Supermassive black holes millions or billions of times the mass of the Sun are found at the center of large galaxies like the Milky Way. But astronomers don’t know how they got there.
They know, however, that galaxies grow by merging. One theory for the formation of these massive black holes is that the early universe was filled with dwarf galaxies with smaller so-called intermediate-mass (IMBH; hundreds to thousands of solar masses) black holes at their centers. Over time, these dwarfs merged or were swallowed up by larger galaxies, their cores combining each time to make a monster in the middle of the final galaxy.
But until recently, only a tiny fraction of dwarf galaxies were known to host massive black holes at their centers. Indeed, classic black hole-hunting techniques, which search for the bright accretion disks around actively feeding black holes (called active galactic nuclei or AGNs), were developed with larger galaxies and their black holes outside. mind, and are not suitable for finding black holes in dwarf galaxies. But now, a recent study published in The Astrophysical Journal and led by scientists at the University of North Carolina at Chapel Hill has discovered a previously “hidden” population of black holes using an improved classification system.
A new point of view
Searching for black holes using traditional techniques in 2014, Sheila Kannappan, along with undergraduate students Ashley Bittner and Carlynn Ferguson, identified a particular type of dwarf galaxy for which three traditional diagnostic tests for finding black holes give different answers. Two of the tests indicate that there is AGN, while the third indicates that only star formation is present.
“Dwarf galaxies are more primordial in their composition and have tons of star formation going on,” says Kannappan. “And the emission, or glow, from star formation can rival the emission from AGN, making it hard to tell if an AGN is really there. That’s why tests designed for galaxies giants don’t work as well on them,” she explains. “The particular test that doesn’t recognize the presence of an AGN is sensitive to the abundance of the higher elements we call ‘metals’, which the dwarves don’t don’t have that many.
Thus, Kannappan’s group developed a test that took these characteristics into account. Instead of throwing out galaxies with conflicting test results, they identified them as a new category. Next, team member Chris Richardson of Elon University followed up with computer simulations, which showed agreement between galaxies in the new category and theoretical predictions for the exit of a model dwarf galaxy with an IMBH .
Graduate student Mugdha Polimera – lead author of the study – applied the new screening technique to published spectroscopic measurements for galaxies in two surveys: RESOLVE and ECO. What she found surprised everyone.
More than 80% of active black holes detected in dwarf galaxies turned out to be part of this previously hidden population, meaning they would otherwise have been missed.
“We ran through a list of all possible explanations,” Polimera says, to make sure they were really picking up signals from growing black holes. “Could this be extreme star formation? Could it be another exotic astrophysics, some kind of diffuse gas? In the end, we concluded that what we were seeing most likely came from black holes. »
This study represents almost a decade of work for Kannappan and his group. If their technique proves reliable, it could provide the first concrete evidence that supermassive black holes are built from mergers of dwarf galaxies with IMBHs.
Romeel Davé of the University of Edinburgh, who was not part of the study, is also cautiously optimistic about this new method and the “treasure” of black holes it has discovered. “Theoreticians, like me, are very good at coming up with clever ideas,” says Davé. “But to know if something is right, well, it requires real data. This study provides the first real constraints on how many black holes in dwarf galaxies might exist. »