# Supernovae and their host galaxies -- VI. Normal Type Ia and 91bg-like supernovae in ellipticals

## TL;DR

This study analyzes the distribution and host galaxy properties of normal and 91bg-like Type Ia supernovae in elliptical galaxies, revealing differences in their host galaxy colors and ages that suggest distinct progenitor delay times.

## Contribution

It provides new insights into the environmental differences between normal and 91bg-like SNe Ia in ellipticals, supporting specific progenitor models like He-ignited violent mergers.

## Key findings

- Normal SN hosts are bluer and younger than 91bg-like hosts.
- Normal SNe Ia are associated with galaxies in the Green Valley, indicating residual star formation.
- 91bg-like SNe Ia occur in older, redder ellipticals with less recent star formation.

## Abstract

We present an analysis of the galactocentric distributions of the "normal" and peculiar "91bg-like" subclasses of 109 supernovae (SNe) Ia, and study the global parameters of their elliptical hosts. The galactocentric distributions of the SN subclasses are consistent with each other, and with the radial light distribution of host stellar populations, when excluding bias against central SNe. Among the global parameters, only the distributions of u-r colours and ages are inconsistent significantly between the ellipticals of different SN Ia subclasses: the normal SN hosts are on average bluer/younger than those of 91bg-like SNe. In the colour-mass diagram, the tail of colour distribution of normal SN hosts stretches into the Green Valley - transitional state of galaxy evolution, while the same tail of 91bg-like SN hosts barely reaches that region. Therefore, the bluer/younger ellipticals might have more residual star formation that gives rise to younger "prompt" progenitors, resulting in normal SNe Ia with shorter delay times. The redder and older ellipticals that already exhausted their gas for star formation may produce significantly less normal SNe with shorter delay times, outnumbered by "delayed" 91bg-like events. The host ages (lower age limit of the delay times) of 91bg-like SNe does not extend down to the stellar ages that produce significant u-band fluxes - the 91bg-like events have no prompt progenitors. Our results favor SN Ia progenitor models such as He-ignited violent mergers that have the potential to explain the observed SN/host properties.