Narrow absorption lines from intervening material in supernovae

Narrow interstellar absorption features in supernova (SN) spectra serve as valuable diagnostics for probing dust extinction and the presence of circumstellar or interstellar material. In this third paper in a series, we investigate how the strength of narrow interstellar absorption lines in low-resolution spectra varies with SN type and host galaxy properties, on both local and global scales. Using a dataset of over 10 000 spectra from ∼1800 low-redshift SNe, we find that Type Ia SNe (SNe Ia) in passive galaxies exhibit significantly weaker narrow absorption features compared to core collapse SNe (CCSNe) and SNe Ia in star-forming hosts (SNe Ia-SF), suggesting lower interstellar gas content in quiescent environments. Within the star-forming hosts, the Na I D equivalent-width distribution of SNe II is much lower than that of both SNe Ia-SF and stripped-envelope SNe (SE-SNe). This result is somewhat unexpected, since CCSNe are generally associated with star-forming regions and occur deeper within galactic disks, where a stronger line-of-sight extinction would be anticipated. This suggests that the observed behaviour cannot be explained solely by absorption from the integrated interstellar medium (ISM) along the line of sight. Instead, if part of the absorption arises from material near the explosion, the similarity between the Na I D EW distributions of SNe Ia-SF and SE-SNe implies that comparable absorption signatures can emerge from distinct progenitor pathways. Possible explanations include (a) circumstellar material expelled by the progenitor system before explosion or (b) interaction of SN radiation with nearby patchy ISM clouds. Our results highlight the diagnostic power of interstellar absorption features in revealing the diverse environments and progenitor pathways of SNe.