Narrow absorption lines from intervening material in supernovae. IV. Type Ia supernovae: Na I D line strength relating to external material and intrinsic properties

Type Ia supernovae (SNe Ia) are thermonuclear runaways in certain white dwarfs in binary systems. They have been extensively studied, yet their progenitor and explosion mechanisms remain poorly understood. We study a large sample of SNe Ia comparing the narrow interstellar absorption features in their spectra with various photometric and spectroscopic supernova properties, as well as with environmental characteristics. We find that the sodium absorption is significantly stronger in younger, more star-forming and more centrally located SNe Ia, as expected. However, we also show that there is a strong dependence on intrinsic properties that is independent of the environment. In fact, we find strong evidence for two environmental SN Ia populations, an old and a young one, with the young population showing significantly different distributions of sodium strength when divided according to the Si II ejecta velocity, nebular velocity, extinction, E(B-V), and reddening curve, RV. Performing a clustering of the SNe Ia, we recover an old population of SNe with low extinction and normal ejecta velocity, while we confirm that the young population can be subdivided into a group of highly-extincted, high-velocity SNe Ia with much stronger blueshifted sodium absorption, and another of low-extincted, normal-velocity objects with little sodium absorption. We interpret this relation of intervening material with intrinsic properties as evidence for the young SN Ia population, occurring in young and star-forming environments, to have asymmetric radiation that interacts with nearby material, and whose observables depend thus on the viewing angle. Finally, we show that the cosmological mass-step is consistent with these populations.

💡 Research Summary

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This paper presents a comprehensive analysis of narrow Na I D absorption lines in a large sample of Type Ia supernovae (SNe Ia) and investigates how these features relate to both the surrounding interstellar/circumstellar material and the intrinsic properties of the explosions. The authors assembled a homogeneous dataset of 981 SNe Ia (3230 spectra) with well‑sampled photometry and host‑galaxy imaging. An automated pipeline measured the equivalent width (EW) and velocity shift (VEL) of the Na I D doublet, carefully handling low‑resolution spectra, continuum placement, and signal‑to‑noise cuts. When multiple spectra were available for a single object, a bootstrap stacking approach was used to obtain a robust EW.

Host‑galaxy environmental parameters were derived from multi‑band (UV–IR) imaging using both global Kron apertures and local 0.5 kpc apertures. Spectral energy‑distribution fitting yielded stellar mass (M*), stellar age (t_age), star‑formation rate (SFR), specific SFR (sSFR), and dust attenuation (A_V). The projected galactocentric offset (Δα) was also recorded. Supernova photometric properties were obtained with two light‑curve fitters: SiFTO (providing stretch s and colour C) and SNooPy (yielding colour‑stretch s_BV, colour excess E(B‑V), total‑to‑selective extinction ratio R_V, and late‑time colour slope dBV_60). Spectroscopic diagnostics included Si II λ6355 Å maximum velocity (v_max), its temporal gradient (v_grad), and nebular‑phase forbidden‑line velocities (v_neb) from