Hypervelocity Stars III. The Space Density and Ejection History of Main Sequence Stars from the Galactic Center

TL;DR

This paper reports the discovery of new hypervelocity stars ejected from the Galactic Center, analyzes their properties to infer their origins, and estimates the total population and ejection history of such stars.

Contribution

It provides the first comprehensive analysis of the space density, ejection history, and main sequence nature of hypervelocity stars from the Galactic Center.

Findings

Discovered 3 new unbound hypervelocity stars.

Estimated 96 ± 10 HVSs of 3-4 Msun within 100 kpc.

Supported continuous ejection over a burst model.

Abstract

We report the discovery of 3 new unbound hypervelocity stars (HVSs), stars traveling with such extreme velocities that dynamical ejection from a massive black hole (MBH) is their only suggested origin. We also detect a population of possibly bound HVSs. The significant asymmetry we observe in the velocity distribution -- we find 26 stars with v_rf > 275 km/s and 1 star with v_rf < -275 km/s -- shows that the HVSs must be short-lived, probably 3 - 4 Msun main sequence stars. Any population of hypervelocity post-main sequence stars should contain stars falling back onto the Galaxy, contrary to the observations. The spatial distribution of HVSs also supports the main sequence interpretation: longer-lived 3 Msun HVSs fill our survey volume; shorter-lived 4 Msun HVSs are missing at faint magnitudes. We infer that there are 96 +- 10 HVSs of mass 3 - 4 Msun within R < 100 kpc, possibly enough HVSs to constrain ejection mechanisms and potential models. Depending on the mass function of HVSs, we predict that SEGUE may find up to 5 - 15 new HVSs. The travel times of our HVSs favor a continuous ejection process, although a ~120 Myr-old burst of HVSs is also allowed.