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An HI mass function (HIMF) was derived for 751 galaxies selected from thedeep Parkes HI survey across the Zone of Avoidance (HIZOA). HIZOA contains boththe Great Attractor Wall and the Local Void, two of the most extremeenvironments in the local Universe, making the sample eminently suitable toexplore the overall HIMF as well as its dependence on local environment. Toavoid any selection bias because of the different distances of theselarge-scale structures, we first used the two-dimensional stepwisemaximum-likelihood method for the definition of an average HIMF. The resultingparameters of a Schechter-type HIMF for the whole sample are $\alpha =-1.33\pm0.05$, $\log(M_{\rm HI}^*/M_{\odot})=9.93\pm0.04$, and $\phi^* =(3.9\pm0.6)\times 10^{-3}$ Mpc$^{-3}$. We then used the $k$-thnearest-neighbour method to subdivide the sample into four environments ofdecreasing local density and derived the Schechter parameters for eachsubsample. A strong trend is observed, for the slope $\alpha$ of the low-massend of the HIMF. The slope changes from being nearly flat, i.e. $\alpha =-0.99\pm0.19$ for galaxies residing in the densest bin, to the steep value of$\alpha = -1.31\pm0.10$ in the lowest density bin. The characteristic mass,however, does not show a clear trend between the highest and lowest densitybins. We find similar trends in the low-mass slope when we compare the resultsfor a region dominated by the Great Attractor, and the Local Void, which arefound to be over-, respectively underdense by 1.35 and 0.59 compared to thewhole sample.

The HI mass function in the Parkes HI Zone of Avoidance survey