Reconciling the Stellar and Nebular Spectra of High Redshift Galaxies (1605.07186v1)

Abstract: We present a combined analysis of rest-frame far-UV (1000-2000 A) and rest-frame optical (3600-7000 A) composite spectra formed from very deep observations of a sample of 30 star-forming galaxies with z=2.4+/-0.1, selected to be representative of the full KBSS-MOSFIRE spectroscopic survey. Since the same massive stars are responsible for the observed FUV continuum and the excitation of the observed nebular emission, a self-consistent stellar population synthesis model must simultaneously match the details of the far-UV stellar+nebular continuum and-- when inserted as the excitation source in photoionization models-- account for all observed nebular emission line ratios. We find that only models including massive star binaries, having low stellar metallicity (Z_*/Z_{sun} ~ 0.1) but relatively high ionized gas-phase oxygen abundances (Z_{neb}/Z_{sun} ~ 0.5), can successfully match all of the observational constraints. We argue that this apparent discrepancy is naturally explained by highly super-solar O/Fe [4-5 times (O/Fe)_{sun}], expected for gas whose enrichment is dominated by the products of core-collapse supernovae. Once the correct ionizing spectrum is identified, photoionization models reproduce all of the observed strong emission line ratios, the direct T_e measurement of O/H, and allow accurate measurement of the gas-phase abundance ratios of N/O and C/O -- both of which are significantly sub-solar but, as for O/Fe, are in remarkable agreement with abundance patterns observed in Galactic thick disk, bulge, and halo stars with similar O/H. High nebular excitation is the rule at high-z (and rare at low-z) because of systematically shorter enrichment timescales (<<1 Gyr): low Fe/O environments produce harder (and longer-lived) stellar EUV spectra at a given O/H, enhanced by dramatic effects on the evolution of massive star binaries.

• The paper combines far-UV and optical spectra from 30 high redshift galaxies to reconcile stellar and nebular emissions.
• The paper employs advanced stellar population models, including Starburst99 and BPASSv2, to show that binary evolution enhances spectral fits.
• The paper determines distinct metallicity levels—with ~0.1 Z⊙ for stars and ~0.5 Z⊙ for nebular gas—highlighting the role of core-collapse supernovae.

Analysis of Stellar and Nebular Spectra at High Redshift

Recent advancements in understanding high redshift galaxies, specifically those at $z \sim 2.4$, have been markedly enriched by the combined analysis of far-UV and optical spectral data. The paper focuses on a sample of 30 star-forming galaxies, leveraging data collected via the Keck Observatory, to decode patterns in both stellar and ionized gas-phase metallicities. The implications of this paper stem from bridging stellar and nebular emissions to portray an accurate picture of the chemical and physical properties of these galaxies.

The need to engage with composite spectra, capturing both stellar and nebular emissions, speaks to previous limitations in linking the contributions of massive star populations to the nebular characteristics. The authors skillfully utilize Keck/MOSFIRE and Keck/LRIS data, accessing a wavelength range spanning from 1000 to 7000 \AA\ in the rest frame, and constructing a self-consistent model that demands harmony in far-UV and nebular emissions simultaneously.

Key Observations and Insights

A significant aspect of this paper is its reliance on stellar population synthesis models, particularly Starburst99 and BPASSv2, to illuminate the spectra of high redshift galaxies with varying assumptions about star metallicity and evolution. The paper makes strong arguments for the inclusion of binary evolution in these models, noting that such consideration substantially aligns the predictions more closely with observations. The spectral traces of {\ion{C}{4}} and {\ion{He}{2}} prove especially revealing, contributing to better fits with the obsolescence of single-star models that fail to account for the observed nebular ionization states.

Despite solar abundance ratios being assumed within most models, the authors recommend a decoupling of stellar metallicity from ionized gas-phase abundances. Through the best models, they conclude a stellar metallicity around 0.1 $Z_{\odot}$ and a nebular metallicity of approximately 0.5 $Z_{\odot}$. This distinction underscores the discrepancies attributed to enhancements in O/Fe due to core-collapse supernovae, which predominantly mark the enrichment epochs from which these massive stars recently emerged.

The findings reveal a notable equivalence between the N/O abundance patterns in these galaxies and localized extragalactic HII regions, bolstering the claim that these patterns may be universally applicable, irrespective of redshift. Such consistencies strengthen the proposition that the stellar FUV and EUV spectra, particularly those accommodating binary evolution, adequately recapture the nebular properties indicative of these high-energy environments.

Implications and Future Directions

The implications of this work are extensive, contributing to how we understand both the formation conditions and subsequent evolution of galaxies during key cosmic epochs. The results emphasize the necessity of incorporating updated astrophysical models that account for the full stellar lifecycle including binary interactions, given their prominent impact in shaping ionizing spectra.

From a practical standpoint, this paper calls for more comprehensive modeling of non-solar abundance ratios in synthesizing stellar populations, paving the way to more accurately reflect the diverse chemical landscapes of early galaxies. This approach also hints at the challenges faced by upcoming observational strategies, such as those planned with the JWST, which will dive deeper into these high-energy stages across cosmic history.

In summary, the paper astutely advances the dual analysis of stellar and nebular spectra, resulting in a substantial contribution to the field of galaxy formation and evolution paper. By advocating for more nuanced models that embrace the complexity of high-energy environments, this research enables clearer insights into the past and ongoing processes that sculpt the universe.