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 stee
p 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 Ias 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.
We present an optical nebular spectrum of the nearby Type Ia supernova 2011fe, obtained 981 days after explosion. SN 2011fe exhibits little evolution since the +593 day optical spectrum, but there are several curious aspects in this new extremely lat
e-time regime. We suggest that the persistence of the $sim5800$~AA feature is due to Na I D, and that a new emission feature at $sim7300$~AA may be [Ca II]. Also, we discuss whether the new emission feature at $sim6400$~AA might be [Fe I] or the high-velocity hydrogen predicted by Mazzali et al. The nebular feature at 5200~AA exhibits a linear velocity evolution of $sim350$ $rm km s^{-1}$ per 100 days from at least +220 to +980 days, but the lines shape also changes in this time, suggesting that line blending contributes to the evolution. At $sim 1000$ days after explosion, flux from the SN has declined to a point where contribution from a luminous secondary could be detected. In this work we make the first observational tests for a post-impact remnant star and constrain its temperature and luminosity to $T gtrsim 10^4$ $rm K$ and $L lesssim 10^4$ $rm L_{odot}$. Additionally, we do not see any evidence for narrow H$alpha$ emission in our spectrum. We conclude that observations continue to strongly exclude many single-degenerate scenarios for SN 2011fe.
