Extragalactic HI Surveys

TL;DR

This review summarizes major extragalactic HI line surveys, highlighting their technical design, key findings on HI source populations, and the role of HI data in understanding galaxy evolution and cosmology.

Contribution

It provides a comprehensive overview of the design, results, and implications of recent large-scale HI surveys, emphasizing their impact on galaxy and cosmological studies.

Findings

HI surveys have established statistical properties of HI sources.

Differences between optical and HI galaxy clustering are significant.

HI data help resolve conflicts between observations and theoretical models.

Abstract

We review the results of HI line surveys of extragalactic sources in the local Universe. In the last two decades major efforts have been made in establishing on firm statistical grounds the properties of the HI source population, the two most prominent being the HI Parkes All Sky Survey (HIPASS) and the Arecibo Legacy Fast ALFA survey (ALFALFA). We review the choices of technical parameters in the design and optimization of spectro-photometric "blind" HI surveys, which for the first time produced extensive HI-selected data sets. Particular attention is given to the relationship between optical and HI populations, the differences in their clustering properties and the importance of HI-selected samples in contributing to the understanding of apparent conflicts between observation and theory on the abundance of low mass halos. The last section of this paper provides an overview of currently ongoing and planned surveys which will explore the cosmic evolution of properties of the HI population.