Formation of massive seed black holes via collisions and accretion

TL;DR

This paper proposes a new pathway for forming massive seed black holes through stellar collisions in primordial star clusters, emphasizing the roles of gas accretion and stellar dynamics in overcoming previous formation challenges.

Contribution

It introduces a model where stellar collisions in primordial clusters lead to massive black hole seeds, highlighting the importance of gas accretion and stellar evolution effects.

Findings

Collision fraction increases to about 10% with gas accretion effects.

Massive objects of 10^4 - 10^5 solar masses can form despite initial fragmentation.

Primordial star clusters are viable sites for supermassive black hole seeds.

Abstract

Models aiming to explain the formation of massive black hole seeds, and in particular the direct collapse scenario, face substantial difficulties. These are rooted in rather ad hoc and fine-tuned initial conditions, such as the simultaneous requirements of extremely low metallicities and strong radiation backgrounds. Here we explore a modification of such scenarios where a massive primordial star cluster is initially produced. Subsequent stellar collisions give rise to the formation of massive (10^4 - 10^5 solar mass) objects. Our calculations demonstrate that the interplay between stellar dynamics, gas accretion and protostellar evolution is particularly relevant. Gas accretion onto the protostars enhances their radii, resulting in an enhanced collisional cross section. We show that the fraction of collisions can increase from 0.1-1% of the initial population to about 10% when compared to gas-free models or models of protostellar clusters in the local Universe. We conclude that very massive objects can form in spite of initial fragmentation, making the first massive protostellar clusters viable candidate birth places for observed supermassive black holes.