Statistical distribution of HI 21cm intervening absorbers as potential cosmic acceleration probes

TL;DR

This paper statistically analyzes the distribution of HI 21cm absorbers to evaluate their potential as probes for cosmic acceleration, providing predictions for upcoming radio surveys.

Contribution

It introduces a statistical framework using Kernel Density Estimation to predict the number of detectable HI 21cm absorbers in future surveys, considering various observational limitations.

Findings

FAST can detect about 80 H21As under optimal conditions.

ASKAP can detect about 500 H21As under optimal conditions.

SKA1-Mid can detect about 600 H21As under optimal conditions.

Abstract

Damped Lyman-$\alpha$ Absorber (DLA), or HI 21cm Absorber (H21A), is an important probe to model-independently measure the acceleration of spectroscopic velocity ($v_\mathrm{S}$) via the Sandage-Loeb (SL) effect. Confined by the shortage of DLAs and Background Radio Sources (BRSs) with adequate information, the detectable amount of DLAs is ambiguous in the bulk of previous work. After differing the acceleration of scale factor ($\ddot{a}$) from the first order time derivative of spectroscopic velocity ($\dot{v}_\mathrm{S}$), we make a statistical investigation of the amount of potential DLAs in the most of this paper. Using Kernel Density Estimation (KDE) to depict general redshift distributions of BRSs, observed DLAs and a DLA detection rate with different limitations (1.4GHz flux, HI column density and spin temperature), we provide fitted multi-Gaussian expressions of the three components and their 1$\sigma$ regions by bootstrap, with a proportional constant of H21As in detected DLAs, leading to the measurable number predictions of H21As for FAST, ASKAP and SKA1-Mid in HI absorption blind survey. In our most optimistic condition ($F_\mathrm{1.4GHz}$>10mJy, $N_\mathrm{HI}>2\times10^{20}\mathrm{cm^{-2}}$ and $T_\mathrm{S}$>500K), the FAST, AKSAP and SKA1-Mid would probe about 80, 500 and 600 H21As respectively.