Environmental dependence of the HI mass function in the ALFALFA 70%
  catalogue
  We search for environmental dependence of the HI mass function in the ALFALFA
70% catalogue. The catalogue is split into quartiles of environment density
based on the projected neighbour density of neighbours found in both SDSS and
2MRS volume limited reference catalogues. We find the Schechter function 'knee'
mass to be dependent on environment, with the value of $\log
({M_{*}/\mathrm{M_{\odot}}})$ shifting from $9.81 \pm 0.02$ to $10.00 \pm 0.03$
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
HI-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.
