HI in XMD Galaxies III. GMRT observations of BCG HS0822+3542
  We present Giant Metrewave Radio Telescope (GMRT) HI 21-cm line data for the smallest known eXtremely Metal Deficient (XMD) blue compact galaxy (BCG) HS0822+3542. From HST imaging it has been suggested that HS0822+3542 actually consists of two still smaller (~ 100pc sized) ultra-compact dwarfs that are in the process of merging. The brighter of these two putative ultra compact dwarfs has an ocular appearance, similar to that seen in galaxies that have suffered a penetrating encounter with a smaller companion. From our HI imaging we find that the gas distribution and kinematics in this object are similar to that of other low mass galaxies, albeit with some evidence for tidal disturbance. On the other hand, the HI emission has an angular size ~25 times larger than that of the putative ultra-compact dwarfs. The optical emission is also offset from the centre of the HI emission. HS0822+3542 is located in the Lynx-Cancer void, but has a nearby companion LSB dwarf galaxy SAO0822+3545. In light of all this we also consider a scenario where the optical emission from HS0822+3542 comes not from two merging ultra-compact dwarfs but from multiple star forming regions in a tidally disturbed galaxy. In this model, the ocular appearance of the brighter star forming region could be the result of triggered star formation. 
Jayaram N. Chengalur, S.A. Pustilnik, J.-M. Martin, A.Y. Kniazev