Decoding the Single-peaked HI Spectra of Low Redshift Post-starburst Galaxies

TL;DR

This study analyzes the shapes of HI spectra in low redshift post-starburst galaxies, revealing that most have centrally concentrated gas rather than unsettled HI, which may relate to their rapid star formation cessation.

Contribution

It introduces the $K$-$A_{F}$ spectral diagnostic plane to distinguish between central concentration and unsettled HI in galaxies using integrated spectra.

Findings

Most PSBs show high spectral concentration ($K$) indicating central gas buildup.

The $K$-$A_{F}$ plane effectively differentiates between settled and unsettled HI.

Most PSBs are dominated by centrally concentrated HI rather than unsettled gas.

Abstract

Recent observations with the Five-hundred-meter Aperture Spherical Telescope (FAST) have revealed abundant reservoirs of neutral hydrogen (HI) in low redshift post-starburst galaxies (PSBs), raising the question of why star formation ceases rapidly in these systems. In this study, we present a detailed analysis of the shape of the integrated HI spectra of 67 PSBs. We find that PSBs exhibit significantly higher HI spectral concentration values ($K$) compared to a matched sample from xGASS, and are more comparable to those of starburst galaxies. By extending our analysis to spatially resolved HI data from THINGS and ATLAS$^{\rm 3D}$, we show that both centrally concentrated HI distributions and dynamically unsettled HI can effectively increase $K$, while non-axisymmetric structures only contribute to the scatter of the $K$ distribution. Distinguishing between central concentration and dynamically unsettled gas as the origin of high $K$ can be achieved by measuring the spectral asymmetry ($A_{\rm F}$), making the $K$-$A_{\rm F}$ plane a powerful diagnostic tool for identifying galaxies with unsettled HI using integrated spectra alone. Based on their location in the $K$-$A_{\rm F}$ plane, we find that most PSBs are not dominated by unsettled HI, but rather exhibit elevated central gas concentration. Both modes of gas redistribution in PSBs may eventually contribute to their quenching.