On the Spatial Distribution and the Origin of Hypervelocity Stars

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.

The Galactic Center S-Stars and the Hypervelocity Stars in the Galactic Halo: Two Faces of the Tidal Breakup of Stellar Binaries by the Central Massive Black Hole?

“…7 Note that Brown et al (2012b) recently reported five new unbound HVSs discovered in the Galactic halo and re-analyzed the HVSs previously discovered. According to this new study, there are 17 unbound HVSs in the northern sky and they are still consistent with being located on two planes revealed by Lu et al (2010). That is, one of the disk planes is consistent with the CWS disk plane, while the other disk plane is more consistent with the warped outer part of the CWS disk and slightly deviates from the Narm plane.…”

“…Lu et al 2009;Bartko et al 2009). The planes of the host disks are assumed to be the same as the two planes that best fit the observations, i.e., (l, b) = (311 • , −14 • ) and (176 • , −53 • ), respectively, and these two planes are consistent with the CWS disk plane and the plane of the northern arm (Narm) of the mini-spiral in the GC (or the outer warped part of the CWS disk; Lu et al 2010;Zhang et al 2010). 7 The injection rates from these two disks are assumed to be the same.…”

Kinematics in galactic tidal tails

“…Stars ejected by one or more black holes in the GC should appear on the sky in an approximately homogeneous distribution or in a ring-like structure (Levin 2006). However, a preferred ejection direction as found in the data is not naturally explained with this mechanism (however, see Lu et al 2010).…”

Segue-2 Limits on Metal-Rich Old-Population Hypervelocity Stars in the Galactic Halo

“…For example, Perets et al (2007) demonstrate that, for reasonable assumptions about the mass function and old-star binary fraction, the ejection rate of 1 M stars should be approximately 0.5 Myr −1 (but see Yu & Tremaine 2003), and could be substantially enhanced by the presence of a secondary massive perturber as envisioned by, e.g., Polnarev & Rees (1994), Baumgardt et al (2006), and Levin (2006) and not included in the Kenyon et al (2008) calculations. Lu et al 2010 have suggested that the HVS population is emerging from the disk at the GC, which is thought to have a top-heavy initial mass function (IMF; Bartko et al 2010). If this is correct, the top-heavy IMF in this region could account for our limits.…”