Letters of Intent received in 2021

LoI 2023-2150
Black holes: From the first stars to Supermassive black holes in the first quasars

Topics

* Formation and initial mass function of the first stars and first black holes

* Single and binary first stars, their remnants and their environments

* Formation of very massive and supermassive primordial stars and black holes

* Tracking black holes from first stars to the first galaxies

* Models for the origins of the first quasars from the first galaxies

* Black holes as probes of the first billion years

Rationale

The evolution of the early Universe from the epoch of cosmic recombination to the phase of reionization of the intergalactic hydrogen remains elusive.  Understanding this transition is of utmost importance and is a topic at the forefront of current astronomical research.  Beyond understanding the origins of the elements heavier than helium, deposited in the interstellar medium by exploding massive stars, a compelling question is understanding when, where, and how the supermassive black holes (SMBHs) powering the first quasars at redshift as high a z~7  formed and grew to giant sizes in a billion years or less, and how and when the bulk of the underlying population of SMBHs formed  and evolved until present, hand in hand with the formation and assembly of galaxies. The observation of the X-ray background combined with the local census of SMBHs, ubiquitous in the bright galaxies, indicates that SMBHs grew via efficient accretion from “black hole seeds” on yet unknown origins, formed between  the recombination and reionization epochs. These black holes might be the relic of the first stars, for which we do not know their initial mass function and hence the initial mass function of the first black holes. Alternatively they may have formed as a result of  runaway stellar collisions in dense star clusters leading to the formation of an intermediate mass black hole, or in the direct collapse of massive pristine gas clouds embedded in strong radiation stellar fields.  But seeds may have origins from primordial black holes formed in phase transitions during the radiation dominated era.  A wide spectrum of black holes is likely to exist in between the stellar and the SMBHs, and  recent results (e.g., from lensing) suggest the presence of roaming intermediate-mass black holes in dwarf galaxies.

Black holes themselves emit no electromagnetic radiation unless when they accrete.  But, since 2015 we have a new window to observe black holes directly: this occurs when they merge emitting gravitational waves.  Although still in its infancy, the detection of gravitational waves allows us, in principle, to probe large cosmic volumes as the gravitational wave signals are unaffected by extinction due to intervening matter due to their weak coupling.  Already-planned future space missions, such as LISA, and ground-based detectors such as Einstein Telescope and Cosmic Explorer, will allow us to detect signals  from coalescing black holes of tens to millions of solar masses, at high redshift, in order to discover how the black hole formed and in turn how the first stars formed jointly with the first galaxies.
 
Detailed studies to “weigh” the black hole in the centre of the Milky Way were recognised by the recent Nobel Prize in Physics to Ghez and and Genzel.  The first direct imaging of a supermassive black hole by the Event Horizon Telescope was a remarkable success of global coordination, engineering, and science, forever changing the picture of a black hole in the public beyond the movie “Interstellar.”  These observations, jointly with those from LIGO/Virgo  hint for the universality of black holes, over a huge mass spectrum.  But when and how SMBHs as we see them today did form remains a mystery.  Key questions are:  Which is the parent population of the SMBHs?  Is there a link between the stellar and giant black holes?  Is there a unique seeding process or many, depending on the environment and cosmic epoch?  How can we recover information on the masses and spins of the early seed black holes erased by accretion?

The era between the formation of the first black holes and the time of the first quasars coincides with the epoch between recombination, which left us the cosmic microwave background, and the time of re-ionisation.  Whereas gravitational waves from that time can reach us without absorption, it is the electromagnetic radiation released that leads to reionization of the Universe as we see it today.  Black hole growth in galaxies and galaxy assembly may have played a key role in providing the radiation that fueled this transition.  Experiments such as the Murchison Widefield Array (MWA) and future Square Kilometre Array (SKA) are setting out to uncover this transition, contributing their own unique avenue toward understanding the first black holes and their early lives.  The next generation of extremely large telescopes (ELT),  as well as space-borne telescopes (JWST) will provide us unprecedented data toward faint and remote objects.
 
Now is the time for such a meeting, in the wake of the recent breakthroughs, the current wave of new observations, and upcoming flood of new opportunities.  Australia is a good location for such a meeting, with both major trans-institutional and multinational Australian-based centres (Astro3D and OzGrav) supporting this meeting and its science.  Australia is one of the locations of SKA, and a place from which one could, in principle, actually see the Galactic centre.  As part of the Symposium program and organisation we will aim to engage Australian Aboriginal Astronomy and include outreach to schools and teachers. The meeting would be held in the local summer (February) before the start of Semester 1 at Australian Universities.