ZTF SN Ia DR2 follow-up: Characterization of subluminous Type Ia supernovae in the ZTF DR2 full sample

Abstract

The Zwicky Transient Facility Data Release 2 (ZTF DR2) includes a total of 3,628 Type Ia supernovae (SNe~Ia), providing the largest and most complete sample of spectroscopically confirmed SNe~Ia at low redshift to date. In this paper, we present a photometric and spectroscopic analysis of 124 subluminous SNe~Ia, the largest sample of spectroscopically classified subluminous SNe~Ia observed with a single instrument, comprising 87 91bg-like, 12 86G-like, 18 04gs-like, and 7 02es-like events. We complement the published DR2 SALT2 light-curve parameters with new parameters obtained using template-based fits from SNooPy. Expansion velocities and pseudo-equivalent widths pEW of key spectral features are measured using Spextractor, and spectral averages are constructed for each subluminous subtype, binned by phase. We also analyze the host galaxy environments, both global and local, in terms of $g - z$ color, stellar mass, and directional light radius $d_{DLR}$. 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. (2006) diagram, with the exception of 02es-like events, which show lower Si II 5972 pEW values. All subluminous subtypes tend to occur in more massive, redder host galaxies, and in the reddest local environments. Notably, 91bg- and 86G-like SNe~Ia explode at significantly larger normalized galactocentric distances. Finally, we identify the $pEW$ of the blended Ti II+Si II+Mg II absorption feature at 4300~A, along with s_BV, as robust and sufficient indicators for subclassifying subluminous SNe~Ia.