New model by IIA to help chalk out black hole archaeology

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BENGALURU: Researchers from

the Indian Institute of Astrophysics

, have developed a model that can help chalk out

black hole archaeology

, where one can look into the past and redraw the

black hole

properties at the formation from the present-day values as initial conditions.
All massive galaxies have supermassive black holes (SMBHs) at their centers, whose gravitational potential dominates stellar or gas motions.
Black holes (BHs) grow their mass and spin by accretion of gas, the capture of stars, and by the merger activity of galaxies while the


torque drives a powerful jet from the black hole drawing from its rotational energy and causing a spin-down of the hole.
These processes constitute the evolution model of the central black hole. The connection of the SMBHs to their host galaxies is evidenced by the strong correlation between the mass of SMBH (M•) and velocity dispersion (σ) of the stars in the rest of the galaxy, far from the BH.
Now, researchers from the Indian Institute of Astrophysics (IIA), have modeled the evolution of this relation which can help build demographics of the BHs and scenarios involving stellar capture. The study on cosmic spin and mass evolution of black holes by D Bhattacharyya and A Mangalam from IIA, has been published in

the Astrophysical Journal

, 2020.
“...The study can help chalk out black hole archaeology, where one can look into the past and redraw the black hole properties at the formation from the present-day values as initial conditions, and also estimate SMBH seed properties at formation from stellar-mass black holes via the stellar capture process,” the department of science and technology (DST) said. IIA is an autonomous institute under the DST.
The key novel aspects of the model, DST said, were the relativistic inputs to the stellar capture theory — stars can get tidally disrupted or can get directly captured depending on the mass of the BH — and including all known contributions of gas, stellar, electromagnetic torque, and mergers in detail for calculating the evolution.
“Each effect is included individually, before finally deriving the complete evolution. The model gives important insights into black hole physics and galactic structure, as the study of the evolution of BHs and the M• − σ relations are very important from both theoretical and observational points of view,” DST added.
While the BH grows its mass, the gas accretion process stops after it reaches a saturated mass due to a feedback of the luminosity on the infalling gas. Before saturation, accretion dominates the growth, and subsequently, stellar capture and mergers take over with roughly equal contributions. For stellar capture, a power-law in

stellar density

in the cusp around the black hole was assumed in a relativistic framework.
The IIA team assumed the merger activity effective for redshift less than 4, and the electromagnetic torque, which contributes to the spin-down process, is included self-consistently. The calculations involving both symbolic and numeric operations were performed using the High-performance computing facility of IIA.
The model of the joint evolution of BH mass, spin, and M•–σ relations developed by IIA researchers throw light on the co-evolution of the BH and its environment from the time of formation.

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