It’s a mystery that’s puzzled astronomers for years. Which came first, supermassive black holes or the galaxies that surround them?
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Every time astronomers look farther out in the Universe, they discover new mysteries. These mysteries require all new tools and techniques to understand. These mysteries lead to more mysteries. What I’m saying is that it’s mystery turtles all the way down.
One of the most fascinating is the discovery of quasars, understanding what they are, and the unveiling of an even deeper mystery, where do they come from?
As always, I’m getting ahead of myself, so first, let’s go back and talk about the discovery of quasars.
Back in the 1950s, astronomers scanned the skies using radio telescopes, and found a class of bizarre objects in the distant Universe. They were very bright, and incredibly far away; hundreds of millions or even billion of light-years away. The first ones were discovered in the radio spectrum, but over time, astronomers found even more blazing in the visible spectrum.
The astronomer Hong-Yee Chiu coined the term “quasar”, which stood for quasi-stellar object. They were like stars, shining from a single point source, but they clearly weren’t stars, blazing with more radiation than an entire galaxy.
Over the decades, astronomers puzzled out the nature of quasars, learning that they were actually black holes, actively feeding and blasting out radiation, visible billions of light-years away.
But they weren’t the stellar mass black holes, which were known to be from the death of giant stars. These were supermassive black holes, with millions or even billions of times the mass of the Sun.
As far back as the 1970s, astronomers considered the possibility that there might be these supermassive black holes at the heart of many other galaxies, even the Milky Way.
In 1974, astronomers discovered a radio source at the center of the Milky Way emitting radiation. It was titled Sagittarius A*, with an asterisk that stands for “exciting”, well, in the “excited atoms” perspective.
This would match the emissions of a supermassive black hole that wasn’t actively feeding on material. Our own galaxy could have been a quasar in the past, or in the future, but right now, the black hole was mostly silent, apart from this subtle radiation.
Astronomers needed to be certain, so they performed a detailed survey of the very center of the Milky Way in the infrared spectrum, which allowed them to see through the gas and dust that obscures the core in visible light.
They discovered a group of stars orbiting Sagittarius A-star, like comets orbiting the Sun. Only a black hole with millions of times the mass of the Sun could provide the kind of gravitational anchor to whip these stars around in such bizarre orbits.
Further surveys found a supermassive black hole at the heart of the Andromeda Galaxy, in fact, it appears as if these monsters are at the center of almost every galaxy in the Universe.
But how did they form? Where did they come from? Did the galaxy form first, and cause the black hole to form at the middle, or did the black hole form, and build up a galaxy around them?