The influence of atomic alignment on absorption and emission spectroscopy

TL;DR

This paper investigates how atomic alignment caused by anisotropic radiation and magnetic fields significantly affects astrophysical spectroscopic measurements, emphasizing the need for correction in interstellar gas analysis.

Contribution

It provides a comprehensive analysis of atomic alignment effects on spectroscopy, including correction expressions and practical application to interstellar medium studies.

Findings

Atomic alignment influences spectral line ratios and physical parameter estimates.

Corrections for atomic alignment are essential for accurate spectroscopic analysis.

Synthetic observations demonstrate the detectability of these effects with current instruments.

Abstract

Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in the universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionisation fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of PDRs in $\rho$ Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability.