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Register a new userGrowth of Supermassive Black Hole Seeds in ETG Star-Forming Progenitors: Multiple Merging of Stellar Compact Remnants via Gaseous Dynamical Friction and Gravitational Wave Emission
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We propose a new mechanism for the growth of supermassive black hole (BH) seeds in the star-forming progenitors of local early-type galaxies (ETGs) at $zgtrsim 1$. This envisages the migration and merging of stellar compact remnants (neutron stars and stellar-mass BHs) via gaseous dynamical friction toward the central high-density regions of such galaxies. We show that, under reasonable assumptions and initial conditions, the process can build up central BH masses of order $10^4-10^6, M_odot$ within some $10^7$ yr, so effectively providing heavy seeds before standard disk (Eddington-like) accretion takes over to become the dominant process for further BH growth. Remarkably, such a mechanism may provide an explanation, alternative to super-Eddington accretion rates, for the buildup of billion solar masses BHs in quasar hosts at $zgtrsim 7$, when the age of the Universe $lesssim 0.8$ Gyr constitutes a demanding constraint; moreover, in more common ETG progenitors at redshift $zsim 2-6$ it can concur with disk accretion to build such large BH masses even at moderate Eddington ratios $lesssim 0.3$ within the short star-formation duration $lesssim$ Gyr of these systems. Finally, we investigate the perspectives to detect the merger events between the migrating stellar remnants and the accumulating central supermassive BH via gravitational wave emission with future ground and space-based detectors such as the Einstein Telescope (ET) and the Laser Interferometer Space Antenna (LISA).
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Super-massive black holes weighing up to $sim 10^9 , mathrm{M_{odot}}$ are in place by $z sim 7$, when the age of the Universe is $lesssim 1 , mathrm{Gyr}$. This implies a time crunch for their growth, since such high masses cannot be easily reached in standard accretion scenarios. Here, we explore the physical conditions that would lead to optimal growth wherein stable super-Eddington accretion would be permitted. Our analysis suggests that the preponderance of optimal conditions depends on two key parameters: the black hole mass and the host galaxy central gas density. In the high-efficiency region of this parameter space, a continuous stream of gas can accrete onto the black hole from large to small spatial scales, assuming a global isothermal profile for the host galaxy. Using analytical initial mass functions for black hole seeds, we find an enhanced probability of high-efficiency growth for seeds with initial masses $gtrsim 10^4 , mathrm{M_{odot}}$. Our picture suggests that a large population of high-$z$ lower-mass black holes that formed in the low-efficiency region, with low duty cycles and accretion rates, might remain undetectable as quasars, since we predict their bolometric luminosities to be $lesssim 10^{41} , mathrm{erg , s^{-1}}$. The presence of these sources might be revealed only via gravitational wave detections of their mergers.
Recent numerical simulations reveal that the isothermal collapse of pristine gas in atomic cooling haloes may result in stellar binaries of supermassive stars with $M_* gtrsim 10^4 mathrm{M}_{odot}$. For the first time, we compute the in-situ merger rate for such massive black hole remnants by combining their abundance and multiplicity estimates. For black holes with initial masses in the range $10^{4-6} mathrm{M}_{odot}$ merging at redshifts $z gtrsim 15$ our optimistic model predicts that LISA should be able to detect 0.6 mergers per year. This rate of detection can be attributed, without confusion, to the in-situ mergers of seeds from the collapse of very massive stars. Equally, in the case where LISA observes no mergers from heavy seeds at $z gtrsim 15$ we can constrain the combined number density, multiplicity, and coalesence times of these high-redshift systems. This letter proposes gravitational wave signatures as a means to constrain theoretical models and processes that govern the abundance of massive black hole seeds in the early Universe.
We present the detection of four far-infrared fine-structure oxygen lines, as well as strong upper limits for the CO(2-1) and [N II] 205 um lines, in 3C 368, a well-studied radio-loud galaxy at z = 1.131. These new oxygen lines, taken in conjunction with previously observed neon and carbon fine-structure lines, suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and extended star formation. A starburst dominated by O8 stars, with an age of ~6.5 Myr, provides a good fit to the fine-structure line data. This estimated age of the starburst makes it nearly concurrent with the latest episode of AGN activity, suggesting a link between the growth of the supermassive black hole and stellar population in this source. We do not detect the CO(2-1) line, down to a level twelve times lower than the expected value for star forming galaxies. This lack of CO line emission is consistent with recent star formation activity if the star-forming molecular gas has low metallicity, is highly fractionated (such that CO is photodissociated through much of the clouds), or is chemically very young (such that CO has not yet had time to form). It is also possible, though we argue unlikely, that the ensemble of fine structure lines are emitted from the region heated by the AGN.
More than two hundred supermassive black holes (SMBHs) of masses $gtrsim 10^9,mathrm{M_{odot}}$ have been discovered at $z gtrsim 6$. One promising pathway for the formation of SMBHs is through the collapse of supermassive stars (SMSs) with masses $sim 10^{3-5},mathrm{M_{odot}}$ into seed black holes which could grow upto few times $10^9,mathrm{M_{odot}}$ SMBHs observed at $zsim 7$. In this paper, we explore how SMSs with masses $sim 10^{3-5},mathrm{M_{odot}}$ could be formed via gas accretion and runaway stellar collisions in high-redshift, metal-poor nuclear star clusters (NSCs) using idealised N-body simulations. We explore physically motivated accretion scenarios, e.g. Bondi-Hoyle-Lyttleton accretion and Eddington accretion, as well as simplified scenarios such as constant accretions. While gas is present, the accretion timescale remains considerably shorter than the timescale for collisions with the most massive object (MMO). However, overall 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 crucial role in determining the final mass of the SMSs. We find that the problem is highly sensitive to the initial conditions and our assumed recipe for the accretion, due to the highly chaotic nature of the problem. The key variables that determine the mass growth mechanism are the mass of the MMO and the gas reservoir that is available for the accretion. Depending on different conditions, SMSs of masses $sim10^{3-5} ,mathrm{M_{odot}}$ can form for all three accretion scenarios considered in this work.
In this paper, we explore the mechanisms that regulate the formation and evolution of stellar black hole binaries (BHBs) around supermassive black holes (SMBHs). We show that dynamical interactions can efficiently drive in-situ BHB formation if the SMBH is surrounded by a massive nuclear cluster (NC), while orbitally segregated star clusters can replenish the BHB reservoir in SMBH-dominated nuclei. We discuss how the combined action of stellar hardening and mass segregation sculpts the BHB orbital properties. We use direct N-body simulations including post-Newtonian corrections up to 2.5 order to study the BHB-SMBH interplay, showing that the Kozai-Lidov mechanism plays a crucial role in shortening binaries lifetime. We find that the merging probability weakly depends on the SMBH mass in the $10^6-10^9{rm ~M}_odot$ mass range, leading to a merger rate $Gamma simeq 3-8$ yr$^{-1}$ Gpc$^{-3}$ at redshift zero. Nearly $40%$ of the mergers have masses in the BH mass gap, $50-140{rm ~M}_odot$, thus indicating that galactic nuclei are ideal places to form BHs in this mass range. We argue that gravitational wave (GW) sources with components mass $m_1>40{rm ~M}_odot$ and $m_2<30{rm ~M}_odot$ would represent a strong indicator of a galactic nuclei origin. The majority of these mergers could be multiband GW sources in the local Universe: nearly $40%$ might be seen by LISA as eccentric sources and, a few years later, as circular sources by LIGO and the Einstein Telescope, making decihertz observatories like DECIGO unique instruments to bridge the observations during the binary inspiral.
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