Consistent Gas-Phase Temperatures and Metallicities from UV and Optical Nebular Emission: A Reliable Foundation from z=0 to Cosmic Dawn

Abstract

The rest-frame UV spectra of star-forming galaxies are increasingly important as they become one of the primary windows to probe the physical properties of cosmic dawn (z>8) galaxies with the James Webb Space Telescope. However, the systematic discrepancies between UV and optical gas-phase metallicity measurements remain poorly understood in the local universe, partly due to challenges in achieving precise comparisons between UV and optical spectra for the same objects. In this work, we introduce a novel method that leverages the HeII 1640 and HeII 4686 nebular emission lines to achieve accurate aperture and reddening corrections between UV and optical spectra. Here we apply this method to three nearby Blue Compact Dwarf (BCD) galaxies. Our results demonstrate that this approach enables precise measurements, with electron temperatures ($T_e$) derived from UV and optical spectra exhibiting closer agreement compared to previous studies, and O/H abundance agreeing within 0.1 dex. However, two BCDs appear to have lower UV-based electron temperatures $T_{e~1666} < T_{e~4363}$, in contrast to expectations from the temperature fluctuation model. We consider a variety of possible explanations for these unphysical temperatures - differential dust attenuation, aperture differences, and spatial extent of emission lines - but no suitable cause is identified. These findings suggest a complex gaseous environment associated with star formation, and underscore the need for additional observations to further investigate the nature of HeII nebular emission and address the systematic issues between UV and optical nebular properties. Nonetheless, the close empirical agreement of these results indicates that UV- and optical-based nebular temperature and abundance measurements can be reliably compared within 0.1 dex, providing a solid foundation for evolutionary studies from the local Universe to cosmic dawn.

Figures (7)

• Figure 1: Images of the three blue compact dwarf galaxies studied in this work. Each panel shows a SDSS u, g, r false-color image, the HST/ COS 2$^{\prime\prime}$5 aperture (circles), and the Keck/ ESI 1$^{\prime\prime}$$\times$ 20$^{\prime\prime}$ slit (rectangles). The 2$^{\prime\prime}$5 COS aperture size corresponds to projected diameters of 0.23 kpc for SB 2, 0.92 kpc for SB 82, and 2.31 kpc for SB 182 Senchyna2017. Near-UV target acquisition images from COS for all three objects can be found in Senchyna2017. All three objects are dwarf galaxies containing one primary H ii region, from which the majority of nebular line emission is captured by both the COS and ESI apertures.
• Figure 2: Keck/ESI optical spectra of the three objects presented in this work. Prominent emission features, epecially the ones useful for this work, are labeled and marked with vertical dashed lines. All relevant features are clearly detected, including nebular He ii$\lambda 4686$. A broad He ii component is visible in SB 82.
• Figure 3: Rest-frame Keck/ESI spectra with the best-fit Gaussian profile and residual for select emission lines used in this analysis for each target. The x-axis range is 10 Å for all spectra presented in this figure and the y-axis is at a scale of $10^{-17}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$. We adopt a double Gaussian profile for He ii$\lambda 4686$ in SB 82, representing both the broad stellar and narrow nebular components. The broad component spans $\gtrsim~100$ Å so it is not shown at the scale of this figure, but can be seen in Figure \ref{['fig:opt_flux_fits_full']}. All other lines are adequately fit with a single narrow component.
• Figure 4: Binned HST/COS spectra, associated 1$\sigma$ error spectra, best-fit Gaussian models, and residual spectra for our targets SB 2, SB 82 and SB 182. SB 2 and SB 82 have 10 orbits of COS data Senchyna2022, while SB 182 has only a single orbit Senchyna2017. Above, $z$ is the redshift as measured from the observed UV spectra. $f_{1640}$, $f_{1661}$ and $f_{1666}$ are the measured flux and 1$\sigma$ uncertainty for He ii$\lambda 1640$, O iii] $\lambda 1661$ and O iii] $\lambda 1666$ respectively. Milky Way interstellar absorption features (e.g., at rest wavelengths of approximately 1663 Å for SB 2 and 1642 Å for SB 82) were masked and excluded from the fitting. All emission lines used in our analysis are well-detected and unaffected by Milky Way absorption.
• Figure 5: Our measurements of $T_{e~1666}$ versus $T_{e~4363}$ for the objects in our sample, along with measurements from Mingozzi_etal_2022 which used a different methodology. Two galaxies in our sample (SB 2 and SB 182) have $T_{e~1666}$$<$$T_{e~4363}$, whereas temperature fluctuations predict the opposite result. Our sample displays a tight distribution with standard deviation scatter of 515 K ($4.2\%$ mean absolute percent offset), while the Mingozzi_etal_2022 sample has a larger scatter of 1557 K ($9.2\%$ mean absolute percent offset). The improved precision demonstrates success in reducing the uncertainties associated with UV attenuation corrections using our He ii method (see Section \ref{['sec:UVreddening']}).