Apparently dwarf galaxies have been hiding supermassive black holes or at least massive blackholes. When you’re a dwarf galaxy, you can only hide so much.

In a new paper appearing in The Astrophysical Journal, researchers led by the student-professor pair Magdha Polimera and Sheila Kannappan have uncovered evidence of massive black holes in star-forming dwarf galaxies. These 1,000-100,000 solar mass black holes are being caught in the act of consuming gas and dust, and the light released from material on its way toward their event horizons is largely lost to the light of surrounding star formation, but with careful study, this team was able to tease apart the specific colors of light from star formation and from stardust consumption.

Researchers have been studying dwarf galaxies near our Milky Way for decades, searching high and low for evidence of these middling size black holes, but so far definitive evidence just hasn’t been found. This could, in part, be due to the old age of most of these systems: older dwarf galaxies don’t have a lot of material to feed black holes. By looking at more active dwarf systems, they had a lot more interference to deal with but also had that chance to catch activity. As explained by Kannappan: Just like fireflies, we see black holes only when they’re lit up — when they’re growing — and the lit-up ones give us a clue to how many we can’t see.

Star formation is just a phase galaxies grow through. If – as this team did – you look at a bunch of star-forming dwarf galaxies and find the majority show signs of having massive black holes, you know that the majority of dwarf galaxies, in general, should have massive black holes.

These results are super pleasing. Researchers had been struggling to understand how merging dwarf galaxies could build supermassive black holes as they combined into larger galaxies. If they didn’t start out with larger black holes that could merge together, some mechanism for building a supermassive black hole from scratch was needed. Now, that problem was solved.

More Information

UNC press release

“RESOLVE and ECO: Finding Low-metallicity z ∼ 0 Dwarf AGN Candidates Using Optimized Emission-line Diagnostics,” Mugdha S. Polimera et al., 2022 May 24, The Astrophysical Journal