Standardizing reverberation-mapped H$\hat{I}\pm$ and H${\hat{I}}^2$ active galactic nuclei using radius--luminosity relations involving monochromatic and broad H$\hat{I}\pm$ luminosities

Abstract

We test the standardizability of a homogeneous sample of 41 lower-redshift ($0.00415\le z\le 0.474$) active galactic nuclei (AGNs) reverberation-mapped (RM) using the broad H$\hat{I}\pm$ and H${\hat{I}}^2$ emission lines. We find that these sources can be standardized using four radius$-$luminosity ($R-L$) relations incorporating H$\hat{I}\pm$ and H${\hat{I}}^2$ time delays and monochromatic and broad H$\hat{I}\pm$ luminosities. Although the $R-L$ relation parameters are well constrained and independent of the six cosmological models considered, the resulting cosmological constraints are weak. The measured $R-L$ relations exhibit slightly steeper slopes than predicted by a simple photoionization model and steeper than those from previous higher-redshift H${\hat{I}}^2$ analyses based on larger datasets. These differences likely reflect the absence of high-accreting sources in our smaller, lower-redshift sample, which primarily comprises lower-accreting AGNs. The inferred cosmological parameters are consistent within 2$\ddot{I}ƒ$ (or better) with those from better-established cosmological probes. This contrasts with our earlier findings using a larger, heterogeneous sample of 118 H${\hat{I}}^2$ AGNs, which yielded cosmological constraints differing by $\gtrsim 2\ddot{I}ƒ$ from better-established cosmological probes. Our analysis demonstrates that sample homogeneity$-$specifically, the use of a consistent time-lag determination method$-$is crucial for developing RM AGNs as a cosmological probe.

34 pages, 18 figures, 9 tables, accepted for publication in PRD