Ancient black holes hidden away in deep space have left behind nuclear clues about the first-ever stars, according to Professor Raffaella Schneider from the Department of Physics of Sapienza University of Rome, Italy, who leads a team of stellar archaeologists.
What exactly is a black hole and how can astronomers observe them?
‘A black hole is a natural product of stellar evolution – when a massive star exhausts its nuclear fuel it ends its life and collapses, reaching such a high density that not even the light is able to escape. We cannot directly see black holes, but we can see the enormous gravitational influence they have on their surroundings and the electromagnetic emission from the gas that feeds the black hole.’
You are the principal investigator of the FIRST project, funded by the European Research Council (ERC), which investigates the nature and properties of the first stars and galaxies – why is this important?
‘My project deals with the very first period of evolution of the structure of the universe – the first billion years of its life. Our aim is to have a better understanding of the first stars that formed, the nature of the first galaxies and the nature of the first black holes. Huge black holes are found in the nucleus of present-day galaxies but these black holes have considerably larger masses than we would expect from stellar evolution – and therefore they are fundamental actors in galaxy evolution. We want to understand why galaxies host these huge early black holes at their centre and what effect they have on the evolution of the galaxy.’
What conclusions do you hope to draw from your observations?
‘We want to provide the link between stellar mass black holes formed by a collapsing star and the nuclear black holes found at the centre of galaxies. We observed at early cosmic times that huge black holes with masses comparable to the masses we observe in the present-day universe are already in place in galaxies only a billion years after the Big Bang – when the universe had reached about 5 % of its present age. So clearly one of the aspects we were interested in trying to address was to provide a theoretical framework to understand the early growth of these super massive black holes and how this relates to the properties of the first stars and the evolution of the first galaxies.’
What are the challenges involved in travelling so far back in time to observe what was happening at the dawn of the universe?
‘From an observational point of view we had to use the best available telescopes. We use data coming from very deep observations from both the Hubble Space Telescope and from ground-based telescopes. The deeper you go the higher the chance you can observe these objects that are very distant from us.
‘From a theoretical point of view when we want to predict or interpret the observations done with these big telescopes we have to run simulations. These can help us to predict the properties of the first galaxies, the first black holes and the consequences these have on galaxies we observe now.’
Can understanding the evolution of the universe in its infancy tell us more about the universe we live in today?
‘Yes, absolutely. We’re trying to build up a coherent theoretical framework to explore the phase of the universe that is still inaccessible to direct observation, allowing us to predict the properties of the galaxies that we do observe, even if they are distant, and also helping to prepare ourselves for a time when more powerful telescopes will allow us to explore even more distant times.’
‘We want to understand why galaxies host these huge early black holes at their centre and what effect they have on the evolution of the galaxy.’
Prof. Raffaella Schneider, Sapienza University of Rome, Italy
What have been your most exciting findings as part of the project?
‘We have been looking for fossilised remnants, as it were, of these early phases of the evolution of the universe. When these first stars exploded they polluted the environment with the heavy metal elements that they have produced. This was the first time this has happened in the history of the universe. Our theoretical understanding is that during the Big Bang the conditions did not allow for the formation of atoms that are significantly heavier than helium. So carbon, oxygen, nitrogen and all the other heavy elements that we know are fundamental for life were synthesised in the interior of stars. The first generation of stars made this fundamental transition from a universe dominated by light atomic nuclei like hydrogen and helium to heavy atomic nuclei like carbon and oxygen. That polluted gas left clear fingerprints on the surface of the stars that we can hope to decode today.’
How has the FIRST project been involved in mapping out data from previous observations of the universe?
‘We have built a theoretical model to connect early star formation to present-day observations – this spans something like 13 billion years of cosmic evolution and you have to do it in a way that is as representative as possible of what happened to our galaxy when it formed.
‘Data that has already been collected through observations in the halo of our own galaxy have provided us with some samples of these very ancient chemically pristine stars that have the imprints of the products of the first supernovae (explosions) on their surface. It is very interesting to have a model which allows you to link these stars that we can observe today with what happened very early on.’
Have there been any other promising findings?
‘Another strand of our project is a discipline called stellar archaeology. By looking at the nuclear synthetic fingerprints on the remnants of stars we may be able to reconstruct the number of stars that formed in the first place in the universe across the different mass ranges. This is very important – we will probably never be able to observe these stars directly, but having an idea of the distribution of their masses means we can explore questions such as the number of black holes they leave behind.’
Have there been any particular highlights of the FIRST project?
‘One thing that was very interesting and has received a lot of attention was the fact that we have tried to give an explanation of the lack of observation of the progenitors (ancestors) of these huge black holes that we observe in the universe’s first billion years. This is something that has frustrated observers for years. We developed different models and what we have found is that these very faint, very small progenitors of super massive black holes grow in a very intermittent way – this is a possible explanation for the lack of observation as the chance of observing them when they are shining is very small. We are involved in an observational proposal with one of the best telescopes available today, the Chandra X-ray satellite, and we are getting close to finding the right combination to be able to catch these black holes while growing. It’s very recent and very exciting.’
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