We all know that there is a massive black hole at the center of our galaxy. It is called Sagittarius A* (abbreviated Sgr A*) and has a mass of 4 million Suns. We got to see a radio image of it a few weeks ago, showing its accretion disc.
So, we know it’s there. Astronomers can chart its actions as it sometimes captures matter and they can see how it affects nearby stars.
Astronomers are still trying to understand how Sgr A* formed.
The answer seems to be that it consists of small black holes, especially those from so-called dwarf galaxies. According to a paper published last week The Astrophysical Journal There is an entire treasure trove of them, by astronomers at the University of North Carolina at Chapel Hill.
These things are sitting inside many dwarfs and may provide a missing link to the evolution of supermassive black holes in larger galaxies.
Huge (and Supermassive) Black Holes and Their Lairs
So, let’s dwell on this a little more, starting with supermassive black holes.
They lurk in the hearts of many, many galaxies. These monsters have millions or even billions of solar masses. How did they get so big?
The answer covers a topic we see in astronomy and planetary science: the hierarchical model. This is a fancy way of saying that small things make big things.
For example, planets start out as dust particles that clump together to form rocks that clump together to form asteroids that collide to form planets that shine on each other.
Galaxy formation also has its own hierarchical model. What makes one one of those stellar cities? The Milky Way galaxy began as a collection of gas in the early universe.
That gas created stars, which evolved, died, and dispersed their material to help form new generations of stars (and their planets).
In many ways, dwarf galaxies are more like primitive galaxies than evolved spirals and ellipticals.
Okay, so we’ve simplified things here to take a look at a complex topic that covers entire textbooks. And, before we get to galaxy mergers.
grow galaxies out of small
Let’s take a closer look at the Milky Way’s past. It has an extensive merger history that goes back billions of years. It began as an infant (perhaps it was a dwarf) about 14 billion years ago. Other smaller ones merged into it.
Eventually, we found the home galaxy we all know and love today. (And don’t forget that it will, in fact, merge with the Andromeda Galaxy in a few billion years.)
So, those little ones that merged to form the present-day Milky Way; Chances are good there were some dwarves. They are the smaller cousins of the larger spiral and elliptical. A typical one contains perhaps a thousand to a billion stars and sports an irregular shape.
Their stars are what astronomers call “metal-poor” (meaning they are mostly hydrogen and helium). And, these strange little galaxies revolve around some of the larger galaxies like Firefly. Sometimes they are caught and even swallowed.
The Milky Way has about 20 or so orbits around it. One – the Sagittarius Dwarf – is chatting and cannibalizing as you’re reading this. It has traveled through our galaxy many times.
It seems that such dwarf galaxies may have what are 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 Tiny Spaces
The North Carolina team actually got several such dwarves. It all started when he raised the question: Where do supermassive black holes come from?
The answer appears to be that they grow from collisions with other black holes. This makes sense in a hierarchical model way.
Small stellar-mass black holes can collide, especially in crowded environments (such as dwarf galaxies or densely populated clusters). After all, they make up more massively.
Such “growing black holes” are seen in large, bright galaxies, but what about dwarfs? Can they have? If they do, how abundant are they in such small galaxies? And, could they be the key to understanding the evolution of supermassive black holes?
To find answers to all those questions, a team led by UNC-Chapel Hill faculty members Sheela Kannappan and Mugdha Polymera set to work.
They analyzed galaxy data from several surveys to search for evidence of a growing black hole. The team looked for bright emissions, like those you’d see around star formation or black hole accretion disks.
Their data came from the Sloan Digital Sky Survey, as well as a Local Volume (RESOLVE) and Resolved Spectroscopy of 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 surveys of brighter, larger galaxies because their emissions are not (or are not) well understood.
It turns out, they are a treasure trove for black hole research.
Black hole revealed by fast emission
The clue was in the strong emission of the regions around those black holes.
Knappan likened the discovery of this black hole to a familiar source of light at certain places on Earth.
“Like fireflies, we see black holes only when they burn up — when they’re growing — and burned ones give us a clue as to how many we can’t see,” she said.
Essentially, Knapan and team are talking about dwarf galaxies that have active black holes at their heart (in other words, active galactic nuclei).
Of course, there may be other reasons for the dwarf galaxy’s strong emissions. For example, massive star formation can occur in dwarfs. That activity also causes bright spectral emission.
“We all panicked,” said Polymera. “The first question on my mind was: Have we missed a way that only extreme star formation can explain these galaxies?”
Polymera spent years researching any alternative explanations for these dwarf galaxies AGN. Leaving out all other possibilities, growing black holes are the best fit for the data.
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 effects of the North Carolina research, Sgr A* is very likely to grow as our Milky Way. Not only did its previous mergers merge the stars, but each dwarf could also bring along its own growing black hole.
They had to go somewhere, didn’t they? So, why won’t they lean on each other to add to the greatness of Sgr A*?
“The black holes we have found are the basic building blocks of supermassive black holes like our own Milky Way,” Kannappan said. “We want to learn a lot about them.”
This article was originally published by Universe Today. Read the original article.