NGC 3603 stacked nebular spectrum

Rather, different extinction curves can be trialled and the most appropriate one selected based on a minimum chi^2^ procedure. 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 E(B-V).

A series of extinction curves were trialled ranging from R(V)=2 to R(V)=8. The appropriate value of R(V) was adopted based on the minimum 2 procedure. 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 R(V)=4.8+/-1.06, larger than the Galactic average of 3.1.

We find a typical value of E(B-V)=0.64+/-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 KrIII and SeIV, and molecular H_2_ emission lines.

This high S/N spectrum can act as a helpful template for identifying nebular emission lines. Using ratios of hydrogen recombination lines, we calculated the value of R(V), E(B-V) and A(V) for > 200 lines of sight across NGC 3603. An extinction curve with a typical value R(V)=4.8+/-1.06 is required to explain the colour excess observed in the nebular spectra.

This corresponds to a typical E(B-V)=0.64+/-0.27. This is significantly lower than what has been found in previous extinction studies of NGC 3603.