Astronomers find hidden trove of massive black holes
The newly discovered massive black holes reside in dwarf galaxies, where their radiation competes with the light of abundant young stars. Credit: NASA & ESA/Hubble, artistic conception of black hole with jet by M. Polimera.

A team led by researchers at the University of North Carolina at Chapel Hill has found a previously overlooked treasure trove of massive black holes. The life story of the supermassive black hole at the center of our own galaxy is offered by the newly discovered black holes.

The merging of many smaller dwarf galaxies is believed to have built up the giant spiral galaxy, the Milky Way. The dwarf galaxies that are seen in the southern sky are merging into the Milky Way. Each dwarf that falls in may bring with it a central massive black hole, tens or hundreds of thousands of times the mass of our sun, potentially destined to be swallowed by the Milky Way.

A key gap in our understanding of how black holes and galaxies grow together is how often dwarf galaxies contain a massive black hole. New research published in the Astrophysical Journal shows that massive black holes are more common in dwarf galaxies than previously thought.

This result really blew my mind because these black holes were previously hiding in plain sight.

Sending mixed messages.

Black holes are usually detected when they are actively growing by eating gas and stardust, which makes them glow intensely.

Professor Kannappan compared black holes to fireflies.

When black holes are lit up, they give us a clue to how many we can see.

Young newborn stars can also light up with high-energy radiation, which is a problem. Astronomers use diagnostic tests to differentiate between black holes and new star formation, using detailed features of the visible light from each galaxy.

The path to discovery began when undergraduate students working with Kannappan tried to apply traditional tests to survey data. The team realized that there were two tests for black holes and one for star formation.

Kannappan said that previous work had rejected ambiguous cases like these from statistical analysis, but he had a hunch they were black holes. She suspected that the third test was more sensitive to the properties of dwarfs than the other two tests, because of their high rate of forming new stars.

Chris Richardson, an associate professor at Elon University, confirmed with his simulations that the mixed-message test results matched what theory would predict for a primordial-composition, highly star-forming dwarf galaxy containing a growing massive black hole.

There is a census of black holes.

The challenge of constructing a new census of growing black holes was taken on by Polimera. She obtained published measurements of visible light features to test for black holes in thousands of galaxies found in two surveys. The surveys include radio and ultraviolet data that is ideal for studying star formation, and they have an unusual design: Unlike most surveys, RESOLVE and ECO are complete inventories of huge volumes of the present-day universe in which dwarf galaxies are abundant.

It was important to me that we did not bias our black hole search towards dwarfs. I was taken aback by the numbers.

The majority of the black holes she found were new.

The result seemed too good, and she wondered if we had missed a way that extreme star formation could explain the galaxies. The team was forced to conclude that the newly identified black holes were real.

Kannappan said they were still pinching themselves. The black holes we have found are the basic building blocks of black holes. We want to learn a lot about them.

More information: Mugdha S. Polimera et al, RESOLVE and ECO: Finding Low-metallicity z ∼ 0 Dwarf AGN Candidates Using Optimized Emission-line Diagnostics, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac6595 Journal information: Astrophysical Journal Citation: Astronomers find hidden trove of massive black holes (2022, May 24) retrieved 24 May 2022 from This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.