Environmental dependence of the H i mass function in the ALFALFA 70% catalogue

We search for environmental dependence of the H i mass function in the Arecibo Legacy Fast ALFA survey (ALFALFA) 70 per cent catalogue. The catalogue is split into quartiles of environment density based on the projected neighbour density of neighbours found in both Sloan Digital Sky Survey (SDSS) and 2MASS Redshift Survey (2MRS) volume-limited reference catalogues. We find the Schechter function ‘knee’ mass to be dependent on environment, with the value of log (M */M⊙) shifting from 9.81 ± 0.02 to 10.00 ± 0.03 between the lowest and highest density quartiles. However, this dependence was only observed when defining environment based on the SDSS reference catalogue, not 2MRS. We interpret these results as meaning that the local environment is the dominant cause of the shift in M *, and that the larger scales that 2MRS probes (compared to SDSS) are almost irrelevant. In addition, we also use a fixed aperture method to probe environment, and find tentative evidence that H i-deficiency depresses the value of M * in the highest density regions. We find no significant dependence of the low-mass slope on environment in any test, using either method. Tensions between these results and those from the literature, are discussed and alternative explanations are explored.