The HI reservoir in central spiral galaxies and the implied star formation process

TL;DR

This study reveals that central spiral galaxies retain abundant atomic hydrogen gas even when their star formation rates decline, indicating that star formation cessation is linked to molecular hydrogen supply, not atomic hydrogen availability.

Contribution

It provides new evidence that atomic hydrogen reservoirs are common in central spirals regardless of star formation activity, highlighting the different roles of HI and H2 in galaxy evolution.

Findings

HI gas mass fraction is constant across different sSFR at fixed stellar mass.

Decline in star formation is associated with halted H2 supply, despite abundant HI.

Central spirals maintain significant atomic gas even with low star formation rates.

Abstract

The cold interstellar medium (ISM) as the raw material for star formation is critical to understanding galaxy evolution. It is generally understood that galaxies stop making stars when, in one way or another, they run out of gas. However, here we provide evidence that central spiral galaxies remain rich in atomic gas even if their star formation rate and molecular gas fraction have dropped significantly compared to "normal" star-forming galaxies of the same mass. Since HI is sensitive to external processes, here we investigate central spiral galaxies using a combined sample from SDSS, ALFALFA, and xGASS surveys. After proper incompleteness corrections, we find that the key HI scaling relations for central spirals show significant but regular systematic dependence on stellar mass. At any given stellar mass, the HI gas mass fraction is about constant with changing specific star formation rate (sSFR), which suggests that HI reservoir is ubiquitous in central spirals with any star formation status down to M* ~ 10^9 Msun. Together with the tight correlation between the molecular gas mass fraction and sSFR for galaxies across a wide range of different properties, it suggests that the decline of SFR of all central spirals in the local universe is due to the halt of H2 supply, though there is plenty of HI gas around. These hence provide critical observations of the dramatically different behavior of the cold multi-phase ISM, and a key to understand the star formation process and quenching mechanism.