What drives the redshift evolution of strong emission line ratios?

Fuyan Bian*, Lisa J. Kewley, Brent Groves, Michael A. Dopita

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    19 Citations (Scopus)


    We study the physical mechanisms that cause the offset between low-redshift and high-redshift galaxies on the [OIII] λ5007/H β versus [NII] λ6584/H α 'Baldwin, Phillips & Terlevich' (BPT) diagram using a sample of local analogues of high-redshift galaxies. These highredshift analogue galaxies are selected from the Sloan Digital Sky Survey. Located in the same region on the BPT diagram as the ultraviolet selected galaxies at z ∼ 2, these highredshift analogue galaxies provide an ideal local benchmark to study the offset between the local and high-redshift galaxies on the BPT diagram.We compare the nitrogen-to-oxygen ratio (N/O), the shape of the ionizing radiation field, and ionization parameters between the highredshift analogues and a sample of local reference galaxies. The higher ionization parameter in the high-redshift analogues is the dominant physical mechanism driving the BPT offset from low- to high-redshift, particularly at high [N II] λ6584/H α. Furthermore, the N/O ratio enhancement also plays a minor role to cause the BPT offset. However, the shape of the ionizing radiation field is unlikely to cause the BPT offset because the high-redshift analogues have a similar hard ionizing radiation field as local reference galaxies. This hard radiation field cannot be produced by the current standard stellar synthesis models. The stellar rotation and binarity may help solve the discrepancy.

    Original languageEnglish
    Pages (from-to)580-585
    Number of pages6
    JournalMonthly Notices of the Royal Astronomical Society
    Issue number1
    Publication statusPublished - 1 Mar 2020


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