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2nd 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. These systems are often remnants of past tidal interactions and/or could be more extreme cousins of very low surface brightness galaxies. Galaxy evolution models have demonstrated that the majority of HI clouds without optical counterparts are likely the result of galaxy-galaxy interactions and have evolved from tidal streams. While star formation is relatively well-understood in "regular" galaxies, the process of star formation in low surface brightness galaxies is still an area of active research. 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 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. The bright optical source near the cloud is an intervening foreground star: there is no optical emission evident from the HI 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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