We all know that a huge 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 were able to see an X-ray image of it a few weeks ago, showing its accretion disk.
So we know it’s there. Astronomers can trace its actions as it occasionally gobbles up material and they can see how it affects nearby stars.
What astronomers are still trying to figure out is how Sgr A* formed.
The answer seems to involve smaller black holes, in particular those of the so-called dwarf galaxies. According to a article published last week in The Astrophysical Journal by astronomers at the University of North Carolina at Chapel Hill, there’s quite a treasure trove of them there.
These things are found inside many dwarfs and may provide a missing link in the growth of supermassive black holes in large galaxies.
Massive (and supermassive) black holes and their lairs
So, let’s dig a little deeper into that, starting with supermassive black holes.
They hide in the heart of many galaxies. These monsters have millions or billions of solar masses. How did they get so big?
The answer involves a subject we see throughout astronomy and planetary science: hierarchical patterns. It’s a fancy way of saying that big things are created from small things.
For example, planets begin as grains of dust that stick together to form rocks that collide to form asteroids that collide to create planetesimals that fuse together to form planets.
Galaxy formation also has its own hierarchical pattern. What creates one of these star cities? Galaxies like the Milky Way began as a collection of gases in the early Universe.
This gas formed stars, which evolved, died, and spilled their materials to help create new generations of stars (and their planets).
In many ways, dwarf galaxies are more like primordial galaxies than evolved spirals and ellipticals.
Alright, so we’ve simplified things here to give an overview of a complex topic that takes up entire textbooks. And that’s before we even get to galaxy mergers.
Cultivate a large galaxy from small ones
Let’s take a closer look at the past of the Milky Way. It has a long history of mergers, dating back billions of years. He started as a child (maybe he was a dwarf) about 14 billion years ago. Other cubs merged with him.
Eventually, we had the home galaxy we all know and love today. (And let’s not forget that it will actually merge with the Andromeda Galaxy in a few billion years.)
So those little guys that merged to form the current Milky Way; there’s a good chance that some are dwarves. They are the smaller cousins of the larger spirals and ellipticals. A typical type 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 strange little galaxies swarm around bigger ones like fireflies. Sometimes they are even caught and gobbled up.
The Milky Way has about 20 or more orbiting around it. One – the Sagittarius dwarf – is cannibalizing itself as you read this. He has made the journey through our galaxy many times.
It seems that dwarf galaxies like this could have what are called “growing black holes” as part of their structures. How do we know? Astronomers have found ways to survey the nearby Universe to search for candidate dwarf galaxies with such growing black holes.
Find black holes in all small places
The North Carolina team actually found a number of these midgets. It all started when they asked the question: where do supermassive black holes come from?
The answer seems to be that they grow by collisions with other black holes. This makes sense in a hierarchical model.
Small stellar-mass black holes could collide, especially in crowded environments (like a dwarf galaxy or a densely populated cluster). Eventually, they form more massive ones.
Such “growing black holes” are seen in large, bright galaxies, but what about dwarfs? Could they have them? If so, what is their abundance in such small galaxies? And could they be the key to understanding the growth of supermassive black holes?
To get answers to all these questions, a team led by UNC-Chapel Hill faculty members Sheila Kannappan and Mugdha Polimera set to work.
They analyzed galaxy data from multiple surveys to look for evidence of black hole growth. The team looked for bright emissions like the ones you would see indicating star formation or around black hole accretion disks.
Their data comes from the Sloan Digital Sky Survey, as well as the REsolved Spectroscopy of a Local VolumE (RESOLVE) and the Environmental COntext Catalog (ECO).
They found evidence of growing black holes in a significant percentage of dwarf galaxies. These galaxies are sometimes “thrown out” from studies of brighter and larger galaxies because their emissions are not (or were not) well understood.
Turns out they’re a treasure trove for black hole research.
Light Emissions Reveal Black Holes
The clue was in the strong emissions emitted by the regions around these black holes.
Kannappan likened this black hole discovery to a light source familiar here in some places on Earth.
“Like fireflies, we only see black holes when they’re lit – when they grow – and lit ones give us a clue to how many we can’t see,” she added. said.
Essentially, Kannapan and the team are talking about dwarf galaxies with active black holes at their cores (in other words, active galactic nuclei).
Of course, there are other reasons why a dwarf galaxy might have strong emissions. For example, dwarfs could have massive bursts of star formation. This activity also causes light spectral emissions.
“We all got nervous”, Polimera said. “The first question that came to mind was: Have we missed a way that extreme star formation alone could explain these galaxies?”
Polimera has spent years researching alternative explanations for these dwarf galaxy AGNs. After ruling out all other possibilities, black hole growth fits the data best.
Implications for the Growth of 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* most likely grew like our galaxy. Not only did his past mergers intermix stars, but each dwarf could also have brought his own growing black hole.
They had to go somewhere, didn’t they? So why wouldn’t they (pardon the pun) gravitate towards each other to add to the greatness of Sgr A*?
“The black holes we found are the building blocks of supermassive black holes like the one in our own Milky Way,” Kannappan said. said. “There are so many things we want to learn about them.”
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