Monstrous clumps of matter lurk at the centre of almost every galaxy, sucking in anything that comes their way. Holding the centre of galaxies together with their immense gravitational pull, so strong that not even light can escape, it is no wonder black holes are a constant source of inspiration for works of science fiction. In real life, black holes are just as exciting, however, and Dr Julie Hlavacek-Larrondo, an astrophysicist at the Universté de Montréal, has dedicated her career to studying their mysteries.
We have never known more about the Universe than we do today, yet there remain a number of mysteries in astrophysics that we still cannot explain. Supermassive black holes, which range from the mass of a hundred thousand of our Suns to over a billion, potentially hold the answer to some of these mysteries. This is what makes them such an important phenomenon to study, says Dr Hlavacek-Larrondo. “The goal of my work is to determine the role supermassive black holes play in the formation and evolution of galaxies,” she says. “In other words, to understand galaxies – the building blocks of our Universe – we must first understand black holes.”
Supermassive black holes
The most massive supermassive black holes can be found in the middle of galactic clusters, giant clusters of galaxies that stick together throughout the Universe. These huge black holes drag in anything that comes their way, and in the process, they also emit powerful jets of radiation and particles. Sometimes these jets can extend to reach beyond the edges of the galaxies at the centre of which the black holes lie. By studying these jets in as many different galaxies as possible, astronomers can learn more about the black holes behind them and how they shape the galaxies.
Dr Hlavacek-Larrondo studies these jets directly using telescopes on and in orbit around the world. These include space telescopes like the Chandra X-ray Observatory and the Hubble Space Telescope alongside ground-based telescopes like the Jansky Very Large Array, a group of telescopes working together to act as one huge telescope in central Mexico.
Part of Dr Hlavacek-Larrondo’s work involves looking at what is called black hole feedback. This is the idea that black holes play an important part in determining how a galaxy ends up looking. For example, over two decades ago, it was discovered that the mass of the central black hole is related to the number of stars in a galactic bulge, the central part of a galaxy, yet the reasons for this link are still not fully understood.
In a paper in 2012, Dr Hlavacek-Larrondo and a team of astronomers looked at the jets coming from supermassive black holes in galaxies located at the centres of galaxy clusters, more distant than ever before. The paper found the black holes in these galaxy clusters formed relatively early in the history of the Universe and have been the same for billions of years. “My major contributions have been to study black hole feedback in the most distant clusters of galaxies, demonstrating that such feedback has been important for over half of the age of the Universe,” says Dr Hlavacek-Larrondo. “I have also provided evidence that some black holes may be significantly more massive than previously thought – the titans among giants.”
In papers in 2012 and more recently 2017, Dr Hlavacek-Larrondo looked into the properties of these titans, the biggest black holes, including how bright the radio waves and X-rays are that they give off. The paper found that the established relationship between black hole mass and galaxy size does not hold for extremely large galaxies.
Capturing better quality images of galaxy clusters than have been seen before is another key aspect of Dr Hlavacek-Larrondo’s work. For example, in 2017 she was author of a paper that presented clearer images of the Perseus galaxy cluster (see top left image), more than 250 million light-years from Earth, which revealed new features astronomers had not seen before. At the centre of the cluster, there is a pool of extremely fast particles that emit radio waves, a structure known as a mini-halo.
The halo was the largest ever seen, more than 1.3 million light-years in diameter. At these distances away from the centre, particles are expected to stop producing radio waves, so the observations were a bit of a mystery. The new images showed the jets from the black hole at the centre of the cluster were helping to keep the particles producing radio waves, by giving them a kick of energy. This helped to solve the mystery as to why the halo was so large and provided more insight into the way black holes can affect the structure of the clusters of galaxies they inhabit.
Women in science
While studying black holes is her main passion, Dr Hlavacek-Larrondo also feels strongly about increasing diversity in science. She is half Chilean and half Tcheque, was born in Calgary, Canada but raised in Montreal. As a young girl, Dr Hlavacek-Larrondo was never exposed to the false belief that science was not for girls; her mother was a scientist herself. “Having been raised by two extraordinary women, my mother and grandmother, I have always believed that women and men can contribute equally to science,” says Dr Hlavacek-Larrondo. “I think this is a key reason that led me to science.”
Growing up, I had not realised the importance that mentors can have in your life and in the decisions you take. Now, looking back, I realise that I probably would not have been able to accomplish so much without their constant support. My mother and grandmother – the most important mentors in my life – taught me two very important things that I continue to apply every day and that no doubt had a very important influence in my career: always work very hard, and especially, don’t be afraid to seize opportunities.
Was there a moment when you realised black holes would be a fascination throughout your career?
Yes and no. The extreme Universe has always fascinated me: extreme gravity around neutron stars and black holes, the mysterious (and still unknown) nature of dark matter, etc. So yes, in a way, I always knew I wanted to study objects like black holes. However, it was during my undergraduate studies in physics that I started to realise just how fascinating black holes were. In particular, I remember reading a press release about a black hole so powerful that its jet was not only able to pierce through its host galaxy, but that it was able to reach and plough through a neighbouring galaxy.
To what extent do the different categorisations of galaxies (elliptical, irregular, spiral-armed, barred etc.) depend on the presence and nature of their central black hole?
Actually, not that much. We are pretty certain that every elliptical galaxy hosts a supermassive black hole at its centre, and that this black hole plays a fundamental role in shaping its properties. However, spiral-arms, bars, etc are thought to be caused by interactions of the galaxy with, for example, a small neighbouring galaxy. This causes instabilities that eventually lead to the creation of spiral-arms, bars, irregular morphologies, so not related to the black hole.
Do you think there’s any hope of ever understanding what lies beyond the event horizon of a black hole?
I sure hope so. For now, our understanding of physics is not good enough to describe what goes on inside the event horizon, but maybe one day it will be good enough. And hopefully, it will be a woman who makes this discovery hehe!
Dr Hlavacek-Larrondo’s work aims to understand the role that supermassive black holes play in the formation and evolution of galaxies.
Julie Hlavacek-Larrondo is funded through the Natural Sciences and Engineering Research Council (NSERC) via the Canada Research Chair programme and the Discovery Grant programme. She is also funded through the Fonds de recherche nature et technologies through the team grant programme.
Julie Hlavacek-Larrondo pursued a bachelor’s degree in physics at Université de Montréal (2004 – 2007) and a master’s degree in astrophysics at the same institution (2007 – 2009). She was then awarded a full scholarship at the University of Cambridge to pursue a PhD in astronomy (King’s College; 2009 – 2012). Afterwards, she was offered several prestigious postdoctoral fellowships, including the Einstein fellowship from NASA which she took to Stanford University (2012 – 2014). Currently, she is an assistant professor in physics at the Université de Montréal, and holds a Canada Research Chair in Observational Astrophysics of Black Holes (only eight astronomy Chairs have been allocated across the country). She is married to a mathematics high-school teacher and has one son.
Dr Julie Hlavacek-Larrondo
Assistant Professor, Canada Research Chair
Département de physique
Université de Montréal
PO Box 6128 Centre-Ville
Montreal QC H3C 3J7