A Spectroscopic Search for Dormant Black Holes in Low-Metallicity Binaries

The discovery of the massive black hole (BH) system Gaia BH3 in pre-release Gaia DR4 data suggests that wide BH binaries with luminous companions may be significantly overrepresented at low metallicities. Motivated by this finding, we initiated a spectroscopic survey of low-metallicity stars exhibiting elevated RUWE values in Gaia DR3 using the FEROS and APF spectrographs. We identified promising BH binary candidates as objects with instantaneously measured radial velocities (RVs) that were very different from their mean RVs reported in Gaia DR3. Thus far, we have observed over 500 targets, including a nearly complete sample of stars with [Fe/H] < ‑1.5, RUWE > 2, and G < 15. Our search yielded one promising target exhibiting slow acceleration and an RV more than 98 km/s different from its DR3 mean RV, as well as dozens of other BH candidates with smaller RV discrepancies. We quantified the sensitivity of our search using simulations, demonstrating that it recovers at least half of the BH companions within our selection criteria.

Read more about this research project in the associated publication here.