Comparison of HI and optical redshifts of galaxies - The impact of redshift uncertainties on spectral line stacking
Monthly Notices of the Royal Astronomical Society
Astrophysics, Cosmology and Gravity Centre (ACGC), Astronomy Department, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa; Oxford Astrophysics, Denys Wilkinson Building, University of Oxford, Kehle Rd, Oxford OX1 3RH, United Kingdom; Physics Department, University of the Western Cape, 7535 Cape Town, South Africa
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 neutral hydrogen (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 Sloan Digital Sky Survey and Arecibo Legacy Fast ALFA survey to investigate the relationship between redshifts derived from optical spectroscopy and 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-1, 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.© 2013 The Authors.