A Census of Active Galactic Nuclei Identified by the Mid-infrared [Ne v] Line at $\boldsymbol{z \leq 0.025}$

TL;DR

This study uses the [Ne v] emission line to identify local AGN at z ≤ 0.025, revealing a population of heavily obscured and previously undetected AGN missed by traditional methods, and providing insights into their properties.

Contribution

It introduces a [Ne v]-based selection method for local AGN, uncovering a significant fraction of obscured and unique AGN not identified by other techniques.

Findings

Identified 103 AGN using [Ne v] emission, representing 18% of the local galaxy population.

Discovered ~15% of AGN are missed by all classical detection methods.

Found that [Ne v]-selected AGN tend to have lower black hole masses and higher Eddington ratios.

Abstract

We present a census of local active galactic nuclei (AGN) at a redshift of $z\leq0.025$ selected using the high-ionization [Ne v] $\lambda14.32\,\mu$m emission line from the Infrared Database of Extragalactic Observables from Spitzer (IDEOS). We identify 103 sources with detected [Ne v] emission, which we regard as AGN within the volume. This sample represents $\sim18\%$ of the galaxy population within this redshift range, consistent with AGN fractions derived using other selection techniques. We investigate the biases and properties of this [Ne v]-selected AGN sample by comparing it with traditional AGN selection methods based on hard X-ray, optical, and mid-infrared colors. We find that our selection significantly misses AGN with underdeveloped narrow line regions (NLRs), which account for approximately half of the AGN identified by NLR-independent methods. However, approximately $\sim10\%$ of our sample are undetected in optical diagnostics, while $\sim40\%$ are missed by hard X-rays and $\sim70\%$ by infrared continuum. Notably, $\sim15\%$ of our AGN are missed by all classical methods, constituting a population of previously unidentified AGN revealed solely by the [Ne v] emission line. Based on our analysis, we show that this line can efficiently select heavily Compton-thick and host-dominated AGN systems. Our analysis also yields mean bolometric luminosities of $\log(L_{\rm bol}/{\rm erg~s^{-1}})=44.5\pm0.7$, black hole masses of $\log(M_{\rm BH}/M_{\odot})=7.3\pm0.6$, and Eddington ratio of $\lambda_{\rm Edd}=0.15\pm0.11$. Our sample harbors AGN with comparable luminosities but systematically lower-mass black holes accreting at higher Eddington ratios than those in the hard X-ray-selected sample. This suggests that our AGN may represent local analogs of the rapidly growing SMBH population prevalent at cosmic noon.