A recent study utilizing data from NASA’s Chandra X-ray Observatory indicates that most smaller galaxies may not harbor supermassive black holes at their centers. This research challenges the widely held belief that nearly all galaxies contain these colossal cosmic phenomena.
The study analyzed a sample of over 1,600 galaxies, including NGC 6278 and PGC 039620, over more than two decades. Researchers found that smaller galaxies, like PGC 039620, exhibit significantly fewer signs of supermassive black holes compared to their larger counterparts. In contrast, larger galaxies, such as NGC 6278, which has a mass similar to the Milky Way, frequently display evidence of these giant black holes.
According to Fan Zou of the University of Michigan, who led the study, “It’s important to get an accurate black hole head count in these smaller galaxies. It’s more than just bookkeeping. Our study gives clues about how supermassive black holes are born.” The findings were published in The Astrophysical Journal.
The researchers focused on the X-ray emissions from galaxies, which occur as material falls onto black holes, producing X-rays through friction. The analysis revealed that more than 90% of massive galaxies, including those with a mass comparable to the Milky Way, contain supermassive black holes. However, smaller galaxies, particularly those with masses less than three billion solar masses—similar to the Large Magellanic Cloud—typically lack the bright X-ray sources indicative of black holes.
Two primary explanations account for this absence of X-ray emissions in smaller galaxies. The first suggests that the fraction of galaxies hosting supermassive black holes is significantly lower among these less massive galaxies. The second posits that the X-ray emissions from any black holes present are too faint for detection by Chandra.
Co-author Elena Gallo, also from the University of Michigan, emphasized the team’s conclusion: “We think, based on our analysis of the Chandra data, that there really are fewer black holes in these smaller galaxies than in their larger counterparts.”
To corroborate their findings, Zou and colleagues examined both hypotheses regarding the lack of X-ray sources in smaller galaxies. The amount of gas falling onto a black hole dictates its brightness in X-rays. Smaller black holes are expected to attract less gas, resulting in fainter emissions that are often undetectable. The researchers confirmed this expectation, but additionally identified an unexpected deficit of X-ray sources in less massive galaxies, suggesting that many of these galaxies do not possess any black holes at their centers.
This conclusion has significant implications for understanding the formation of supermassive black holes. Two primary theories exist: the first proposes that giant gas clouds collapse directly into massive black holes, while the second suggests that smaller black holes form from the collapse of massive stars and subsequently merge to create larger black holes. Co-author Anil Seth from the University of Utah noted, “The formation of big black holes is expected to be rarer, in the sense that it occurs preferentially in the most massive galaxies being formed, so that would explain why we don’t find black holes in all the smaller galaxies.”
The study lends support to the theory that giant black holes are born with substantial mass right from their inception. If the alternative theory were accurate, the researchers would have expected smaller galaxies to exhibit a similar proportion of black holes as larger ones.
Moreover, this research may also influence projections regarding black hole mergers resulting from collisions between dwarf galaxies. A reduced number of black holes could lead to fewer detectable gravitational waves in the future, as well as a lower occurrence of black holes disrupting stars within dwarf galaxies.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, oversees the Chandra program, while the Smithsonian Astrophysical Observatory manages science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.
This study represents a significant advancement in our understanding of the relationship between galaxy size and the presence of supermassive black holes, painting a more nuanced picture of the cosmos.
