BPT Diagram: Galaxy & AGN Diagnostics

• The BPT diagram is an optical diagnostic tool that distinguishes star formation from AGN activity using logarithmic ratios of key emission lines.
• It employs ratios of [N II]/Hα and [O III]/Hβ to classify galaxies into star-forming, AGN, or composite categories with precision.
• Advanced applications include spatially resolved mapping and adaptations for high-redshift studies to probe AGN feedback and chemical evolution.

The Baldwin‐Phillips‐Terlevich (BPT) diagram is an optical emission‐line diagnostic originally introduced in 1981 to distinguish between the dominant ionization mechanisms in galaxies. It uses specific logarithmic ratios of strong nebular lines to separate star formation from active galactic nucleus (AGN) activity and related phenomena. The diagram has become not only a standard classification tool for nearby galaxies but also a basis for advanced studies in high‐redshift surveys, spatially resolved spectroscopy, and even feedback processes in the interstellar medium (ISM).

1. Emission Lines and Diagnostic Ratios

The traditional BPT diagram is constructed using ratios of the following strong emission lines:

• [O III] λ5007/Hβ: Sensitive to the hardness of the ionizing radiation.
• [N II] λ6584/Hα: Sensitive to gas-phase metallicity and secondary nitrogen production.

In many studies the ratios are expressed logarithmically as $$x = \log \left(\frac{[\mathrm{NII}]}{\mathrm{H}\alpha}\right) \quad \text{and} \quad y = \log \left(\frac{[\mathrm{OIII}]}{\mathrm{H}\beta}\right).$$ These ratios are largely insensitive to interstellar extinction due to the close wavelength proximity of Hα and [N II] as well as Hβ and [O III].

2. Construction of the BPT Diagram and Classification Criteria

In the classical scheme, galaxies are plotted in the (x, y) plane and classified using empirically and theoretically derived demarcation curves. One widely used empirical division is given by Kewley et al. (2013), whose equation $$\log \left(\frac{[\mathrm{OIII}]}{\mathrm{H}\beta}\right) = \frac{0.61}{\log\left(\frac{[\mathrm{NII}]}{\mathrm{H}\alpha}\right) - 0.02 - 0.1833} + 1.2 + 0.03$$ separates galaxies into a star-forming locus (below the curve) and AGN-dominated regions (above the curve). Sources falling in between, sometimes termed “composites,” indicate contributions from both star formation and AGN activity.

3. Applications in Galaxy and AGN Diagnostics

The BPT diagram serves as a powerful diagnostic in multiple contexts:

• Lensed Quasar Searches: By measuring the line ratios in emission from lensed sources, surveys (e.g., SuGOHI) identify probable lensed quasars when the flux ratios place the source above the empirical AGN boundary on the diagram. For instance, a source with undetected [N II] and a high [O III]/Hβ ratio is interpreted as a likely lensed quasar, whereas others falling below the demarcation are consistent with star-forming galaxies.
• Double-Peaked Emission Lines: In systems that exhibit double-peaked narrow emission lines, the BPT diagram is applied separately to the red and blue components. This enables categorizing dual AGN candidates (if both components lie in the AGN region), dual star-forming systems, or mixed cases.
• AGN Feedback Studies: Spatially resolved BPT mapping (using, for example, HST narrow-band imaging) reveals intricate ionization structures in galaxy cores. In galaxies like NGC 3393, the classic BPT applied on a pixel-by-pixel basis has uncovered a well-defined Seyfert bicone surrounded by a thin LINER cocoon, a signature of layered AGN-driven feedback processes in the circumnuclear ISM.

4. Probing Physical Conditions and Evolution

Because the line ratios depend on the ionization parameter, metallicity, and excitation spectrum, the BPT diagram provides insight into the physical conditions of ionized gas:

• Metallicity Sensitivity: Higher metallicity increases [N II]/Hα while low-ionization [O III]/Hβ tends to decrease. This behavior allows the diagram to trace chemical evolution and calibrate mass-metallicity relations.
• Ionization Parameter and Radiation Hardness: Variations in the ionization parameter (U) and the hardness of the ionizing spectrum (from massive young stars or AGN) shift the galaxy positions on the diagram. Theoretical modeling and simulations (e.g., using CLOUDY with outputs from cosmological simulations) have shown that offsets observed at high redshift can result primarily from selection biases toward high-mass, metal-rich galaxies along with subtle adjustments in N/O ratios.
• Spatial Trends: When applied at high resolution, the BPT diagram has provided evidence that AGN feedback and shocks cause spatially distinct regions with different excitation properties, emphasizing the utility of the diagram beyond integrated spectra.

5. Limitations and Extensions

Despite its widespread application, the BPT diagnostic has inherent limitations:

• Redshift Constraint: Beyond $z \sim 0.5$ the [N II] and Hα lines redshift out of the optical window. To extend diagnostics to higher redshifts (up to $z \sim 1.4$ and beyond), complementary diagrams such as the TBT (Trouille, Barger & Tremonti) and MEx (Mass-Excitation) diagrams have been developed. These substitute [Ne III]/[O II] ratios and broadband colors or stellar mass to maintain sensitivity to AGN activity.
• AGN Completeness: The BPT diagram reliably identifies AGN only when the narrow-line region is sufficiently luminous. Studies have shown that up to 40% of X-ray-selected AGN possess intrinsically weak narrow lines such that they cannot be placed on the BPT diagram and, therefore, may be misclassified as star-forming.
• Composite and Mixed Activity: The ambiguous region between pure star formation and AGN (composite) requires careful interpretation. Advanced techniques—such as spatially resolved mapping and statistical methods that quantify the perpendicular distance from the star-forming ridge—provide a continuous measure of AGN-like activity rather than a strict binary classification.

6. Extensions Through Spatially Resolved and Alternative Methods

Recent advances have integrated the BPT framework with:

• High-Resolution Imaging: HST imaging at sub-100 pc resolution allows construction of resolved BPT maps, uncovering detailed excitation gradients and features such as the Seyfert bicone and LINER cocoon in AGN hosts.
• Emission Line Decomposition: In large surveys (e.g., SDSS-V combined with eROSITA), decomposing broad and narrow components of emission lines prevents misclassification. Using only the narrow-line fluxes shifts the galaxy distribution upward on the BPT diagram, resulting in better alignment with X-ray properties and black hole mass indicators.
• Data-Driven Approaches: Methods that employ principal component analysis (PCA) on full visible spectra extend the BPT classification to galaxies with weak or undetected individual lines. Kernel density estimation in a reduced-dimension latent space can yield a probability mapping that mirrors traditional BPT classes but is more robust to data limitations.

7. Conclusion

The BPT diagram remains an indispensable tool in extragalactic astrophysics for disentangling the ionization sources in galactic nuclei. Its use—which spans from identifying lensed quasars to probing AGN feedback and tracking chemical evolution—exemplifies how simple line ratio diagnostics can yield deep physical insight. While limitations such as redshift constraints and sensitivity issues necessitate complementary diagnostics and advanced modeling, the BPT diagram continues to serve as a foundational reference point in both observational and theoretical studies of galaxy evolution.