Spectroscopic Confusion: Its Impact on Current and Future Extragalactic HI Surveys

TL;DR

This paper models spectroscopic confusion in HI surveys, showing its effects on galaxy mass functions and predicting lower detection counts for future surveys, emphasizing the importance of survey design for accurate results.

Contribution

It introduces a comprehensive model for spectroscopic confusion, quantifies its impact on HI mass functions, and revises detection estimates for upcoming surveys.

Findings

Confusion biases the HI mass function, increasing the knee mass and steepening the faint end slope.

Impact on existing surveys like ALFALFA and HIPASS is comparable to random errors.

Predicted detection counts for SKA-precursor surveys are 60-75% of previous estimates.

Abstract

We present a comprehensive model to predict the rate of spectroscopic confusion in HI surveys, and demonstrate good agreement with the observable confusion in existing surveys. Generically the action of confusion on the HI mass function was found to be a suppression of the number count of sources below the `knee', and an enhancement above it. This results in a bias, whereby the `knee' mass is increased and the faint end slope is steepened. For ALFALFA and HIPASS we find that the maximum impact this bias can have on the Schechter fit parameters is similar in magnitude to the published random errors. On the other hand, the impact of confusion on the HI mass functions of upcoming medium depth interferometric surveys, will be below the level of the random errors. In addition, we find that previous estimates of the number of detections for upcoming surveys with SKA-precursor telescopes may have been too optimistic, as the framework implemented here results in number counts between 60% and 75% of those previously predicted, while accurately reproducing the counts of existing surveys. Finally, we argue that any future single dish, wide area surveys of HI galaxies would be best suited to focus on deep observations of the local Universe (z < 0.05), as confusion may prevent them from being competitive with interferometric surveys at higher redshift, while their lower angular resolution allows their completeness to be more easily calibrated for nearby extended sources.