Comparing absorption from classic tracers of the diffuse interstellar medium at optical and radio wavelengths along the same sightlines

TL;DR

This study compares optical and radio absorption line profiles along the same sightlines to better understand the properties and kinematics of the diffuse interstellar medium, revealing correlations and differences among various tracers.

Contribution

It provides a comparative analysis of optical and radio tracers of the diffuse interstellar medium along the same sightlines, highlighting their kinematic and abundance relationships.

Findings

Optical atoms are heavily depleted compared to Solar abundance.

HI and HCO+ optical depths correlate well with E(B-V).

CaII traces warmer, broader-lined gas, behaving differently from other cold neutral medium tracers.

Abstract

% aims To compare information gained from radio and optical absorption line profiles from the diffuse interstellar medium along the same sightline % methods We compare new and existing 21cm HI and 3.4mm HCO+ profiles with profiles of the optical tracers CaII, NaI, and KI from an unpublished thesis of Tappe (2004) %results The atoms traced optically are all heavily depleted compared to a Solar abundance and only the integrated optical depths of HI and HCO+ correlate well with E(B-V). HCO+ is the species with by far the most limited kinematic distribution and the narrowest lines followed in order by KI, HI, NaI and CaII. CaII behaves separately in both column density and kinematics because it samples broader-lined warmer gas. Tracers of the cold neutral medium NaI, KI, HI and HCO+ share the same kinematic space statistically without correlating to nearly the same extent in abundance. N(NaI) and N(KI) are correlated, as are the integrated optical depths of HI and HCO+ but abundance correlations between optical and radio tracers are not seen. In the only direction with a measured CH+ profile, a 2 km/s velocity shift between CH+ and CH, usually interpreted as the sign of shocked gas, is mimicked in the shift of HI relative to HCO+. CH and HCO+ appear in the ratios N(CH)/N(HCO+)=14.6 and 21.1 along the two sightlines with optically-measured N(CH),compared with a mean of 12 determined previously at radio and submillimeter wavelengths.