(Photo: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO), STScI, K. Whitaker et al.)
In a joint effort with ALMA, the Hubble telescope has discovered six huge, rare, ancient galaxies that date from the universe’s most prolific period of star formation. But they’re running on empty. These galaxies have run out of the raw material required to form stars, and we don’t know why.
The galaxies were found as part of the poetically named REQUIEM program. “REQUIEM” is a weapons-grade backronym for Resolving QUIEscent Magnified Galaxies At High Redshift. It’s intended to turn what used to be a nuisance into an asset: REQUIEM’s goal is to use massive foreground galaxy clusters as natural telescopes, through the power of gravitational lensing. Strong gravitational lensing can distort things, it’s true, but it can also magnify them. According to NASA, “When an early, massive, and very distant galaxy is positioned behind such a cluster, it appears greatly stretched and magnified, allowing astronomers to study details that would otherwise be impossible to see.”
“Finding a chance alignment like this one is incredibly rare and requires searching the entire sky,” said study author Christina Williams. “Without the fortuitous alignment and strong gravitational lensing, we would not have been able to observe these extremely faint galaxies.”
Even the effect of strong lensing wouldn’t be enough, though, without the “exquisite resolution” of Hubble and ALMA working in tandem. Astronomers used the Hubble telescope to determine the galaxies’ locations. Then, using ALMA, they looked for the cold dust used as a proxy for the existence of cold hydrogen gas.
Seeing these galaxies, though, created more questions than it answered. Chief among them is what happened to all that dust and gas. The galaxies also appear to have epitomized “live fast, die young,” forming their stars in a burst of early, rapid activity that died out while other galaxies were just getting started.
“At this point in our universe, all galaxies should be forming lots of stars. It’s the peak epoch of star formation,” explained lead study author Kate Whitaker. “So what happened to all the cold gas in these galaxies so early on?”
In 4.5 billion years or so, the Milky Way and Andromeda are expected to collide. When they do, there is expected to be another wave of star formation. But similar shakeups don’t seem to have had the same effect on these strange, dead galaxies. They don’t perk up and start making stars after accreting gas from their surroundings, nor even after collisions with other galaxies.
Whitaker meets these deep questions with enthusiasm: “Did a supermassive black hole in the galaxy’s center turn on and heat up all the gas? If so, the gas could still be there, but now it’s hot. Or it could have been expelled and now it’s being prevented from accreting back onto the galaxy. Or did the galaxy just use it all up, and the supply is cut off? These are some of the open questions that we’ll continue to explore with new observations down the road.”
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