The LBT $Y_{\rm p}$ Project III: LUCI Spectra of Metal-Poor Nebulae

Abstract

Accurately determining the elemental abundances of a low metallicity nebula strongly depends on measuring the density (n$_e$) and temperature (T$_e$) of the gas. Because these two parameters are inherently degenerate when derived solely from H and He recombination lines, we rely on the density-sensitive HeI $\lambda$10830 line to assist in resolving this issue, especially for accurate He abundances. To facilitate this, we present near-IR (NIR) LUCI spectra of 48 low-metallicity targets from the Large Binocular Telescope (LBT) and homogeneously reduce them using Pypeit as part of the LBT $Y_{\rm p}$ Project. IR spectra require special care, and we wavelength calibrate by-hand using the bright OH emission lines, carefully apply proper telluric corrections, and co-add the spectra of LUCI1 and LUCI2 on a resampled grid to ensure accurate results. We use a Gaussian profile to fit the emission lines and measure the fluxes relative to Paschen-gamma (P$\gamma$), resulting in HeI $\lambda$10830 to P$\gamma$ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR HeI $\lambda$10830 fluxes in low metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of $\widetilde{\sigma} = 0.08$, reduce the overall uncertainties in helium abundance estimates for individual targets. The increased size of the high-quality sample enables searching for systematic uncertainties and improves the reliability of the helium abundance determinations used to infer the primordial helium abundance ($Y_{\rm p}$).