Requiem for a Weeping, Doomed Galaxy

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Astronomy can be bittersweet. We’ve recently caught a glimpse of a tiny galaxy, some 54 million light-years away that is in the process of dying. As its life draws to a close, it’s weeping with tears of stars and ultraviolet light, which stream out for hundreds of thousands of light-years into the bleak emptiness of intergalactic space…

Well OK, so maybe that’s all a bit melodramatic and melancholy, but the interesting fact remains that this is possibly the first time we’ve actually witnessed the death of a galaxy in detail. It’s known as IC 3418, part of the Virgo galaxy cluster, and its death knell is important news for astronomers.

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You see, extragalactic astronomy is a difficult art; looking at objects so distant and faint that it’s difficult to see them clearly can make it difficult to immediately recognize what we’re seeing out there. Couple that with the fact that we’re literally gazing out into the unknown. It’s a big universe out there, and we still understand only a very little of it. So when Jeffrey Kenney, of Yale University, recognized what he might be looking at it was an understandably exciting realization.

“We think we’re witnessing a critical stage in the transformation of a gas-rich dwarf irregular galaxy into a gas-poor dwarf elliptical galaxy — the depletion of its lifeblood,” Kenney explained. Galaxies need all of their gas to form stars. Without that gas, their star formation ceases, leaving them with a population of steadily aging stars that will eventually all die off.

Astronomers categorize galaxies by the colors of light they emit. When undergoing a healthy amount of star formation, galaxies appear blue; young, hot stars shine brightly in blue to ultraviolet wavelengths of light. Older galaxies whose star formation days are over, however, appear much more red, leading some to refer to them as “red and dead.”

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But the actual transition, where a galaxy loses its gas and star formation stops (technically referred to as ‘quenching’) has been elusive to spot. “Until now, there has been no clear example of this transformation happening,” said Kenney, highlighting the importance of this discovery.

Ultraviolet Tears

Those ‘tears’ you can see in the image above actually show the way in which the galaxy is dying (incidentally, the scientists involved in this study are no less melodramatic, referring to them as fireballs). It’s undergoing a process known as ram pressure stripping.

Intergalactic space is by no means empty. Instead, it’s full of a thin plasma made up mostly of hydrogen ions, known as the intergalactic medium, buffeted about by the gravitational pull of nearby galaxies. While it may be at pressures that are unfathomably low compared to what we’re used to on Earth, it’s enough that any galaxy feels some pressure as it drifts through intergalactic space — it’s perhaps easiest to think of it as an intergalactic wind.

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In the midst of galaxy clusters (containing numerous large galaxies and even more numerous dwarfs), that intergalactic plasma becomes denser — usually being referred to, technically, as ‘intracluster medium’. Heated to extreme temperatures by pure gravitational energy, this hot plasma shines brightly in x-rays. As gravity accelerates entire galaxies towards the center of these clusters, they feel this intergalactic wind more strongly.

Eventually, it becomes too much, and as a galaxy falls towards the center of a cluster, its gas and dust are literally blown away. Only the heavier objects, like stars, are unaffected by this process, leaving a galaxy which has undergone ram pressure stripping in this way with no gas from which to form new stars.

In the case of IC 3418, those tears aren’t actually tears at all, they’re huge globules of gas that have been blown out of the galaxy. And all of that gas is still forming stars! Newly formed, orphaned stars born in intergalactic space illuminate the globules in ultraviolet. Kenney and his colleagues note this as a sign of active ram pressure stripping occurring.

However, since its collision with the Virgo cluster, this is almost certainly the last star formation that this ill fated galaxy is ever going to see. No new stars have formed in the core of IC 3418 for over 200 million years.

While poor little IC 3418 is not a new discovery by any means, the paper entitled “Transformation of a Virgo Cluster Dwarf Irregular Galaxy by Ram Pressure Stripping: IC3418 and its Fireballs” (due to be submitted to the Astrophysical Journal) is the first to argue the case that it is, in fact, doomed.

“It’s gratifying to find a clear example of an important process in galaxy evolution,” continued Kenney, true to the spirit of any good scientist. “I enjoy digging through evidence to assemble a story about what happens to galaxies. I’ve come to think of myself as an intergalactic forensic pathologist — someone who studies the bodies of galaxies seeking evidence of traumatic events responsible for the present state of the galaxy.”

Image credit: NASA/JPL-Caltech/SDSS

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