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3rd of September 2021
ASKAP WALLABY dark galaxies
by Wong et al.
Previous HI (neutral hydrogen) surveys have revealed that ‘dark galaxies’ -- HI-dominated systems with no optical counterparts -- are very rare. Wong et al. present ASKAP WALLABY (Widefield ASKAP L-band Legacy All-sky Blind surveY) observations of two ‘dark’ HI sources (with HI masses of a few times 10^8 solar masses and no known stellar counterpart) that reside within 400 kiloparsecs of NGC 1395, the most massive early-type galaxy in the Eridanus group of galaxies. The authors investigate whether these ‘dark’ HI sources have resulted from past tidal interactions, or whether they are an extreme class of low surface brightness galaxies, and conclude that both scenarios are possible, and are not mutually exclusive. The team identified three analogues of candidate primordial ‘dark’ HI galaxies within the TNG100 cosmological, hydrodynamic simulation. All three model analogues are dark matter-dominated, have assembled most of their mass 12–13 billion years ago, and have not experienced much evolution until cluster infall 1–2 billion years ago. These results suggest that the upcoming ASKAP large area HI surveys will have a significant impact on our understanding of low surface brightness galaxies and the physical processes that shape them.

The left image shows the HI column density maps of one of the dark HI clouds overlaid on a deep optical image stack from DR8 and DR9 of the DECam Legacy Survey. While the HI contours lie on top of a bright optical galaxy, the redshift of that galaxy reveals it is a background object unrelated to the foreground neutral hydrogen cloud. The blue circle in the bottom-left represents the 30-arcsecond ASKAP synthesised beam. The right image shows the HI spectra of the cloud. The vertical lines mark the maximum velocity width of the HI profiles.

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