Exploring the origin of cold gas and star formation in a rare population of strongly bulge-dominated early-type Galaxies

TL;DR

This study investigates a rare class of strongly bulge-dominated early-type galaxies with large HI reservoirs, revealing their external gas origins and recent star formation activity despite their quiescent classification.

Contribution

It provides the first detailed analysis of HI-rich sBDEs, highlighting their unique properties, environment, and star formation characteristics compared to typical early-type galaxies.

Findings

Most HI-rich sBDEs are quiescent despite large HI reservoirs.

Star-forming sBDEs have higher star formation efficiency and older stellar populations.

HI profiles in star-forming sBDEs are narrower and more concentrated.

Abstract

We analyze the properties of a rare population, the strongly bulge-dominated early-type galaxies (referred to as sBDEs) with significant HI gas, using the databases from the FAST All Sky HI survey (FASHI) and the Arecibo Legacy Fast ALFA (ALFALFA) survey. We select the sBDEs from the Sloan Digital Sky Survey (SDSS) and cross-match with the FASHI-ALFALFA combined HI sample, resulting in 104 HI-rich sBDEs. These sBDEs tend to have extremely high HI reservoirs, which is rare in previous studies such as ATLAS$^{3D}$. 70% of the selected sBDEs are classified as quiescent galaxies, even though they have a large HI reservoir. We study the properties of these sBDEs from five main aspects: stellar population, gas-phase metallicity, stacked HI spectra, environment, and spatially resolved MaNGA data. The majority of HI-rich sBDEs appear to show lower gas-phase metallicity and are located in significantly lower-density environments, suggesting an external origin for their HI gas. We find that star-forming sBDEs exhibit statistically higher star formation efficiency and slightly older stellar populations compared to normal star-forming galaxies, suggesting a recent star formation on Gyr-timescale. They also show narrower and more concentrated HI profiles compared to control star-forming galaxies, which may explain their higher star formation efficiency.