Distant Gas Halo Could Unlock Secrets of Galactic Formation

Very young galaxies are dim, making them difficult to study. However, astronomers from the University of Wisconsin-Milwaukee have used the recently improved W. M. Keck Observatory on Maunakea, Hawaii to take a look at a galaxy called Q2343-BX418. The team thinks the gas halo around Q2343-BX418 could reveal a great deal about how galaxies formed in the early universe.

This object is 10 billion light years away, so it’s actually quite old by now, but it serves as an excellent analog for studying younger galaxies that are too dim. This observation wouldn’t have been possible if not for the latest upgrade to the Keck Observatory. The team led by Dawn Erb used the Keck Cosmic Web Imager (KCWI), a wide-field spectrograph that’s perfect for this sort of examination. Specifically, the team analyzed the “Lyman alpha emission” of the gas halo. Astronomers get a spectrum for every pixel in the image.

As a galaxy begins to coalesce around a central mass (often a black hole), it pulls in gas from the surrounding space. According to Erb, most of the normal matter in the universe is stuck in diffuse gas clouds in between galaxies. The gas halo around a young galaxy is where that matter enters the system. BX418 has a particularly visible gas halo (see artist’s rendering above), making it possible to learn about the galaxy formation that occurred in the early universe.

Observations indicate that the gas halo around BX418 extends about 75,000 light years in all directions, making it significantly larger than the galaxy itself. Over time, that halo should condense and add to the mass of the galaxy to fuel the formation of stars and planets. The light from BX418 is 10 billion years old, so it may have gone on to become a galaxy very much like ours. But all we can see is the embryonic form — the team determined the halo’s velocity and density, leading to a 3D model of the gas behavior over time.

Erb and her colleagues are careful to note in the study that this is all based on a single galaxy so far. Additional targets should be identified for further analysis to either support or refute this model. In time, we could use these discoveries to learn about how the first galaxies formed many billions of years ago.

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