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arXiv · 2512.13345

Standardizing reverberation-mapped H$\beta$ active galactic nuclei using time-averaged radius$-$luminosity relations with 5100\,\AA\,, broad H$\beta$, and narrow \OIII\ luminosities

Shulei Cao·Amit Kumar Mandal·Michal Zajaček·Bharat Ratra
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Abstract

Active galactic nuclei (AGN) have been studied as alternate probes in cosmology due to their large and stable luminosities and broad redshift range. Previously it was shown that higher-redshift AGN that were reverberation-mapped (RM) using broad Mg\,\textsc{ii} and C\,\textsc{iv} lines are standardizable and yield weak cosmological constraints that are consistent with those from better-established probes. In contrast, lower-redshift AGN that were reverberation-mapped using the broad H$\beta$ line exhibited tensions with the standard cosmological model, in particular they preferred currently decelerating cosmological expansion. Here we study the standardizability of a homogeneous RM H$\beta$ sample of $\sim 100$ AGN (over redshifts $0.00308 \leq z \leq 0.8429$), whose H$\beta$ time delays and three luminosity tracers (at 5100\,\AA\,, broad H$\beta$, and narrow [O\,\textsc{iii}]) are averaged over several epochs. We find that this averaged sample is standardizable using three $R-L$ relations. While for luminosities corresponding to 5100\,\AA\, and the broad H$\beta$ line the cosmological constraints prefer currently decelerating cosmological expansion, the cosmological parameters for the narrow [O\,\textsc{iii}] luminosity are more consistent with those from better-established probes and they are in agreement with currently accelerating cosmological expansion. This demonstrates for the first time that narrow-line region [O\,\textsc{iii}] can be utilized for AGN standardization and cosmological constraints. Selecting proper photoionizing flux proxies for the broad-line region is thus crucial in studies of RM AGN standardizability.

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Shulei Cao, Amit Kumar Mandal, Michal Zajaček, Bharat Ratra. 2025-12-15. Standardizing reverberation-mapped H$\beta$ active galactic nuclei using time-averaged radius$-$luminosity relations with 5100\,\AA\,, broad H$\beta$, and narrow \OIII\ luminosities. https://arxiv.org/abs/2512.13345

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