Interstellar extinction correction in ionised regions using HeI lines

TL;DR

This paper introduces a new method to correct for interstellar extinction in ionised regions using HeI lines, which can be more reliable than traditional Hα/Hβ ratios, especially in extended nebulae.

Contribution

The authors develop a HeI-based extinction correction method that accounts for triplet states, improving accuracy over traditional hydrogen line methods.

Findings

HeI lines provide compatible extinction estimates with Hβ-based methods.

Using both HeI and Hβ lines reduces the error in extinction correction.

The method is applied successfully to the 30 Doradus nebula using MUSE data.

Abstract

The logarithmic extinction coefficient, c(H$\beta$), is usually derived using the H$\alpha$/H$\beta$ ratio for case B recombination and assuming standard values of electron density and temperature. However, the use of strong Balmer lines can lead to selection biases when studying regions with different surface brightness, such as extended nebulae, with the use of single integral field spectroscopy observations, since, in some cases, the H$\alpha$ line can be saturated in moderate to long exposures. In this work, we present a method to derive extinction corrections based only on the weaker lines of HeI, taking into account the presence of triplet states in these atoms and its influence on recombination lines. We have applied this procedure to calculate the extinction of different regions of the 30 Doradus nebula from MUSE integral-field spectroscopy data. The comparison between helium and hydrogen c(H$\beta$) determinations has been found to yield results fully compatible within the errors and the use of both sets of lines simultaneously reduces considerably the error in the derivation.