Ancient galaxy's halo holds clues to its formation and evolution

The inflow and outflow of gas from galaxies' halos directly affects star formation.
By Laurel Kornfeld | Nov 22, 2018
Thanks to a new instrument installed at the W.M. Keck Observatory in Mauna Kea, Hawaii, researchers successfully detected an unusual type of light being emitted by a small, ancient galaxy approximately 10 billion light years away.

An analog for even younger and more distant galaxies too faint to be observed, galaxy Q2343-BX418 gives scientists insight into the formation and evolution of the universe's earliest galaxies.

Researchers led by Dawn Erb of the University of Wisconsin-Milwaukee,Charles Streidel of Caltech, and Yuguang Chen, also of Caltech, used the newly-installed Keck Cosmic Web Imager (KCWI) to conduct a spectral analysis of the gases in the young galaxy's halo, the extended spherical region of stars and gas beyond its central structure.

This was not the first observation of BX418, which the researchers have observed over many years with various Keck Observatory instruments.

During observations with KCWI, they discovered the galaxy's halo is giving off a specific type of light known as Lyman alpha emission.

"In the last several years, we've learned that the gaseous halos surrounding galaxies glow with a particular ultraviolet wavelength called Lyman alpha emission. There are a lot of different theories about what produces this Lyman alpha emission in the halos of galaxies, but at least some of it is probably due to light that is originally produced by star formation in the galaxy being absorbed and re-emitted by gas in the halo," Erb explained.

A better understanding of halo gases will provide scientists with insight into the stars forming within the galaxy.

"Most of the ordinary matter in the universe isn't in the form of a star or planet, but gas. And most of that gas exists not in galaxies, but around and between them," she emphasized.

Gases both enter and exit galaxies through their halos. Inflowing gas acts as fuel for star formation while outflowing gas suppresses it.

"So, understanding the complex interactions happening in this gaseous halo is key to finding out how galaxies form stars and evolve," Erb said.

Designed to study the cosmic web, the faint gases that connect galaxies, KCWI enabled the researchers to observe this galaxy with an unprecedented level of detail.

By taking the spectra of the Lyman alpha emission in the galaxy's halo, the scientists were able to trace the gas, calculate its velocity, and produce a 3D map of its structure.

As a next step, they plan to study other ancient galaxies to determine whether this one's halo is typical or anomalous.

Findings of the study have been published in The Astrophysical Journal Letters.



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