An Aperture synthesis Study of the Irregular Galaxy Holmberg II.

The Cambridge Half-Mile telescope has been used to map the H I distribution and velocity field of Ho II with an angular resolution of $${2}^{\prime}\,\times\,{2}^{\prime}\cdot1$$ and a velocity resolution $$16\,\text{km}\,\text{s}^{-1}$$. The HI disc is 25–50 per cent larger than the Holmberg dimensions and the H I surface density is strongly peaked at the centre. The rotational velocity field is regular out to a radius 7 kpc and the total mass is $$(4\,\pm\,1)\,\times\,{10}^{9}\,{M}_{\odot}$$. The internal dynamics of two bright central condensations imply that these are gravitationally bound, consistent with the fact that they are near sites of star formation. The hydrogen structure of Ho II is similar to that of the irregular galaxy IC 10.