Comparison of HI and optical redshifts of galaxies - The impact of redshift uncertainties on spectral line stacking

TL;DR

This study compares optical and HI redshifts, showing their agreement and analyzing how redshift uncertainties impact spectral line stacking, providing guidelines for future survey planning to improve detection sensitivity.

Contribution

It demonstrates the relationship between optical and HI redshifts and quantifies how redshift errors affect spectral stacking, offering practical guidelines for survey design.

Findings

Optical and HI redshifts agree with negligible systematic offset.

Redshift errors below ~150 km/s minimally impact stacked profile width.

Guidelines are provided for the number of spectra needed for desired sensitivity.

Abstract

Accurate optical redshifts will be critical for spectral co-adding techniques used to extract detections from below the noise level in ongoing and upcoming surveys for HI, which will extend our current understanding of gas reservoirs in galaxies to lower column densities and higher redshifts. We have used existing, high quality optical and radio data from the SDSS and ALFALFA surveys to investigate the relationship between redshifts derived from optical spectroscopy and neutral hydrogen (HI) spectral line observations. We find that the two redshift measurements agree well, with a negligible systematic offset and a small distribution width. Employing simple simulations, we determine how the width of an ideal stacked HI profile depends on these redshift offsets, as well as larger redshift errors more appropriate for high redshift galaxy surveys. The width of the stacked profile is dominated by the width distribution of the input individual profiles when the redshift errors are less than the median width of the input profiles, and only when the redshift errors become large, ~150 km/s, do they significantly affect the width of the stacked profile. This redshift accuracy can be achieved with moderate resolution optical spectra. We provide guidelines for the number of spectra required for stacking to reach a specified mass sensitivity, given telescope and survey parameters, which will be useful for planning optical spectroscopy observing campaigns to supplement the radio data.