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arXiv · 2310.06011

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

Chul Chung·Suk-Jin Yoon·Seunghyun Park·Seunghyeon An·Junhyuk Son·Hyejeon Cho·Young-Wook Lee
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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.

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Chul Chung, Suk-Jin Yoon, Seunghyun Park, Seunghyeon An, Junhyuk Son, Hyejeon Cho, Young-Wook Lee. 2023-10-09. On the root cause of the host `mass-step' in the Hubble residuals of type Ia supernovae. https://arxiv.org/abs/2310.06011

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