This two-panel graphic represents the results of a new study that suggests that smaller galaxies do not contain supermassive black holes nearly as often as larger galaxies do. The new study, as described in our latest press release, analyzed over 1,600 galaxies that have been observed with NASA’s Chandra X-ray Observatory over two decades. The findings imply that most smaller galaxies likely do not have supermassive black holes in their centers, which is in contrast with the common idea that nearly every galaxy has one of these giant black holes within their cores.

The two galaxies shown here, NGC 6278 and PGC 039620, are representative of the galaxies in the study. In optical and X-ray images (X-rays are purple), both galaxies are seen in optical light data from the Sloan Digital Sky Survey. The insets contain just the X-ray data from Chandra. NGC 6278 is roughly the same size as our home galaxy and has X-rays detected from its core. PGC 03620 on the other hand is a smaller galaxy — only a fraction of the size of the Milky Way — and does not show any evidence of an X-ray source.

NGC 6278 and PGC 039620, labeled. Credit: X-ray: NASA/CXC/SAO/F. Zou et al.; Optical: SDSS; Image Processing: NASA/CXC/SAO/N. Wolk

The presence of bright X-rays in the centers of galaxies like NGC 6278 is a clear signature that there is a supermassive black hole in the center. As material falls onto black holes, it is heated by friction and produces X-rays. Many of the massive galaxies in the study contain bright X-ray sources in their centers.

However, most smaller galaxies in the study, including PGC 039620, did not have these unambiguous black hole signals. Galaxies with masses less than three billion Suns — about the mass of the Large Magellanic Cloud, a close neighbor to the Milky Way — usually do not contain detectable X-ray sources in their centers.

The researchers considered two possible explanations for this lack of X-ray sources. The first is that the fraction of galaxies containing massive black holes is much lower for these less massive galaxies. The second is that the amounts of X-rays produced by matter falling onto these black holes is so faint that Chandra cannot detect it.

To reach their conclusion, the team colleagues considered both possibilities for the lack of X-ray sources in small galaxies in their large Chandra sample. The amount of gas falling onto a black hole determines how bright or faint they are in X-rays. Because smaller black holes are expected to pull in less gas than larger black holes, they should be fainter in X-rays and often not detectable. The researchers confirmed this expectation.

However, the researchers found that an additional deficit of X-ray sources is seen in less massive galaxies beyond the expected decline from decreases in the amount of gas falling inwards. This additional deficit can be accounted for if many of the low mass galaxies simply don't have any black holes at their centers. The team’s conclusion was that the drop in X-ray detections in lower mass galaxies reflects a true decrease in the number of black holes located in these galaxies.

This result could have important implications for understanding how supermassive black holes form. There are two main ideas: in the first a giant gas cloud directly collapses into a black hole, which contains thousands of times the Sun’s mass from the start. The other idea is that supermassive black holes instead come from much smaller black holes, created when massive stars collapse.

A paper describing these results has been published in The Astrophysical Journal and is available here. The authors of the study are Fan Zou (University of Michigan), Elena Gallo (University of Michigan), Anil Seth (University of Utah), Edmund Hodges-Kluck (NASA Goddard Space Flight Center), David Ohlson (University of Utah), Tomasso Treu (University of California, Los Angeles), Vivienne Baldassare (Washington State University), Niel Brandt (Penn State University), Jenny Greene (Princeton University), Piero Madau (University of California, Santa Cruz), Dieu Nguyen (University of Michigan), Richard Plotkin (University of Nevada), Amy Reines (Montana State University), Alberto Sesana (University of Milano-Bicocca, Italy), Jong-Hak Woo (Seoul National University, Korea), and Jiafeng Wu (Xiamen University, China).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.