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 M, of H i distributed over as much as 30 kpc, i.e., substantially larger 8 1.5 # 10 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 ∼ atoms cm , which is well below the canonical 20 1.8 # 10 star formation threshold of ∼1021 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 M, and a dark-to-luminous mass 9 7 # 10 ratio of 110. 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. Subject headings: galaxies: clusters: general — galaxies: dwarf — galaxies: formation — galaxies: individual (HIJASS J1021 6842) — radio lines: galaxies