Supernova Cousins: The Intrinsic Dispersion and Environmental Dependence of Type Ia Supernovae in Galaxy Groups with TITAN

RESUMO

At low redshift, scatter in the Type Ia supernova (SN Ia) Hubble diagram is increased by peculiar velocities (PVs) and correlations between SN distances and their host-galaxy environments, limiting the precision of cosmological distance measurements. We investigate these effects using 1086 spectroscopically-confirmed SNe Ia from the Type Ia Supernova Trove from ATLAS in the Nearby Universe (TITAN) sample in the redshift range $0.01<z<0.05$. We identify 314 group-associated SNe, including 80 SN "cousins" occurring in different galaxies within 32 common groups. We first examine whether using the group-averaged redshifts of SNe in galaxy groups can mitigate PV-induced scatter; however, we find that the Hubble residual scatter increases slightly from $0.184^{+0.017}_{-0.009}$ mag using individual host-galaxy redshifts to $0.200^{+0.016}_{-0.015}$ mag using group-averaged redshifts. For the sample of SN cousins, we next analyze pairwise differences in their distances and compare their scatter to the Hubble residual scatter of the full SN sample. For analysis variants that limit line-of-sight differences in distance modulus between cousins, we find $\sim$30-40% lower scatter from SN cousins at significances of 2.3-2.8$\sigma$. We also find tentative evidence that these differences in scatter may be driven by the youngest groups, with a 2$\sigma$ reduction in scatter for groups with high fractions of spiral galaxies. These results suggest that SN cousins may provide a novel route for increasing SN Ia distance precision and understanding the role of host-galaxy and group properties on SN Ia distance measurements.