Concurrent infall of satellites Collective effects changing the overall picture

Abstract

Context. In recent years, high-quality observational data have allowed researchers to undertake an extensive analysis of the orbit of several Milky Way satellite galaxies, with the aim to constrain its accretion history. Although various research groups have studied the orbital decay of a satellite galaxy embedded inside a dark matter halo, a large variety of new physical processes have been proven to play an important role in this process, but its full scope not yet understood. Aims. Our goal is to assess whether the orbital history of a satellite galaxy remains unchanged during a concurrent sinking. For this purpose, we analyzed the impact that the internal structure of the satellites and their spatial distribution inside the host halo may have on the concurrent sinking process due to both mass loss and the combined effect of self-friction – as processes that have not been studied before for the concurrent sinking of satellites. Methods. We set up a set of N-body simulations that includes multiple satellites that are sinking simultaneously into a host halo and we compared them with models that include a single satellite. Results. The main result of our work is that the satellite’s accretion history differs from the classical isolated view when we consider the collective effects. Furthermore, the accretion history of each satellite strongly depends on the initial configuration, the number of satellites present in the halo at the time of infall, and the internal properties of each satellite. We observe that compact satellites in a flat configuration fall slower than extended satellites that have lost mass, showing a non-reported behavior of dynamical self-friction; the latter is reinforced by analytical expressions that describe the orbital decay through different approaches for the dynamical friction, including (or not) the mass loss and radial dependence of the satellite. In particular, we find that such effects are maximized when satellites are located in a flat configuration. Here, we show that in a flat configuration similar to the observed vast polar structure, deviations in the apocenters can be of about 30% with respect to the isolated case, and up to 50% on the eccentricities. Conclusions. Overall, we conclude that ignoring the collective effects produced by the concurrent sinking of satellite galaxies may lead to large errors in the determination of the merger progenitor properties, making it considerably more challenging to trace back the accretion event. Timing constraints on host density profile may be modified by the effects discussed in this paper.