The Ursa Major Association of Galaxies. VI: A relative dearth of gas-rich dwarf galaxies

We determined the HI mass function of galaxies in the Ursa Major association of galaxies using a blind VLA-D array survey, consisting of 54 pointings in a cross pattern, covering the centre as well as the outskirts of the Ursa Major volume. The calculated HI mass function has best-fitting Schechter parameters θ^* = 0.19+/-0.11 Mpc^{-3}, log(M^*_{HI}/M_{\odot}) = 9.8+/-0.8 and α = -0.92+/-0.16. The high-mass end is determined by a complementary, targeted WSRT survey, the low-mass end is determined by the blind VLA survey. The slope is significantly shallower than the slopes of the HIPASS (α = -1.37+/-0.03+/-0.05) and ALFALFA (α = -1.33+/-0.02) HI mass functions, which are measured over much larger volumes and cover a wider range of cosmic environments: There is a relative lack of low HI mass galaxies in the Ursa Major region. This difference in the slope strongly hints at an environmental dependence of the HI mass function slope.