We all know that a humongous black hole exists at the center of our galaxy. It's called Sagittarius A* (Sgr A* for short) and it has the mass of 4 million suns. We got to see a radio image of it a few weeks back, showing its accretion disk.
So, we know it's there. Astronomers can chart its actions as it gobbles up matter occasionally and they can see how it affects nearby stars.
What astronomers are still trying to understand is how Sgr A* formed.
The answer looks like it involves smaller black holes, especially ones from so-called dwarf galaxies. According to a paper published this past week in The Astrophysical Journal by astronomers at the University of North Carolina at Chapel Hill, there's a whole treasury of them out there.
These things are sitting inside many dwarfs and may provide a missing link to the growth of supermassive black holes in larger galaxies.
Massive (and Supermassive) Black Holes and their Lairs
So, let's dig into this a bit more, starting with supermassive black holes.
They lurk in the hearts of many, many galaxies. These monsters have millions or billions of solar masses. How did they get to be so big?
The answer involves a topic that we see across astronomy and planetary science: hierarchical models. That's a fancy way of saying that big things are created from smaller things.
For example, planets get started as dust grains that stick together to make rocks that slam together to make asteroids that collide to create planetesimals that glom onto each other to make planets.
Galaxy formation has its own hierarchical model, too. What creates one of those stellar cities? Galaxies like the Milky Way started out as a collection of gas in the early Universe.
That gas formed stars, which evolved, died, and spread their materials out to help create new generations of stars (and their planets).
In many senses, dwarf galaxies are more like the primordial galaxies than they are the evolved spirals and ellipticals.
Okay, so we simplified things here to give a look at a complex topic that takes up entire textbooks. And, that's even before we get to galaxy mergers.
Growing a Big Galaxy from Little Ones
Let's look at the Milky Way's past more closely. It has an extensive merger history, going back billions of years. It started as an infant (maybe it was a dwarf) some 14 billion years ago. Other little ones merged with it.
Eventually, we got the home galaxy we all know and love today. (And let's not forget that it will, in fact, merge with the Andromeda Galaxy in a few billion years.)
So, those little guys that merged to make the current Milky Way; chances are good some were dwarfs. They're the little cousins of the big spirals and ellipticals. A typical one has maybe a thousand to a billion stars and sports an irregular shape.
Their stars are what astronomers call "metal-poor" (meaning they're mostly hydrogen and helium). And, these weird little galaxies swarm around some larger ones like fireflies. Sometimes they even get caught and swallowed up.
The Milky Way has about 20 or so of them orbiting around it. One – the Sagittarius Dwarf – is getting cannibalized as you read this. It's made the trip through our galaxy numerous times.
It seems that dwarf galaxies like this one could have what's called "growing black holes" as part of their structures. How do we know this? Astronomers found ways to survey the nearby Universe to look for candidate dwarf galaxies with such growing black holes.
Finding Black Holes in All the Small Places
The North Carolina team actually found a number of such dwarfs. It all began when they posed the question: where do supermassive black holes come from?
The answer seems to be they grow by collisions with other black holes. That makes sense in a hierarchical model way.
Small stellar-mass black holes could collide, particularly in crowded environments (like a dwarf galaxy or a thickly settled cluster). Eventually, they form more-massive ones.
Such "growing black holes" are seen in big, bright galaxies, but what about the dwarfs? Could they have them? If they do, how abundant are they in such small galaxies? And, could they be key to understanding the growth of supermassive black holes?
To get answers to all those questions, a team led by UNC-Chapel Hill faculty members Sheila Kannappan and Mugdha Polimera got to work.
They analyzed galaxy data from several surveys to hunt for evidence of growing black holes. The team looked for bright emissions like those you'd see indicating star formation or around black hole accretion disks.
Their data came from the Sloan Digital Sky Survey, plus the REsolved Spectroscopy of a Local VolumE (RESOLVE) and Environmental COntext Catalog (ECO).
They found evidence of growing black holes in a significant percentage of dwarf galaxies. These galaxies sometimes get "tossed out" of surveys of brighter, bigger galaxies because their emissions aren't (or weren't) well-understood.
It turns out, they are a treasure trove for black hole research.
Bright Emissions Reveal Black Holes
The clue was in the strong emissions the regions around those black holes give off.
Kannappan compared this black hole discovery to a familiar source of light here in some places on Earth.
"Just like fireflies, we see black holes only when they're lit up – when they're growing – and the lit-up ones give us a clue to how many we can't see," she said.
Essentially, Kannapan and the team are talking about dwarf galaxies with active black holes at their hearts (in other words, active galactic nuclei).
Of course, there are other reasons why a dwarf galaxy could have strong emissions. For example, the dwarfs could have massive spurts of star formation going on. That activity causes bright spectral emissions, too.
"We all got nervous," Polimera said. "The first question to my mind was: Have we missed a way in which extreme star formation alone could explain these galaxies?"
Polimera spent years researching any alternative explanations for these dwarf galaxy AGNs. After excluding all the other possibilities, growing black holes fit the data the best.
Implications for Growing Black Hole Monsters
The discovery of growing black holes in dwarf galaxies brings us back to the Milky Way and its central black hole.
Based on the implications of the North Carolina research, Sgr A* very likely grew as our galaxy did. Not only did its past mergers mingle stars, but each dwarf could also have brought along its own growing black hole.
They had to go somewhere, right? So, why wouldn't they gravitate (excuse the pun) to each other to add to the greatness of Sgr A*?
"The black holes we've found are the basic building blocks of supermassive black holes like the one in our own Milky Way," Kannappan said. "There's so much we want to learn about them."