By looking for a single line in the spectrum of light emitted from dwarf galaxies, Montana State University astronomer Mallory Molina found evidence of supermassive black holes in the centers of 81 dwarf galaxies — irregularly-shaped galaxies between 10 and 100 times smaller than our Milky Way.
Molina says that by looking for this particular signature of black holes in X-ray, astronomers stand a better chance of finding black holes that would otherwise be hidden by the bright glow of star formation in their galaxies. And by finding more black holes, especially the relatively small ones that squat at the center of dwarf galaxies, astronomers may find clues to how the very first black holes formed in the early universe.
What’s New? — Molina examined more than 46,000 galaxies from the Sloan Digital Sky Survey. They were looking for light emitting a particular wavelength called [Fe X] λ 6374 — or iron-10 — which signals the presence of ionized iron. At the luminosity Molina viewed in, it’s the product of very high-energy events — the kind usually found in hot X-rays from a black hole’s corona, or in powerful cosmic winds flowing outward from a black hole. Ordinary stars don’t emit photons that energetic.
“So if you see this line, it’s a pretty good indicator that there’s a black hole present,” Molina says.
In total, Molina and their colleagues witnessed 81 dwarf galaxies with supermassive black holes. About half of them showed other features that generally point to the presence of a black hole — meaning the other half would have otherwise escaped detection. Sometimes those features were visible in the optical spectrum (the range of light our eyes can see), but others showed up only in the radio or X-ray wavelengths.
Light emitted at the wavelength of iron-10 could give astronomers a way to find black holes in galaxies where they’re normally hidden. When astronomers look at visible light from dwarf galaxies, the black holes they find tend to be large ones, and they tend to be in the middle of older, redder dwarf galaxies with less active star formation. That rules out the majority of dwarf galaxies, because most of them tend to be hotbeds of star formation.
The 81 galaxies where Molina found black holes by looking for iron-10, however, tend to be lower in mass, and they tend to be blue — a trademark of galaxies where a lot of new stars are being born. According to Molina, iron-10 helped them detect a population of black holes in smaller dwarf galaxies where astronomers had never seen black holes before. This could, in turn, help astronomers understand how galaxies form.
“The ways that we traditionally search for black holes in massive galaxies are only basically finding the easiest to find black holes in dwarf galaxies,” Molina says. “By adopting these new techniques, you can understand that it's not just the very massive red end of the dwarf galaxy population that host black holes.”
Here’s the Background — The idea of looking for black holes by the iron-10 emission spectra came to Molina when they were looking at data around two dwarf galaxies, J12 20+3020 and Mrk 709S — the former discovered in 2014 and the latter in 2020. Astronomers pieced together via radio waves that each had a supermassive black hole, and on further investigation, found that both emitted light in iron-10 frequencies.
“This led me to wonder: could I actually use Fe X to help find black holes in dwarf galaxies that would otherwise be hidden by star formation?” Molina says. So they put together the sample of 46,000 galaxies from SDSS, which observes the entire night sky in New Mexico to catalog objects over a period of several years.
Finding black holes in the centers of other galaxies is often a challenge for astronomers. They only emit light faintly, mostly from infalling matter, which is a fraction of the light put out by stars in those galaxies. It’s like trying to see a flashlight beam against the blinding glare of a spotlight, according to Molina.
But if the flashlight beam is a different color than the spotlight, it’s much easier to pick out, even if the spotlight is very bright. The iron-10 emission line is, essentially, a flashlight of a different color for astronomers to look for even in bright galaxies bursting with the glow of newborn stars.
Why It Matters — The relatively low-mass supermassive black holes at the heart of dwarf galaxies may be our best clues about how the oldest black holes in the universe formed. They’re about 10 percent the size of the black hole at the center of our galaxy and could inform how supermassive black holes and galaxies merge over time.
“Unfortunately, we can't just go and directly detect the first generation of black holes in the early universe,” Molina says, because those galaxies are so distant that they show up as single points of light even to our most powerful current telescopes.
At the moment, most astrophysicists see two possible origin stories for the universe’s first generation of black holes:
- The first option is that black holes didn’t exist until the first massive stars formed, lived long enough to burn up all inner fuel, and collapsed to form black holes. Those black holes then drew in more and more mass, gradually growing to supermassive size and pulling together whole galaxies around themselves.
- The second option is that in the earliest moments of the universe, black holes didn’t need the mass of an entire collapsed star to form. According to this theory, there was so much pressure and heat in the early universe that a much smaller mass could be compressed into a black hole. Those primeval black holes would have drawn in more mass, gradually growing and building galaxies in the process.
In other words, the big question about the universe’s first black holes is: how big a “seed” did it take to grow a black hole in the early universe? The answer to that question could tell us whether stars had to form before black holes.
Smaller black holes at the center of dwarf galaxies today could suggest that black holes can, in fact, form from smaller masses. Or, as Molina put it, their work “hints that smaller seeds might be a more reasonable scenario for formation in the early universe.”
They emphasized that their results don’t settle the debate one way or the other, however. That’s going to take more data from a new generation of telescopes.
What’s next? — Dwarf galaxies with black holes aren’t unusual. Molina suspects that a lot more dwarf galaxies are hiding iron-10 emission signatures that the Sloan Digital Sky Survey’s telescope just didn’t have high enough resolution to spot.
“‘How many dwarf galaxies host black holes?’ is kind of the million-dollar question in this field,” said Molina.
It’s a question that the next generation of telescopes, like the James Webb Space Telescope, could help answer.
“We're all kind of holding our breath waiting for these new telescopes. We're kind of reaching a limit of what we can do with traditional methods that we're using right now,” said Molina.
Abstract — The massive black hole (BH) population in dwarf galaxies (MBH ≲ 105M⊙) can provide strong constraints on the origin of BH seeds. However, traditional optical searches for active galactic nuclei (AGNs) only reliably detect high-accretion, relatively high-mass BHs in dwarf galaxies with low amounts of star formation, leaving a large portion of the overall BH population in dwarf galaxies relatively unexplored. Here, we present a sample of 81 dwarf galaxies (M⋆ ≤ 3 × 109M⊙) with detectable [Fe x]λ6374 coronal line emission indicative of accretion onto massive BHs, only two of which were previously identified as optical AGNs. We analyze optical spectroscopy from the Sloan Digital Sky Survey and find [Fe x]λ6374 luminosities in the range L[Fe x] ≈ 1036–1039 erg s−1, with a median value of 1.6 × 1038 erg s−1. The [Fe x]λ6374 luminosities are generally much too high to be produced by stellar sources, including luminous Type IIn supernovae (SNe). Moreover, based on known SNe rates, we expect at most eight Type IIn SNe in our sample. That said, the [Fe x]λ6374 luminosities are consistent with accretion onto massive BHs from AGNs or tidal disruption events (TDEs). We find additional indicators of BH accretion in some cases using other emission line diagnostics, optical variability, and X-ray and radio emission (or some combination of these). However, many of the galaxies in our sample only have evidence for a massive BH based on their [Fe x]λ6374 luminosities. This work highlights the power of coronal line emission to find BHs in dwarf galaxies missed by other selection techniques and to probe the BH population in bluer, lower-mass dwarf galaxies.