Dwarf galaxies like NGC 5264 are thought to host smaller versions of the supermassive black holes found at the centers of all massive galaxies. Credit: ESA/Hubble and NASA
It is well known that all massive galaxies like the Milky Way They host supermassive black holes millions to billions of times the mass of the Sun at their centers. These galaxies and their black holes are intertwined, and the evolution of one significantly affects the evolution of the other.
But what about lower mass galaxies, like dwarf ones? Astronomers are still unsure whether these galaxies follow a similar trend, hosting lower-mass black holes called simply MBHs (massive black holes) that weigh between thousands and millions of solar masses. Because dwarf galaxies (and therefore their central black holes, if they exist) are fainter, they are more difficult to study. But dwarf galaxies offer extremely valuable clues for learning about the conditions of the early universe, because the population of modern dwarfs allows astronomers to work backwards to determine how the first galaxies were “seeded”—that is, how their black holes and black holes were born. whether these black holes were more or less massive relative to the total mass of the galaxy.
A number of questions still remain: Do all low-mass galaxies host massive black holes, or is it possible that some do not have a central black hole at all? Also, how many low-mass galaxies have black holes that are actively feeding? How do these black holes affect their host galaxies? And do MBHs in low-mass galaxies accumulate matter in the same way as their supermassive counterparts?
Answering these questions will allow astronomers to better understand how galaxies evolve.
Searching for massive black holes
While at the Max Planck Institute for Extraterrestrial Physics in Germany, Riccardo Arcodia (now at MIT's Kavli Institute for Astrophysics and Space Research) set out to answer the last question: “study the accretion mode of low-mass galaxies, which We thought it was the easiest problem of all! he said during a presentation Tuesday about the work at the 244th Meeting of the American Astronomical Society in Madison, Wisconsin.
Actively feeding black holes shine brightly across the electromagnetic spectrum, allowing astronomers to detect them in many ways. When a black hole attracts material, that material forms an accretion disk, which emits infrared and visible light from further away and whose brightness varies over time. Feeding black holes also emit high-energy X-rays, which come from a corona located near the black hole.
Multiwavelength astronomy allows researchers to identify objects using different types of light, which can offer a complementary way to observe the universe. In this study, Arcodia and his team identified for the first time dwarf galaxies whose centers showed telltale signs of MBH through variations in optical and infrared light. They then looked for these galaxies in the eROSITA all-sky survey, carried out at X-ray wavelengths by the Spectrum-Roentgen-Gamma mission.
“The advantage here is that since we know that there is a black hole and that it is active… by just studying the X-rays of this sample, we should only worry about the accretion mode,” Arcodia explained.
What they found was surprising: of more than 200 galaxies that showed signs of an MBH in visible or infrared light, only 17 emitted X-rays.
That's strange, because accreting black holes ought emit X-rays. Particularly because “the predicted study. paperwhich has been accepted for publication in Astronomy and Astrophysicsin a Press release.
In other words, “we could have detected an X-ray MBH if it were there… but we didn't,” Arcodia explained. Aside from the 17 detections, the rest of the galaxies only had X-ray emissions consistent with a normal galaxy that does not contain an accreting black hole, even though astronomers knew one was there. ought be present depending on other wavelengths.
So, Arcodia said, most of the low-mass galaxies they observed must contain little bright Accumulation of black holes that are not feeding in the way astronomers would expect.
Different environment, different accretion
This disparity, he said, could be due to one of two things. First, perhaps MBHs from low-mass galaxies simply do not produce X-rays as effectively as their higher-mass counterparts. Or secondly, perhaps MBHs in low-mass galaxies don't accumulate matter in the same way as black holes in more massive galaxies. This idea, Arcodia added, “is what I'm leaning toward, so effectively no selection of a single band,” or observation in a given wavelength light regime, “would be representative of the entire population. It is necessary to combine them all to have a clear idea” of how MBHs act in low-mass galaxies before astronomers can move on to answer the question of whether all dwarf galaxies host MBHs.
There are several reasons to be suspicious of this latter conclusion, Arcodia said. Unlike larger galaxies, dwarf galaxies are more clustered, with their stars and gas more unevenly distributed, and therefore cannot feed the black hole in the same way as in a more massive galaxy, where most of the material is concentrated in the center. Additionally, studies have found that MBHs in dwarf galaxies tend to be displaced from the center of the galaxy, further affecting the nearby material available to feed the black hole. Both factors could affect the magnetic field near the black hole, which in turn could have an impact on the production of X-rays.
Finally, there is evidence that lower-mass galaxies experience shorter, more transient accretion events (such as the black hole interacting with or even devouring a single star) that are not as long-lasting as the long-term accretion that can be sustained. in larger galaxies, again with more material available to funnel into a black hole.
So even though astronomers understand the relationships between massive galaxies and their supermassive black holes quite well, this work shows that “dwarf galaxies and low-mass galaxies are much more complicated than massive ones,” Arcodia said.
