Joint Optical and Infrared Observations of N and O Reveal the Dust-Obscured Gas in Haro 3

Abstract

Accurate chemical compositions of star-forming regions are a critical diagnostic tool to characterize the star formation history and gas flows which regulate galaxy formation. However, the abundance discrepancy factor (ADF) between measurements from the "direct" optical electron temperature ($T_e$) method and from the recombination lines (RL) represents $\sim0.2$ dex systematic uncertainty in oxygen abundance. The degree of uncertainty for other elements is unknown. We conduct a comprehensive analysis of O$^{++}$ and N$^+$ ion abundances using optical and far-infrared spectra of a star-forming region within the nearby dwarf galaxy Haro 3, which exhibits a typical ADF. Assuming homogeneous conditions, the far-IR emission indicates an O abundance which is higher than the $T_e$ method and consistent with the RL value, as would be expected from temperature fluctuations, whereas the far-IR N abundance is too large to be explained by temperature fluctuations. A two-phase analytical model reveals that differential dust obscuration associated with temperature inhomogeneity is likely required to explain all the emission line ratios, and that the total oxygen metallicity of two phases is consistent with the RL metallicity. Our findings underscore the critical importance of resolving the cause of abundance discrepancies and understanding the biases between different metallicity methods. This work represents a promising methodology, and we identify further approaches to address the current dominant uncertainties.

Figures (2)

• Figure 1: Top-left: Fields of view of the Keck/KCWI (cyan) and SOFIA/FIFI-LS (magenta) observations, and contours of the [O$\;$] 88 $\mu\mathrm{m}$ emission map from Herschel/PACS (yellow), overlaid on the color composite image constructed from Sloan Digital Sky Survey images in g, r and i filters. Top-right: Extracted spectra after continuum subtraction (blue) and the corresponding fitted line profiles (orange) of several important emission features for this work. Bottom: The full Keck/KCWI optical spectrum of Haro 3 in the 2$" .\,$25 aperture observed from the BL-4500 and RL-7250 grating setups. A subset of prominent emission features are marked in red.
• Figure 2: Left: An example of PSF matching between the [O$\;$] $\lambda5007$ PNB image from Keck/KCWI (top-left) and the [O$\;$] 88 $\mu\mathrm{m}$ PNB image from Herschel/PACS (bottom-left). The KCWI PNB image has been convolved with a 2D Gaussian kernel (top-right) to match the PACS PNB image's PSF. The resulting residual is shown in the bottom-right panel. The red dashed circles indicate the apertures from which the 1D spectra were extracted. Right: Modeling of dust attenuation based on the observed flux ratios of hydrogen Balmer and Paschen lines. In the top panel, the data points represent the correction factors needed to align the observed flux ratios with those predicted by Case B recombination. The best-fit attenuation models are represented by solid curves, while the shaded areas denote their 1$\sigma$ uncertainties. The blue (orange) color indicates the 1D spectra before (after) PSF matching. The steep slope at the lower wavelengths is primarily due to contributions from stellar absorption. The lower panel shows the fractional deviations between the observed and best-fit flux ratios, for the same data points. Data points near unity indicate a good agreement with the attenuation model.