Environment, Ram Pressure, and Shell Formation in Holmberg II

Abstract

Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation, are presented. These were extracted from the multi-configuration dataset of Puche et al. (1992). H I is detected to radii over 16 ′ or 4R 25 , almost a factor of two better than previous studies. The total H I mass M HI = 6.44 × 10 8 M ⊙ . The integrated H I map has a comet-like appearance, with a large but faint component extending to the northwest and the H I appearing compressed on the opposite side. This suggests that HoII is affected by ram pressure from an intragroup medium (IGM). The velocity field shows a clear rotating disk pattern and a rotation curve corrected for asymmetric drift was derived. However, the gas at large radii may not be in equilibrium. Puche et al. (1992) multi-configuration data were also reanalyzed and it is shown that they overestimated their fluxes by over 20%.The rotation curve derived for HoII is well defined for r ∼ < 10 kpc. For 10 ∼ < r ∼ < 18 kpc, however, velocities are only defined on the approaching side, such that this part of the rotation curve should be used with caution. An analysis of the mass distribution, using the whole extent of this rotation curve, 1 Hubble Fellow

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