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

TL;DR

The study addresses the primordial helium abundance $Y_{p}$ from low-metallicity H II regions, where the $T_e$-$n_e$ degeneracy weakens abundance determinations. It uses the density-sensitive He I $λ10830$ line to constrain $n_e$ and break the degeneracy, leveraging 48 LUCI near-infrared spectra reduced with PypeIt, wavelength-calibrated via OH skylines and tied to Paschen-$γ$ for robust flux ratios $F(10830)/F(Pγ)$. This expanded, high-quality dataset improves helium-abundance determinations by enabling systematic-error assessments and reducing line-ratio uncertainties to about $\tilde{\sigma}\approx0.08$. The results strengthen $Y_{p}$ inferences within the standard cosmological model and provide a robust empirical foundation for Big Bang nucleosynthesis tests.

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 $λ$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$γ$), resulting in HeI $λ$10830 to P$γ$ ratios consistent with previous studies. As a result, this work significantly expands the available dataset of NIR HeI $λ$10830 fluxes in low metallicity galaxies. These high-quality measurements, where we find a median flux ratio uncertainty of $\widetildeσ = 0.08$, 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 ($Y_{\rm p}$).

Figures (4)

• Figure 1: The bright, night-sky OH lines seen in the top panels are removed when sky-subtraction is performed, showing the success of the algorithm. Note that in the IR, Pypeit performs sky-subtraction by fitting the residuals produced by image-differencing. This is the cause of the black traces.
• Figure 2: Top: Telluric model generated from PypeIt. Bottom: Comparison of our original un-corrected spectrum for UM420 ($z \sim 0.058$) in black with our corrected spectrum in red.
• Figure 3: Examples of our final, post-processed spectra from LUCI1, LUCI2, and LUCI1+2 showing our fits to the line profiles.
• Figure 4: Left: Flux ratio of He I $\lambda$10830 to P$\gamma$ from this work as diamonds and izot2014 as stars. Right:EMPRESS25 as circles compared to this work in diamonds. J2104-0035 has no counterpart in EMPRESS25 as they did not detect emission lines in their observations of this target. Targets from Hsyu2020 are shown as squares. Both panels are color-coded by temperature and shared observations between studies are bolded and labeled. The colored horizontal lines show constant temperatures and densities calculated with a nominal value of $\mathrm{y}^{+}$ = 0.085 while the hatched region shows the nonphysical regime.