Tousif Islam and Casey Lam

From the calendar

Merger remnants of binary black-hole systems can receive are coil—also referred to as a “kick”—with velocities reaching up to ~5000 km/s. Remnants that receive large kicks are likely to escape their host environments, such as active galactic nuclei, nuclear star clusters, or globular clusters, whereas remnants with smaller kicks are expected to be retained and may undergo hierarchical mergers, potentially leading to the formation of more massive black holes. Several black-hole masses inferred from LIGO–Virgo–KAGRA observations already suggest possible hierarchical-merger origins. Accurately modeling recoil velocities is therefore crucial for understanding the growth of black-hole populations. However, existing kick models are either too restrictive in their parameter-space coverage or exhibit significant inaccuracies. In this work, we construct an accurate kick model that spans mass ratios from 1:1 to 200:1, leveraging a large set of state-of-the-art numerical relativity simulations for comparable-mass binaries and perturbative black-hole simulations for extreme mass ratios, combined with analytical insights from post-Newtonian theory and machine-learning techniques such as normalizing flows.

There are expected to be 10^7 - 10^9 stellar-mass black holes (BHs) in the Milky Way. Only about 30 have been found, and nearly all of them are X-ray binaries, which are an intrinsically rare type of BH system. The majority of Galactic BHs are expected to be either isolated or in wide binaries. Since they do not produce "loud" accretion signals like X-ray binaries, they have eluded detection until very recently. I will discuss how to find isolated and wide binary BHs, the selection effects introduced in the process, and how we can start placing observational constraints on BH population properties. Characterizing the Galactic BH population will ultimately enable us to understand a broad range of astrophysics: from the evolution and death of massive stars, to binary interactions, mergers, and disruptions, to providing context for the population of merging BHs found via gravitational waves.