THE COSMIC BPT DIAGRAM: CONFRONTING THEORY WITH OBSERVATIONS

By Lisa J. Kewley, C. Maier, K. Yabe, K. Ohta, M. Akiyama, M. Dopita, T. Yuan. 2013

162 citations recorded by the source corpus.

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 interstellar medium 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 active galactic nuclei (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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/0b8adbfb3d68d96fb5d197bb31a421d63e911895

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In Astronomy and Astrophysics

1. P. Hopkins
2. C. Conroy
3. Naveen A. Reddy
4. A. Shapley
5. L. Hernquist
6. P. V. van Dokkum
7. R. Dav'e
8. C. Steidel

In Instrumentation

1. Naveen A. Reddy
2. A. Shapley
3. C. Steidel
4. P. V. van Dokkum
5. C. Conroy
6. M. Franx
7. M. Pettini
8. D. Erb

Citations in the corpus

• Ionization state of inter-stellar medium in galaxies: evolution, SFR-M * -Z dependence, and ionizing photon escape (2013, background): “Nakajima et al. (2013) have pointed out a possibility that fesc positively correlates with ionization parameter, since an optically thin nebula could show a high [O III]/[O II] ratio (e.g. Giammanco, Beckman & Cedrés 2005; Brinchmann et al. 2008; Kewley et al. 2013b).”
• PHYSICAL PROPERTIES OF EMISSION-LINE GALAXIES AT z ∼ 2 FROM NEAR-INFRARED SPECTROSCOPY WITH MAGELLAN FIRE (2014, background): “The elevated [O iii]/Hβ ratios in the classical BPT diagram could be at least partially due to AGN activity in such galaxies (Trump et al. 2011, 2013; Kewley et al. 2013a).”
• THE GRISM LENS-AMPLIFIED SURVEY FROM SPACE (GLASS). I. SURVEY OVERVIEW AND FIRST DATA RELEASE (2015, background): “High-redshift galaxies are often offset from the local star forming locus toward that of AGN (Mannucci et al. 2010; Kewley et al. 2013; Shapley et al. 2014; Steidel et al. 2014).”
• THE MOSDEF SURVEY: ELECTRON DENSITY AND IONIZATION PARAMETER AT z ∼ 2.3 (2015, background): “It has been proposed that the position of high-redshift galaxies in the [O iii]/Hβ vs. [N ii]/Hα diagram can be explained by a systematically harder ionizing spectrum compared to that of local galaxies with similar metallicity (Kewley et al. 2013a; Steidel et al. 2014).”
• STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE (2014, methodology): “Expectations for the behavior of high-redshift AGN in the BPT plane have been explored in some detail by Kewley et al. (2013a), who pointed out that AGN in low metallicity hosts (which appear to be extremely rare at z ' 0) could conceivably be found with high [OIII]/Hβ but low [NII]/Hα.”
• GRB hosts through cosmic time - VLT/X-Shooter emission-line spectroscopy of 96 γ-ray-burst-selected galaxies at 0.1 (2015, methodology): “…we plot the logarithmic space density of 330000 local (z ∼ 0) SDSS (Abazajian et al. 2009) galaxies from the MPA-JHU catalogue (e.g., Kauffmann et al. 2003), as well as the redshift-dependent classification lines between AGN and star-forming galaxies as gray dashed lines (Kewley et al. 2013b).”
• Cosmic X-ray surveys of distant active galaxies (2015, background): “…identification of broad emission lines; measuring the emission-line gas conditions from emission-line ratios; e.g., Veilleux and Osterbrock 1987; Kewley et al 2013) and the identification of the large-scale structure environment in which they reside (e.g., identifying “filaments” of galaxies…”
• Chemical abundances in high-redshift galaxies: a powerful new emission line diagnostic (2016, background): “It is likely that a change in both ionization parameter and in the ISM pressure is responsible for this difference (Brinchmann et al. 2008; Liu et al. 2008; Hainline et al. 2009; Bian et al. 2010; Nakajima et al. 2012; Kewley et al. 2013a).”
• Nuclear activity versus star formation: emission-line diagnostics at ultraviolet and optical wavelengths (2015, background): “Interestingly, the ultraviolet observations of active galaxies in Figs 5 and 6 tend to favor AGN models with high ionization parameter, logUS ∼ −1, while the optical observations of SDSS galaxies in Figs 2–4 were compatible with a much larger range in US (a similar result was already noted by Groves”
• The Observer's Guide to the Gamma-Ray Burst-Supernova Connection (2016, background): “This offset is often attributed to harder ionization fields, higher ionization parameters or changes in the ISM properties [293, 294, 295].”
• SDSS IV MaNGA – spatially resolved diagnostic diagrams: a proof that many galaxies are LIERs (2016, methodology): “…lines in the BPT diagrams have been the main classification tool to distinguish star formation from other sources of ionisation in several studies based on the SDSS and more distant galaxy samples (Kauffmann et al. 2003b; Tremonti et al. 2004; Kewley et al. 2006, 2013; Shapley et al. 2015).”
• Spectroscopy of superluminous supernova host galaxies. A preference of hydrogen-poor events for extreme emission line galaxies (2014, methodology): “In the region of interest for H-poor SLSN hosts, an increase in q leads to an increase in the [O III]/Hβ ratio and we have marked this on our graph with a qualitative arrow similar to Kewley et al. (2013b).”
• De re metallica: the cosmic chemical evolution of galaxies (2018, methodology): “3.5.4), and could also be differential for the various methods (Kewley et al., 2013b,a; Steidel et al., 2014; Kewley et al., 2015; Shapley et al., 2015; Strom et al., 2017a; Kashino et al., 2017).”