They are known as Active Galactic Nuclei (aka. quasars), the core regions of galaxies that are so bright that they temporarily outshine all the stars in the galactic disk combined. This is the result of the Supermassive Black Holes (SMBHs) at their centers, which accelerate infalling gas and dust in their accretion disks to near the speed of light. This produces intense radiation across the electromagnetic spectrum, from visible light and infrared to microwaves and X-rays. For decades, astronomers have known that SMBHs reside at the centers of many massive galaxies, and the same was thought to be true of dwarf galaxies.
However, scientists have since learned that many do not have black holes at their center, highlighting the need for a proper census of dwarf galaxies. At a recent press conference at the 247th meeting of the American Astronomical Society in Phoenix, Arizona, astronomers from the Harvard & Smithsonian Center for Astrophysics (CfA) and the University of North Carolina at Chapel Hill presented the most comprehensive census of active galactic nuclei (AGN) to date. Their results showed that AGNs are more common in dwarf galaxies than many previous surveys suggested.
The team's census contained over 8,000 nearby galaxies, which they reassessed for signs of black hole activity. This consisted of grouping these nearby galaxies by mass and analysing the latest optical, infrared, and X-ray observations to detect the faintest signs of black hole activity. While previous surveys have generally found about 10 AGNs per 1,000 dwarf galaxies (or 1%), the new census yielded values of about 20-50 (2 to 5%). While this is significantly less than what has been observed in medium-sized galaxies (16-27%) and large galaxies (20-48%), the new results indicate that AGNs are more common than previously thought.
In summary, their results suggest that AGNs are mass-dependent and that their frequency increases sharply among galaxies of mass similar to that of the Milky Way. They also reinforce recent findings, an international team of astronomers made using data from NASA's Chandra X-ray Observatory.
"The intense jump in AGN activity between dwarf galaxies and mid-sized, or transitional galaxies tells us something important is changing between the two," said Mugdha Polimera, a CfA astronomer and the lead author of the new census, explained in a CfA press release. "It could be a shift in the galaxies themselves, or a sign that we’re still not catching everything in the smaller ones and need better detection methods. Either way, it’s a new clue we can’t ignore."
An integral part of their work was suppressing the glare from star formation, which had obscured emissions from accreting black holes in previous surveys. However, their results still have a margin of uncertainty where fainter accreting black holes are involved. As a result, these new percentages are approximate, and future observations are likely to change this census. Said Sheila J. Kannappan, a professor of physics and astronomy at UNC and co-author of the new census:
Cutting through the glare of star formation reveals massive black holes that have slipped under the radar in dwarfs, but we're still trying to figure out why black holes are suddenly more common in galaxies like our own. We believe that the Milky Way was formed from many smaller galaxies that merged, so the dwarf galaxies' massive black holes should have merged to form the Milky Way's supermassive black hole. These results are essential for testing models of black hole origins and their role in shaping galaxies.
This census provides the clearest picture yet of how likely galaxies of different sizes are to host active black holes, and offers astronomers a stronger starting point for understanding black hole formation and growth. It also demonstrates how future studies can benefit from cutting through the glare of star formation to see what's happening at the center of nearby galaxies. The team is currently releasing the study's processed measurements to allow other researchers to confirm and expand on their results.
Further Reading: CfA