[Paper Review] A comprehensive chemical abundance analysis of the extremely metal poor Leoncino Dwarf galaxy (AGC 198691)

Erik Aver, Danielle A. Berg+6 more|arXiv (Cornell University)|Sep 1, 2021

Galaxies: Formation, Evolution, Phenomena74 references41 citations

TL;DR

This study presents a comprehensive chemical abundance analysis of the extremely metal-poor dwarf galaxy AGC 198691 using high-resolution LBT/MODS spectroscopy, enabling the first reliable determination of sulfur, argon, and helium abundances. The oxygen abundance is confirmed at 12 + log(O/H) = 7.06 ± 0.03, and the helium abundance is consistent with predictions, yielding a primordial helium abundance of Yp = 0.2448 ± 0.0033, though the faintness of the galaxy limits its impact on improving Yp precision.

ABSTRACT

We re-examine the extremely metal-poor (XMP) dwarf galaxy AGC 198691 using a high quality spectrum obtained by the LBT's MODS instrument. Previous spectral observations obtained from KOSMOS on the Mayall 4-m and the Blue Channel spectrograph on the MMT 6.5-m telescope did not allow for the determination of sulfur, argon, or helium abundances. We report an updated and full chemical abundance analysis for AGC 198691, including confirmation of the extremely low "direct" oxygen abundance with a value of 12 + log(O/H) = 7.06 $\pm$ 0.03. AGC 198691's low metallicity potentially makes it a high value target for helping determine the primordial helium abundance ($Y_p$). Though complicated by a Na I night sky line partially overlaying the He I $\lambda$5876 emission line, the LBT/MODS spectrum proved adequate for determining AGC 198691's helium abundance. We employ the recently expanded and improved model of Aver et al. (2021), incorporating higher Balmer and Paschen lines, augmented by the observation of the infrared helium emission line He I $\lambda$10830 obtained by Hsyu et al. (2020). Applying our full model produced a reliable helium abundance determination, consistent with the expectation for its metallicity. Although this is the lowest metallicity object with a detailed helium abundance, unfortunately, due to its faintness (EW(H$\beta$) $<$ 100 AA) and the compromised He I $\lambda$5876, the resultant uncertainty on the helium abundance is too large to allow a significant improvement on the measurement of $Y_p$.

Motivation & Objective

• To perform a comprehensive chemical abundance analysis of the extremely metal-poor dwarf galaxy AGC 198691, which had previously lacked reliable measurements of sulfur, argon, and helium abundances.
• To improve the determination of the primordial helium abundance (Yp) by including AGC 198691 in a regression of helium abundance versus metallicity.
• To assess the utility of faint, extremely metal-poor galaxies as targets for constraining Yp, given observational challenges such as low signal-to-noise and spectral line contamination.
• To validate and apply the updated helium abundance modeling framework from Aver et al. (2021), incorporating higher Balmer and Paschen lines and the infrared He I λ10830 line.

Proposed method

• Utilized high-resolution, broad wavelength coverage spectroscopy from the Large Binocular Telescope's Multi-Object Double Spectrograph (LBT/MODS) to obtain high-quality emission line data for AGC 198691.
• Applied the improved helium abundance modeling framework from Aver et al. (2021), which includes enhanced treatment of collisional excitation, underlying stellar absorption, and radiative transfer for blended lines such as He I λ3889 and H8.
• Incorporated the infrared He I λ10830 emission line flux from Hsyu et al. (2020) to strengthen the helium abundance determination.
• Performed a Monte Carlo-based fitting procedure to evaluate uncertainties and model-data consistency, accounting for physical and observational systematics.
• Conducted a regression of helium abundance versus oxygen abundance across 17 XMP galaxies, including AGC 198691, to extrapolate to zero metallicity and derive Yp.
• Used quality cuts and statistical analysis to ensure robustness, with χ² reduced to 8.1 for the final regression.

Experimental results

Research questions

• RQ1 What are the sulfur, argon, and helium abundances in the extremely metal-poor galaxy AGC 198691, given its previously unmeasured emission lines?
• RQ2 How does the oxygen abundance in AGC 198691 compare to prior estimates, and what is its significance for the metallicity of the most pristine galaxies?
• RQ3 Can the improved helium abundance model, incorporating higher Balmer and Paschen lines and the He I λ10830 line, yield a reliable helium abundance for AGC 198691 despite its faintness and spectral contamination?
• RQ4 To what extent does including AGC 198691 in a Yp regression improve the precision of the primordial helium abundance determination?
• RQ5 What are the observational selection criteria that maximize the contribution of XMP galaxies to Yp determination, given the challenges of faintness and spectral degradation?

Key findings

• The oxygen abundance in AGC 198691 is confirmed at 12 + log(O/H) = 7.06 ± 0.03, consistent with it being one of the most metal-poor galaxies known.
• This is the first reliable determination of sulfur and argon abundances in AGC 198691, made possible by the high-resolution and broad wavelength coverage of LBT/MODS.
• The helium abundance was successfully determined using the expanded model from Aver et al. (2021), incorporating higher Balmer and Paschen lines and the He I λ10830 line, yielding a consistent result with expectations for its metallicity.
• The inclusion of AGC 198691 in a regression of helium abundance versus oxygen abundance yields a primordial helium abundance of Yp = 0.2448 ± 0.0033, in good agreement with the SBBN prediction of 0.2469 ± 0.0002.
• Despite the consistency, the large uncertainty in AGC 198691’s helium abundance—due to its faintness (EW(Hβ) < 100 Å) and contamination of the He I λ5876 line—limits its contribution to improving Yp precision.
• The study concludes that faint XMP galaxies, even with extremely low metallicity, are poor targets for Yp improvement; higher surface brightness objects (e.g., EW(Hβ) > 200 Å) should be prioritized.