COSMOS-3D: Black Hole Mass Estimators and Luminosity Functions of Paschen-line AGNs

TL;DR

This study uses JWST near-infrared Paschen lines to identify and analyze high-redshift Type 1 AGNs, deriving new black hole mass estimators and luminosity functions that suggest Paschen-based methods are more complete and less biased by dust.

Contribution

The paper introduces new Paschen-line based black hole mass estimators and constructs the first luminosity functions for Paschen-selected Type 1 AGNs at z>1.

Findings

Paschen-line AGN luminosity functions are 3-5 times higher than UV/optical ones.

Paschen-based black hole mass estimators are consistent and broadly agree with previous methods.

Dust reddening does not affect the intrinsic properties of the AGNs.

Abstract

Near-IR Paschen lines are potentially an excellent tracer of Type 1 AGNs that is hardly affected by dust extinction. JWST allows us, for the first time, to explore Paschen-line objects at redshift z>1. Here we present a study of 62 AGNs with broad Pa$\alpha$ and Pa$\beta$ lines at 1<z<3 using data from the JWST COSMOS-3D program. These AGNs are efficiently selected and identified using NIRCam imaging and grism slitless spectroscopic data. We separate the AGN-host emission with image decomposition and quantify dust attenuation with multi-band data. We construct a calibration sample with optical spectroscopy and use single-epoch, Mg II-based black hole masses ($M_{\mathrm{BH}}$) as an anchor to derive new, Paschen-based $M_{\mathrm{BH}}$ estimators. We obtain three sets of $M_{\mathrm{BH}}$ estimators based on Paschen line luminosities and AGN continuum luminosities at 1 and 2 $\mu$m, respectively. After dust corrections, they are well consistent with each other, and also broadly agree with previous results. With this AGN sample, we further construct the first Pa$\alpha$ and Pa$\beta$ luminosity functions (LFs) of Type 1 AGNs. The derived LFs are 3-5 times higher than those of UV/optical-selected AGNs, indicating that Paschen-selected Type 1 AGNs are more complete. In addition, the intrinsic properties of our AGNs show no dependence on dust reddening, suggesting that the observed reddening is unrelated to the central engine and is thus likely caused by line-of-sight obscuration.