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 Very Large Array (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 theta* = 0.19 +/- 0.11 Mpc(-3), log M-HI/M-circle dot* = 9.8 +/- 0.8, and alpha = -0.92 +/- 0.16. The high-mass end is determined by a complementary, targeted Westerbork Synthesis Radio Telescope (WSRT) survey, and the low-mass end is determined by the blind VLA survey. The slope is significantly shallower than the slopes of the HI Parkes All Sky Survey (alpha= -1.37 +/- 0.03 +/- 0.05) and Arecibo Legacy Fast Arecibo L-band Feed Array (alpha = -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-H I mass galaxies in the Ursa Major region. This difference in the slope strongly hints at an environmental dependence of the HI mass function slope.