Galaxies are massive collections of stars, gas, and dust and most have supermassive black holes at their centers. Past studies have shown that supermassive black holes could form as black holes drift towards their galactic centers from dynamical friction, merging and collecting stars as well as any local material that falls in. Recent JWST observations have shown evidence that some of the earliest galaxies could have formed supermassive black holes first, challenging prior theories of which came first.
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Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode, we're going to talk about a chicken and egg problem in astronomy, that is, which came first, the galaxy or the supermassive black holes at their centers.
Our own galaxy is about 12 billion years-old and at its center is a black hole of nearly 5 million times the mass of our sun, but it's rather small compared to many other galaxies. Our neighborhood spiral galaxy, Andromeda, has a central black hole that's 100 million solar masses. The central black hole of Messier-87 is six and a half billion solar masses, and that in the galaxy TON-618 is an amazing 66 billion times the mass of our sun.
Where did these giant black holes come from?
Prevalent theories had posited that the galaxy came first — at least in some rough form. Very massive stars in that galaxy would use up their fuel and collapse into black holes. Then these black holes would merge and become much larger — and as they get larger, their gravity becomes stronger — and they could collect even more mass.
While black holes are not vacuum cleaners sucking in everything around them, it can be a useful analogy for their gravity. Imagine you begin with a small mini-vac — cleaning up dust — but with everything you collect, the vacuum becomes stronger and the nozzle larger. Pretty soon you can begin vacuuming up small pebbles — which add mass more quickly — and then larger pebbles. Pretty soon your vacuum is so strong you can begin vacuuming up people, then boulders, then buildings and so on. So while black holes cannot just suck in everything around them — if our sun became a black hole with the same mass, our orbit wouldn't change — if something happens to fall within its strong gravity and gets pulled in, the black hole's gravity becomes stronger. That in turn makes objects that were initially safe from the black hole fall into the black hole and so on. Of course once all the nearby objects are absorbed, the black hole runs out of material to collect, but at the centers of galaxies, there's lots of material so they can grow very large.
A 15-year study completed in 2008 observed 28 stars moving around the black hole at the center of our galaxy — with one star completing an entire orbit. None of those stars would naturally be sucked into the black hole, but as those stars pass near to their neighbors they can exchange orbital energy — with one star gaining and the other star losing. Then there's a chance that the star losing orbital energy could pass close enough to the black hole to be consumed. Such circumstances would slowly grow the black hole, but it has been thought that earlier in a galaxy's evolution, such interactions are much more common, leading to more material falling into the center — or so the story went. Chickens eventually evolved into egg layers.
However, a new study using the James Webb Space Telescope data has upended that idea. As JWST looks at the early universe, it continues to find supermassive black holes.
How could such massive objects form so early on?
Two papers are shedding light on the notion that perhaps the egg came first. The first is by Dr. Roberto Mialino of the University of Cambridge and collaborators published in monthly notices of the Royal Astronomical Society. In it, they measure the mass of a black hole and the metallicity — or chemical enrichment — in a galaxy just 700 million years after the universe was created. The black hole is nearly 32 million times the mass of our sun, but the surrounding gas is 250 times less chemically enriched. The significance is that supernova that form black holes enrich the chemistry, making it very difficult to make such a massive black hole without increasing the ratio of heavier elements. As such, an alternative formation method is needed.
The second paper, published in Nature and including Dr. Mialino, measured properties of a galaxy in our young universe — one of the so-called little red dots we discussed in a previous episode. In this galaxy, they were able to measure both the mass of its black hole and the total mass of its stars, and they found that the black hole was more than twice as massive as all of its stars combined. That surely means that the black hole of about 40 million solar masses must have formed prior to this early galaxy.
From this evidence, it seems the egg needs another source besides the chicken.