A recent study utilizing data from NASA’s Chandra X-ray Observatory has indicated that most smaller galaxies may not host supermassive black holes at their centers. The research examined a sample of over 1,600 galaxies collected over two decades, challenging the prevailing notion that nearly every galaxy contains such massive black holes.
The study focused on two galaxies, NGC 6278 and PGC 039620, representing a broader analysis of galaxies ranging from more than ten times the mass of the Milky Way down to dwarf galaxies. These findings, published in The Astrophysical Journal, suggest that the prevalence of supermassive black holes is significantly lower in smaller galaxies compared to their larger counterparts.
Research led by Fan Zou of the University of Michigan emphasizes the importance of accurately counting black holes in these less massive galaxies. Zou stated, “It’s more than just bookkeeping. Our study gives clues about how supermassive black holes are born. It also provides crucial hints about how often black hole signatures in dwarf galaxies can be found with new or future telescopes.”
As matter falls into black holes, it generates heat and emits X-rays, which serve as indicators of their presence. The study noted that many larger galaxies exhibited bright X-ray sources at their centers, a clear sign of supermassive black holes. In contrast, the researchers found that more than 90% of massive galaxies, including those comparable in mass to the Milky Way, contained these black holes, while smaller galaxies typically lacked such discernible signatures.
Galaxies with masses under three billion solar masses—similar to the mass of the Large Magellanic Cloud—often did not demonstrate bright X-ray emissions. The researchers proposed two explanations for this phenomenon: either a lower fraction of these smaller galaxies possess supermassive black holes, or the X-rays generated by matter falling onto these black holes are too faint for Chandra to detect.
Co-author Elena Gallo, also from the University of Michigan, expressed confidence in the conclusion drawn from the Chandra data, stating, “We think there really are fewer black holes in these smaller galaxies than in their larger counterparts.” The analysis confirmed that smaller black holes are expected to have less gas falling into them, resulting in fainter X-ray emissions.
The research team discovered an additional deficit of X-ray sources in lower-mass galaxies beyond what could be explained by decreased gas inflow. This additional decline implies that many low-mass galaxies may simply lack black holes altogether. As a result, the study concludes that the reduced number of X-ray detections in smaller galaxies reflects a genuine decrease in the presence of supermassive black holes.
The implications of this study extend to our understanding of how supermassive black holes form. Two primary theories exist: one posits that a giant gas cloud directly collapses into a black hole with a mass several thousand times that of the Sun, while the other suggests that supermassive black holes evolve from smaller black holes formed during the collapse of massive stars. Co-author Anil Seth from the University of Utah noted, “The formation of big black holes is expected to be rarer… that would explain why we don’t find black holes in all the smaller galaxies.”
This research lends support to the theory that giant black holes are born already possessing substantial mass. Had the alternate theory been valid, a similar fraction of black holes would be expected in both smaller and larger galaxies. The findings also have substantial implications for the rates of black hole mergers resulting from collisions between dwarf galaxies. A reduced number of black holes would lead to fewer sources of gravitational waves detectable by future observatories like the Laser Interferometer Space Antenna.
NASA’s Marshall Space Flight Center manages the Chandra program, while the Smithsonian Astrophysical Observatory oversees its science and flight operations. As research continues, the insights gained from the Chandra X-ray Observatory will enhance our understanding of the universe’s structure and the formation of its most enigmatic features.
