We all know that there is a huge black hole in the center of our galaxy. It is called Sagittarius A * (Sgr A * for short) and has a mass of 4 million soles. We saw a radio picture of him a few weeks ago, showing his accretion disk.
So we know it’s there. Astronomers can trace their actions as they devour matter from time to time and see how it affects nearby stars.
What astronomers are still trying to understand is how Sgr A * formed.
The answer seems to involve smaller black holes, especially those in so-called dwarf galaxies. According to an article published last week in The Astrophysical Journal by astronomers at the University of North Carolina at Chapel Hill, there is a treasure trove.
These things are found within many dwarfs and can provide a missing link to the growth of supermassive black holes in larger galaxies.
Massive (and supermassive) black holes and their burrows
So let’s dig a little deeper into that, starting with supermassive black holes.
They hide in the hearts of many, many galaxies. These monsters have millions or billions of solar masses. How did they get so big?
The answer involves a topic we see through astronomy and planetary science: hierarchical models. This is an elegant way of saying that big things are created from smaller things.
For example, planets begin as dust grains that come together to make rocks that collide to make asteroids that collide to create planetesimals that glow at each other to make planets.
The formation of the galaxy also has its own hierarchical pattern. What creates one of these stellar cities? Galaxies like the Milky Way began as a collection of gas in the early universe.
This gas formed stars, which evolved, died, and spread their materials to help create new generations of stars (and their planets).
In many ways, dwarf galaxies are more like primitive galaxies than evolved spirals and ellipticals.
Okay, we’ve simplified things here to take a look at a complex topic that covers whole textbooks. And this is even before galaxy mergers are reached.
Growing a large galaxy from the smallest
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. It started when I was a child (maybe a dwarf) about 14 billion years ago. Other little ones merged with him.
Finally, we have the home galaxy that we all know and love today. (And let’s not forget that, in fact, it will merge with the Andromeda Galaxy in a few billion years.)
So, those guys who merged to make the current Milky Way; there is a good chance that some were dwarfs. They are the little cousins of the big spirals and ellipticals. A typical one has between a billion and a billion stars and has an irregular shape.
Its stars are what astronomers call “metal-poor” (that is, they are mostly hydrogen and helium). And these strange galaxies swarm around larger ones like light worms. Sometimes they are even caught and swallowed.
The Milky Way has about twenty orbiting it. One, the Sagittarius dwarf, is interacting and cannibalizing himself as you read this. He has traveled the 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 that? Astronomers have found ways to study the nearby universe to look for candidate dwarf galaxies with growing black holes.
Find black holes in all small places
The North Carolina team found a number of these dwarfs. It all started when the question arose: where do supermassive black holes come from?
The answer seems to be growing by collisions with other black holes. This makes sense in a hierarchical model.
Small black holes of stellar mass could collide, especially in crowded environments (such as a dwarf galaxy or a densely settled cluster). Finally, they form more massive.
These “growing black holes” are seen in large, bright galaxies, but what about dwarfs? Could they have them? If so, how abundant are they in such small galaxies? And, could they be key to understanding the growth of supermassive black holes?
To answer all these questions, a team led by UNC-Chapel Hill professors Sheila Kannappan and Mugdha Polymer set to work.
They analyzed galaxy data from various surveys to look for evidence of black hole growth. The team looked for bright broadcasts like the ones you see that indicate star formation or around black hole accretion disks.
Their data came from the Sloan Digital Sky survey, in addition to the resolved spectroscopy of a local volume (RESOLVE) and the environmental context catalog (ECO).
They found evidence of the growth of black holes in a significant percentage of dwarf galaxies. These galaxies are sometimes “thrown” out of the polls of brighter and larger galaxies because their emissions are not (or were not) well understood.
It turns out they are a treasure trove for black hole research.
Bright emissions reveal black holes
The track was in the strong emissions that the regions emit around these black holes.
Kannappan compared this discovery of the black hole with a known light source here in some places on Earth.
“Like light worms, we only see black holes when they are lit, when they grow, and the lighted ones give us a clue of how many we can’t see,” he said.
Essentially, Kannapan and the team talk about dwarf galaxies with active black holes in the heart (i.e., active galactic nuclei).
Of course, there are other reasons why a dwarf galaxy could have high emissions. For example, dwarfs could have massive bursts of star formation. This activity also causes bright spectral emissions.
“We all got nervous,” Polimera said. “The first question I thought was, have we lost a way that only extreme star formation could explain these galaxies?”
Polymer spent years researching any alternative explanations for these dwarf galaxy AGNs. After ruling out all other possibilities, growing black holes fit the data better.
Implications for the growth of black hole monsters
The discovery of growing black holes in dwarf galaxies takes us back to the Milky Way and its central black hole.
From the implications of North Carolina research, Sgr A * probably grew as our galaxy. Not only did their past mergers shuffle stars, but each dwarf could also have brought its own growing black hole.
They had to go somewhere, right? So why not gravitate (forgive the pun) between them to add to the greatness of Sgr A *?
“The black holes we found are the basic blocks of supermassive black holes like our own Milky Way,” Kannappan said. “There’s a lot of desire to learn about them.”
This article was originally published by Universe Today. Read the original article.