Formation of Supermassive Black Holes in the Early Universe
Abstract
The aim of the work presented in this thesis is to understand the formation and growth of the seeds of the supermassive black holes in early universe. Supermassive black holes (SMBH) with masses larger than 108MSun have been observed when the Universe was only 800 Myr old. The formation and accretion history of the seeds of these supermassive black holes are a matter of debate. We consider the scenario of massive seed black hole formation which allows gas to directly collapse into a black hole (DCBH) of similar mass. Considering this scenario, we show that the mass function of SMBH after such a limited time period with growing formation rate paired with super-Eddington accretion can be described as a broken power-law with two characteristic features. Another possible pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs). We explore how SMSs with mass of the order of 103-5MSun could be formed via gas accretion and runaway stellar collisions in high-redshift, metal-poor nuclear star clusters. These SMSs could grow into a few times 109MSun supermassive black holes observed at z~7 via Eddington-accretion. We explore physically motivated accretion scenarios, e.g. Bondi–Hoyle–Lyttleton accretion and Eddington accretion, as well as simplified scenarios such as a constant accretion. While gas is present, the accretion timescale remains considerably shorter than the timescale for collisions with the most massive object. However, the timescale for collisions between any two stars in the cluster can become comparable or shorter than the accretion timescale, hence collisions still play a relevant role. In addition, initial collisions will speed up the accretion process particularly in the Eddington and Bondi scenario. We show that the upper mass limit of the initial mass function (IMF) affects the maximum mass of the final mass distribution in the cluster in the case of constant accretion, while it is not statistically relevant for Eddington or Bondi accretion. This is even the case in spite of the fact that the upper IMF cut-off sets the accretion rate. For Bondi accretion, which scales as the square of the accretor mass, the initial accretion rate is higher and the most massive object forms earlier for a larger upper-mass cutoff. The initial compactness of the cluster is important in case of constant accretion rates and for the Eddington scenario, while it does not significantly affect the mass of the most massive object in the Bondi scenario. The Bondi scenario can potentially produce the most massive objects with 105MSun, but it also implies a long timescale without relevant growth. Depending on the conditions, massive objects of 103-5MSun can form for all three accretion scenarios considered. However, mass loss due to stellar winds could be an important limitation for the formation of the SMSs and affect the final mass, especially in high-metallicity environments. In this thesis, we study the effect of mass loss driven by stellar winds on the formation and evolution of SMSs in dense NSCs using idealised N-body simulations. We adopt theoretical mass loss rates from the literature. Considering different accretion scenarios, we have studied the effect of the mass loss rates over a wide range of metallicities Z*=[.001-1] ZSun and Eddington factors fEdd = L*/LEdd = 0.5, 0.7, & 0.9. The final masses of the SMSs are determined by the adopted accretion scenario, metallicity, and luminosity. For the accretion rate of 10-4MSun/yr , SMSs of masses ~103-4MSun can be formed for all adopted values of Z* and fEdd. For the accretion rate of 10-4MSunyr-1 , SMSs of masses ~ 103-4MSun can be formed for all adopted values of Z* and fEdd, except for Z* = ZSun and fEdd = 0.7 or 0.9. For Eddington accretion SMSs of masses of the order of 103MSun can be formed in low metallicity environments with Z< 0.01 ZSun. The most massive SMSs of masses of the order of 105MSun can be formed for Bondi-Hoyle accretion in environments with Z< 0.5 ZSun.