Old-Population Hypervelocity Stars from the Galactic Center: Limits from the SDSS

TL;DR

This study uses SDSS data to set limits on the ejection rate of old-population hypervelocity stars from the Galactic Center, finding a low ejection rate consistent with a normal mass function.

Contribution

It provides the first constraints on old-population HVS ejection rates from the Galactic Center using a large stellar sample.

Findings

Upper limit on ejection rate of ~45/Myr for old-population HVS

Detected 4 candidate bound stars returning toward the GC

Found 2 candidate unbound stars unlikely from the GC

Abstract

We present limits on the ejection of old-population HVS from a sample of over 290,000 stars selected from the SDSS. We derive the speed at the solar circle from the measured positions and radial velocities by assuming a radial orbit and adopting a simple isothermal model of the Galactic halo, which enables us to identify candidate bound and unbound ejectees. We find 4 candidate bound F-stars from this sample, all with negative Galactocentric radial velocity (i.e., returning toward the GC). We additionally find 2 candidate unbound stars (one F and one G), however, existing proper motion measurements make these unlikely to be emerging from the GC. These data place an upper limit on the rate of ejection of old-population stars from the GC of ~45/Myr. Comparing to the rate for more massive B-star ejectees of ~0.5/Myr, our limit on the rate of ejection of old-population HVS shows that the mass function at the GC is not bottom-heavy and is consistent with being normal. Future targeted surveys of old-population HVS could determine if it is indeed top-heavy.