Spectral characterisation of the extinction properties of NGC 3603 using JWST NIRSpec

Context. A necessary ingredient in understanding the star formation history of a young cluster is knowledge of the extinction towards the region. This has typically been done by making use of the colour-difference method with photometry, or similar methods utilising the colour-colour diagram. Aims. The colour-excess can be independently determined by studying the decrements of the recombination lines produced by the nebular gas. Having access to many recombination lines from the same spectral series removes the need of adopting an extinction curve. Methods. Using the Micro-Shutter Assembly (MSA) on board the Near InfraRed Spectrograph (NIRSpec), multi-object spectroscopy was performed, yielding 600 nebular spectra from the Galactic massive star formation region NGC 3603. The recombination line intensity ratios were used to determine independent values of colour-excess. Results. The extinction characteristics of NGC 3603 are similar to other Galactic HII regions like Orion, as well as starburst regions such 30 Doradus in the Large Magellanic Cloud, in that we find a relatively large value of total-to-selective extinction of 4.8 $\pm$ 1.06, larger than the Galactic average of 3.1. We find a typical value for the colour-excess of 0.64 $\pm$ 0.27, significantly lower than values determined in previous studies. We also present a stacked nebular spectrum with a typical continuum S/N = 70. This spectrum highlights the recombination lines of the HII region, several s-process elements such as Kr III and Se IV, and molecular H2 emission lines. Conclusions. Using ratios of hydrogen recombination lines, we calculated the total-to-selective extinction, colour-excess and visual extinction for > 200 lines of sight across NGC 3603. An extinction curve with a total-to-selective extinction of 4.8 $\pm$ 1.06 was found, corresponding to a colour-excess of 0.64 $\pm$ 0.27.