How are supermassive black holes born? That is the question astronomer Elena Maria Rossi is trying to answer. But how do you investigate something you cannot see?
Supermassive black holes are the largest black holes in the universe and can be found at the center of galaxies including our own Milky Way. They are at least a million times more massive than the Sun. 'I want know where and how they are born,' says Elena Rossi, Professor of Theoretical Astrophysics. 'That is one of the big open questions in astrophysics. Understanding how supermassive black holes form and grow, will help us understand how galaxies, and ultimately ourselves, are formed.'
Elena Rossi held her inaugural lecture 'The academic I meant to be' on 23 May, in which she looked back on her career and looked ahead to future research.
Black holes are completely dark, and nothing, not even light, can escape from them. As a result, they cannot be seen . But astronomers can observe them because of the influence black holes have on their environment. Rossi's research group uses three different methods to study supermassive black holes. Each offers a unique perspective on these invisible giants.
Method 1: Like Marie Curie
Instead of looking directly at the centre of galaxies - a region that is difficult to study because of dust and stars - Rossi looks at stars ejected from galaxies. In big data from, for example, the Gaia satellite, she looks for these so-called hypervelocity stars. By analysing their properties, such as age and chemical composition, Rossi gains more information about how black holes grow. 'It's just like what Marie Curie did when she wanted to understand the atom. She didn't look at the atom, but at particles that were emitted. We do the same with stars.'
Method 2: Flares
Sometimes a star is pulled too close to a black hole and is torn apart. This releases an enormous amount of light: a flare. This event betrays the presence of a black hole. Rossi's team is developing a statistical method to extract information about the black hole from these flashes of light.
'This will allow us to make a map of black holes in the universe, with their masses and properties,' Rossi explains. This map will help to test theories about the formation of supermassive holes. There are two competing scenarios: black holes are born small from the first generation of stars, or they are born already large, simultaneously with galaxies.
Method 3: Gravitational waves
For centuries, astronomers universe used light and other forms of electromagnetic radiation to study the universe . Gravitational waves have offered a new way of looking at the universe, since they were detected by LIGO (Laser Interferometer Gravitational-Wave Observatory) in 2015. These ripples in space-time that Albert Einstein predicted existed 100 years ago can be caused by a massive pair of objects in accelerated motion around each other, such as two merging black holes. Gravitational waves can be compared to a stone thrown into water: where the stone touches the water, a ripple is created, which continues to expand.
Rossi is investigating whether some of these signals come from the centres of galaxies where supermassive black holes also occur. 'If we can demonstrate that, we will learn more about the environment of black holes: how much gas there is, how many other black holes there are and how they interact with each other.' To do this, she has developed a statistical method.
A new ESA space mission
She is also working with many other scientists on the LISA satellite (launch planned for 2035), which will be able to measure gravitational waves in space. 'I am now looking at signals from about 10 billion years after the Big Bang. With LISA, we will be able to look further back in time, to just after the Big Bang. I'm like an archaeologist. LISA will allow us to look right into the past and say how black holes are born. It's fantastic to be part of this.'
Image: An illustration of a supermassive black hole.
