ZTF SN Ia DR2 follow-up

We find that all subluminous SNe~Ia (except the 02es-like subtype) are intrinsically red. This is evident by separating extrinsic from intrinsic color components. Since SALT2 is not trained on subluminous SNe Ia, it compensates for their redder colors by inflating the c parameter, thus extending the luminosity- width relation to negative values of x1.

As expected, all subluminous SNe Ia fall within the Cool region of the Branch et al. (2006PASP..118..560B) diagram, with the exception of 02es-like events, which show lower SiII 5972 pEW values. All subluminous subtypes tend to occur in more massive, redder host galaxies, and in the reddest local environments within their stellar mass bins. Notably, 91bg- and 86G-like SNe Ia explode at significantly larger normalized galactocentric distances.

Finally, we identify the $pEW$ of the blended TiII+ SiII+MgII absorption feature at 4300{AA}, along with s_BV_, as robust and sufficient indicators for subclassifying subluminous SNe Ia.