A new study using NASA’s Chandra X-ray Observatory suggests that many small galaxies may not contain supermassive black holes at their centers after all.
This finding challenges the long-standing idea that almost every galaxy in the universe hosts one of these enormous objects.
Supermassive black holes are millions or even billions of times heavier than the Sun and are known to sit at the centers of large galaxies like our Milky Way.
For years, astronomers assumed this was true for nearly all galaxies, regardless of size. But the new research paints a more complicated picture, especially for smaller, dwarf galaxies.
The research team analyzed data from more than 1,600 galaxies observed by Chandra over the past 20 years.
These galaxies ranged from giants more than ten times the mass of the Milky Way to tiny dwarf galaxies with only a small fraction of its stars. By studying this large and diverse sample, the scientists were able to look for patterns linking galaxy size to the presence of black holes.
Astronomers usually detect black holes by looking for X-rays. When gas and dust fall toward a black hole, they heat up and emit powerful X-ray radiation. In large galaxies, Chandra often detects a bright X-ray source right at the center, which is a strong sign of a supermassive black hole.
In fact, the study found that more than 90% of massive galaxies, including those similar in size to the Milky Way, show this clear signal.
Smaller galaxies told a very different story. Galaxies with masses below about three billion times the mass of the Sun, roughly comparable to the Large Magellanic Cloud near the Milky Way, usually did not show bright X-ray sources at their centers. After carefully analyzing the data, the researchers concluded that only about 30% of dwarf galaxies are likely to host supermassive black holes.
At first, the team considered whether black holes in small galaxies might simply be harder to detect. Smaller black holes are expected to pull in less gas, making them fainter in X-rays and easier to miss. While this effect does explain part of the drop in detections, it does not account for all of it. Even after taking this into account, there were still fewer X-ray signals than expected.
The most likely explanation, the researchers say, is that many small galaxies simply do not have central black holes at all. This finding offers important clues about how supermassive black holes form. One idea is that they grow slowly from small black holes left behind by dying stars. Another is that they are born large, forming directly from the collapse of huge clouds of gas in the early universe. The new results support the second idea, suggesting that giant black holes form more easily in massive galaxies and are much rarer in small ones.
This discovery also has broader consequences. If fewer dwarf galaxies contain black holes, there will be fewer black hole mergers when such galaxies collide, leading to fewer gravitational wave signals for future space missions to detect. It may also mean fewer dramatic events where black holes tear stars apart in small galaxies.
By showing that size matters when it comes to black holes, the Chandra study adds an important new piece to the puzzle of how galaxies and black holes grew together over cosmic time.
Source: KSR.
