NASA Study Reveals Small Galaxies Lack Supermassive Black Holes

Recent findings from NASA’s Chandra X-ray Observatory indicate that many smaller galaxies may not host supermassive black holes at their centers. This challenges the prevailing belief that nearly all galaxies contain these enormous cosmic entities. The research examined a comprehensive sample of 1,600 galaxies over the course of more than two decades, revealing significant differences in black hole presence between smaller and larger galaxies.

The study focused on two galaxies, NGC 6278 and PGC 039620, which illustrate the results of this extensive analysis. While NGC 6278, comparable in size to the Milky Way, shows evidence of a supermassive black hole, PGC 039620 does not exhibit similar characteristics. The research team, led by Fan Zou from the University of Michigan, utilized X-ray signatures to identify the presence of black holes, concluding that smaller galaxies typically lack these distinct markers.

Over the years, astronomers have utilized Chandra to explore a wide range of galaxies. The study analyzed various galaxy masses, ranging from over ten times that of the Milky Way to dwarf galaxies with substantially lower masses. A paper detailing these findings was published in The Astrophysical Journal.

“It’s important to get an accurate black hole head count in these smaller galaxies,” said Zou. “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 materials fall into black holes, they generate heat and emit X-rays. The majority of massive galaxies in the study displayed bright X-ray sources, indicating the presence of supermassive black holes. The research revealed that more than 90% of massive galaxies, including those with the mass of the Milky Way, contain these black holes. Conversely, smaller galaxies, particularly those with less than three billion solar masses, showed a significant lack of bright X-ray sources.

Researchers considered two hypotheses regarding the absence of X-ray sources in smaller galaxies. One possibility is that a lower percentage of these galaxies harbor massive black holes. Alternatively, the X-rays produced by matter falling onto black holes could be so faint that Chandra cannot detect them.

Based on their analysis, Zou and colleagues concluded that smaller galaxies indeed contain fewer supermassive black holes than their larger counterparts. The team determined that the amount of gas falling onto black holes influences their brightness in X-rays. Since smaller black holes are expected to attract less gas, they are often fainter and harder to detect.

The researchers also noted a significant additional deficit of X-ray sources in less massive galaxies beyond what would be expected from the reduced gas intake. This suggests that many low-mass galaxies do not have black holes at their centers.

This finding could reshape our understanding of how supermassive black holes form. One prevailing theory posits that giant gas clouds collapse into black holes with masses thousands of times greater than the Sun. Alternatively, supermassive black holes may evolve from smaller black holes formed by the collapse of massive stars.

“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,” noted Anil Seth, a co-author from the University of Utah. “That would explain why we don’t find black holes in all the smaller galaxies.”

The study supports the theory that giant black holes are formed with substantial initial masses, rather than arising from smaller black holes. If the latter theory were accurate, a similar proportion of smaller galaxies would likely host black holes.

These findings also have implications for the rates of black hole mergers resulting from dwarf galaxy collisions. A lower number of black holes would lead to fewer gravitational wave sources detectable by future instruments like the Laser Interferometer Space Antenna. Additionally, the reduced presence of black holes tearing apart stars in dwarf galaxies could further impact our understanding of cosmic events.

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

As research continues, these revelations about smaller galaxies may pave the way for future studies into the nature of black holes and their role in the universe.