Asymmetries in stellar streams induced by a galactic merger

TL;DR

The paper investigates how a major galactic merger perturbs pre-existing tidal streams and develops a new cumulative-density asymmetry metric to quantify leading-trailing differences. Using collisionless N-body simulations of a Milky Way–like host with 36 globular-cluster streams and a 1:10 merger, it shows merger-induced asymmetries appear after coalescence and can persist for about 2 Gyr, though population-averaged length asymmetries are modest due to non-synchronized stream responses. The authors compare a density-based approach with traditional length measures and demonstrate that the cumulative-density metric robustly captures local underdensities and gaps caused by the merger, especially in wide-orbit streams perturbed early. They also discuss degeneracies with other perturbative processes (bars, GMCs, subhaloes) and caution that merger history must be considered when mapping dark matter with streams. Overall, the work highlights the transient yet detectable fossil signatures of mergers in the Milky Way’s tidal streams and their implications for interpreting stream morphologies in the context of dark matter.

Abstract

Stellar streams are sensitive to perturbations from, e.g., giant molecular clouds, bars and spiral arms, infalling dwarf galaxies, or globular clusters which can imprint gaps, clumps, spurs, and asymmetries in tails. In addition to these effects, the impact of a galactic major merger on a population of stellar streams remains to be explored. Here, we focus on the emergence and longevity of asymmetries between the leading and trailing tails of streams caused by such interactions. We run collisionless N-body simulations of a Milky Way-like galaxy hosting 36 globular cluster streams and merging with a perturber galaxy. We propose a new asymmetry metric to quantify the structural differences between both tails from their respective cumulative density profiles. We find that the over- and under-densities along streams induced by the merger depend on the orbital characteristics of their progenitors. The non-simultaneity of this effect from stream to stream implies that global asymmetry signatures are less prominent than in individual cases. These population-averaged imprints remain detectable over only 2.5 Gyr but asymmetric signatures can persist over much longer periods for individual streams with wide orbits that have been perturbed prior to coalescence. We thus caution that the interpretation of streams' morphology in the context of dark matter mapping is strongly subject to degeneracies and should be performed considering the merger history of the host.

Figures (9)

• Figure 1: Spatial distribution of stellar streams in the merger case (top row) and reference case (bottom row) at four different timesteps: 2.00 Gyr (before the merger), 2.22 Gyr (shortly after the satellite's first crossing of the Milky Way disk), 3.00 Gyr, and 5.00 Gyr. Each colour represents a different stellar stream. The black cross marks the centre of mass (COM) of the Milky Way-type galaxy, and the red cross marks the COM of the satellite galaxy (only in the merger case). All panels show projections in the $x-z$ plane, with coordinates centered on the Milky Way COM.
• Figure 2: Projection of a stellar stream from our simulations in its progenitor’s orbital plane at $t=3$ Gyr, for the reference run (top) and the merger run (bottom). The Milky Way’s centre of mass is at $(x,y)=(0,0)$ and the progenitor is located at x=0. The solid curves are the 1-DREAM fits to the stream (blue for the reference case, red for the merger case). The panels below each map display the corresponding density profiles along the stream.
• Figure 3: First row: Three snapshots of one stream in the reference simulation, shown at the progenitor’s pericentre, apocentre, and at a time when the length asymmetry reaches one of its maxima. The black cross marks the Milky Way’s centre of mass and the blue line the progenitor’s trajectory. Second row: Time evolution of the progenitor’s distance to the Milky Way’s centre of mass. Third row: Time evolution of the length of the leading and trailing arm. Fourth row: Normalized asymmetry between the leading and trailing arm lengths. The three vertical dotted lines mark the times of the snapshots shown in the top row. All time-dependent curves are smoothed with a 1D Gaussian filter with $\sigma=2$.
• Figure 4: Top row: Fraction of stream particles recovered by the 1-DREAM algorithm as a function of time. Bottom row: Median asymmetry between the leading and trailing arm lengths versus time; shaded envelopes indicate the $\pm1\sigma$ robust standard deviation. The dash-dotted lines mark the first (2.1 Gyr) and second (2.8 Gyr) passages of the satellite.
• Figure 5: Normalized cumulative fraction of particles along the leading (solid) and trailing (dashed) arms of the same stream shown in Fig. \ref{['Fig:Ter8_maps']}, computed from 2 kpc outward from the progenitor. Blue curves correspond to the reference simulation and red curves to the merger simulation.