Searching for stars ejected from the Galactic Centre in DESI

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

^★ Corresponding author: verberne@strw.leidenuniv.nl

Received: 13 June 2025
Accepted: 29 July 2025

Abstract

Dynamical interactions between stars and the supermassive black hole Sgr A* at the Galactic Centre (GC) may result in stars being ejected into the Galactic halo. While recent fast ejections by Sgr A* have been identified in the form of hypervelocity stars (hundreds to thousands of km/s), it is also believed that the stellar halo contains slower stars, ejected over the last few billion years. In this study we used the first data release of DESI to search for these slower GC ejecta, which are expected to stand out from the stellar halo population thanks to their combined high metallicity ([Fe/H] ≳ 0) and low vertical angular momentum (L_Z), whose distribution should peak at zero. Our search did not yield a detection but allowed us to place an upper limit on the ejection rate of stars from the GC of ~2.8 × 10⁻³ 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 those that invoke mergers of Sgr A* with other massive black holes in the last few billion years.