Interstellar Sodium and Calcium Absorption toward SN 2011dh in M51

TL;DR

This study uses high-resolution spectroscopy of SN 2011dh in M51 to analyze interstellar sodium and calcium absorption, revealing complex gas structures and low extinction along the line of sight.

Contribution

It provides detailed velocity and composition data of interstellar gas in M51 and the Milky Way toward SN 2011dh, highlighting the gas distribution and properties in the galaxy.

Findings

Multiple absorption components detected in Na I D and Ca II lines.

M51 gas components span over 140 km/s, beyond H I emission range.

Low Na I D absorption indicates minimal foreground and host galaxy extinction.

Abstract

We present high-resolution echelle observations of SN 2011dh, which exploded in the nearby, nearly face-on spiral galaxy M51. Our data, acquired on three nights when the supernova was near maximum brightness, reveal multiple absorption components in Na I D and Ca II H and K, which we identify with gaseous material in the Galactic disk or low halo and in the disk and halo of M51. The M51 components span a velocity range of over 140 km s^-1, extending well beyond the range exhibited by H I 21 cm emission at the position of the supernova. Since none of the prominent Na I or Ca II components appear to coincide with the peak in H I emission, the supernova may lie just in front of the bulk of the H I disk. The Na I/Ca II ratios for the components with the most extreme positive and negative velocities relative to the disk are ~1.0, similar to those for more quiescent components, suggesting that the absorption originates in relatively cool gas. Production scenarios involving a galactic fountain and/or tidal interactions between M51 and its companion would be consistent with these results. The overall weakness of Na I D absorption in the direction of SN 2011dh confirms a low foreground and host galaxy extinction for the supernova.