Theoretical Evolution of Optical Strong Lines across Cosmic Time

Abstract: We use the chemical evolution predictions of cosmological hydrodynamic simulations with our latest theoretical stellar population synthesis, photoionization and shock models to predict the strong line evolution of ensembles of galaxies from z=3 to the present day. In this paper, we focus on the brightest optical emission-line ratios, [NII]/H-alpha and [OIII]/H-beta. We use the optical diagnostic Baldwin-Phillips-Terlevich (BPT) diagram as a tool for investigating the spectral properties of ensembles of active galaxies. We use four redshift windows chosen to exploit new near-infrared multi-object spectrographs. We predict how the BPT diagram will appear in these four redshift windows given different sets of assumptions. We show that the position of star-forming galaxies on the BPT diagram traces the ISM conditions and radiation field in galaxies at a given redshift. Galaxies containing AGN form a mixing sequence with purely star-forming galaxies. This mixing sequence may change dramatically with cosmic time, due to the metallicity sensitivity of the optical emission-lines. Furthermore, the position of the mixing sequence may probe metallicity gradients in galaxies as a function of redshift, depending on the size of the AGN narrow line region. We apply our latest slow shock models for gas shocked by galactic-scale winds. We show that at high redshift, galactic wind shocks are clearly separated from AGN in line ratio space. Instead, shocks from galactic winds mimic high metallicity starburst galaxies. We discuss our models in the context of future large near-infrared spectroscopic surveys.

• The paper predicts the evolution of BPT diagnostic line ratios from redshift 3 to today by integrating hydrodynamic and photoionization simulations.
• The paper reveals how metallicity and ionization conditions drive shifts in galaxy positions on the BPT diagram, distinguishing star formation from AGN activity.
• The paper outlines predictions for near-infrared surveys to detect emission signatures from star formation, AGN, and shock-induced ionization across cosmic epochs.

Theoretical Evolution of Optical Strong Lines across Cosmic Time

The paper presents a comprehensive investigation into the theoretical evolution of optical strong lines in galaxies across cosmic time, focusing on the emission-line ratios of Nitrogen ([N II]) and Oxygen ([O III]) as well as their diagnostics on the Baldwin-Phillips-Terlevich (BPT) diagram. This work synthesizes chemical evolution predictions from cosmological hydrodynamic simulations with integrated stellar population synthesis, photoionization, and shock models to project the evolution of these optical line ratios from redshift $z = 3$ to the present day.

Key Highlights

• BPT Diagram Evolution: The study uses the BPT diagnostic diagram as a critical tool to probe the spectral properties of galaxy ensembles, distinguishing star-forming galaxies from those hosting Active Galactic Nuclei (AGN). It recognizes a "mixing sequence" formed by galaxies containing both star-formation and AGN processes, which evolves significantly over cosmic time due to the metallicity sensitivity of optical emission lines.
• Metallicity and Ionization Effects: The position and trajectory of galaxy populations on the BPT diagram are strongly influenced by the metallicity and ionization conditions within each galaxy. The models incorporate various metallicity regimes to delineate their impact on optical line ratios, providing critical insights into the chemical evolution within galaxies and how it influences observable characteristics.
• Galaxy Properties across Different Redshifts: The paper simulates expected changes in galaxy properties using four distinct redshift windows, selected to align with capabilities of contemporary and forthcoming near-infrared spectrographs. These predictions consider varying ionization conditions and stellar population synthesis scenarios over different eras.
• Implications for Future Observations: By predicting the appearance of the BPT diagram across redshift windows, the paper highlights the potential observational signatures that upcoming large near-infrared spectroscopic surveys might reveal. It suggests that the shifts in line ratios could distinguish between contributions from star-formation, AGN, and shock-induced ionization in galaxies.

Implications and Future Directions

The theoretical framework and simulation-based predictions set forth in this paper offer profound implications for understanding galaxy evolution. Practically, this research assists in optimizing the design and interpretative strategies for the application of near-infrared spectroscopic surveys. Observational models tailored from these theoretical predictions can refine methods to classify galaxies accurately, heterogeneously across cosmic epochs, allowing for better mapping of chemical and ionization state histories in the universe.

Furthermore, the analysis provides groundwork for distinguishing environmental and internal mechanisms shaping galaxy evolution. Future research could focus on adding further granularity to these predictions by incorporating data from upcoming surveys, modifying initial assumptions regarding AGN activity, and exploring a broader range of shock velocities and abundances. Expanding the suite of models to account for variable initial mass functions or different star formation rates could also enhance future theoretical insights.

The synthesis between theoretical modeling and observational data, crystallized in this paper, sets a pivotal point for advancing understanding in galaxy evolution studies. The ability to decipher complex emissions from ancient cosmic epochs and understand their evolution to the present day will continue to refine paradigms of cosmic structure formation and the dynamic history of the universe.