Detection Rate of Molecular Gas in Elliptical Galaxies: Constraints on Galaxy-Formation Theories

In order to constrain parameters in galaxy formation theories, especiallythose for a star formation process, we investigate cold gas in ellipticalgalaxies. We calculate the detection rate of cold gas in them using asemi-analytic model of galaxy formation and compare it with observations. Weshow that the model with a long star formation time-scale (~20 Gyr) isinconsistent with observations. Thus, some mechanisms of reducing the mass ofinterstellar medium, such as the consumption of molecular gas by star formationand/or reheating from supernovae, are certainly effective in galaxies. Ourmodel predicts that star formation induced when galaxies in a halo collide eachother reduces the cold gas left until the present. However, we find that thereduction through random collisions of satellite (non-central) galaxies in meanfree time-scale in a halo is not required to explain the observations. This mayimply that the collisions and mergers between satellite galaxies do not occurso often in clusters or that they do not stimulate the star formation activityas much as the simple collision model we adopted. For cD galaxies, thepredicted detection rate of cold gas is consistent with observations as long asthe transformation of hot gas into cold gas is prevented in halos whosecircular velocities are larger than 500 km s^-1. Moreover, we find that thecold gas brought into cDs through captures of gas-rich galaxies is little. Wealso show that the fraction of galaxies with observable cold gas should besmall for cluster ellipticals in comparison with that for field ellipticals.Comment: 6 pages, accepted by PAS