Environmental dependence of the HI mass function in the ALFALFA 70% catalogue

TL;DR

This study investigates how the environment influences the HI mass function in the ALFALFA catalogue, revealing that the characteristic mass varies with local density when using SDSS data, but not with 2MRS, and suggesting local environment as the key factor.

Contribution

It demonstrates the environmental dependence of the HI mass function's knee mass using different reference catalogues and methods, highlighting the importance of local environment over larger scales.

Findings

The Schechter function 'knee' mass depends on environment when using SDSS data.

No significant change in low-mass slope with environment.

Tentative evidence links HI-deficiency to lower $M_{*}$ in dense regions.

Abstract

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.