The discovery of the oldest supermassive black hole

Abstract

<p indent="0mm">On December 20, 2021, the website of <italic>Science News</italic> summarized “6 surprising records science set in 2021”, including the oldest black hole discovered by astronomers. These new findings extend the limits of scientific research and the imagination of human beings. Here we present a brief introduction to the discovery of the oldest black hole in the early Universe and its importance in science, and give some prospects of the related studies in the future. Quasars are luminous active galactic nuclei with supermassive black holes (SMBHs) in their centers, accreting the surrounding gas and emitting huge power. Distant quasars are unique tracers to study the formation of the earliest SMBHs and the history of cosmic reionization. On January 14, 2021, in a paper appeared in the <italic>Astrophysical Journal Letters</italic>, an international team led by Dr. Feige Wang, Dr. Jinyi Yang and Prof. Xiaohui Fan at University of Arizona reported the discovery of a luminous quasar J0313-1806 at redshift 7.64. This discovery breaks the former redshift record, 7.54 for the quasar J1342+0928, making J0313-1806 the most distant quasar discovered so far, with a distance of about 13 billion light years from us. The near-infrared spectra taken with several largest ground based telescopes reveal the existence of a black hole of 1.6 billion solar masses in the center of J0313-1806. It becomes the oldest black hole in the early Universe, with the age of only 670 million years after the Big Bang. How can a black hole grow to 1.6 billion solar masses within 670 million years (only about 5% of the universe’s current age)? The discovery of the oldest black hole provides the most serious challenge to the current understanding of the black hole formation and growth. SMBHs are thought to grow from smaller seed black holes that accrete the surrounding matter. The popular idea is that the seed black holes formed through the collapse of the first generation massive stars (or population III stars), but this can only make the seed black holes up to a few hundred solar masses in most cases, or up to a few thousand solar masses for the extreme cases in the dense stellar clusters. However, even if we assume that J0313-1806’s seed black hole grew as fast as possible, it would have needed a starting mass of at least 10000 solar masses at redshift 30. Among the theoretical models currently known, only the direct collapse black hole (DCBH) model involving the vast amounts of primordial cold hydrogen gas can provide a possible explanation. In addition, J0313-1806 also shows many important features. The rest-frame UV spectrum exhibits broad absorption troughs, which are thought to be produced by the extremely high-velocity outflows launched from the accretion disk around the central black hole. These outflows have a maximum velocity up to 20% of the speed of light, and can provide strong feedbacks to affect the star formation in the quasar host galaxy due to the extremely high kinetic power. J0313-1806’s host galaxy has a very rapid star formation process, with the star formation rate 200 times higher than the Milky Way. Together with its huge luminosity of 36 trillion times higher than the Sun, J0313-1806 has become one of the most important targets to study the formation and growth of the supermassive black holes, as well as their influences on the host galaxies in the early Universe. There will be more observations on the oldest black hole in the near future. The James Webb Space Telescope (JWST) was successfully launched on December 25, 2021. The team who discovered the oldest black hole has been awarded the JWST time of more than two hundred hours. More surprising discoveries are expected to be revealed by the future JWST observations on the most distant black holes, galaxies and quasars in the epoch of cosmic reionization.