Searching out Black Holes in Distant Galaxies

Black holes are strange beasts in the cosmic zoo. They come in two “types”: stellar and supermassive. It’s now well known that most galaxies have many stellar-mass black holes scattered throughout. They’re usually created by the deaths of stars in supernova explosions. They’re sometimes found in binary systems, where a black hole and a white dwarf or some other type of star are doing an orbital dance with each other.

stellar mass black hole
 An artist’s conception of a stellar-mass black hole (in blue) hat likely formed when a supermassive star collapsed, feeding from material ejected by a nearby star. ESA, NASA and Felix Mirabel)

Meet the Behemoths

The largest black holes, the supermassives, are tucked away in the hearts of galaxies and contain the mass of millions or billions of stars. They spend at least some of their time chowing down on material in their immediate neighborhoods. Most of the supermassive black holes astronomers know about are tucked away in galaxies that themselves are bundled together in clusters. The largest one found so far has the mass of 21 billion suns and holds court in the core of a galaxy in the Coma Cluster. Coma is a huge conglomeration that lies 336 million light-years away from the Milky Way galaxy.

The Coma Cluster of galaxies has one galaxy with the most massive black hole ever detected.
 The heart of the Coma Cluster of galaxies; one of the galaxies contains the most massive black hole ever found. This HST image reveals galaxies and globular clusters associated with the core of the cluster. NASA/ESA/STScI

That wasn’t the only big one out there. Astronomers also found a 17-billion-solar-mass black hole hunkered down in the core of a galaxy called NGC 1600, which itself is in a cosmic backwater where only about 20 galaxies exist. Since most of the really big black holes live in the “big cities” (that is, in well-populated galaxy clusters) finding this one out in the galactic sticks tells astronomers that something strange had to have happened to create it in its current galaxy.

Merging Galaxies and Black Hole Build-ups

So, how does a monster black hole get stashed away in a small-town galaxy cluster? One possible explanation is that it merged with another black hole at some point in the distant past. Early in the history of the universe, galaxy interactions were much more common, building ever-larger ones from smaller ones.

 Galaxy collisions contribute to the formation of supermassive black holes. This Hubble Space Telescope image shows a collision called the Antennae. During the course of the collision, billions of stars will be formed and any central black holes in each of the colliding galaxies can merge to form ever-larger monsters. NASA/ESA.STScI

When two galaxies merge, not only do their stars and gas and dust mingle, but their central black holes (if they have them, and most galaxies do) migrate to the core of the newly formed, more massive galaxy. There, they orbit each other, becoming what’s called a “binary black hole”. Any stars or clouds of gas and dust are in double jeopardy from the gravitational pull of these black holes. However, this material can actually steal momentum from the black holes (provided it doesn’t fall right into them). When that happens, the stars escape, leaving the black holes with less momentum. They start to move closer together, and eventually, they merge to create a behemoth black hole. It continues to grow by gobbling up gas funneled to the core throughout the collision.

Growing a Massive Black Hole

So, how did NGC 1600’s black hole get so massive? The most likely explanation is that it was extremely hungry at one point in its early life, leading it to suck in lots of gas and other material.

That huge appetite may explain also why the host galaxy is in such a small cluster, compared to other supermassive black holes in galaxies in the hearts of much larger clusters. NGC 1600 is the largest, most massive galaxy in its group. It’s also three times brighter than any of the other nearby galaxies. That huge difference in brightness is not something astronomers have seen in other groups.

Most of the galaxy’s gas was consumed long ago when the black hole blazed as a brilliant quasar from material streaming into it that was heated into glowing plasma. In modern times, NGC 1600’s central black hole is relatively quiet. In fact, astronomers called it a “sleeping giant”. That explains why it hadn’t been detected in earlier studies of the galaxy. Astronomers stumbled across this massive monster when they were measuring the velocities of nearby stars. The intense gravitational field of the black hole affects the motions and speeds of stars. Once astronomers were able to measure those speeds, they could then determine the black hole’s mass.


How Do Astronomers Even Find a Black Hole?

Astronomers used special instruments at the Gemini Observatory in Hawai’i to study the light coming from the stars near the black hole in NGC 1600. Some of those stars are circling the black hole, and that motion shows up in the starlight’s fingerprint (called its spectrum). Other stars had motions that seem to suggest that they had once ventured a bit too close to the black hole and were gravitationally flung away in a more-or-less straight line from the galaxy core. This makes sense since Hubble Space Telescope data also showed the core to be very faint. You’d expect that if the black hole were tossing stars away from itself. It’s possible that NGC 1600’s core has ejected enough stars to make 40 billion suns. That tells astronomers there’s a pretty powerful and massive black hole hidden away at the heart of this galaxy, which lies some 209 million light-years from Earth.

Instrument support mechanism for Gemini telescopes.

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