On the spatial distribution and the origin of hypervelocity stars

TL;DR

This paper shows that hypervelocity stars (HVSs) retain information about their progenitors' directions, revealing their origin from the Galactic Center's stellar disks and providing insights into star formation and black hole fueling.

Contribution

It demonstrates that HVSs' ejection directions encode their progenitors' origins, linking HVS spatial distribution to the Galactic Center's stellar disk structures.

Findings

HVSs' ejection directions are nearly anti-parallel to progenitors' directions.

Most HVSs are aligned with two thin disk planes, one matching the CWS disk.

Supports the Galactic Center origin of HVSs and suggests persistent or recurrent stellar disks.

Abstract

Hypervelocity stars (HVSs) escaping away from the Galactic halo are dynamical products of interactions of stars with the massive black hole(s) (MBH) in the Galactic Center (GC). They are mainly B-type stars with their progenitors unknown. OB stars are also populated in the GC, with many being hosted in a clockwise-rotating young stellar (CWS) disk within half a parsec from the MBH and their formation remaining puzzles. In this paper, we demonstrate that HVSs can well memorize the injecting directions of their progenitors using both analytical arguments and numerical simulations, i.e., the ejecting direction of an HVS is almost anti-parallel to the injecting direction of its progenitor. Therefore, the spatial distribution of HVSs maps the spatial distribution of the parent population of their progenitors directly. We also find that almost all the discovered HVSs are spatially consistent with being located on two thin disk planes. The orientation of one plane is consistent with that of the (inner) CWS disk, which suggests that most of the HVSs originate from the CWS disk or a previously existed disk-like stellar structure with an orientation similar to it. The rest of HVSs may be correlated with the plane of the northern arm of the mini-spiral in the GC or the plane defined by the outer warped part of the CWS disk. Our results not only support the GC origin of HVSs but also imply that the central disk (or the disk structure with a similar orientation) should persist or be frequently rejuvenated over the past 200 Myr, which adds a new challenge to the stellar disk formation and provides insights to the longstanding problem of gas fueling into massive black holes.