AOP scientists lauded for ground-breaking computer model
Scientists at Armagh Observatory and Planetarium (AOP) have developed a ground-breaking new computer model which will play a key role in explaining how Supermassive Black Holes are formed as well as having the potential to shape the world of Astrophysics for the next decade.
Lead author, Professor Jorick Vink supported by AOP colleagues, Dr Erin Higgins, Dr. Andreas Sander, and Gautham Sabhahit built the theoretical model which will now be widely used across the scientific community and has already been receiving a lot of worldwide interest.
The model is expected to help advance the solving of the enduring puzzle of the existence of Supermassive Black Holes at Cosmic dawn.
The research by the Armagh scientists will now be published in the Monthly Notices of the Royal Astronomical Society (MNRAS), one of the world’s leading astronomical journals.
In September 2020, the world of astrophysics was shaken by the discovery of a merger between the heaviest stellar black holes ever observed. These were an astounding 66 and 85 times the mass of the sun, and as such heavy black holes were not expected to occur from the death-throes of the largest stars in our Milky Way. The discovery left astronomers baffled.
Led by Professor Vink, the young astronomers in the massive-star group at Armagh Observatory and Planetarium set about working on stellar models of blue supergiants. Remarkably within just a few days, Dr Erin Higgins and PhD student Gautham Sabhahit were, for the first time, able to produce an 85 solar mass black hole on their computers.
Professor Vink said: “This is without doubt the proudest moment of my 20-year career in astronomy. This computer model will have a profound impact on the world of Astrophysics over the next 10 years and I am confident it will help us unlock many more mysteries.
“I am extremely proud of my young colleagues. Erin graduated only last year and had already been making a name for herself in the world of astronomy while Gautham, who is in his second year as a PhD student, has announced himself on the international stage in a very exciting way.
“Andreas helped with our assessment that the mass lost from the outer envelopes due to winds was limited, allowing the large bulk of the envelope mass in stay intact – it was a complete team effort.
“This work is a great example of the world class research which is consistently taking place at Armagh Observatory and Planetarium.”
The research in more detail
At the end of their lives, the most massive stars in the universe are expected to collapse into black holes. These are objects in space that are so heavy that even light cannot escape them.
The detection of a black hole as heavy as 85 times the mass of the sun from the gravitational merger event dubbed GW 190521 in September 2020 seemed to present a fundamental problem as to how the heaviest black holes are formed.
Normally, when stars run out of nuclear fuel they collapse into black holes, but if the mass of the star exceeds 50 times the mass of the sun, the particle physics decides that such massive stars obliterate themselves, leaving nothing behind. So how was it possible that a star blatantly above this critical limit was detected?
The potential resolution put forward by the merger discoverers was that the heavy black hole was itself the merger of originally lower mass black holes.
Using a sophisticated stellar model MESA, astronomers at AOP were for the first time able to show that stars as large as 100 times the mass of the sun can produce heavy black holes of 85 solar masses if these are hosted in galaxies that are still chemically pristine.
The AOP astronomers were able to make blue supergiant stars with cores small enough to remain below the critical mass limit, yet due to its favourable chemistry lose an amount of mass in outflows small enough to produce an “impossible” black hole of 85 solar masses.
The two key points are an appropriate consideration of mixing inside the star, and the outflow physics from massive stars throughout cosmic time.
When the universe was first created in the big bang the chemistry was still pristine. Over time, new generations of stars produced chemical elements, such as the carbon in our bones and the iron in our blood, preventing the formation of the heaviest Black Holes in the hostile environment of our Milky Way.
With this improved understanding of outflows considering the amount of iron “rust” the new models are not only able to make an impossible black hole, but they also provide the maximum black hole mass through cosmic time.
The new stellar models will play a key role to explain how Supermassive Black Holes at the centres of galaxies already formed in the earliest days of our universe.
Up to now, it was believed Supermassive Black Holes are formed from mergers of lower mass black holes.
With the new scenario where a very massive star directly collapses into a really heavy black hole in a more pristine environment, it becomes more likely that the puzzle of the existence of Supermassive Black Holes at cosmic dawn may find a resolution in the next decade.