# An accurate low-redshift measurement of the cosmic neutral hydrogen density

## TL;DR

This study uses spectral stacking of radio data from SDSS galaxies to precisely measure the cosmic neutral hydrogen density at low redshift, confirming its stability over recent cosmic times.

## Contribution

It provides a new, robust estimate of the cosmic HI mass density at low redshift using a carefully calibrated stacking technique and addresses systematic uncertainties.

## Key findings

- Measured $
ho_{HI}$ at $z hickapprox 0.07$ with high precision.
- Found no significant evolution of HI content at low redshift.
- Results are consistent with previous surveys and stacking experiments.

## Abstract

Using a spectral stacking technique, we measure the neutral hydrogen (HI) properties of a sample of galaxies at $z < 0.11$ across 35 pointings of the Westerbork Synthesis Radio Telescope (WSRT). The radio data contains 1,895 galaxies with redshifts and positions known from the Sloan Digital Sky Survey (SDSS). We carefully quantified the effects of sample bias, aperture used to extract spectra, sidelobes and weighting technique and use our data to provide a new estimate for the cosmic HI mass density. We find a cosmic HI mass density of $\Omega_{\rm HI} = (4.02 \pm 0.26)\times 10^{-4} h_{70}^{-1}$ at $\langle z\rangle = 0.066$, consistent with measurements from blind HI surveys and other HI stacking experiments at low redshifts. The combination of the small interferometer beam size and the large survey volume makes our result highly robust against systematic effects due to confusion at small scales and cosmic variance at large scales. Splitting into three sub-samples with $\langle z\rangle$ = 0.038, 0.067 and 0.093 shows no significant evolution of the HI gas content at low redshift.