There is a humongous black hole at the center of our universe. It has a mass of 4 million suns and is called Sagittarius A*. We were able to see a radio image of it a few weeks ago.
We know it is there. Astronomers can see how it affects nearby stars when they chart its actions.
Astronomers are trying to understand how Sgr A* formed.
The answer seems to involve smaller black holes from dwarf galaxies. There is a whole treasury of them out there according to a paper published this past week in The Astrophysical Journal.
There is a missing link between the growth of black holes in larger galaxies and these things sitting inside dwarfs.
Let's start with black holes.
They are in the hearts of many of the universe's stars. These monsters have a lot of solar mass. How did they get so big?
Hierarchical models are a topic we see in astronomy and planetary science. Big things are created from smaller things.
Dust grains that stick together to make rocks that slam together to make asteroids that collide to make planets.
The model of the galaxy formation is different. What makes one of those great cities? The early Universe had a collection of gas.
The gas formed stars, which evolved, died, and spread their materials out to help create new generations of stars.
dwarf galaxies are more similar to the primordial galaxies than they are to the evolved spirals and ellipticals.
We simplified things here to give a look at a topic that takes up a lot of textbooks. We will get to galaxy mergers before that.
Let's take a closer look at the past. It has a long history of mergers. It was an infant 14 billion years ago. Other little ones joined it.
We all know and love the home galaxy. In a few billion years, it will merge with the AndromedaGalaxy.
Some of the little guys that merged to make the current Milky Way were dwarfs. They are cousins of the ellipticals and spirals. A typical one has a thousand to a billion stars and has an irregular shape.
Astronomers call their stars "metal-poor" because they are mostly hydrogen and helium. There are weird little galaxies that swarm around larger ones. Sometimes they get swallowed up.
There are about 20 or so of them around the Milky Way. As you read this, you will see that the Sagittarius Dwarf is interacting and getting cannibalized. It has traveled through our galaxy many times.
It seems that dwarf galaxies like this one could have black holes as part of their structures. Astronomers found ways to look for dwarf galaxies with black holes.
A number of such dwarfs were found by the North Carolina team. When they posed the question, they asked where the black holes come from.
They grow by colliding with other black holes. That makes sense in a hierarchy.
Small stellar-mass black holes can collide in crowded environments.
Black holes are seen in big, bright galaxies, but what about dwarfs? How abundant are they in small galaxies if they have them? Is it possible that they are key to understanding the growth of black holes?
A team led by UNC-Chapel Hill faculty members got to work to get answers to all those questions.
They looked for evidence of black holes in the data. The team looked for bright emissions like those you would see around a black hole.
The data came from the Digital Sky Survey and the REsolved Spectroscopy of a Local VolumE.
They found evidence of black holes in dwarf galaxies. The surveys of brighter, bigger galaxies are thrown out because their emissions aren't well- understood.
They are a great place for black hole research.
The regions around the black holes give off strong emissions.
Kannappan compared the black hole discovery to a source of light on Earth.
Black holes are only seen when they are lit up, and lit-up ones give us a clue to how many we can see.
Kannapan and the team are talking about dwarf galaxies with active black holes at their hearts.
There are other reasons why a dwarf galaxy has strong emissions. The dwarfs could have a lot of star formation. That activity causes bright emissions as well.
The first question to my mind was, "Have we missed a way in which extreme star formation alone could explain these galaxies?"
There were many alternative explanations for the dwarf galaxy AGNs. Black holes fit the data the best.
The discovery of black holes in dwarf galaxies brings us back to the central black hole in the Milky Way.
The implications of the North Carolina research suggest that Sgr A* grew as our galaxy did. Each dwarf could have brought its own black hole from its past mergers.
They had to go somewhere. They would gravitate to each other to add to the greatness of Sgr A*.
Kannappan said that the black holes they have found are the basic building blocks of supermassive black holes.
The article was published by Universe Today. The original article is worth a read.
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