AAS Nova

Selections from 2018: Discovery of Dark Galaxies

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star-forming galaxy
Unlike the massive star-forming galaxy illustrated here, recently discovered "dark galaxies" contain no stars — only large amounts of hydrogen gas. [National Astronomical Observatory of Japan]

Editor’s note: In these last two weeks of 2018, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume in January.

Dark Galaxy Candidates at Redshift ~3.5 Detected with MUSE

Published May 2018

Main takeaway:

A team of scientists led by Raffaella Marino (ETH Zürich, Switzerland) have used the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO’s Very Large Telescope to discover six candidate “dark galaxies”, galaxies that contain a large amount of gas but don’t yet contain any stars.

Why it’s interesting:

We still don’t fully understand what the fuel for the first stars in the universe was — how did the diffuse intergalactic medium first come together to trigger star formation in early galaxies? One theory is that there was an epoch in the early phase of galaxy formation during which galaxies were gas-rich but still inefficient at forming stars. By discovering signs of these dark galaxies in the early universe, Marino and collaborators have added supporting evidence to this theory.

Why we haven’t found these dark galaxies before now:

dark galaxy candidates

Example observations of three of the dark galaxy candidates. Left panels show the presence of hydrogen gas; right panels show the lack of detection of stellar continuum emission. [Adapted from Marino et al. 2018]

Since dark galaxies aren’t forming stars yet, they aren’t emitting easily observable starlight. Marino and collaborators instead searched for another kind of emission to identify candidates: fluorescence of the vast reservoirs of hydrogen gas caused by the ultraviolet radiation of nearby quasars, active galactic centers powered by supermassive black holes. Due to the deep imaging made possible by the MUSE instrument, the authors were able to identify six strong candidates for dark galaxies at redshifts of z > 3.5.

Citation

Raffaella Anna Marino et al 2018 ApJ 859 53. doi:10.3847/1538-4357/aab6aa