Bengaluru: To spot a black hole is tough and to find them in smaller galaxies is even more difficult. But that did not stop Dr Ragadeepika Pucha, a postdoctoral researcher of Indian origin at the University of Utah. Pucha’s team sifted through early data provided by the Dark Energy Spectroscopic Instrument (DESI), an astronomical instrument in Arizona that measures the effect of dark energy on the expansion of the universe.
The team identified 2,500 potentially active black holes in dwarf galaxies, and detected the presence of 300 new intermediate mass black hole candidates. Both findings are the largest and most extensive collection to date.
It triples the existing census of the high energy galactic centres and doubles the number of intermediate black holes, according to a press release by NOIRLab, the US national centre for ground-based, night-time optical and infrared astronomy.
But it is the discovery of intermediate mass black hole candidates that has the team buzzing with excitement. While scientists have confirmed the existence of supermassive black holes (imagine an adult), they could not figure out how they were formed. They theorised that these black holes should have a ‘childhood’ or an intermediate stage.
The theory further assumed that these intermediate mass black holes would be found in the centre of dwarf galaxies rather than massive ones. But the team found that only seventy of these candidates of intermediate mass black holes are actually in dwarf galaxies. Most of them are actually in massive galaxies, Pucha, who traces her roots to Andhra Pradesh, told ThePrint. The findings are a dual achievement in the study of black holes and galaxies. This will help in understanding how black holes are formed.
To make sense of the universe, explorers first have to understand how dwarf galaxies performed and evolved. “That is my area of interest. I want to understand how dwarf galaxies formed and evolved,” she said. Pucha spent the last four years surfing through the extensive data by logging into a supercomputer to get to the present result.
“This is the first time we have a statistical sample of candidates which will help us actually investigate what is happening between galaxies and central black holes, which we did not have until now,” she added.
Pucha’s discovery is being celebrated in Andhra Pradesh as well. “A daughter of Tenali, Guntur, she joins the ranks of illustrious Telugu women who have made a mark on the global stage,” Chief Minister Chandrababu Naidu congratulated her on X.
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Previous estimates showed that only 0.5 percent of galaxies were found to host an active galactic nucleus (AGN)—a region at the centre of a galaxy from which a large amount of energy is pumped.
Now, Pucha and her team have found that two percent of the dwarf galaxies studied host AGN, a four-fold increase from earlier studies. So it looks like scientists might have been missing out on a large number of black holes, that too of low mass and undiscovered.
Their presence can be the first step in the search for the existence of a black hole at the centre of a dwarf galaxy. In addition, the team also identified 300 intermediate mass black hole candidates.
Most black holes are either lightweight or supermassive. Lightweight black holes are less than 100 times the mass of the Sun, and supermassive black holes are more than one million solar masses. There is a difference of four orders of magnitude between them, and intermediate ones have masses in between the first two.
“In science or in nature, you expect a continuous distribution. But this is like a patchy distribution. What is happening in between them? That is the question,” explains Pucha.
With the large population discovered by DESI, scientists now have a new dataset to use to study these cosmic enigmas, according to the press release by NOIRLab. The research, titled Tripling the Census of Dwarf AGN Candidates Using DESI Early Data, is presently in the arXiv, and has been accepted for publication in The Astrophysical Journal.
Finding black holes
A black hole does not give anything out of it due to its strong gravitational pull. This makes it a challenge to find black holes in the vast universe. But scientists have a way to enjoy the “now I see you” moment. This is through observing what happens around a black hole and then confirming the presence of it.
Scientists look at the disturbances around. Then, they assume that if there is a disturbance, there should be a cause for it. So the quicker way to understand the cause is to root through the disturbances. Crudely put, if one hears the hiss among a grass, it is quite obvious to sense the presence of a snake. Likewise, the hiss in the grass can be related to disturbances in the vast universe.
In previous efforts, astronomers set their foot into 500 images from 20 years of NASA’s Hubble Space Telescope observations. They found the presence of black holes by observing the pull on stars in the Omega Centauri globular star cluster, similar to how a hiss in the grass points to the possibility of the presence of a snake.
Later, scientists calculated the mass that pulled the stars to the centre of this globular star cluster. It was 8,200 times that of our Sun—evidence of an intermediate mass black hole.
In the recent study by Pucha, the detected black hole candidates are actively feeding themselves. These centres give constant long burps into surroundings and hence, are easier to spot. The long burps are what scientists identify as a constant release of energy.
“This dramatic activity serves as a beacon, allowing us to identify hidden black holes in these small galaxies,” said Pucha in the press release by NOIRLab.
Just like how the hiss in the grass points to the possibility of a snake, the high energy long burps at the centre of galaxies suggest a possibility of a black hole. “You have to keep finding more assurances that this is a black hole. But now you can still consider them as good candidates for investigation into the details of physics that is going on,” said Pucha.
The search was made using the state-of-the-art instrument DESI, which can capture light from 5,000 galaxies at the same time. Through further search, Pucha and her team expect to capture more clues regarding black holes and dwarf galaxies.
“People expect dramatic discoveries but that is not always the case. You could go one step at a time and get results. They might not be dramatic, but we are still expanding our scientific knowledge as a human community,” said Pucha.
This is an updated version of the report
(Edited by Radifah Kabir)
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