There’s something odd about the black holes we can detect around us, in our galaxy and beyond. It’s not the fact that they devour all matter in their vicinity, or even that despite being technically invisible, they can glow brightly as the gas around their edges heats up. Those things, while strange, fit with what scientists know and understand about these extreme objects. The odd thing about black holes is the sizes they come in: either small, or very large, with very few in between.
That’s puzzling because astronomers observe plenty of smaller black holes dotted throughout galaxies, called stellar mass black holes (because their masses are comparable to stars), and huge great black holes at the center of galaxies, called supermassive black holes, and it seems likely that these giant monsters must have evolved from their smaller brethren. So why don’t we see evidence of these in-between size black holes?
Instruments like the Hubble Space Telescope have found occasional evidence of an intermediate-mass black hole, showing that it is possible for these objects to exist — we just see them weirdly rarely.
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Now, though, using an instrument called the Dark Energy Spectroscopic Instrument or DESI, scientists have discovered a trove of these intermediate black hole candidates. They found 300 promising objects, which is by far the largest group identified to date, and researchers hope to investigate these objects further.
“The technological design of DESI was important for this project, particularly its small fiber size, which allowed us to better zoom in on the center of galaxies and identify the subtle signatures of active black holes,” explained researcher Stephanie Juneau of NSF NOIRLab. “With other fiber spectrographs with larger fibers, more starlight from the galaxy’s outskirts comes in and dilutes the signals we’re searching for. This explains why we managed to find a higher fraction of active black holes in this work relative to previous efforts.”
Another topic the researchers were looking at was the presence of black holes in small galaxies called dwarf galaxies. As the size of the black hole at the center of a galaxy is often related to the size of that galaxy, you might expect that these dwarf galaxies would have smaller black holes — but that wasn’t necessarily the case. The researchers found 2,500 dwarf galaxies with bright, active central regions that indicate an active black hole.
However, only 70 objects appeared on both lists, suggesting that the intermediate black holes can be found in larger galaxies too.
This leaves plenty of questions about how black holes and the galaxies they live in evolve together. “For example, is there any relationship between the mechanisms of black hole formation and the types of galaxies they inhabit?” said lead researcher Ragadeepika Pucha of the University of Utah. “Our wealth of new candidates will help us delve deeper into these mysteries, enriching our understanding of black holes and their pivotal role in galaxy evolution.”