Can radial motions in the stellar halo constrain the rate of change of mass in the Galaxy?

Abstract

ABSTRACT A change in the mass of the Galaxy with time will leave its imprint on the motions of the stars, with stars having radially outward (mass-loss) or inward (mass accretion) bulk motions. Here, we test the feasibility of using the mean radial motion of stars in the stellar halo to constrain the rate of change of mass in the Galaxy, e.g. due to decay of dark matter. In the lambda cold dark matter (ΛCDM) paradigm, the stellar halo is formed by accretion of satellites on to the host galaxy and its mean radial motion 〈VR〉 is eventually expected to be close to zero. But due to incomplete mixing most haloes have substructures and this can lead to non-zero 〈VR〉 in them. Using simulations, we measure the mean radial motion of stars in 13 ΛCDM stellar haloes lying in a spherical shell of radius 30 kpc. For most haloes, the shell motion is quite small, with 75 per cent of haloes having $\langle V_\text{R}\rangle \lesssim 1.2 \:{\rm km}\, {\rm s}^{-1}$. When substructures are removed by using a clustering algorithm, 〈VR〉 is reduced even further, with 75 per cent of haloes having $\langle V_\text{R}\rangle \lesssim 0.6 \:{\rm km}\, {\rm s}^{-1}$. A value of $\langle V_\text{R}\rangle \approx 0.6 \:{\rm km}\, {\rm s}^{-1}$ can be attained corresponding to a galactic mass-loss rate of 2 per cent per Gyr. We show that this can place constraints on dark matter decay parameters such as the decay lifetime and the kick velocity that is imparted to the daughter particle. The advent of all-sky stellar surveys involving millions to billions of stars is encouraging for detecting signatures of dark matter decay.