The Origin of the Gas and Its Low Star Formation Efficiency in Quiescent Galaxies

Quiescent galaxies (QGs) typically have little cold gas to form stars. The discovery of gas-rich QGs challenges our conventional understanding of the evolutionary paths of galaxies. We take advantage of a new catalog of nearby, massive galaxies with robust, uniformly derived physical properties to better understand the origin of gas-rich QGs. We perform a comparative analysis of the cold interstellar medium and star formation properties of carefully matched samples of galaxies with varying degrees of star formation activity and gas richness. QGs with different gas content have virtually identical morphological types, light concentration, mass-size relation, stellar age, dark matter halo mass, and black hole activity. The only distinguishing characteristic is the environment. Gas-rich satellite QGs reside in a lower-density environment than their gas-poor counterparts, as a consequence of which they manage to retain their gas and experience a higher probability of cold gas accretion or gas-rich mergers. The environmental densities of central QGs are similar regardless of their gas content. We suggest that the cold gas resides mainly in the outskirts of the gas-rich QGs, where bars, if present, cannot transport it inward efficiently to fuel central star formation. The prominent bulges in gas-rich QGs stabilize the cold gas from fragmentation and leads to low star formation efficiency.