| 3rd of September 2021 |
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| ASKAP WALLABY dark galaxies |
| by Wong et al. |
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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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