NASA’s Chandra Reveals Smaller Galaxies Lacking Supermassive Black Holes

Recent research from NASA’s Chandra X-ray Observatory indicates that most smaller galaxies do not harbor supermassive black holes at their centers. This finding challenges the prevalent view that nearly every galaxy contains a giant black hole. The study analyzed data from over 1,600 galaxies collected over more than two decades, revealing significant discrepancies in black hole presence based on galaxy size.

The research, detailed in a paper published in The Astrophysical Journal, focused on smaller galaxies, such as PGC 039620, which typically do not exhibit the X-ray signatures associated with supermassive black holes. In contrast, galaxies like NGC 6278, which have masses comparable to the Milky Way, frequently show evidence for giant black holes within their cores.

According to Fan Zou from the University of Michigan, who led the study, understanding the black hole population in smaller galaxies is crucial for theories regarding their formation. “It’s more than just bookkeeping,” Zou stated. “Our study gives clues about how supermassive black holes are born and hints about how often black hole signatures in dwarf galaxies can be found with new or future telescopes.”

As material spirals into black holes, it generates X-rays due to friction. The research team noted that over 90% of massive galaxies—including those with a mass similar to the Milky Way—possess supermassive black holes, indicated by bright X-ray emissions. Conversely, smaller galaxies, particularly those with masses below three billion solar masses, generally lack these illuminating X-ray sources.

The researchers proposed two explanations for this absence of X-ray emissions. One possibility is that the fraction of galaxies hosting massive black holes is significantly lower among less massive galaxies. The second explanation suggests that the X-rays emitted by matter falling onto these black holes might be too faint for detection by Chandra.

Co-author Elena Gallo, also from the University of Michigan, noted, “Based on our analysis of the Chandra data, we believe there are indeed fewer black holes in these smaller galaxies than in their larger counterparts.” The study confirmed that smaller black holes are expected to attract less gas than larger ones, leading to fainter X-ray emissions that often go undetected.

Moreover, the findings revealed an additional deficit of X-ray sources in lower mass galaxies beyond what was anticipated solely from reduced gas inflow. This suggests that many low-mass galaxies may simply lack central black holes altogether.

The implications of this study are profound for our understanding of supermassive black hole formation. Researchers have proposed two primary theories: one posits that a massive gas cloud collapses directly into a black hole with a mass thousands of times that of the Sun, while the other suggests that supermassive black holes evolve from smaller black holes formed when massive stars collapse.

Anil Seth, a co-author from the University of Utah, explained, “The formation of big black holes is expected to be rarer, preferentially occurring in the most massive galaxies. This explains why we don’t find black holes in all smaller galaxies.” The results lend support to the theory that these giant black holes originate already massive, rather than evolving from smaller ones.

This research could also affect predictions regarding black hole mergers, particularly as dwarf galaxies collide. A lower number of black holes would likely result in fewer gravitational wave sources detectable by future observatories, such as the Laser Interferometer Space Antenna. Additionally, the number of black holes disrupting stars in these smaller galaxies would be diminished.

NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program, while the Smithsonian Astrophysical Observatory oversees science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

The findings from this study represent a significant shift in our understanding of galaxy formation and the role of supermassive black holes within the cosmos, emphasizing the importance of continued research in this area.