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.
Today, galaxies are typically clustered together, large in mass, containing a mix of old and young stars.
When two similarly-sized galaxies merge, it triggers a starburst: a massive formation of new stars.
Under the right circumstances, some gas will form stars while the remainder is expelled, lost forever to the intergalactic medium.
Once the gas for forming new stars is used up, the galaxy simply ages as the bluest, most massive stars die off.
Over billions of years, only the redder, dimmer, lower mass stars remain.
We see this general pattern when we look at younger galaxies: they’re smaller, bluer, and filled with younger stars.
Until, that is, the galaxy MACS J2129–1 was discovered back in 2017.
Located behind a massive galaxy cluster, its light gets stretched and magnified by the warping of spacetime.
This galaxy is intrinsically faint and red, with its light arriving only after a 10.8 billion year journey.
It’s gas-poor, compact, and faint, despite having three times the Milky Way’s mass.
Having a galaxy this young and distant, with no new stars, remains an unexplained puzzle for astronomers.
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.