NIRI SV observation 5

Ratios of HeI/H recombination emission lines have a steep dependence on the effective temperature of the stellar UV fields, and thus on the upper limit to the stellar mass function. They also have many advantages over traditional optical and more recent mid- and far-IR probes (based on fine structure lines) as a quantitative diagnostic of the gas excitation. In particular the HeI 4^3D-3^3P transition at 1.701um is superior to other HeI lines as it arises solely from recombination. (It is an IR analogue to the more commonly observed optical transition HeI 447.1nm, but preserves the insensitivity to extinction). Adjacent, and of comparable strength, are the Br-series transitions Br10 (1.737um) and Br11 (1.681um).

Observe the 1.7um HeI and several Br-series lines across the actively star-forming region in a nearby starburst galaxy. From the line ratio the effective temperature of the radiation field and thus an upper mass limit to the present day mass function can be derived. Spatial changes in the MF will indicate intrinisic variations in the initial MFand/or star formation history.