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Starts With A Bang

This One Distant, Red, Gas-Free Galaxy Defies Astronomers’ Expectations

Young galaxies are bright and blue; old galaxies are red and dead. So how did this old, red galaxy form so early?

The observable Universe contains two trillion distinct galaxies, but they all follow the same rules.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal, and observing for longer periods of time won’t improve this issue by very much. Hubble has reached its limits. (HUDF09 AND HXDF12 TEAMS / E. SIEGEL (PROCESSING))

Today, galaxies are typically clustered together, large in mass, containing a mix of old and young stars.

Stephan’s Quintet, also known as Hickson Compact Group 92, consists of four galaxies gravitationally bound together and in the process of merging, along with a smaller, younger, closer galaxy (of a different color) that’s aligned only by chance in the foreground. A galaxy’s gas level, age, and merger history will determine the content of the stars inside. (NASA, ESA, AND THE HUBBLE SM4 ERO TEAM)

When two similarly-sized galaxies merge, it triggers a starburst: a massive formation of new stars.

Galaxy mergers are common, heralded by the formation of new stars that show up as bright, blue regions containing large numbers of young, massive stars. As time goes on, all the gravitationally bound galaxies in groups and clusters will eventually merge into a single galaxy at the core of each bound structure. When major mergers occur, the result is often a giant elliptical, but nobody expected old-looking, red, gas-free galaxies to exist so early in the Universe. (A. GAI-YAM / WEIZMANN INST. OF SCIENCE / ESA / NASA)

Under the right circumstances, some gas will form stars while the remainder is expelled, lost forever to the intergalactic medium.

Galaxies undergoing massive bursts of star formation can outshine even much larger, typical galaxies. M82, the Cigar Galaxy, is gravitationally interacting with its neighbor (not pictured), causing this burst of active, new star formation, which expels gas from its central region. The effects of the stellar winds are clearly visible in red. (NASA, ESA, AND THE HUBBLE HERITAGE TEAM (STSCI/AURA))

Once the gas for forming new stars is used up, the galaxy simply ages as the bluest, most massive stars die off.

Galaxies that have formed no new stars in billions of years and have no gas left inside them are considered ‘red-and-dead.’ A close look at NGC 1277, shown here, reveals that it may be the first such galaxy in our own cosmic backyard. (NASA, ESA, M. BEASLEY (INSTITUTO DE ASTROFÍSICA DE CANARIAS), AND P. KEHUSMAA)

Over billions of years, only the redder, dimmer, lower mass stars remain.

The most widely accepted theory of galaxy formation and evolution says that galaxies formed from “collisions” of smaller structures, which then evolved over the past 8–11 billion years into the galaxies we see today. (GEMINI OBSERVATORY ILLUSTRATION / JON LOMBERG)

We see this general pattern when we look at younger galaxies: they’re smaller, bluer, and filled with younger stars.

Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this ought to be taken to the extreme, and remains valid as far back as we’ve ever seen. The exceptions, when we encounter them, are both puzzling and rare. (NASA AND ESA)

Until, that is, the galaxy MACS J2129–1 was discovered back in 2017.

This is a wide-field view of galaxy cluster MACS J2129–0741, located in the constellation Aquarius. The massive galaxy cluster magnifies, brightens, and distorts the images of remote background galaxies, including the far-distant, dead disk galaxy MACS2129–1, in red at the upper right. (NASA, ESA, M. POSTMAN (STSCI), AND THE CLASH TEAM)

Located behind a massive galaxy cluster, its light gets stretched and magnified by the warping of spacetime.

When an observatory views a strong source of mass, like a quasar, galaxy, or galaxy cluster, it can often find multiple images of the lensed, magnified, distorted background sources due to the bending of space by the foreground mass. The curvature of spacetime affects not only the masses, but the massless photons traveling in the vicinity of the cluster. (ALMA (ESO/NRAO/NAOJ), L. CALÇADA (ESO), Y. HEZAVEH ET AL.; JOEL JOHANSSON)

This galaxy is intrinsically faint and red, with its light arriving only after a 10.8 billion year journey.

A blown-up view of the gravitationally lensed galaxy, MACS2129–1, which rotates extremely rapidly and is completely devoid of new, young stars. (NASA, ESA, AND S. TOFT (UNIVERSITY OF COPENHAGEN) ACKNOWLEDGMENT: NASA, ESA, M. POSTMAN (STSCI), AND THE CLASH TEAM)

It’s gas-poor, compact, and faint, despite having three times the Milky Way’s mass.

This artist’s concept shows what the young, dead, disk galaxy MACS2129–1, right, would look like when compared with the Milky Way galaxy, left. Although three times as massive as the Milky Way, it is only half the size. MACS2129–1 is also spinning more than twice as fast as the Milky Way. Note that regions of Milky Way are blue from bursts of star formation, while the young, dead galaxy is yellow, signifying an older star population and no new star birth. MACS2129–1 appears redder overall because of its cosmic redshift. (NASA, ESA, AND Z. LEVY (STSCI))

Having a galaxy this young and distant, with no new stars, remains an unexplained puzzle for astronomers.

By mapping the matter distribution of the foreground, lensing cluster, astronomers can reconstruct what the galaxy looks like without the effects of the gravitational lens. (NASA, ESA, AND S. TOFT (UNIVERSITY OF COPENHAGEN); ACKNOWLEDGMENT: NASA, ESA, M. POSTMAN (STSCI), AND THE CLASH TEAM)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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