The LBT Y p Project. III. LUCI Spectra of Metal-poor Nebulae

Abstract

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.

The authors' abstract, as published at the source. The Astrophysical Journal, 2026 · DOI ↗