Formation of Massive Black Holes in Dense Star Clusters. Ii. Initial Mass Function and Primordial Mass Segregation

Abstract

A promising mechanism to form intermediate-mass black holes is the runaway merger in dense star clusters, where main-sequence stars collide and form a very massive star (VMS), which then collapses to a black hole (BH). In this paper, we study the effects of primordial mass segregation and the importance of the stellar initial mass function (IMF) on the runaway growth of VMSs using a dynamical Monte Carlo code for N-body systems with N as high as 10 6 stars. Our code now includes an explicit treatment of all stellar collisions. We place special emphasis on the possibility of top-heavy IMFs, as observed in some very young massive clusters. We find that both primordial mass segregation and the shape of the IMF affect the rate of core collapse of star clusters and thus the time of the runaway. When we include primordial mass segregation, we generally see a decrease in core-collapse time (t cc ). Although for smaller degrees of primordial mass segregation this decrease in t cc is mostly due to the change in the density profile of the cluster, for highly mass-segregated (primordial) clusters, it is the increase in the average mass in the core which reduces the central relaxation time decreasing t cc . The final mass of the VMS formed is always close to ∼10 −3 of the total cluster mass, in agreement with previous studies and is reminiscent of the observed correlation between the central BH mass and the bulge mass of the galaxies. As the degree of primordial mass segregation is increased, the mass of the VMS increases at most by a factor of three. Flatter IMFs generally increase the average mass in the whole cluster, which increases t cc . For the range of IMFs investigated in this paper, this increase in t cc is to some degree balanced by stellar collisions, which accelerate core collapse. Thus, there is no significant change in t cc for the somewhat flatter global IMFs observed in very young massive clusters.

The Close Stellar Companions to Intermediate-Mass Black Holes

“…That the mass of most of the IMBHʼs companions is significantly larger than that of a typical cluster star is not surprising. The trend toward energy equipartition in two-body interactions causes stars more massive than the mean mass to sink toward orbits deep in the cluster core, where they are very likely to come into direct interaction (and potentially partnership) with the IMBH (e.g., Gürkan et al 2004;Goswami et al 2012;Leigh et al 2014). Further, more massive secondary objects are likely to replace an existing companion if they do undergo a three-body interaction (Fullerton & Hills 1982;Sigurdsson & Phinney 1993;Mapelli & Zampieri 2014;Samsing et al 2014).…”

Investigating the retention of intermediate-mass black holes in star clusters usingN-body simulations

“…According to this scenario, the most massive stars sink to the centre of a GC even before they become BHs and thus the cluster experiences an early core collapse during which the central density of stars becomes large enough, for massive stars to start rapidly and continuously merging with each other (see also Portegies Zwart et al 2004;Goswami et al 2012). This runaway process very soon leads to the formation of a very massive star (VMS), located close to the centre of the GC.…”

Mid-Infrared Colors of Dwarf Galaxies: Young Starbursts Mimicking Active Galactic Nuclei

“…To understand these difficult-to-observe objects, researchers have turned to observations of nearby dwarf galaxies that may host analogous lower-mass black holes (for a review, see Reines & Comastri 2016). By assembling large samples of low-mass black holes, it may be possible to distinguish between the different proposed theoretical scenarios for their creation: these objects may be remnants of massive Population III stars (Bromm & Yoshida 2011), a result of direct collapse of primordial dense gas (Haehnelt & Rees 1993;Begelman et al 2006;Lodato & Natarajan 2006;van Wassenhove et al 2010), or perhaps they are the end product of very massive stars formed through stellar mergers in dense star clusters (Gürkan et al 2004;Freitag et al 2006;Goswami et al 2012;Giersz et al 2015;Lützgendorf et al 2016).…”