Quasars are extremely luminous galaxies and their light is produced by a supermassive black hole at their center, which is actively feeding on the cosmic gas and dust around it. Previous theories have suggested that quasars and their supermassive black holes are formed when galaxies collide, but a recent doctoral thesis at the University of Turku shows that quasars can form even in the absence of galaxy interactions.
Until now it has been assumed that quasars and their central supermassive black holes are formed in violent intergalactic collisions. Recent observations of quasar companion galaxies within a few gigayears of light travel time from the Earth have revealed a picture that contradicts these merger predictions from previous numerical simulations.
These new observations revealed that it is not necessary for a quasar to interact with another massive galaxy in order to have an active supermassive black hole.
What triggers the supermassive black hole?
The Earth is located in the Milky Way Galaxy, which looks like a giant spiral. About a hundred years ago, observations by a famous astronomer Edwin Hubble provided evidence that there are many other galaxies in the Universe - some look like the Milky Way, but others appear to be elliptical, dwarf, or irregular. These galaxies shine brightly in the wavelengths perceptible to the human eye due to the light from their constituent stars.
A minor fraction of galaxies emit exceptionally bright light that does not come from stars but is produced in a compact area in their centers. This light can outshine the starlight of a galaxy. Such galaxies are called active galaxies or quasars. They are thought to be powered by accretion onto central black holes of colossal size - more than one hundred million times the mass of the Sun.
It is still unknown what role quasars play in the cosmic evolution of galaxies and how the supermassive black hole activity is triggered, but it is assumed that their creation requires collision between two galaxies.
– I was always curious how the environment around a galaxy affects its evolution. Previously, cosmic simulations of two massive spiral galaxies showed that it was theoretically possible to activate quasar activity due to the violent merger processes. But it is important to check models with real observations, Stone says.
However, observing quasars is challenging. Instead, observing galaxies around quasars may offer clues about quasars themselves. To explore the relationship of galaxies with their surrounding cosmic environment, Stone made hands-on observations with remote telescopes and analyzed data from large survey archives.
The key finding is that within several billion light years distance, the giant black holes in active galaxies are triggered due to processes inside their host galaxy. They did not need external influence, such as interaction or merging with a neighboring galaxy.
– The star formation, and other properties of galaxies close to active galaxies are similar to galaxies around normal, inactive galaxies. So it does not matter in the life of a galaxy if its close neighbor is active or not. These results challenge our understanding of quasar triggering, and they are contrary to the existing pictures of simulated clashes between two model galaxies, Stone explains.
They challenge our imagination on how quasars influence the evolution of galaxies around them. New models need to be developed where processes inside a galaxy provide a way to turn on a supermassive black hole and to feed it. For example, some already have shown simulations of instabilities in spiral arms which lead for the gas to be directed to the center of a galaxy to turn it into a quasar, without any influence from external neighbors.
– Even though we imagined the quasars to behave one way, through mergers, which was very exciting, we found out that this is not the full story. It is just like with the Earth-centric theory, a paradigm which was later corrected by a system where planets instead circle around the Sun. The true picture of how the Universe works may be different than what we have imagined so far, but it is definitely immensely more exciting! I look forward to discovering more about quasars in the future, Stone says.
Hands-on observations at the telescopes around the world revealed clues about the quasars
Stone used an observational technique called spectroscopy to gather information of galactic starlight for those galaxies that are neighbors to quasars.
Telescopes, such as the Nordic Optical Telescope, have instruments which are able to collect the starlight of quasar neighbors and split it into many wavelength intervals, similar to how the light splits into a colorful rainbow as it passes through water droplets. The resultant galaxy spectrum is then studied in detail to check how much energy is output at each wavelength. From this information, important parameters are estimated, such as distance to the galaxy, its shape, age, how many stars it is producing, its life history, chemical composition and abundances, and much more.
– I performed hands-on observations to collect part of the data at the Nordic Optical Telescope in the Canary Islands and at the New Technology Telescope in Chile. Both remote observatories have spectacular nature and the starlit night sky is stunning. It is very special to think how the photons, these messengers of light, traveled for billions of years before finding their demise on the collecting device of a telescope. As an observer, you learn to read the language of nature by carefully observing it.
This research was done by observing galaxies with several telescopes: the Nordic Optical Telescope (Canary Islands), the Gran Tecan (Canary Islands), and the New Technology Telescope (Chile). Also, data from large survey archives were used, such as the Sloan Digital Sky Survey and the Galaxy and Mass Assembly spectroscopic survey.
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MSc Maria Babakhanyan Stone defends the dissertation in Astronomy titled “Galaxy evolution through the lens of active galactic nuclei, their host galaxies, and environments: an observational study” at the University of Turku on 11 June 2024 at 12.00 (University of Turku, Quantum, Auditorium, Vesilinnantie 5, Turku).
Opponent: Dr. Anna Wolter (National Institute for Astrophysics, Brera Astronomical Observatory, Italy)
Custos: Professor Emeritus Mauri Valtonen (University of Turku)
Doctoral Dissertation at UTUPub
The audience can participate in the defence by remote access
