Standardizing reverberation-mapped Hα and Hβ active galactic nuclei using radius-luminosity relations involving monochromatic and broad Hα luminosities

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This is the sample of objects used for the analysis to create Figs. 2 and 3. We investigate the possibility of standardizing a homogeneous set of 41 low-redshift (0.00415 ≤ z ≤ 0.474) active galactic nuclei for cosmological applications using reverberation mapping based on the broad Hα and Hβ emission lines. The analysis shows that these objects can be placed on well-defined radius–luminosity relations when employing four variants that combine Hα and Hβ time delays with both monochromatic and integrated Hα luminosities. The derived R–L relations are tightly constrained and remain insensitive to the choice among the six cosmological models considered. However, the resulting cosmological constraints derived from these relations are relatively weak. The best-fitting slopes are somewhat steeper than those expected from simple photoionization arguments and also exceed those reported in previous higher-redshift Hβ-based studies using larger samples. This discrepancy is likely linked to the absence of strongly accreting sources in the present dataset, which is dominated by lower-accretion AGNs. The cosmological parameters inferred from this homogeneous sample are broadly consistent—within approximately 2σ—with results from established cosmological probes. This contrasts with our earlier analysis of a larger but more heterogeneous sample of 118 Hβ reverberation-mapped AGNs, which yielded tensions at the ≳2σ level with standard cosmological constraints. Overall, our results highlight that the internal consistency of the sample, in particular the uniformity of time-lag measurements, is a critical requirement for establishing reverberation-mapped AGNs as reliable cosmological distance indicators. Acknowledgements: Project funded by National Science Centre, Poland under the OPUS call in the Weave programme, UMO-2021/43/I/ST9/01352.