The LBT Y 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 He I λ 10830 line to assist in resolving this issue, especially for accurate He abundances. To facilitate this, we present near-IR (NIR) LBT Utility Camera in the Infrared (LUCI) spectra of 48 low-metallicity targets and homogeneously reduce them using Pypeit as part of the LBT Y 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 coadd 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 γ ), resulting in He I λ 10830 to P γ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR He I λ 10830 fluxes in low-metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of σ ˜ = 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.