Supermassive black holes may have comparatively humble origins

(ORDO NEWS) — We all know that there is a huge black hole at the center of our galaxy. It is called Sagittarius A* (abbreviated as Sgr A*) and has a mass of 4 million suns. We saw a radio image of it a few weeks ago showing its accretion disk.

So we know he’s there. Astronomers can chart its actions when it engulfs matter from time to time, and they can see how it affects nearby stars.

Astronomers are still trying to figure out how Sagittarius A* formed.

The answer looks like it has to do with smaller black holes, especially those from 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’s a treasure trove of them out there.

These things are found inside many dwarfs and could provide the missing link in the growth of supermassive black holes in larger galaxies.

Massive (and supermassive) black holes and their lairs

So let’s dig into this a little more, starting with supermassive black holes.

They lurk in the hearts of many, many galaxies. These monsters have a mass of millions or billions of solar masses. How did they get so big?

The answer has to do with a theme we see in astronomy and planetary science: hierarchical models. It’s a fancy way of saying that big things are made from smaller things.

For example, planets start out as dust particles that stick together to form rocks that collide with each other to create asteroids that collide to create planetesimals that sparkle. on top of each other 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 gas in the early universe.

This gas formed stars that evolved, died, and spread their material, helping create new generations of stars (and their planets). ).

In many ways, dwarf galaxies are more like primordial galaxies than evolved spirals and elliptical galaxies.

Okay, so we’ve simplified here to take a look at a complex topic taking up entire textbooks. And that’s before we even get to the galaxy merger.

Growing a big galaxy from small ones

Let’s take a closer look at the past of the Milky Way. It has an extensive history of mergers going back billions of years. He appeared in infancy (perhaps it was a dwarf) about 14 billion years ago. The other little ones merged with him.

In the end, we got the home galaxy that 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; it is highly probable that some of them were dwarfs. These are the little cousins ​​of the big coils and elliptical trainers. A typical one has a thousand to a billion stars and is irregularly shaped.

Their stars, as astronomers say, are “poor in metals” (meaning that they are mainly composed of hydrogen and helium). And these strange little galaxies swarm around the larger ones like fireflies. Sometimes they are even caught and swallowed.

There are about 20 of them in the Milky Way, orbiting it. One of them, a Sagittarius dwarf, is being cannibalized as you read this. He has traveled through our galaxy many times.

It appears that dwarf galaxies like this one may have so-called “growing black holes” as part of their structure. How can we know? Astronomers have found ways to probe the nearby universe to find a candidate dwarf galaxy with such growing black holes.

Finding Black Holes in all the little places

The North Carolina team actually found several of these dwarfs. It all started when they asked the question: where do supermassive black holes come from?

The answer seems to be that they grow as a result of collisions with other black holes. This makes sense from the point of view of the hierarchical model.

Small stellar mass black holes can collide, especially in crowded environments (such as a dwarf galaxy or a densely populated cluster). In the end, they form more massive ones.

Such “growing black holes” are observed in large, bright galaxies, but what about dwarfs? Can they have them? If so, how many are there in such small galaxies? And could they be the key to understanding the growth of supermassive black holes?

To answer all these questions, a team led by UNC-Chapel Hill faculty Sheila Kannappan and Mugdha Polymera set to work.

They analyzed data on galaxies from several surveys to find evidence for the growth of black holes. The team looked for bright emissions like the ones you see, indicating star formation or around black hole accretion disks.

Their data were obtained from the Sloan Digital Sky Survey as well as from RE resolution local volume spectroscopy (RESOLVE). and the Ecological Context Catalog (ECO).

They found evidence of black hole growth in a significant percentage of dwarf galaxies. These galaxies are sometimes “ejected” from surveys of brighter and larger galaxies because their emission is not (or has not been) well studied.

It turns out that they are a treasure trove of black holes. study.

Bright emissions show Black Holes

The clue was in the strong radiations emitted by the regions around these black holes. off

Kannappan compared this discovery of a black hole to a familiar light source here in some places on Earth.

“Like fireflies, we only see black holes when they’re lit upwards – when they grow – and lit up gives us a clue as to how much we can’t see,” she said.

Essentially, Kannapan and team are talking about dwarf galaxies with active black holes in their cores (in other words, active galactic nuclei).

Of course, there are other reasons why a dwarf galaxy might have strong radiation. For example, dwarfs can experience massive bursts of star formation. This activity also causes bright spectral emission.

“We were all nervous,” Polymera said. “The first question that came to my mind was: have we missed the way that extreme star formation alone could explain these galaxies?”

Polymera has spent years researching any alternative explanations for these AGN dwarf galaxies. After all other possibilities have been ruled out, growing black holes are the best fit to the data.

Consequences for growing monster black holes

The discovery of growing black holes in dwarf galaxies takes us back to the Milky Way and its central black hole.

Based on the results of the North Carolina study, Sagittarius A* most likely grew as our galaxy did. Not only have its past mergers mixed stars, but each dwarf may also have brought its own growing black hole with it.

They had to go somewhere, right? So why don’t they gravitate (sorry for the pun) to each other to increase the greatness of Sgr A*?

“The black holes we found are the basic building blocks of supermassive black holes. like in our Milky Way,” Kannappan said. “There’s so much we want to know about them.”

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