On the root cause of the host `mass-step' in the Hubble residuals of type Ia supernovae

TL;DR

This paper investigates the origin of the host galaxy mass-step in Type Ia supernovae luminosity residuals, attributing it to the bimodal age distribution of host galaxies linked to a nonlinear mass-age relation.

Contribution

It proposes that the host galaxy mass-step arises from the bimodal age distribution of galaxies, connecting it to the red sequence and blue cloud in galaxy populations.

Findings

Bimodal age distribution of SN Ia hosts identified.

Mass-step correlates with the transition between young and old galaxy groups.

Nonlinear mass-age relation explains the observed mass-step.

Abstract

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 ($\sim 10^{9.5}{\rm M}_{\rm sun}$) and an older ($>$ 6 Gyr) group with higher mass ($\sim 10^{10.5}{\rm M}_{\rm sun}$). On the Hubble residual versus host mass plane, the two groups create the mass-step at $\sim 10^{10}{\rm M}_{\rm sun}$. 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.