Synthetic nebular emission from massive galaxies II: ultraviolet-line diagnostics of dominant ionizing sources
Michaela Hirschmann, Stéphane Charlot, Anna Feltre, Thorsten Naab, Rachel S. Somerville, Ena Choi
TL;DR
The paper develops a self-consistent framework to predict optical and UV nebular emission from massive galaxies by post-processing high-resolution cosmological zoom-in simulations with dedicated nebular-emission models for star formation, AGN, and post-AGB stars. It shows that classical optical diagrams lose discriminatory power at metallicities below $0.5Z_ extodot$, especially at $z>1$, and demonstrates the potential of UV diagnostics with newly proposed diagrams. The approach uses Cloudy-based grids Gutkin16, Feltre16, and Hirschmann17, coupled to simulated galaxy properties to produce robust predictions across cosmic time. These results are timely for interpreting JWST spectra of distant galaxies and for constraining the relative roles of star formation and nuclear activity in cosmic reionization.
Abstract
We compute synthetic optical and ultraviolet (UV) emission-line properties of galaxies in a full cosmological framework by coupling, in post-processing, new-generation nebular-emission models with high-resolution, cosmological zoom-in simulations of massive galaxies. Our self-consistent modelling accounts for nebular emission from young stars and accreting black holes (BHs). We investigate which optical- and UV-line diagnostic diagrams can best help to discern between the main ionizing sources, as traced by the ratio of BH accretion to star formation rates in model galaxies, over a wide range of redshifts. At low redshift, simulated star-forming galaxies, galaxies dominated by active galactic nuclei and composite galaxies are appropriately differentiated by standard selection criteria in the classical [OIII]$λ$5007/H$β$ versus [NII]$λ$6584/H$α$ diagram. At redshifts $z \gt 1$, however, this optical diagram fails to discriminate between active and inactive galaxies at metallicities below $0.5\ Z_\odot$. To robustly classify the ionizing radiation of such metal-poor galaxies, which dominate in the early Universe, we confirm 3 previous, and propose 11 novel diagnostic diagrams based on equivalent widths and luminosity ratios of UV emission lines, such as EW(OIII]$λ$1663) versus OIII]$λ$1663/HeII$λ$1640, CIII]$λ$1908/HeII$λ$1640 versus OIII]$λ$1663/HeII$λ$1640, and CIV$λ$1550/CIII]$λ$1908 versus CIII]$λ$1908/CII$λ$2326. We formulate associated UV selection criteria and discuss some caveats of our results (e.g., uncertainties in the modelling of the HeII$λ$1640 line). These UV diagnostic diagrams are potentially important for the interpretation of high-quality spectra of very distant galaxies to be gathered by next-generation telescopes, such as the James Webb Space Telescope.