Extragalactic HI spectra data processing method based onStacking
Abstract
Blind HI survey is a powerful tool for exploring the HI universe. During the extragalactic HI survey, due to the weakness of HI signal and the limitation of telescope sensibility, most HI emissions from galaxies are overwhelmed by noise signals and cannot be directly detected. However, using the technique of spectral stacking, we can increase the the signal-to-noise ratio (SNR) by co-adding a set of spectra if the optical redshifts and position are known. This paper presents a method to derive the integrated HI emission in a galaxy group by stacking all the HI spectra of the member galaxies in the group. This method can generate a stacked spectrum representing the total HI emission of the group. The stacked spectrum have a lower rms noise level and thus higher S/N ratio than that of the individual member galaxies. In some cases, the integrated HI emission shows clear detection (S/N$>$4.5) in the stacked spectrum even though no detection is visible in the spectrum of individual member galaxy in the group. We have also developed a software code to automatically handle the stacking of large amount spectra in galaxy groups. As an example, we use this software to derive the stacked HI spectra of 80 galaxy groups selected from the SDSS galaxy group catalog, of which the HI data are from the ALFALFA HI survey. Such software will be a valuable tool to process the stacking of large mount of HI spectra from the future extragalactic HI survey with the Five hundred meter Aperture Spherical radio Telescope (FAST).
