HISS, a new tool for H I stacking: application to NIBLES spectra

H I stacking has proven to be a highly effective tool to statistically analyse average H I properties for samples of galaxies which may or may not be directly detected. With the plethora of H I data expected from the various upcoming H I surveys with the SKA Precursor and Pathfinder telescopes, it will be helpful to standardize the way in which stacking analyses are conducted. In this work we present a new PYTHON-based package, HISS, designed to stack H I (emission and absorption) spectra in a consistent and reliable manner. As an example, we use HISS to study the H I content in various galaxy sub-samples from the NIBLES survey of SDSS galaxies which were selected to represent their entire range in total stellar mass without a prior colour selection. This allowed us to compare the galaxy colour to average H I content in both detected and non-detected galaxies. Our sample, with a stellar mass range of 10^8 \lt {{ M}}_\star (M_\odot) \lt 10^{12}, has enabled us to probe the H I-to-stellar mass gas fraction relationship more than half an order of magnitude lower than in previous stacking studies.