Q2343-BX418, a young star-forming galaxy located about 10 billion light years away from Earth, is surrounded by an enormous halo of gas, according to new research. This halo extends about 75,000 light-years on either side of the galaxy’s disk and is about ten times the size of the galaxy itself.
An artist’s concept showing the gaseous halo surrounding a galaxy, illuminated by a narrow band of ultraviolet light called Lyman alpha emission. Image credit: T. Klein, University of Wisconsin-Milwaukee.
Q2343-BX418 is an analog for younger galaxies that are too faint to study in detail, making it an ideal candidate for learning more about what galaxies looked like shortly after the birth of the Universe.
The gas halo around this galaxy gives off a special type of light called Lyman alpha emission. This emission acts as a tracer for the gas because its photons are absorbed and re-emitted by hydrogen in the halo, enabling astronomers to study the motions and spatial extent of the gas.
“In the last several years, we’ve learned that the gaseous halos surrounding galaxies glow with Lyman alpha emission,” said Dr. Dawn Erb, an astronomer at the University of Wisconsin-Milwaukee.
“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.”
Dr. Erb and her colleagues used the Keck Cosmic Web Imager (KCWI), a wide field, seeing limited, integral field spectrograph recently installed on the Keck II telescope at W. M. Keck Observatory on Maunakea, Hawaii, to perform a detailed spectral analysis of Q2343-BX418’s gas halo; its properties could offer clues about the stars forming within the galaxy.
“Most of the ordinary matter in the Universe isn’t in the form of a star or a planet, but gas. And most of that gas exists not in galaxies, but around and between them,” Dr. Erb said.
“The halo is where gas enters and exits the system. The gas surrounding galaxies can fuel them; gas from within a galaxy can also escape into the halo. This inflow and outflow of gas influences the fate of stars.”
“The inflow of new gas accreting into a galaxy provides fuel for new star formation, while outflows of gas limit a galaxy’s ability to form stars by removing gas. So, understanding the complex interactions happening in this gaseous halo is key to finding out how galaxies form stars and evolve.”
The astronomers used the KCWI instrument to take spectra of the Lyman alpha emission of Q2343-BX418’s halo.
This allowed them to trace the gas, plot its velocity and spatial extent, then create a 3D map showing the structure of the gas and its behavior.
The new data suggest that the galaxy is surrounded by a roughly spherical outflow of gas and that there are significant variations in the density and velocity range of this gas.
“This analysis is the first of its kind. Because it has only been tested on one galaxy, other galaxies need to be studied to see if these results are typical,” Dr. Erb said.
The findings were published in the July 24, 2018 issue of the Astrophysical Journal Letters (arXiv.org preprint).
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Dawn K. Erb et al. 2018. The Kinematics of Extended Lyα Emission in a Low-mass, Low-metallicity Galaxy at z = 2.3. ApJL 862, L10; doi: 10.3847/2041-8213/aacff6
