ZTF SN Ia DR2 follow-up: Exploring the origin of the Type Ia supernova host galaxy step through Si II velocities

¹ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
² Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁴ Université Lyon, CNRS, IP2I Lyon/IN2P3, UMR 5822, F-69622 Villeurbanne, France
⁵ Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University, Seoul, 03722 Republic of Korea
⁶ Oskar Klein Centre, Department of Astronomy, Stockholm University, SE-10691 Stockholm, Sweden
⁷ Instituto de Ciencias Exactas y Naturales (ICEN), Universidad Arturo Prat, Iquique, Chile
⁸ Department of Physics, Lancaster University, Lancaster, LA1 4YB UK
⁹ The Oskar Klein Centre, Department of Physics, Stockholm University, SE-10691 Stockholm, Sweden
¹⁰ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
¹¹ Lawrence Berkeley National Laboratory, 1 Cyclotron Road MS 50B-4206, Berkeley, CA, 94720 USA
¹² Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA, 94720 USA
¹³ Caltech Optical Observatories, California Institute of Technology, Pasadena, CA, 91125 USA
¹⁴ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA, 91125 USA
¹⁵ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA, 91125 USA
¹⁶ Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL, 60208 USA
¹⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), 1800 Sherman Ave., Evanston, IL, 60201 USA
¹⁸ NSF-Simons AI Institute for the Sky (SkAI), 172 E. Chestnut St., Chicago, IL, 60611 USA

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Received: 11 July 2025
Accepted: 26 October 2025

Abstract

The relation between Type Ia supernovae (SNe Ia) and the stellar masses of their host galaxy is well documented. In particular, Hubble residuals display a distinct luminosity shift based on host mass. This is known as the mass step. This effect is widely used as an additional correction factor in the standardisation of SN Ia luminosities. We investigate the Hubble residuals and the mass step of normal SNe Ia in the context of Si IIλ6355 velocities based on 277 normal SNe Ia that are near their peak in the second data release (DR2) of the Zwicky Transient Facility (ZTF). We divided the sample into high-velocity (HV) and normal-velocity (NV) SNe Ia, separated at 12,000 km s⁻¹. This produced a sample of 70 HV and 207 NV objects. We then explored potential environment- and/or progenitor-related effects by investigating the Si IIλ6355 velocities with parameters such as the light-curve stretch x₁, the colour c, and the host galaxy properties. Although we only find a marginal difference between the Hubble residuals of HV and NV SNe Ia, the NV mass step is 0.149 ± 0.024 mag (6.3σ). The HV mass step is smaller, 0.046 ± 0.041 mag (1.1σ), and is consistent with zero. The difference between the NV and HV mass steps is modest, at ∼2.2σ. Moreover, the clearest subtype difference appears for SNe in central regions (d_DLR < 1), where NV SNe Ia show a large mass step, whereas HV SNe Ia are consistent with no step, yielding a difference of 3.1–3.6σ between NV and HV SNe Ia. We observe a host-colour step for both subtypes. NV SNe Ia show a step of 0.142 ± 0.024 mag (5.9σ), while HV SNe Ia show a step of 0.158 ± 0.042 mag (3.8σ), where the HV SNe Ia step appears to be larger, but the significance is lower because the sample size is smaller. Overall, the NV and HV colour steps are statistically consistent. HV SNe Ia also show modest (∼2.5–3σ) steps in certain subsets, such as those in outer regions (d_DLR > 1), whereas NV SNe display stronger environmental trends. Our results indicate that NV SNe Ia appear to be more environmentally sensitive, particularly in central likely metal-rich and older regions, while HV SNe Ia show weaker and subset-dependent trends. This suggests that applying a universal mass-step correction might introduce biases, and that incorporating refined classifications and/or environment-dependent factors, such as the location within the host, might improve future cosmological analyses beyond the standard x₁ and c cuts.