Encounters Between M33 and Present-Day M31 Satellites Hint at a Previous Group Accretion

TL;DR

This study tests whether two M31 satellites, Pisces and Andromeda XVI, were previously bound to M33 by performing backward orbit integrations in a multi-body, rigid-potential framework that includes MW, M31, LMC, and M33. Using 6D phase-space data and 1,000 Monte Carlo realizations across nine mass combinations for M31 and M33, the authors find that both dwarfs experienced recent pericenters around M33 within ~1.4–2.0 Gyr, typically at distances of ~100–125 kpc and speeds well above M33’s local escape velocity, implying flybys rather than long-term binding. A substantial fraction of orbits also place the dwarfs inside M33’s virial radius during these interactions (roughly 60% for Pisces and 42% for And XVI), and the dwarfs often approach each other within the last ~1 Gyr, suggesting possible past group infall or satellite exchange scenarios. The results are robust to mass variations, though the masses mainly modulate the likelihood and timing of M31 vs. M33 interactions, with important implications for interpreting satellite statistics and the M33 satellite mass function in observational surveys. Improved proper motions and mass estimates will further constrain the accretion history of these dwarfs and their relation to the M31–M33 system.

Abstract

This work investigates whether two known Andromeda (M31) satellites, Pisces (LGS 3) and Andromeda XVI, have interacted with M33, M31's most massive satellite. $Λ$CDM predictions imply a handful of satellite galaxies around M33, yet few M33 satellites have been found and confirmed despite its high mass. We use proper motions combined with backward orbit integration in a semi-analytic potential to constrain plausible interaction scenarios for Pisces and And XVI. Both dwarfs are currently M31 satellites, defined as being inside its virial radius. However, our results show that, in our fiducial mass models, 42% (And XVI) and 60% (Pisces) of dwarf orbits support that they were previously satellites of M33 (i.e., once inside its virial radius). Both dwarfs had fly-by encounters with M33 at relative velocities greater than M33's escape speed within the past 1-2 Gyr. In over 70% of orbits, Pisces and And XVI also had a close approach with each other post-M33 interaction and share an orbital plane, suggesting possible past group accretion. We explore a range of mass combinations for M31 and M33, finding that this primarily regulates the likelihood that the dwarfs were satellites of M33 in the past, while upholding conclusions of recent flybys about M33. These close interactions provide new evidence for past satellite exchange and/or group infall scenarios between M31 and M33. Such interactions also affect comparisons to observational surveys that define satellites primarily by their distance relative to host galaxies.

Figures (7)

• Figure 1: Top: Direct orbital histories and orbital uncertainties between Pisces (left) and And XVI (right) relative to M31. The blue dashed, horizontal line is the virial radius of M31. Middle: Same as the top row, but orbits are shown relative to M33. The dotted horizontal line is the virial radius of M33. Bottom: The measured SFH of Pisces from Hidalgo_2011 and from Savino_2025 for And XVI. For Pisces, there is a clear correlation between the time of pericenter around M33 (1.91 Gyr ago) and a burst of star formation. For And XVI, the SFH uncertainties lead to less conclusive evidence of correlations between the orbital and star formation histories.
• Figure 2: Top: Spatial projection of the direct orbital histories provided in Figure \ref{['fig:orbit_errors']} in the M31-centric frame over the last 6 Gyr. Markers indicate the positions of the dwarfs today. The virial extent of M31 (gray) and M33's (blue) halos is indicated by dashed circles. Pisces and And XVI are within M31's halo but not within M33's halo at present; however, both dwarfs have crossed through M33's halo in the past. Small closed circles show the time of pericenter relative to M33, while the "x" symbols indicate the timing of pericenter between the dwarfs. Similarly, small closed circles on M33's trajectory correlate to the timing of the dwarfs' pericenter around M33. Bottom: Same spatial projection but shown between 1.75 Gyr ago, the time splitting when both dwarfs have a pericenter around M33, and 6 Gyr ago. Both dwarfs clearly reside inside the halo of M33 at the time of pericenter.
• Figure 3: Left: Distance and velocity of both dwarfs with respect to M33 at the time of pericenter. Error bars represent the standard deviations of these quantities from the 1,000 orbit calculations sampling the 6D phase space information measurements in the fiducial mass combination. Open symbols indicate the position and velocity of both dwarfs at the present day. The solid blue curve illustrates M33's escape velocity as a function of distance for the fiducial M33 mass, while the shaded region indicates the escape velocity curve extents for the lower and higher M33 masses. The 1$\sigma$ and 2$\sigma$ contours are also shown for the combined propagation of orbital uncertainties across all mass combinations. Both Pisces and And XVI were previously M33 satellites and have a high likelihood of experiencing a fly-by encounter with M33 in the past. Right: The tidal acceleration, $a_{tidal}$, exerted on Pisces and And XVI from M31 (solid lines) and M33 (dashed lines) as a function of time in the fiducial setups. $a_{tidal}$ is computed between the position of each dwarf relative to M31/M33 as a function of time, compared to tidal acceleration at the virial radius of each dwarf ($R_{vir}=$44 kpc for Pisces and $R_{vir}=$26 kpc for And XVI). The tidal acceleration from M33 is stronger than from M31 for both dwarfs, until the last $\sim$1 Gyr when M31's tidal acceleration becomes more dominant.
• Figure 4: Orbital poles of Pisces and And XVI relative to M31 in Galactic coordinates. Also shown is the orbital pole of M33 relative to M31 and the M31 disk spin. Open symbols show the orbital poles computed with the coordinates in Table \ref{['tab:Pos_vel']}. Closed symbols represent orbital poles calculated using the 1,000 samples drawn from the Monte Carlo scheme used to propagate uncertainties on observational quantities used to derive 6D phase space information. See Section \ref{['subsec:uncertainties']} for more details.
• Figure 5: Orbits of Pisces and And XVI for a fixed M31 mass of $1.5\times10^{12}\, M_{\odot}$. The first and third columns show orbits of each dwarf relative to M31, while the second and fourth columns show orbits relative to M33. Second and fourth columns also indicate the distance and time of pericenter, followed by the [15.9, 84.1] percentile, as well as, the fraction of orbits where the pericenter distance is within the virial radius of M33. From top to bottom, the rows correspond to varying M33 masses such that the top row represents an M33 mass of $1\times10^{11}\,M_{\odot}$, the middle row is for the fiducial M33 mass of $2.5\times10^{11}\,M_{\odot}$, and the bottom for is for an M33 mass of $5\times10^{11}\,M_{\odot}$. The gray lines correspond to 1,000 orbital uncertainties calculated for each combination of M33 and M31 mass.