How many black holes are out there in the Universe? This is one of the most
relevant and pressing questions in modern astrophysics and cosmology. The
intriguing issue has recently been addressed by the SISSA Ph.D. student Alex
Sicilia, supervised by Prof. Andrea Lapi and Dr. Lumen Boco, together with
other collaborators from SISSA and from other national and international
institutions. In a first paper of a series just published in The Astrophysical
Journal, the authors have investigated the demographics of stellar mass black
holes, which are black holes with masses between a few to some hundred solar
masses, that originated at the end of the life of massive stars. According to
the new research, a remarkable amount around 1% of the overall ordinary
(baryonic) matter of the Universe is locked up in stellar mass black holes.
Astonishingly, the researchers have found that the number of black holes
within the observable Universe (a sphere of diameter around 90 billions light
years) at present time is about 40 trillions, 40 billion billions (i.e., about
40 x 1018, i.e. 4 followed by 19 zeros!).
A new method to calculate the number of black holes
As the authors of the research explain: "This important result has been
obtained thanks to an original approach which combines the state-of-the-art
stellar and binary evolution code SEVN developed by SISSA researcher Dr.
Mario Spera to empirical prescriptions for relevant physical properties of
galaxies, especially the rate of star formation, the amount of stellar mass
and the metallicity of the interstellar medium (which are all important
elements to define the number and the masses of stellar black holes).
Exploiting these crucial ingredients in a self-consistent approach, thanks
to their new computation approach, the researchers have then derived the
number of stellar black holes and their mass distribution across the whole
history of the Universe. Alex Sicilia, first author of the study, comments:
"The innovative character of this work is in the coupling of a detailed
model of stellar and binary evolution with advanced recipes for star
formation and metal enrichment in individual galaxies. This is one of the
first, and one of the most robust, ab initio computation of the stellar
black hole mass function across cosmic history."
Origin of most massive stellar black holes
The estimate of the number of black holes in the observable Universe is not
the only issue investigated by the scientists in this piece of research. In
collaboration with Dr. Ugo Di Carlo and Prof. Michela Mapelli from
University of Padova, they have also explored the various formation channels
for black holes of different masses, like isolated stars, binary systems and
stellar clusters. According to their work, the most massive stellar black
holes originate mainly from dynamical events in stellar clusters.
Specifically, the researchers have shown that such events are required to
explain the mass function of coalescing black holes as estimated from
gravitational wave observations by the LIGO/Virgo collaboration.
Lumen Boco, co-author of the paper, comments: "Our work provides a robust
theory for the generation of light seeds for (super)massive black holes at
high redshift, and can constitute a starting point to investigate the origin
of 'heavy seeds', that we will pursue in a forthcoming paper.
Prof. Andrea Lapi, Sicilia's supervisor and coordinator of the Ph.D. in
Astrophysics and Cosmology at SISSA, adds: "This research is really
multidisciplinary, covering aspects of, and requiring expertise in stellar
astrophysics, galaxy formation and evolution, gravitational wave and
multi-messenger astrophysics; as such it needs collaborative efforts from
various members of the SISSA Astrophysics and Cosmology group, and a strong
networking with external collaborators."
Alex Sicilia's work occurs in the context of an Innovative Training Network
Project "BiD4BESt -- Big Data Application for Black Hole Evolution Studies"
co-PIed by Prof. Andrea Lapi from SISSA (H2020-MSCAITN-2019 Project 860744),
that has been funded by the European Union with about 3.5 million Euros
overall; it involves several academic and industrial partners, to provide
Ph.D. training to 13 early stage researchers in the area of black hole
formation and evolution, by exploiting advanced data science techniques.
Reference:
Alex Sicilia, Andrea Lapi, Lumen Boco, Mario Spera, Ugo N. Di Carlo, Michela
Mapelli, Francesco Shankar, David M. Alexander, Alessandro Bressan, Luigi
Danese. The Black Hole Mass Function Across Cosmic Times. I. Stellar Black
Holes and Light Seed Distribution. The Astrophysical Journal, 2022; 924 (2):
56
DOI: 10.3847/1538-4357/ac34fb
Tags:
Space & Astrophysics