Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging

TL;DR

This study uses deep Hubble imaging to confirm that some Type Ia supernovae in galaxy clusters lack host galaxies, indicating they originate from intracluster stars and highlighting the importance of deep imaging for understanding their progenitors.

Contribution

First deep Hubble imaging confirms that some hostless SNe Ia originate from intracluster stars, refining understanding of their progenitors and environments.

Findings

Two SNe Ia have no associated host galaxy, confirming intracluster origin.

One SN Ia is near a faint galaxy with a chance alignment probability.

A faint source may be a globular cluster or dwarf galaxy, affecting SN Ia rate estimates.

Abstract

We present deep Hubble Space Telescope imaging at the locations of four, potentially hostless, long-faded Type Ia supernovae (SNe Ia) in low-redshift, rich galaxy clusters that were identified in the Multi-Epoch Nearby Cluster Survey. Assuming a steep faint-end slope for the galaxy cluster luminosity function ($\alpha_d=-1.5$), our data includes all but $\lesssim0.2\%$ percent of the stellar mass in cluster galaxies ($\lesssim0.005\%$ with $\alpha_d=-1.0$), a factor of 10 better than our ground-based imaging. Two of the four SNe Ia still have no possible host galaxy associated with them ($M_R>-9.2$), confirming that their progenitors belong to the intracluster stellar population. The third SNe Ia appears near a faint disk galaxy ($M_V=-12.2$) which has a relatively high probability of being a chance alignment. A faint, red, point source coincident with the fourth SN Ia's explosion position ($M_V=-8.4$) may be either a globular cluster (GC) or faint dwarf galaxy. We estimate the local surface densities of GCs and dwarfs to show that a GC is more likely, due to the proximity of an elliptical galaxy, but neither can be ruled out. This faint host implies that the SN Ia rate in dwarfs or GCs may be enhanced, but remains within previous observational constraints. We demonstrate that our results do not preclude the use of SNe Ia as bright tracers of intracluster light at higher redshifts, but that it will be necessary to first refine the constraints on their rate in dwarfs and GCs with deep imaging for a larger sample of low-redshift, apparently hostless SNe Ia.