Abstract
Abstract Approximately 200 supermassive black holes (SMBHs) have been discovered within the first ∼gigayear after the Big Bang. One pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). A possible obstacle to this scenario is that the collapsing gas fragments and forms a cluster of main-sequence stars. Here, we raise the possibility that stellar collisions may be sufficiently frequent and energetic to inhibit the contraction of the massive protostar, avoiding strong UV radiation driven outflows, and allowing it to continue growing into an SMS. We investigate this scenario with semianalytic models incorporating star formation; gas accretion; dynamical friction from stars and gas; stellar collisions; and gas ejection. We find that when the collapsing gas fragments at a density of ≲3 × 1010 cm−3, the central protostar contracts due to infrequent stellar mergers, and in turn photoevaporates the remaining collapsing gas, resulting in the formation of a ≲104 M ⊙ object. On the other hand, when the collapsing gas fragments at higher densities (expected for a metal-poor cloud with Z ≲ 10−5 Z ⊙ with suppressed H2 abundance) the central protostar avoids contraction and keeps growing via frequent stellar mergers, reaching masses as high as ∼105–106 M ⊙. We conclude that frequent stellar mergers represent a possible pathway to form massive BHs in the early universe.
Highlights
IntroductionApproximately two hundred supermassive black holes (SMBHs) have been discovered with masses of 109 Me at redshift z 6 (e.g., Fan et al 2001; Willott et al 2010; Mortlock et al 2011; Venemans et al 2013; De Rosa et al 2014; Wu et al 2015; Jiang et al 2016; Banados et al 2018; Matsuoka et al 2018, 2019; Izumi et al 2019; Onoue et al 2019; Shen et al 2019; Wang et al 2019; Yang et al 2019, and references therein)
To investigate how stars form, migrate inward, and crash into the central star, and how they are affected by feedback, we use a semianalytical model incorporating the effects of star formation, protostellar evolution, gas accretion, dynamical friction by stars and gas, collisions, and gas ejection (Figure 1)
We find that for efficient growth via stellar bombardment, the formation of a high-density star cluster is required in order to enhance the inward acceleration by stellar dynamical friction
Summary
Approximately two hundred supermassive black holes (SMBHs) have been discovered with masses of 109 Me at redshift z 6 (e.g., Fan et al 2001; Willott et al 2010; Mortlock et al 2011; Venemans et al 2013; De Rosa et al 2014; Wu et al 2015; Jiang et al 2016; Banados et al 2018; Matsuoka et al 2018, 2019; Izumi et al 2019; Onoue et al 2019; Shen et al 2019; Wang et al 2019; Yang et al 2019, and references therein). Growth from stellar-mass BH remnants of Population III stars (e.g., Madau & Rees 2001; Abel et al 2002; Heger & Woosley 2002; Tan & McKee 2004; Volonteri & Rees 2006; McKee & Tan 2008; Yoshida et al 2008; Clark et al 2011b; Greif et al 2011; Hirano et al 2014; Susa et al 2014; Stacy et al 2016) to SMBHs is difficult because the gas accretion rate is suppressed by radiative and kinetic feedback processes (Whalen et al 2004; Alvarez et al 2009; Milosavljevic et al 2009; Tanaka & Haiman 2009; Tanaka et al 2012; Regan et al 2019) and growth by mergers is made inefficient by large recoil induced by gravitational wave emission during mergers, which unbinds the merger remnant BHs from the shallow potential wells of their early hosts (Haiman 2004) These difficulties have motivated several alternative pathways.
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