We know that virtually all large galaxies, like our own Milky Way, have supermassive black holes at their center. We also strongly suspect that these huge monsters with millions or billions of times the mass of the Sun may have grown from small “seed” black holes called intermediate-mass black holes – or IMBHs – which have thousands to hundreds of thousands of solar masses.
We also know that smaller galaxies, called dwarf galaxies, contain large black holes, most likely IMBHs. But do all dwarf galaxies have them, or some percentage, or what? It’s hard to say. When a black hole is actively feeding, gobbling up interstellar matter, that matter becomes hellishly hot and bright, making it easy to spot. However, dwarf galaxies also tend to make stars at a high rate, which also emit a lot of light and can mimic the appearance of a bright, nourishing black hole.
A new method recently developed by a team of astronomers tweaks an older method to separate the two processes and does a much better job of finding active black holes than the older method. And it revealed a veritable treasure trove of black holes in nearby dwarf galaxies [link to paper].
The methods used here are subtle. Unlike the Sun, which emits light at all wavelengths in a continuum, gas clouds in space emit light at very specific wavelengths – think of them as colors – which astronomers call lines. If you want details, I’ve written about this process in a previous article and cover it in detail in my episode of Crash course in astronomy: light. Each element in a gas cloud emits light in a set of narrow colors, and this acts like a fingerprint that tells us that the element is there, as well as things like its quantity, temperature, density and After.
Matter swirling in a black hole and clouds of star-forming gas emit these lines, and it’s a long and somewhat complicated chain of measurements needed to tell the two apart, by looking at the ratios of line intensities. emitted by oxygen, hydrogen, nitrogen, and sulfur, for example. There’s a standard set of line ratios used to look at dwarf galaxies and see if they have active black holes versus lots of star formation, and what astronomers have found is that this method doesn’t work. not good if a dwarf galaxy is really fertile — making a lot of stars at a high rate — or if the galaxy has less than normal amounts of heavy elements. Or both.
The fact is that this is the case for many nearby dwarf galaxies! The standard method therefore does not work well and potentially misses many active black holes in these small nearby galaxies. So in a nutshell, they tried to use a different set of line ratios and applied it to a deep study of the sky that basically looks at all the dwarf galaxies within a certain distance of us.
What they found was surprising: many galaxies identified as forming stars using the old method actually make lots of stars and host an active black hole. The old method estimated that about 1% of all nearby dwarf galaxies were like this, but the new method shows that they are actually 3-16%! It is much more. Even better, they found that nearly all newly discovered dual-function dwarf galaxies have low heavy element counts, a clear indication that this new method has the edge over the old one.
They were also able to create many subcategories of galaxies, including those with different types of black hole activity, which may depend on the orientation at which we view the material surrounding it. It’s also a big step forward, helping astronomers understand the detailed dynamics of what’s happening at the heart of dwarf galaxies.
All of this is important for two main reasons. The first is that dwarf galaxies are everywhere, but are faint enough to be hard to see at great distances. Categorizing those we see nearby will help astronomers understand those at greater distances that are more difficult to study.
The other is that we think large galaxies grow partly because they eat up dwarf galaxies. This often happened in the early Universe when the galaxies were closer together, but it still happens today – literally today, since we see their remnants in the Milky Way. If we want to understand how large galaxies are born, grow, evolve, and transform into the mighty structures we see now, we need to understand the humbler dwarf galaxies. It’s a good step in the right direction.