CLEAR: High-Ionization [Ne V] ${\lambda}$3426 Emission-line Galaxies at $1.4 <z< 2.3$

TL;DR

This study investigates high-ionization [Ne V] emission-line galaxies at redshifts 1.4 to 2.3, revealing their likely active galactic nuclei origins and exploring the nature of their energetic ionizing sources using multi-wavelength data.

Contribution

It provides the first detailed analysis of [Ne V] emitters at these redshifts, combining UV/optical, X-ray, and infrared data to understand their ionization mechanisms and AGN activity.

Findings

Most [Ne V] galaxies are consistent with AGN ionization.

[Ne V] luminosities resemble those of $z\,\sim 1$ QSOs.

X-ray properties suggest a possible soft X-ray excess or complex geometry.

Abstract

We analyze a sample of 25 [Ne V] $\lambda$3426 emission-line galaxies at $1.4<z<2.3$ using Hubble Space Telescope/Wide Field Camera 3 G102 and G141 grism observations from the CANDELS Lyman-$\alpha$ Emission at Reionization (CLEAR) survey. [Ne V] emission probes extremely energetic photoionization (97.11-126.21 eV), and is often attributed to energetic radiation from active galactic nuclei (AGN), shocks from supernova, or an otherwise very hard ionizing spectrum from the stellar continuum. In this work, we use [Ne V] in conjunction with other rest-frame UV/optical emission lines ([O II] $\lambda\lambda$3726,3729, [Ne III] $\lambda$3869, H$\beta$, [O III] $\lambda\lambda$4959,5007, H$\alpha$+[N II] $\lambda\lambda$6548,6583, [S II] $\lambda\lambda$6716,6731), deep (2--7 Ms) X-ray observations (from Chandra), and mid-infrared imaging (from Spitzer) to study the origin of this emission and to place constraints on the nature of the ionizing engine. The majority of the [Ne V]-detected galaxies have properties consistent with ionization from AGN. However, for our [Ne V]-selected sample, the X-ray luminosities are consistent with local ($z\lesssim 0.1$) X-ray-selected Seyferts, but the [Ne V] luminosities are more consistent with those from $z\sim 1$ X-ray-selected QSOs. The excess [Ne V] emission requires either reduced hard X-rays, or a $\sim$0.1 keV excess. We discuss possible origins of the apparent [Ne V] excess, which could be related to the ``soft (X-ray) excess'' observed in some QSOs and Seyferts, and/or be a consequence of a complex/anisotropic geometry for the narrow line region, combined with absorption from a warm, relativistic wind ejected from the accretion disk. We also consider implications for future studies of extreme high-ionization systems in the epoch of reionization ($z \gtrsim 6$) with JWST.