Rest-frame UV single-epoch black hole mass estimates of low-luminosity AGN at intermediate redshifts

TL;DR

This study evaluates UV-based single-epoch black hole mass estimators at intermediate redshifts, confirming their reliability and calibrating their accuracy against optical methods across a broad range of AGN luminosities.

Contribution

It extends the comparison of UV and optical virial BH mass estimators to higher redshifts and luminosities, providing updated calibration and assessing their scatter and systematic uncertainties.

Findings

MgII correlates tightly with Balmer lines (scatter ~0.25 dex)

CIV and CIII] estimators are viable after luminosity trend correction

Intrinsic scatter between Balmer and carbon estimators is ~0.37 dex

Abstract

The ability to accurately derive black hole (BH) masses at progressively higher redshifts and over a wide range of continuum luminosities has become indispensable in the era of large-area extragalactic spectroscopic surveys. In this paper we present an extension of existing comparisons between rest-frame UV and optical virial BH mass estimators to intermediate redshifts and luminosities comparable to the local H$\beta$ reverberation mapped active galactic nuclei (AGN). We focus on the MgII, CIV, and CIII] broad emission lines and compare them to both H$\alpha$ and H$\beta$. We use newly acquired near-infrared spectra from the FMOS instrument on the Subaru telescope for 89 broad-lined AGN at redshifts between 0.3 and 3.5, complemented by data from the AGES survey. We employ two different prescriptions for measuring the emission line widths and compare the results. We confirm that MgII shows a tight correlation with H$\alpha$ and H$\beta$, with a scatter of ~0.25 dex. The CIV and CIII] estimators, while showing larger scatter, are viable virial mass estimators after accounting for a trend with the UV-to-optical luminosity ratio. We find an intrinsic scatter of ~0.37 dex between Balmer and carbon virial estimators by combining our dataset with previous high redshift measurements. This updated comparison spans a total of 3 decades in BH mass. We calculate a virial factor for CIV/CIII] logf(CIV/CIII])=0.87 with an estimated systematic uncertainty of ~0.4 dex and find excellent agreement between the local reverberation mapped AGN sample and our high-z sample.