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

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB Carrer de Can Magrans s/n E-08193 Barcelona, Spain
² Institut d’Estudis Espacials de Catalunya (IEEC) E-08034 Barcelona, Spain
³ School of Physics, Trinity College Dublin, The University of Dublin Dublin 2, Ireland
⁴ Department of Physics, Lancaster University Lancaster LA1 4YB, UK
⁵ Oskar Klein Centre, Department of Physics, Stockholm University SE-10691 Stockholm, Sweden
⁶ Instituto de Ciencias Exactas y Naturales (ICEN), Universidad Arturo Prat Iquique, Chile
⁷ School of Physics and Astronomy, University of Birmingham Birmingham, UK
⁸ Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University Seoul 03722, Republic of Korea
⁹ Institut für Physik, Humboldt-Universität zu Berlin Newtonstr. 15 12489 Berlin, Germany
¹⁰ Univ. Lyon, Univ. Claude Bernard Lyon 1, CNRS, IP2I Lyon/IN2P3, UMR 5822 F-69622 Villeurbanne, France
¹¹ Oskar Klein Centre, Department of Astronomy, Stockholm University SE-10691 Stockholm, Sweden
¹² Caltech Optical Observatories, California Institute of Technology Pasadena CA 91125, USA
¹³ IPAC, California Institute of Technology 1200 E. California Blvd Pasadena CA 91125, USA
¹⁴ Division of Physics, Mathematics and Astronomy, California Institute of Technology Pasadena CA 91125, USA

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Received: 16 June 2025
Accepted: 19 December 2025

Abstract

The Zwicky Transient Facility Data Release 2 (ZTF DR2) includes a total of 3628 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 Type Ia supernova 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. We measured the expansion velocities and pseudo-equivalent widths (pEW) of key spectral features using SPEXTRACTOR. Next, the spectral averages were constructed for each subluminous subtype, binned by phase. We also analyzed the host galaxy environments, both global and local, in terms of g − z color, stellar mass, and directional light radius (d_DLR). We found that all subluminous SNe Ia (except the 02es-like subtype) are intrinsically red. This is made evident when we separate the extrinsic color components from intrinsic ones. Since SALT2 has not been trained on subluminous SNe Ia, it compensates for their redder colors by inflating the c parameter, thereby 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, PASP, 118, 560) diagram, with the exception of 02es-like events, which display lower Si IIλ5972 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 identified the pEW of the blended Ti II+Si II+Mg II absorption feature at 4300 Å, along with s_BV, as robust and sufficient indicators for subclassifying subluminous SNe Ia.