Searching for stars ejected from the Galactic Centre in DESI

TL;DR

This study searches for stars ejected from the Galactic Centre using DESI data, focusing on slower stars with high metallicity and low angular momentum, but finds no such stars and sets an upper ejection rate limit.

Contribution

First to use DESI data to search for slow, metal-rich stars ejected from the Galactic Centre, providing model-independent upper limits on ejection rates over billions of years.

Findings

No detection of GC ejected stars in DESI data.

Sets an upper limit on ejection rate of ~2.8×10⁻³ yr⁻¹.

Constraints on models involving black hole mergers at the GC.

Abstract

Dynamical interactions between stars and the super massive black hole Sgr A* at the Galactic Centre (GC) may eject stars into the Galactic halo. While recent fast ejections by Sgr A* have been identified in the form of hypervelocity stars (hundreds to thousands km/s), it is also expected that the stellar halo contains slower stars, ejected over the last few billion years. In this study, we use the first data release of DESI to search for these slower GC ejecta, which are expected to stand out from the stellar halo population for their combined high metallicity (${\rm [Fe/H]}\gtrsim0$) and small values of their vertical angular momentum ($L_Z$), whose distribution should peak at zero. Our search does not yield a detection, but allows us to place an upper limit on the ejection rate of stars from the GC of $\sim2.8\times10^{-3}$ yr$^{-1}$ over the past ~5 Gyr, which is ejection model independent. This implies that our result can be used to put constraints on different ejection models, including that invoking mergers of Sgr A* with other massive black holes in the last last few billion years.