Supermassive black holes millions to billions of times the mass of the Sun are found at the centers of large galaxies like the Milky Way. But astronomers don’t know how they got there.

However, they do know that galaxies grow by merging. One theory for how these massive black holes form is that the early universe was riddled with dwarf galaxies with smaller so-called intermediate-mass black holes (IMBHs; hundreds to thousands of solar masses) at their centers. Over time, these dwarfs merged or were swallowed by larger galaxies, their cores combining each time to build a monster in the midst of the final galaxy.

But until recently, only a small fraction of dwarf galaxies were known to harbor massive black holes at their centers. This is because classical black hole-hunting techniques, which search for the bright accretion disks around actively feeding black holes (called active galactic nuclei, or AGN), were developed with the largest galaxies and their relatives in mind. black holes, 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 uncovered a previously “hidden” population of black holes with the help of an improved classification scheme.

A new perspective
While searching for black holes using traditional techniques in 2014, Sheila Kannappan, along with undergraduates 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 say that there is an AGN, while the third says that there is only star formation.

“Dwarf galaxies are more primordial in composition and have tons of star formation going on,” says Kannappan. “And the emission, or brightness, from star formation can compete with the emission from the AGN, making it hard to tell if there really is an AGN. This is why tests designed for giant galaxies don’t work as well in them,” he 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’, that the dwarves don’t have that much.

So Kannappan’s group developed a test that took these features into account. Instead of ruling out galaxies with conflicting test results, they identified them as a new category. Team member Chris Richardson of Elon University then followed up with computer simulations, which showed an agreement between the galaxies in the new category and the theoretical predictions for the output of a dwarf galaxy model 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 studies: RESOLVE and ECO. What he found surprised everyone.