HI in extremely metal deficient galaxies - III. Giant Metrewave Radio Telescope observations of blue compact galaxy HS 0822+3542

International audienceWe present the results of Giant Metrewave Radio Telescope HI 21-cm line observations of the extremely metal deficient (XMD) blue compact galaxy (BCG) HS 0822+3542. HS 0822+3542 is the smallest known XMD galaxy; from Hubble Space Telescope (HST) imaging, it has been suggested that it actually consists of two still smaller (~100 pc 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. HS 0822+3542 is located in the nearby Lynx-Cancer void, but has a nearby companion low surface brightness dwarf galaxy SAO 0822+3545. In light of all this, we also consider a scenario where the optical emission from HS 0822+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