On the Root Cause of the Host "Mass Step" in the Hubble Residuals of Type Ia Supernovae

It is well established that the Hubble residuals of Type Ia supernovae (SNe Ia) show the luminosity step with respect to their host galaxy stellar masses. This "mass step" is taken as an additional correction factor for the SN Ia luminosity standardization. Here we investigate the root cause of the mass step and propose that the bimodal nature of the host age distribution is responsible for the step. In particular, by using the empirical nonlinear mass-to-age relation of local galaxies, we convert the mass function of SN Ia hosts to their age distribution. We find that the age distribution shows clear bimodality: a younger (<6 Gyr) group with lower mass (similar to 10(9.5)M(Sun)) and an older (> 6 Gyr) group with higher mass (similar to 10(10.5)M(Sun)). On the Hubble residual versus host-mass plane, the two groups create the mass step at similar to 10(10)M(Sun). This leads us to conclude that the host galaxy mass step can be attributed to the bimodal age distribution in relation to a nonlinear relation between galaxy mass and age. We suggest that the mass step is another manifestation of the old "red sequence" and the young "blue cloud" observed in the galactic color-magnitude diagram.