Recent findings from NASA’s Chandra X-ray Observatory suggest that most smaller galaxies do not harbor supermassive black holes at their centers. This conclusion stems from an analysis of over 1,600 galaxies observed during more than 20 years of the Chandra mission. This research challenges the prevailing notion that nearly every galaxy contains a supermassive black hole.
The study focused on two specific galaxies, NGC 6278 and PGC 039620, which represent a sample analyzed for the presence of supermassive black holes. While larger galaxies, such as NGC 6278, which is comparable in size to the Milky Way, frequently showed evidence of giant black holes at their cores, many smaller galaxies exhibited little to no indication of such phenomena.
According to Fan Zou, the lead researcher from the University of Michigan, “It’s important to get an accurate black hole head count in these smaller galaxies. Our study gives clues about how supermassive black holes are born.” This insight is crucial for understanding the formation and evolution of black holes, particularly in dwarf galaxies.
The team’s investigation revealed that more than 90% of massive galaxies, including those with a mass similar to that of the Milky Way, contain supermassive black holes. In stark contrast, smaller galaxies with masses under three billion solar masses, akin to the Large Magellanic Cloud, typically lacked the bright X-ray sources associated with black holes.
Two primary hypotheses emerged regarding the absence of X-ray signals in smaller galaxies. The first proposes that fewer of these less massive galaxies contain black holes. The second suggests that even if black holes exist, the X-ray emissions produced by material falling into them are so faint that they escape detection by Chandra.
Co-author Elena Gallo, also from the University of Michigan, emphasized the implications of these findings, stating, “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.” The study’s results indicate that the brightness of a black hole in X-rays correlates with the amount of gas it accretes. Since smaller black holes are expected to attract less gas, they are inherently fainter and often undetectable.
The researchers confirmed their expectation regarding the correlation between black hole size and X-ray brightness. They identified an additional deficit of X-ray sources in smaller galaxies that could not solely be accounted for by reduced gas accretion. This finding leads to the conclusion that many low-mass galaxies may simply not possess supermassive black holes.
The results of this study hold significant implications for the understanding of how supermassive black holes originate. There are two primary theories surrounding black hole formation. One theory posits that giant gas clouds collapse directly into black holes, starting at masses thousands of times that of the Sun. The other theory suggests that supermassive black holes develop from smaller black holes created by the collapse of massive stars.
Co-author Anil Seth from the University of Utah explained, “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.” This supports the idea that giant black holes are born with substantial mass, rather than evolving from smaller progenitors.
The implications of this study may extend beyond black hole formation to the frequency of black hole mergers resulting from dwarf galaxy collisions. A lower number of black holes in smaller galaxies suggests fewer sources of gravitational waves that could be detected by future observatories, including the Laser Interferometer Space Antenna. Furthermore, the number of black holes disrupting stars in dwarf galaxies would also be reduced.
NASA’s Chandra program is managed by the Marshall Space Flight Center in Huntsville, Alabama. The science operations for the Chandra X-ray Center are run by the Smithsonian Astrophysical Observatory from Cambridge, Massachusetts, while flight operations are conducted from Burlington, Massachusetts.
The findings have been detailed in a paper published in The Astrophysical Journal, contributing to the ongoing exploration of our universe and the enigmatic nature of black holes.
