Discovery of Galactic center ejected star in DESI DR1

TL;DR

This study identifies DESI-312 as a compelling Galactic Centre ejected star using a six-dimensional search that combines DESI DR1 spectroscopy and Gaia DR3 astrometry, with spectro-photometric distances to build full phase-space information. Through backward orbit integration in a realistic Milky Way potential and a metallicity filter, DESI-312 stands out as the sole GC ejecta candidate, featuring a GC ejection velocity of about $698^{+35}_{-27}$ km s$^{-1}$ and supersolar metallicity of $ m [Fe/H]\approx0.27$, implying a Hills mechanism origin. The star appears to be an old, solar-mass main-sequence or early subgiant, enabling detailed chemical tagging that could illuminate the GC’s chemical composition and formation history. The findings demonstrate the power of DESI+Gaia data for GC-ejected star searches and establish a framework applicable to upcoming surveys (e.g., 4MOST, WEAVE) and future Gaia releases, with the potential to reveal the GC’s chemical fingerprint and dynamical history through bound and unbound HVS populations.

Abstract

Hypervelocity stars (HVSs) are stars ejected from the Galactic Centre (GC) through tidal interactions with the central supermassive black hole. Formed in the immediate vicinity of Sgr~A$^\ast$, these stars are accelerated to velocities high enough to escape the GC and be observable in the Galactic halo. Using spectroscopy from the Dark Energy Spectroscopic Instrument (DESI) and astrometry from Gaia, we conducted a six-dimensional search for HVSs and identified a compelling candidate, hereafter DESI-312, whose bound trajectory can be confidently traced back to the GC. The star resides in the inner halo and exhibits supersolar metallicity ([Fe/H] $= 0.27\pm 0.09$), distinct from other known stellar populations with radial orbits. Its inferred GC ejection velocity of $698^{+35}_{-27}$ is consistent with a Hills mechanism ejection, supporting an origin in the innermost regions of the Milky Way. We considered alternative origins for the star, including disk ejections from young clusters and globular clusters, but these scenarios fail to explain both its orbit and metallicity. Unlike previously identified A- and B-type HVSs, DESI-312 is a $\sim 1\,M_{\odot}$ star on the main sequence or early subgiant branch, thus enabling a detailed chemical analysis of its atmosphere and offering a rare window - unobscured by dust and crowding - into the composition of the central regions of the Galaxy.

Figures (10)

• Figure 1: Fraction of realizations (out of 1,000 total) whose first Galactic-plane crossing falls within a 1 $\mathrm{\,kpc}$-radius circle around the GC, shown as a function of $\mathrm{\,[Fe/H]}$ for DESI candidates. S5-HVS1 is overplotted in orange for reference s5hvs1. The only candidate found, star DESI-312, is shown in red.
• Figure 2: Comparison between the best-fitting model spectrum (red) and the observed DESI spectra (black) for DESI-312, both provided as part of the first data release of the DESI stellar catalogue DESIMW. From top to bottom, the panels show the red, blue, and z arms of the spectra. The close agreement between model and data gives no suggestion of a problematic fit or unreliable atmospheric parameters.
• Figure 3: Corner plot of the posterior distributions for stellar mass $M_\star$, $\log_{10}(\mathrm{Age/yr})$, $\mathrm{[Fe/H]}$, distance $d$, and reddening $E(B{-}V)$. Green contours are from the photometry-only fit; blue contours include photometry and the DESI rvspecfit measurements of $T_{\mathrm{eff}}$ and $\log g$. Contours indicate the 68%, 95%, and 99.7% credible regions; diagonal panels show the corresponding 1D marginalized posteriors. Titles show the median and 16th and 84th percentiles for the spectro-photometric fit.
• Figure 4: Hertzsprung–Russell diagram of DESI-312. The orange curves show 1000 samples from the posterior of the spectro-photometric model (see Figure \ref{['fig:posterior_stellar']}). Best fit isochrone is shown as a dashed line.
• Figure 5: Example orbit of DESI-312 integrated over the median lifetime in the XZ (top) and XY plane (bottom), in the Galactic Cartesian coordinate system. The orbit is backward integrate for 8.05 Gyr, given by the median on the age posterior for the spectro-photometric model described in \ref{['subsec:phot_fit']}.