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Astro Seminar

Keep the black holes merging: cluster escape speed and the curious case of GW190412

Speaker: Davide Gerosa (Birmingham)
Date: Wednesday 27 May 2020
Time: 14:00
Venue: Zoom

Pair instabilities in supernovae might prevent the formation of black holes with masses between ∼50 and ∼130 solar masses. Gravitational-wave observations in this mass range would need to be explained with other astrophysical mechanisms besides stellar collapse. Highly spinning black holes might also require not-so-standard explanations, as current models of stellar evolution tend to predict slowly rotating remnants. Multiple generations of black-hole mergers provide a possible way to populate these "mass gap" and "spin gap". However, this interpretation requires an astrophysical environment with a sufficiently large escape speed to retain merger remnants and prevent them from being ejected by gravitational-wave recoils. We show that a single LIGO/Virgo observation of a black hole in the pair-instability mass gap implies that its progenitors grew in an environment with escape speed >50 km/s. This is larger than the escape speeds of most globular clusters, requiring denser and heavier environments such as nuclear star clusters or disks-assisted migration in galactic nuclei. We then apply this line of reasoning to the recently announced event GW190412, and show that (i) its occurrence is a real dilemma for current astrophysical formation models, and (ii) a multiple-generation scenario is indeed promising to explain the observed features.