“Starved” galaxies in the early cosmos


Composite image (Hubble / ALMA) of the galaxy cluster MACSJ 0138, enlarged by a gravitational lens. (Image: ALMA (ESO / NAOJ / NRAO) / S. Dagnello (NRAO), STScI, K. Whitaker et al)

The early days of our cosmos were actually a phase of intense growth: new stars and galaxies were formed in quick succession. But there are some early galaxies that, despite their enormous size, simply died – their star formation almost completely stopped for reasons that have not yet been clarified. Astronomers have now investigated six of these extinct prehistoric galaxies with the Hubble space telescope and the radio antennas of the ALMA observatory. The recordings revealed: The cause of the end of star formation was apparently a drastic lack of raw materials. The galaxies lacked the cold gas from which new stars normally arise. Why, however, is still a mystery.

Around two to three billion years after the Big Bang, our universe went through a high phase of activity: New stars appeared everywhere at a rapid pace and galaxies grew to enormous masses. “These most massive galaxies in the universe lived fast and violently and produced their stars in a remarkably short time,” explains lead author Katherine Whitaker of the University of Massachusetts at Amherst. The raw material for this, cold molecular hydrogen gas, was abundant and drove the formation of stars. But in recent years astronomers have discovered some early, massive galaxies that are different: They are largely inactive and their star formation has almost come to a standstill. “For some reason, they just shut down,” says Whitaker. Observations suggest that around three billion years after the Big Bang, up to half of the massive galaxies in the cosmos could have been affected by this extinction.

Enlarged twice

To find out why these early galaxies suddenly went out after a brilliant start, Whitaker and her colleagues took a closer look at six of these galaxies. As part of the REQUIEM project, they used a combination of powerful man-made telescopes and a natural magnifying glass: gravitational lenses. In these, the gravity of a massive foreground galaxy amplifies and magnifies the light of its more distant “conspecifics”. “When a distant galaxy barely forms new stars, it becomes so faint that it becomes difficult or even almost impossible to examine it more closely with a single telescope,” explains co-author Justin Spilker of the University of Texas at Austin. “REQUIEM solves this problem by examining galaxies that are magnified by a gravitational lens.”

The six galaxies now examined have such a gravitational lens and are around twelve billion light years away. The research team was able to visualize the details of these primeval structures with the help of the Hubble space telescope and the radio telescopes of the Atacama Large Millimeter / submillimeter Array (ALMA). While the Hubble images depict the galaxies in the near-infrared to ultraviolet range of light and thus show the distribution of stars, for example, the radio and microwave radiation captured by ALMA make the dust and thus the raw material for star formation visible.

Not enough cold gas

The combined images revealed something surprising: “We originally assumed that these extinct galaxies kind of stepped on the brakes as they formed stars,” says Whitaker. It has been suggested that the transformation of cold gas into compact stars – that is, star formation itself – was somehow disrupted in these galaxies. “But in our new study we found out that the early galaxies did not step on the brakes, but that their fuel ran out,” explains the astronomer. The observations showed that these massive early galaxies hardly contained any raw material for star formation. Their cold molecular gas content was more than ten times lower than what the models suggest was normal, the team reports. “These are the first measurements of the distribution of cold gas in distant galaxies at rest and the first measurements of this kind outside the local cosmos,” says Whitaker.

These results suggest that for some reason the early galaxies failed to refill their “tank” – even though there was actually plenty of gas in the early cosmos. “We still don’t understand why this happened,” says co-author Christina Williams of the University of Arizona. “Possible explanations could be that the gas flow into these galaxies has been cut off or that the supermassive black hole in the center of the galaxy gives off so much energy that the galactic gas remains too hot.” The latter could also explain why a large part of these early extinct galaxies it did not manage to restart star formation even in the course of further development, the astronomers speculate. “We still have a lot to learn about why the most massive galaxies in the universe formed so early and then stopped star formation even though enough gas was available,” says Whitaker. The current observations are only a first small step towards this.

Source: Katherine Whitaker (University of Massachusetts, Amherst) et al., Nature, doi: 10.1038 / s41586-021-03806-7

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