VLA imaging of the intriguing H i cloud HIJASS J1021+6842 in the M81 group

We present VLA H I 21 cm observations of HIJASS J1021+6842, which has been discovered in the direction of the M81 Group. Our synthesis imaging reveals that the H I is distributed over a larger angular extent and velocity range than the single-dish discovery observations. Assuming that HIJASS J1021+6842 is at the distance of the M81 Group, we detect 1.5 × 10⁸ M _⊙ of H I distributed over as much as 30 kpc, i.e., substantially larger than the biggest dwarf galaxies in the same group. At the depth of our imaging, the H I appears to be confined to at least seven clouds. Peak H I column densities are ∼1.8 × 10²⁰ atoms cm^-2, which is well below the canonical star formation threshold of ∼10 ²¹ atoms cm^-2 and therefore consistent with the fact that no optical counterpart has as yet been identified. A gradient in velocity is observed across the extent of the detected H I; assuming that the object is gravitationally bound we derive a dynamical mass of 7 × 10⁹ M_⊙ and a dark-to-luminous mass ratio of >10. Alternatively, a tidal origin may also result in the observed velocity gradient, which would lead to a considerably lower dynamical mass. Given the above properties and the absence of evidence of a stellar population, HIJASS J1021+6842 is unique amongst the other systems in the M81 Group.