The Cosmic BPT Diagram: Confronting Theory With Observations

Abstract

We compare a large sample of galaxies between 0.5 < z < 2.6 with theoretical predictions for how the optical diagnostic line ratios in galaxy ensembles change as a function of cosmic time. We show that star forming galaxies at high redshift (z > 1.5) are consistent with a model in which the ISM conditions are more extreme at high redshift than seen in the global spectra of local galaxies. We speculate that global spectra of our high redshift galaxies may be dominated by H II regions similar to the extreme clumpy, dense star-forming complexes in the Antennae and M82. The transition to local-type conditions occurs between 0.8 < z < 1.5. We conclude that classification schemes developed for local samples should not be applied at high redshift (z ≥ 1.5). We use our theoretical models to derive a new redshift-dependent classification line that utilizes the standard optical diagnostic line ratios [O III]/Hβ and [N II]/Hα. Our new line can be used to separate star-forming galaxies from AGN between z = 0 to z ∼ 3.5. We anticipate that our redshift-dependent optical classification line will be useful for future large surveys with near-infrared multi-object spectrographs. We apply our classification line to a sample of gravitationally lensed galaxies at z ∼ 2.5. Although limited by small numbers, we show that our classification line is consistent with the position of AGN that have been independently confirmed via other methods.

First light: exploring the spectra of high-redshift galaxies in the Renaissance Simulations

“…In our data, all points lie below the AGN boundary for S ii (Kauffmann et al 2003) and the AGN boundary for N ii (Kewley et al 2001). Though some objects graze the boundary in the N ii diagram, there is some evidence that this is a feature of star-formation dominated spectra for high-redshift objects (Kewley et al 2013).…”

Are long gamma-ray bursts biased tracers of star formation? Clues from the host galaxies of theSwift/BAT6 complete sample of bright LGRBs

“…We therefore chose the lower branch solution for these two galaxies. We note that the reliability of diagnostics that are based on [O iii] and [N ii] emission lines at high redshifts have been questioned (Kewley et al 2013;Shapley et al 2015). However, this uncertainty is not expected to affect our z < 1 sample study.…”

On R − W1 as a Diagnostic to Discover Obscured Active Galactic Nuclei in Wide-Area X-Ray Surveys

“…Alternatively, candidate narrow-lined AGNs at z0.5-1 can be identified based on signatures at other wavelengths (e.g., infrared color, infrared/optical colors, X-ray emission) and followedup with ground-based infrared spectroscopy to observe the traditional rest-frame BPT emission lines, where the cosmic BPT diagram can then differentiate between AGNs and star-forming galaxies (Kewley et al 2013a(Kewley et al , 2013bKartaltepe et al 2015).…”