Peering into the Chaos at the Cores of Colliding Galaxies

Astronomers predict that in about four billion years, our very own Milky Way Galaxy will collide with its neighbor, the Andromeda Galaxy. Although the thought of galaxies running into each other brings visions of havoc and fiery collisions, the truth is that since galaxies are mostly empty space, not a whole lot is likely to happen to stars like our Sun, comfortably housed on the periphery of the galaxy. The galactic centers though—that’s another story, and one that work by Dr. Michael Koss at Eureka Scientific, Inc., is helping to shed light on.

Like many galaxies, the Milky Way is home to an enormous black hole at its center. While gimmicky science fact books like to tell you that we can’t see black holes (since not even light can escape their deadly pull) that’s not strictly true. Certainly, we can’t see beyond the event horizon, an imaginary line denoting the closest matter can get without being sucked in, but black holes are actually some of the brightest objects in the Universe! That’s because, before matter is pulled into the singularity, it must lose a large amount of angular momentum and gravitational potential energy. As the matter falls closer and closer to the black hole, that energy re-manifests itself as kinetic energy, heat—and light.

This is an artist’s rendition of a black hole caught in the act of sucking matter in. Gas and dust spirals in towards the black hole, forming what is known as an accretion disk. This black hole also emits jets of high-energy particles (seen in blue), but the really interesting part is the X-radiation that bathes the accretion disk in light and allows astronomers to measure key parameters of this system.
Image Credit: NASA/JPL-Caltech

That light is a powerful tool for astronomers, who can judge by its brightness how rapidly the black hole is growing (technically speaking, its accretion rate). However, when the Milky Way’s black hole eventually approaches and merges with Andromeda’s, alien observers aren’t likely to be able to observe what happens: the gravitational shakeup of a galaxy merger tends to kick up large amounts of dust and gas, and this obscures the black holes’ dance from prying eyes. Unfortunately, it also means that Earth-based scientists have historically had very little luck testing their models of black hole mergers.

Koss realized, though, that many dusty interacting galaxies shine brightly in the X-ray spectrum, which is not blocked as strongly by interstellar dust. Working on a hunch that those galaxy mergers in possession of such luminous cores might be concealing a pair of black holes, he started a survey of nearby galaxies. Using 10 years’ worth of data from the Burst Alert Telescope, an instrument on NASA’s Neil Gehrels Swift Telescope, he painstakingly combed through previously-observed galaxy interactions and tagged those that were particularly luminous in the X-ray spectrum—481 in all.

Although X-rays are able to pierce the dust clouds surrounding the galaxy cores, they produce blurrier images than Koss wanted to work with. Instead, he dug into archives from the Hubble Space Telescope, looking for his tagged galaxies imaged in near-infrared light, which also shines through interstellar dust. The 96 galaxies he couldn’t find in Hubble’s archives he imaged using the Keck Observatory in Hawai’i, which uses a special laser system called adaptive optics to counteract the blurring effect of the atmosphere, allowing him to achieve a similar image quality as he might from space.

The results were beautifully clean, showing stunning images of the galaxy nuclei associated with black holes in the late stages of a merger. Although galaxy mergers last billions of years, models tell us that the fastest black hole growth takes place in the last 10 or 20 million years—a blink of the cosmic eye. Some of the images Koss uncovered show pairs of black holes only 3000 light years apart, representing a snapshot of a very late-stage merger. (For reference, the Milky Way’s and Andromeda’s black holes are separated by a distance on the order of 2.5 million light years.) He was also able to see old stellar populations within the galaxies, which are invisible in X-ray light.

(Click to Enlarge)
This beautiful collage is a sampling of the images Koss used in his study. Each image labeled with the galaxy name was taken in the visible/UV spectrum. Notice in particular the horizontal band of dust obscuring a large part of the uppermost galaxy. In contrast, the redder images show the same galaxies taken in the near-infrared about 10 times sharper, which allows their luminous cores to be seen clearly.
Image Credit: NASA, ESA, and M. Koss (Eureka Scientific, Inc.);
W. M. Keck Observatory; Panoramic Survey Telescope and Rapid Response System

Since these mergers take place on incredibly long astronomical timescales, we won’t be able to follow their progression in our human lifetimes. However, Koss is still able to use his study to check astronomers’ models for this type of merger by piecing together the individual snapshots of different stages, to get a sense of the long-term evolution. “Models of galaxy merger simulations had shown that the peak of black hole growth is supposed to happen right before the two galaxy nuclei and black holes merge because it’s the most violent phase,” Koss says. Ultimately, they did find those “hidden mergers”, in keeping with the models’ prediction.

Even so, he’s not entirely content. “In some way our study is a bit like an archeologist finding some dinosaur bones and trying to reconstruct how dinosaurs lived with very little information to go on,” he says. He dreams of tracing the motions of all the dust and gas in these galaxy collisions, even watching matter fall into the black holes. Unfortunately, we simply don’t have the spatial resolution needed for a study of this sort—and even if we did, his hypothetical video would have to be tens of millions of years long to capture even the last stages of a black hole merger!

Instead, he’s looking to a couple of other techniques to hone his research. The first is called Integral Field Spectroscopy, and it combines all of the advantages of traditional spectroscopy with astrophotography. By taking an entire spectrum of light for each pixel (rather than the sum total of photons, as with regular photography), astronomers will be able to measure key indicators of the mergers’ dynamics, including the elements present, rotational velocity, and the mass of the black holes.

This in turn will help refine the models predicting the final inspiral of the black holes as they finally collapse inward and become one. These last moments will likely release gravitational waves strong enough to be measured on Earth; if that happens, Koss wants to be ready with estimates for the timeframe and strength of the potential gravitational waves for his galaxies.

Finally, with the launch of the James Webb Space Telescope—and the advent of next-generation ground-based telescopes like the Thirty Meter Telescope and the European Extremely Large Telescope—Koss hopes to collect images at higher resolution. Perhaps even more importantly, the James Webb Space Telescope is so sensitive that it will allow teams like Koss’ to study galaxies farther away in space and time, where black hole growth is thought to be the greatest.

It’s hard to know which of these leads will provide the next big clue into galaxy evolution, but whatever it is will help us piece together other galaxies’ pasts—and our own’s future.

—Eleanor Hook

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