2017A&A...606A.111F

Context. Type Ia supernovae (SNe Ia) can be used to address numerous questions in astrophysics and cosmology. Due to their well known spectral and photometric properties, SNe Ia are well suited to study gas and dust along the lines-of-sight to the explosions. For example, narrow NaI D and CaII H&K absorption lines can be studied easily, because of the well-defined spectral continuum of SNe Ia around these features.
Aims. We aim to study the gas and dust along the line-of-sight to iPTF16abc, which occurred in an unusual location, in a tidal arm, 80kpc from centre of the galaxy NGC 5221.
Methods. Using a time-series of high-resolution spectra, we have examined narrow NaI D and CaII H&K absorption features for variations in time, which would be indicative for circumstellar (CS) matter. Furthermore, we have taken advantage of the well known photometric properties of SNe Ia to determine reddening due to dust along the line-of-sight.
Results. From the lack of variations in NaI D and CaII H&K, we determine that none of the detected absorption features originate from the CS medium of iPTF16abc. While the NaI D and CaII H&K absorption is found to be optically thick, a negligible amount of reddening points to a small column of interstellar dust.
Conclusions. We find that the gas along the line-of-sight to iPTF16abc is typical of what might be found in the interstellar medium (ISM) within a galaxy. It suggests that we are observing gas that has been tidally stripped during an interaction of NGC 5221 with one of its neighbouring galaxies in the past ∼ 10⁹yr. In the future, the gas clouds could become the locations of star formation. On a longer time scale, the clouds might diffuse, enriching the circum-galactic medium (CGM) with metals. The gas profile along the line-of-sight should be useful for future studies of the dynamics of the galaxy group containing NGC 5221.