The LBT Y p Project. II. MODS Spectra, Physical Conditions, and Oxygen Abundances in Local Metal-poor Nebulae

Abstract Empirically measuring the primordial He mass fraction, Y p , requires a significant number of low-metallicity nebulae with direct constraints on He/H and O/H abundances. This technique requires high-fidelity measurements of the gas-phase physical conditions, namely the electron temperature ( T e ) and density ( n e ). To this end, we present deep rest-optical spectroscopy for a sample of 62 low-metallicity (≲20% solar O/H) galaxies acquired using the Multi-Object Double Spectrographs on the LBT as part of the LBT Y p Project. We discuss new fitting methods that recover the intensity of up to 61 H and He recombination lines, of which, up to 26 will be used to determine gas-phase He abundances, and we examine the emission line properties of the LBT Y p Project sample. We assess different scaling relations in the low-metallicity interstellar medium (ISM), finding that n e [Ar iv ] measured in 31 targets is systematically larger than n e [S ii ] or n e [O ii ]. The larger densities are insufficient to significantly bias T e [O iii ] or the O/H abundance. T e [S iii ] and T e [O iii ] are strongly correlated over a range of ∼10 4 K with very low scatter, and we calibrate new T e [S iii ]– T e [O iii ] scaling relations for use in other low-metallicity environments. We examine different T e measured in the low-ionization gas, finding significant scatter compared to T e [O iii ]. The precision direct O/H derived in this analysis (median uncertainty ∼4%) are consistent with prior literature measurements, albeit with relatively large scatter. These data provide a key component necessary to empirically measure Y p and the abundance patterns of other elements in the ISM.