Sextans' cold substructures as a dynamical judge: Core, Cusp or MOND?

TL;DR

This study uses N-body simulations to investigate how the longevity of cold stellar substructures in Sextans dwarf galaxy constrains its dark matter profile, favoring cored halos over cuspy ones and exploring MOND implications.

Contribution

It provides new constraints on the dark matter profile of Sextans by analyzing the survival of stellar substructures through detailed simulations.

Findings

Cold substructures survive longer in cored halos than in cuspy ones.

Stellar clusters in cuspy halos are disrupted within ~5 Gyr unless very close to the center.

In MOND, extended stellar clumps can survive for a Hubble time but require an unrealistic mass-to-light ratio.

Abstract

The cold dark matter model predicts cuspy dark matter halos. However, it has been found that, in some low-mass galaxies, cored dark halos provide a better description of their internal dynamics. Here we give constraints on the dark halo profile in the Sextans dwarf spheroidal galaxy by studying the longevity of two cold kinematic substructures detected in this galaxy. We perform N-body simulations of a stellar clump in the Sextans dwarf galaxy, including a live dark matter halo and the main stellar component. We find that, if the dark halo is cuspy, stellar clumps orbiting with semi-major axis ~400 pc are disrupted in ~5 Gyr, even if the clump is initially as compact stellar cluster with a radius of r_c=5 pc. Stellar clusters in an initial orbit with semi-major axis \leq 250 pc may survive to dissolution but their orbits decay towards the center by dynamical friction. In contrast, the stellar clumps can persist for a Hubble time within a cored dark matter halo, even if the initial clump's radius is as extended as r_c=80 pc. We also study the evolution of the clump in the MONDian context. In this scenario, we find that even an extended stellar clump with radius r_c=80 pc survives for a Hubble time, but an unrealistic value for the stellar mass-to-light ratio of 9.2 is needed.