Discovery of an isolated RELHIC candidate: J1351+0039

We report the discovery of J1351+0039, a new isolated dark galaxy candidate identified through a blind ̋I survey with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) combined with deep optical imaging from the DESI Legacy Survey. The object displays a compact ̋I structure with a systemic velocity of ( LSR = 429.0 ± 1.4 , -1 ) and an inferred ̋I mass of ( (4.5 ± 1.7) 10^ 5 , ̊m M_⊙ ) at a distance of 3.6 ± 0.7 Mpc. No optical counterpart was detected in the DESI images, with a g-band surface brightness limit of 28.1 mag arcsec -2 , corresponding to a stellar mass limit of ( M_⋆ łesssim 1.2 10^ 5 , ̊m M_⊙ ). The dynamical mass is estimated to be ( M_ dyn ≈ 1.3 , M_⊙ ), yielding a dark-to-baryonic mass ratio of ( ∼ 200 ), implying that J1351+0039 is overwhelmingly dominated by dark matter. While weak rotation was detected in J1351+0039, the kinematics are dominated by internal velocity dispersion, confirming pressure support and reminiscent of reionization-limited ̋I clouds (RELHICs). J1351+0039 stands out as one of the most promising isolated RELHIC candidates, offering a unique laboratory for studying baryonic physics and dark matter in the absence of star formation. 10^ 8