An H I mass function ( HIMF ) was derived for 751 galaxies selected from the deep Parkes H I survey across the Zone of Avoidance ( HIZOA ) .
HIZOA contains both the Great Attractor Wall and the Local Void , two of the most extreme environments in the local Universe , making the sample eminently suitable to explore the overall HIMF as well as its dependence on local environment .
To avoid any selection bias because of the different distances of these large-scale structures , we first used the two-dimensional stepwise maximum-likelihood method for the definition of an average HIMF .
The resulting parameters of a Schechter-type HIMF for the whole sample are \alpha = -1.33 \pm 0.05 , \log ( M _ { HI } ^ { * } / M _ { \odot } ) = 9.93 \pm 0.04 , and \phi ^ { * } = ( 3.9 \pm 0.6 ) \times 10 ^ { -3 } Mpc ^ { -3 } .
We then used the k -th nearest-neighbour method to subdivide the sample into four environments of decreasing local density and derived the Schechter parameters for each subsample .
A strong trend is observed , for the slope \alpha of the low-mass end of the HIMF .
The slope changes from being nearly flat , i.e . \alpha = -0.99 \pm 0.19 for galaxies residing in the densest bin , to the steep value of \alpha = -1.31 \pm 0.10 in the lowest density bin .
The characteristic mass , however , does not show a clear trend between the highest and lowest density bins .
We find similar trends in the low-mass slope when we compare the results for a region dominated by the Great Attractor , and the Local Void , which are found to be over- , respectively underdense by 1.35 and 0.59 compared to the whole sample .