The Clustering Characteristics of HI-Selected Galaxies from the 40% ALFALFA Survey

TL;DR

This paper analyzes the clustering properties of HI-selected, gas-rich galaxies from the ALFALFA survey, revealing their weak clustering and bias characteristics, and providing constraints for galaxy formation models.

Contribution

It presents the largest sample of HI-selected galaxies to date and measures their real-space correlation function, bias, and deviations from pure power-law clustering.

Findings

HI galaxies are the most weakly clustered known.

Measured correlation length r_0 = 3.3 h^{-1} Mpc.

HI galaxies show severe antibias on small scales.

Abstract

The 40% Arecibo Legacy Fast ALFA (ALFALFA) survey catalog (\alpha.40) of approximately 10,150 HI-selected galaxies is used to analyze the clustering properties of gas-rich galaxies. By employing the Landy-Szalay estimator and a full covariance analysis for the two-point galaxy-galaxy correlation function, we obtain the real-space correlation function and model it as a power law, \xi(r) = (r/r_0)^(-\gamma), on scales less than 10 h^{-1} Mpc. As the largest sample of blindly HI-selected galaxies to date, \alpha.40 provides detailed understanding of the clustering of this population. We find \gamma = 1.51 +/- 0.09 and r_0 = 3.3 +0.3, -0.2 h^{-1} Mpc, reinforcing the understanding that gas-rich galaxies represent the most weakly clustered galaxy population known; we also observe a departure from a pure power law shape at intermediate scales, as predicted in \Lambda CDM halo occupation distribution models. Furthermore, we measure the bias parameter for the \alpha.40 galaxy sample and find that HI galaxies are severely antibiased on small scales, but only weakly antibiased on large scales. The robust measurement of the correlation function for gas-rich galaxies obtained via the \alpha.40 sample constrains models of the distribution of HI in simulated galaxies, and will be employed to better understand the role of gas in environmentally-dependent galaxy evolution.