We Can Finally See the "Super Halos' Around Young Galaxies

by Kastalia Medrano
A. Angelich (NRAO/AUI/NSF)

For the first time, astronomers have observed the “super halos” that surround early versions of Milky Way-like galaxies. Made of clouds comprised of carbon and hydrogen gas tens of thousands of light-years across, the halos have always been too overwhelmed by brighter light from the quasars behind them to detect. But now that a new generation of powerful telescopes has finally registered the halos, entire new datasets for understanding the evolution of early galaxies are suddenly opening up.

One team of researchers realized back in the early 2000s that it could be possible to use different galaxies’ own carbon emissions to detect the elusive halos — but they didn’t yet have a way to act on it. When the quasar’s light passes through the system, it’s so much brighter than the halos themselves that it blows them out and registers them undetectable to a lesser instrument. Then, in 2013, the Atacama Large Millimeter Array (ALMA) became operational.

“We recognized at that time that it was going to take a first-class facility like ALMA to really make this work,” Xavier Prochaska, a scientist from the University of California, Santa Cruz, tells Inverse. “So we waited and planned and strategized for the next 12 years or so while the facility was being built.”

On Thursday, Prochaska and his colleagues published a paper detailing new findings on super halos in the journal Science. The researchers used ALMA to observe the halos around a pair of galaxies about 12 billion lightyears away, with quasars another half a billion lightyears behind. As the quasar’s light travels toward us through the halo, it illuminates the spectral signature of the gas and allows ALMA to detect it in absorption. So naturally, the researchers had assumed the galaxy would be found in the same location.

“The biggest surprise when we looked at the data was, ‘holy cow, these galaxies are not where we expected them to be,” Prochaska says. “Hell, no. [They’re] four or five times the size of the galaxy away, was way off [from the quasars’ location]. To be honest, we’re still struggling with it, but that’s what new data does.”

Because these galaxies resemble our own Milky Way, there are substantial implications behind what we can learn about the origin of our neck of the cosmos. We can now see that these galaxies are already rotating in the manner of spiral galaxies like the Milky Way. We can estimate the galaxies’ size, something we couldn’t do before. We can better estimate their mass. We can resolve the dynamics, and more directly estimate how many stars they contain. And now that we know that the halos are far more extensive in these early galaxies than imagined, we can better speculate as to how they evolve over time.

The carbon atoms, which are brighter and appear more offset from the quasar light, thus allowing the researchers to disentangle the two, are actually only a trace amount of the atoms in the universe. The far more dominant element is hydrogen, and the researchers believe there are probably massive quantities of it in the halos as well, but we still don’t have the technology to observe it.

“It’s funny that we’re using this [minute amount of carbon] because the most abundant atom in the universe is hydrogen,” Prochaska says. “But that’s still beyond out grasp to directly image, at least for now.”

Gas surrounding high redshift galaxies has been studied through observations of absorption line systems toward background quasars for decades. However, it has proven difficult to identify and characterize the galaxies associated with these absorbers due to the intrinsic faintness of the galaxies compared to the quasars at optical wavelengths. Utilizing the Atacama Large Millimeter/Submillimeter Array, we report on detections of [C II] 158μm line and dust continuum emission from two galaxies associated with two such absorbers at a redshift of z ∼ 4. Our results indicate that the hosts of these high-metallicity absorbers have physical properties similar to massive star-forming galaxies and are embedded in enriched neutral hydrogen gas reservoirs that extend well beyond the star-forming interstellar medium of these galaxies.
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