arXiv (accepted for publication in ApJ):
Based on a new H I survey using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), combined with the Pan-STARRS1 images, we identified an isolated H I cloud without any optical counterpart, named FAST J0139+4328...These findings provide observational evidence that FAST J0139+4328 is an isolated dark dwarf galaxy with a redshift of z = 0.0083.
Moreover, this disk galaxy has an extremely low absolute magnitude (M_B >-10.0 mag).
This is an insanely low absolute magnitude. For reference, a normal absolute magnitude for a galaxy like this might be around 10.
Furthermore, we obtained that the H I mass of this galaxy is (8.3±1.7)e7 SM, and the dynamical mass to total baryonic mass ratio is 47±27, implying that dark matter dominates over baryons in FAST J0139+4328.
That's a substantial error bar--I wonder if future observations can constrain that much better.
And they got a hit: the radio waves emitted by a cloud of HI 94 million light-years away were consistent with a rotating disk galaxy, without the optical light expected of one. Follow-up observations in infrared and ultraviolet revealed a faint smattering of stars.
"This is the first time that a gas-rich isolated dark galaxy has been detected in the nearby Universe," the researchers write.
There are a few other dark galaxy candidates, namely HI 1225+01 (ADS) and HI1232+20 (arXiv).
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