BPT Analysis in Extragalactic Research

• Baldwin-Phillips-Terlevich (BPT) analysis is a spectroscopic diagnostic that uses optical emission-line ratios to distinguish between ionization from star formation and AGN activity.
• It leverages ratios like [O III]/Hβ and [N II]/Hα to classify galaxies, even when some lines are detected only as upper limits.
• The methodology is critical in lens surveys, enabling secure quasar identification and providing insights into galaxy evolution despite observational limitations.

The Baldwin-Phillips-Terlevich (BPT) analysis is a cornerstone spectroscopic technique in extragalactic astrophysics, providing a robust emission-line diagnostic for distinguishing the dominant ionizing source in galaxies—whether intense star formation or active galactic nucleus (AGN) activity. Originally developed to interpret integrated galaxy spectra, the BPT framework leverages characteristic optical emission line ratios sensitive to the physical conditions and excitation mechanisms within ionized gas clouds. Over four decades, BPT diagnostics have become essential for classifying extragalactic sources, revealing the astrophysical origin of lensed objects, and interpreting large spectroscopic surveys.

1. Principles and Construction of the BPT Diagram

The classical BPT diagram utilizes two fundamental emission-line ratios: log([N II] λ6584 / Hα) on the x-axis and log([O III] λ5007 / Hβ) on the y-axis. These ratios exploit the differing hardness of stellar and AGN ionizing continua:

• [O III]/Hβ is a tracer of ionization parameter and radiation field strength.
• [N II]/Hα reflects gas-phase metallicity and the degree of secondary nucleosynthesis.

Discrimination between star-forming and AGN-ionized sources is performed using empirical and theoretical demarcations, notably the boundary: $$\log\left(\frac{[\mathrm{O\,III}]}{\mathrm{H}\beta}\right) = \frac{0.61}{\log([\mathrm{N\,II}]/\mathrm{H}\alpha) - 0.02 - 0.1833} + 1.2 + 0.03$$ as adopted from Kewley et al. (2013). Galaxies above this curve are classified as AGN hosts, while those below are dominated by star formation.

2. Application in the Spectroscopic Analysis of Lensed Sources

BPT diagnostics are applied to near-infrared (NIR) spectra for the spectral classification of strongly lensed source galaxies. Emission lines—[N II] λ6584, Hα, [O III] λ5007, and Hβ—are isolated in X-shooter NIR spectra, with fluxes measured via Gaussian profile fitting. Notably, in the referenced SuGOHI survey, three lensed candidates yielded the necessary lines for BPT analysis: HSCJ022622$-$042522, HSCJ023817$-$054555, and HSCJ144320$-$012538. For several, [N II] was not directly detected; only upper limits on its flux were available, resulting in censored constraints in resultant BPT placement.

The BPT diagram was constructed for these sources, plotting their measurements (or limiting positions). Figure~\ref{fig:bpt} in the paper juxtaposes the candidates relative to the AGN/SF demarcation, enabling direct morphological and spectroscopic classification in one diagnostic space.

3. Classification Outcomes and Diagnostic Power

The spectral classification via BPT analysis revealed that:

• HSCJ022622$-$042522 is uniquely placed above the AGN/SF dividing curve, even when only an upper limit for [N II] is used. This robustly indicates an AGN, specifically a lensed quasar.
• HSCJ023817$-$054555 and HSCJ144320$-$012538 fall below the AGN separation, classifying them as consistent with star-forming galaxies.
• A fourth source, HSCJ141136$-$010216, did not have BPT-accessible lines ([N II], Hα not detected); Lyα width was used for its classification, finding it to be a probable Lyman-α emitter (not AGN).

The resultant BPT classifications are summarized as follows:

Name	[OIII], Hβ, Hα Detected?	[NII]	BPT Placement	Classification
HSCJ022622$-$042522	Yes	Upper limit	Above AGN line	Likely AGN/quasar
HSCJ023817$-$054555	Yes	Upper limit	Below AGN line	Star-forming galaxy
HSCJ144320$-$012538	Yes	Upper limit	Below AGN line	Star-forming galaxy
HSCJ141136$-$010216	No ([NII]/Hα unavailable)	--	-- (Lyα only)	LAE, not AGN

This approach demonstrates the BPT diagram’s diagnostic power for robustly identifying lensed quasars. The method's reliability persists even when line fluxes are limited to upper bounds, provided line ratio constraints remain physically informative.

4. Methodological Limitations and Caveats

Several limitations are inherent in BPT-based classification in this observational context:

• Narrow-line Non-detections: Absence of [N II] detections—common in faint, distant lensed sources—reduces the discriminating power, as classification relies on upper limits.
• Sample Incompleteness: Only three out of six followed-up lens candidates possessed sufficient emission line detections for BPT mapping. Remaining sources without all four lines or only weaker lines (e.g., Lyα) cannot be placed in the standard diagram.
• Statistical Constraints: The small sample and upper limit measurement on [N II], especially for HSCJ022622$-$042522, result in conservative classifications; yet, placement above the AGN division even with an upper limit remains compelling.
• Line Sensitivities: The BPT formalism presupposes all lines are measured to comparable depth and fidelity; practical application to faint lensed sources can yield incomplete (censored) data vectors.

5. Scientific Implications for Gravitational Lens Surveys

The adoption of BPT analysis in the context of gravitationally lensed source identification provides critical leverage for the scientific use of lens surveys:

• Secure Quasar Identification: Only through the BPT diagram could HSCJ022622$-$042522 be definitively identified as a lensed quasar, supporting its inclusion in cosmological/galaxy evolution analyses that require bona fide AGN.
• Population Demographics: The approach enables statistical statements about the nature of lensed sources—distinguishing AGN from star-forming galaxy lenses, which is vital for sub-sample construction in downstream cosmological or astrophysical studies.
• Limitations in Purely Morphological Classification: While image morphology and lens modeling suggest a quasar in HSCJ022622$-$042522, only the BPT diagram provides a quantitative spectroscopic confirmation.

6. Role of BPT Analysis in Future Wide-field Imaging Surveys

BPT diagnostics are poised to remain fundamental as lensing candidate identification scales with future surveys. The demonstrated ability to apply BPT ratios even from upper limits or incomplete line sets maximizes extraction of physical insight given common spectroscopic limitations in high-z/faint lens systems. These methods reinforce the necessity of high-fidelity, NIR spectroscopic follow-up for critical line ratio measurement and motivate further improvements in emission-line extraction from noisy spectra in lens confirmation workflows.

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In summary, the BPT analysis, via the [O III]/Hβ versus [N II]/Hα diagram, was critical for confirming the AGN (quasar) nature of HSCJ022622$-$042522 in the referenced lensed quasar survey. This result underscores the BPT diagram’s essential role in robustly classifying lensed sources in wide-field imaging surveys, with the principal limitation being line sensitivity—most notably, the frequent necessity to rely on upper limits for [N II] in faint or high-redshift sources.