Kinematics of the dwarf irregular galaxy GR8

Abstract

We present deep, high velocity resolution (~ 1.6 km s ⁻¹) Giant Meterwave Radio Telescope HI 21 cm synthesis images for the faint ( M_B ~ −12.1) dwarf irregular galaxy GR8. We find that the velocity field of the galaxy shows a clear systematic large scale pattern, with a maximum amplitude ~10 km s ⁻¹. Neither pure rotation, nor pure radial motion alone can fit the observed velocity field; however a combination of radial and circular motions can provide a reasonable fit. The most natural interpretation is that the neutral ISM, in addition to rotating about the center, is also expanding outwards, as a result of energy input from the ongoing star formation in the galaxy. Support for this interpretation comes from the fact that the pressure in the HII regions in the galaxy is known to be substantially (~ 55 times) higher than the average pressure in the gas disk. It is, however, also possible that the velocity field is the result of the gas swirling inwards, in which case GR8 could be in the process of formation via the merger of subgalactic clumps.