Hypervelocity stars from the Andromeda galaxy

Blake D Sherwin,Ryan M O'Leary,Abraham Loeb

doi:10.1111/j.1365-2966.2008.13097.x

Abstract

Hypervelocity stars (HVSs) discovered in the Milky Way (MW) halo are thought to be ejected from near the massive black hole (MBH) at the galactic centre. In this paper we investigate the spatial and velocity distributions of the HVSs which are expected to be similarly produced in the Andromeda galaxy (M31). We consider three different HVS production mechanisms: (i) the disruption of stellar binaries by the galactocentric MBH; (ii) the ejection of stars by an in-spiraling intermediate mass black hole; and (iii) the scattering of stars off a cluster of stellar-mass black holes orbiting around the MBH. While the first two mechanisms would produce large numbers of HVSs in M31, we show that the third mechanism would not be effective in M31. We numerically calculate 1.2*10^6 trajectories of HVSs from M31 within a simple model of the Local Group and hence infer the current distribution of these stars. Gravitational focusing of the HVSs by the MW and the diffuse Local Group medium leads to high densities of low mass (~ solar mass) M31 HVSs near the MW. Within the virialized MW halo, we expect there to be of order 1000 HVSs for the first mechanism and a few hundred HVSs for the second mechanism; many of these stars should have distinctively large approach velocities (< -500 km/s). In addition, we predict ~5 hypervelocity RGB stars within the M31 halo which could be identified observationally. Future MW astrometric surveys or searches for distant giants could thus find HVSs from M31.

Highlights

Hypervelocity stars (HVSs) travel at such high speeds that they are not gravitationally bound to the galaxy from which they originate
In addition to the bound and expelled stars, stars released under certain angles with a small range of velocities ≈650–900 km s−1 were gravitationally pulled into the Milky Way (MW) or towards the intergalactic axis
(i) The differences between the galactic centres of M31 and the MW imply that the BHC model for HVS production would not be effective in M31

Summary

Introduction

Hypervelocity stars (HVSs) travel at such high speeds that they are not gravitationally bound to the galaxy from which they originate. The HVSs have typical speeds of 550 km s−1. There is evidence for a bound population of stars similar to the HVSs but with speeds slightly lower than the escape velocity of the MW (Brown et al 2007a). The observations of MW HVSs indicate that all but one of these stars originate from the centre of the galaxy.. The observations of MW HVSs indicate that all but one of these stars originate from the centre of the galaxy.1 This is understood if HVSs are ejected by the massive black holes (MBHs) which inhabit the centres of galaxy bulges The observations of MW HVSs indicate that all but one of these stars originate from the centre of the galaxy. This is understood if HVSs are ejected by the massive black holes (MBHs) which inhabit the centres of galaxy bulges

Methods

Results

Discussion

Conclusion