A simple non-parametric method for resolving merged doublet lines: Insights into complex kinematics and outflows

TL;DR

This paper introduces a simple, non-parametric technique for resolving merged doublet emission lines in astronomical spectra, enabling better analysis of complex kinematics and outflows without complex fitting procedures.

Contribution

The paper presents a novel, fast, and robust non-parametric method that resolves merged doublet lines using known line ratios and wavelength differences, avoiding traditional fitting assumptions.

Findings

Successfully recovers line profiles in simulated data

Maintains low noise levels in extracted lines

Effective on real AGN [OIII] doublets

Abstract

Doublet line emission and absorption is common in astronomical sources (e.g. [OIII], [OII], NaD, MgII). In many cases, complex kinematics in the emitting source can cause the doublet lines to merge, making characterisation of the source kinematics challenging. Here, we present a non-parametric method for resolving merged doublet emission when the line ratio and wavelength difference is known. The method takes as input only the line ratio and wavelength difference, using these quantities to resolve the components of the doublet without resorting to fitting (e.g. using multiple Gaussians) or making any assumptions about the components' line profiles (save that they are the same for both components). The method is simple, fast and robust. It is also ideal for visualisation. We show that the method recovers line profiles of merged emission lines in simulated data. We also show, using simulated data and mathematical analysis, that the method does not significantly increase noise levels in the extracted lines, and is robust to background contamination. We demonstrate the strength of the method by applying it to strongly merged [OIII] 5007/4959~{\AA} in Active Galactic Nuclei (AGN). A python implementation of the method is provided in the Appendix.