We know that essentially all large galaxies, like our Milky Way, have supermassive black holes at their centers. We also strongly suspect that these massive monsters millions or billions of times the mass of the Sun may have grown out of smaller “seed” black holes called intermediate-mass black holes – or IMBHs – that are thousands to hundreds of thousands of solar masses.
We also know that smaller galaxies, called dwarf galaxies, have huge black holes, probably IMBHs. But do all dwarf galaxies have them, or some percentage, or what? That’s hard to say. When a black hole is actively feeding, devouring interstellar material, that matter gets hellishly hot and bright, making it easy to spot. However, dwarf galaxies also tend to produce stars at a high rate, which also emits a lot of light and can mimic the appearance of a bright, feeder 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 old one. And revealed a veritable 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 on a continuum, clouds of gas in space emit light at very specific wavelengths – think of them as colors – that astronomers call lines. If you want some details, I wrote about this process in a previous article and I cover it in detail in my episode of Crash Course Astronomy: Light. Each element in a cloud of gas emits light in a narrow array of colors, and this acts like a fingerprint that tells us that the element is there, as well as how much there is, how hot it is, what the density is, and more.
Both matter spinning in a black hole and clouds of gas forming stars emit these lines, and it’s a long and somewhat complicated chain of measurements needed to distinguish the two by looking at the proportions of the intensities of the lines emitted by oxygen, hydrogen, nitrogen. , and sulfur, for example. There is a standard set of line ratios used to look at dwarf galaxies and see if they have active black holes versus a lot of star formation, and what astronomers have found is that this method doesn’t work well if a dwarf galaxy is actually being fecund – making too many stars at a high rate – or if the galaxy has a lower than normal amount of heavy elements in it. Or both.
The fact is that this is the case for many nearby dwarf galaxies! So the standard method is not working well and potentially missing a lot of active black holes in these small nearby galaxies. So, in a nutshell, they tried using a different set of line ratios and applied it to a deep sky survey that essentially observes all dwarf galaxies at a certain distance from us.
What they found was surprising: many galaxies identified as star-forming using the old method are actually producing many stars and hosting 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 actually make up 3-16%! This and much more. Even better, they found that nearly all of the newly discovered dual-function dwarf galaxies have a low proportion of heavy elements, a clear indication that this new method has an advantage 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 in which we see material around them. This is also a big step, helping astronomers understand the detailed dynamics of what is happening in the hearts of dwarf galaxies.
All of this is important for two main reasons. One is that dwarf galaxies are everywhere, but they are faint enough that it is difficult to see them at great distances. Categorizing the ones we see nearby will help astronomers understand which ones are farther away and harder to study.
The other is that we think large galaxies grow in part due to ingestion of dwarf galaxies. This happened very early in the Universe, when the galaxies were closer together, but it still happens today – literally today, as we see their remains in the Milky Way. If we want to understand how large galaxies are born, grow, evolve, and morph into the mighty structures we see now, we need to understand the humblest dwarf galaxies. This is a good step in the right direction.