing a c/a of 0.155 ± 0.032 and RMS thickness of 21.0 ± 4.2 kpc, comparable in quality to the GPoA (N_sat = 19, c/a = 0.107 ± 0.005).
• The M31 plane with N_sat = 18 is viewed nearly face-on from the MW and is approximately orthogonal to the GPoA, indicating a distinct structural plane.
• Plane quality peaks at N_sat = 19 for the MW’s VPOS-3 and N_sat = 18 for M31’s secondary peak, as indicated by stable normal vectors and minimal change in c/a and RMS thickness beyond these values.
• The plane in M31 with N_sat = 18 is more robust than the Ibata-Conn-14 plane (N_sat = 14), being both more populated and thinner (c/a = 0.155 vs. 0.125) and having lower distance uncertainty impact.
• The MW’s classical plane (N_sat = 11) and the VPOS-3 (N_sat = 24) are confirmed as high-quality structures, with VPOS-3 showing a c/a of 0.209 ± 0.002 and RMS thickness of 19.9 ± 0.3 kpc.
• Correlation tests show that satellite mass does not determine membership in the best-quality planes, indicating that mass is not a driver of planar alignment in the MW or M31 systems.