Quantifying the contamination from nebular emission in NIRSpec spectra of massive star forming regions

TL;DR

This paper models and quantifies nebular emission contamination in NIRSpec spectra of star-forming regions, focusing on 30 Doradus, to improve data calibration and interpretation of stellar spectra.

Contribution

It develops a 3D nebular emission model for 30 Doradus and uses simulations to assess contamination impact on stellar spectra in JWST NIRSpec observations.

Findings

Nebular contamination can be subtracted with about 0.8% error.

Simulation results help optimize NIRSpec observation strategies.

Quantifies calibration uncertainties due to nebular emission.

Abstract

The Near InfraRed Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) includes a novel micro shutter array (MSA) to perform multi object spectroscopy. While the MSA is mainly targeting galaxies across a larger field, it can also be used for studying star formation in crowded fields. Crowded star formation regions typically feature strong nebular emission, both in emission lines and continuum. In this work, nebular emission is referred to as nebular contamination. Nebular contamination can obscure the light from the stars, making it more challenging to obtain high quality spectra. The amount of the nebular contamination mainly depends on the brightness distribution of the observed `scene'. Here we focus on 30 Doradus in the Large Magellanic Cloud, which is part of the NIRSpec GTO program. Using spectrophotometry of 30 Doradus from the Hubble Space Telescope (HST) and the Very Large Telescope (VLT)/SINFONI, we have created a 3D model of the nebular emission of 30 Doradus. Feeding the NIRSpec Instrument Performance Simulator (IPS) with this model allows us to quantify the impact of nebular emission on target stellar spectra as a function of various parameters, such as configuration of the MSA, angle on the sky, filter band, etc. The results from these simulations show that the subtraction of nebular contamination from the emission lines of pre-main sequence stars produces a typical error of $0.8\%$, with a $1\sigma$ spread of $13\%$. The results from our simulations will eventually be compared to data obtained in space, and will be important to optimize future NIRSpec observations of massive star forming regions. The results will also be useful to apply the best calibration strategy and to quantify calibration uncertainties due to nebular contamination.