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

We calculated the detection rate of cold gas in elliptical galaxies using a semi-analytic model of galaxy formation, and compared it with observations. We have shown that a model with a long star-formation time-scale ( ∼ 20 Gyr) is inconsistent with observations. Thus, some mechanisms of reducing the mass of interstellar medium, such as the consumption of molecular gas by star formation and/or reheating from supernovae, are certainly effective in galaxies. Our model predicts that star formation is induced when galaxies in a halo collide with each other and it reduces the cold gas left until the present. However, we find that the reduction through random collisions of satellite (non-central) galaxies in the mean-free time-scale in a halo is not required to explain the observations. For cD galaxies, the predicted detection rate of cold gas is consistent with observations as long as the transformation of hot gas into cold gas is prevented in halos whose circular velocities are larger than 500;km; s⁻¹. Moreover, we find that the cold gas brought into cDs through capturing gas-rich galaxies is small. We also show that the fraction of galaxies with observable cold gas should be small for cluster ellipticals in comparison with that for field ellipticals. Our fiducial models and the models with a large reheating efficiency can reproduce the observations well, although a comparison with a larger and complete sample of elliptical galaxies will constrain the physical parameters in galaxy-formation theories more strictly.