These objects are recognized as active galactic nuclei (AGN), also termed quasars—the core regions of galaxies so luminous that they temporarily outshine the entire stellar disk combined. This brilliance stems from supermassive black holes (SMBHs) at their centers, which propel infalling gas and dust within their accretion disks to near light speed. This action generates intense radiation across the entire electromagnetic spectrum, ranging from visible light and infrared emissions to microwaves and X-rays. For decades, astronomers understood that SMBHs reside in the centers of many massive galaxies, and it was generally assumed the same held true for dwarf galaxies.
However, researchers later discovered that a number of dwarf galaxies lack central black holes, which underscores the necessity for a thorough census of these smaller galaxies. At a recent press conference during 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 what is currently the most comprehensive survey of AGN to date. Their findings indicated that AGN are present in dwarf galaxies more frequently than many prior investigations had suggested.
During their study, the team examined over 8,000 nearby galaxies, re-evaluating them for tell-tale signs of black hole activity. This involved categorizing these proximate galaxies by mass and scrutinizing recent optical, infrared, and X-ray observations to detect the faintest traces of black hole action. While earlier research typically identified roughly 10 AGN per 1,000 dwarf galaxies (or 1%), the new study yielded figures around 20 to 50 (ranging from 2% to 5%). Although this is considerably lower than the rates observed in medium-sized galaxies (16-27%) and large galaxies (20-48%), the new results point to a higher prevalence of AGN than previously believed.
Their results reveal a mass dependency for active galactic nuclei, with their incidence rate sharply increasing among galaxies whose mass is comparable to that of the Milky Way. Furthermore, these observations lend support to recent conclusions drawn by an international team of astronomers utilizing data from NASA’s Chandra X-ray Observatory.
“The sharp jump in AGN activity between dwarf galaxies and intermediate-sized, or transition, galaxies tells us that significant changes are occurring across this boundary,” explained Mugdha Polimeria, an astronomer at CfA and lead author of the new research, in a CfA press release. “This could signify an alteration within the galaxies themselves, or it might be evidence that we are still failing to detect everything in smaller galaxies and need superior detection methods. Regardless, it is a new clue we cannot disregard.”
A crucial element of their work involved mitigating the glare caused by star formation, which in previous surveys had masked the radiation emanating from accreting black holes. Nevertheless, their findings still carry a degree of uncertainty when dealing with the faintest accreting black holes. Consequently, these newly derived percentages are estimates, and future observations are likely to refine this figure. Sheila J. Kannappan, a professor of physics and astronomy at the University of North Carolina and a co-author of the new assessment, commented, “By looking beyond the noise of star formation, we are finding massive black holes that have previously evaded notice in dwarf galaxies, but we are still working to understand why black holes suddenly become much more common in galaxies similar to ours. We hypothesize that the Milky Way formed from the merger of numerous smaller galaxies, meaning the massive black holes in dwarf galaxies must have merged to form the Milky Way’s supermassive black hole. These results are vital for testing models concerning the origins of black holes and their role in galaxy evolution.”
This census offers the clearest picture yet of the likelihood of finding active black holes across different galaxy sizes, providing astronomers with a more robust starting point for understanding the processes of black hole formation and growth. It also demonstrates how future investigations can benefit by effectively filtering out stellar activity to discern what is occurring at the centers of nearby galaxies. The team is currently making the processed survey data public so that other researchers can validate and build upon these findings.
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