Tracing AGN-Galaxy Co-Evolution with UV Line-Selected Obscured AGN

TL;DR

This work presents the first UV-line-selected obscured AGN sample with complete X-ray-to-radio coverage in COSMOS, derived from [Ne V] 3426Å and CIV 1549Å lines. By performing X-ray spectral analyses and extensive SED fitting with CIGALE (including X-ray and radio modules), the authors derive robust host and AGN properties for 184 sources and compare them to Simba hydrodynamical analogues, revealing that these line selections identify transient, obscured accretion phases. The [Ne V] and CIV samples trace different evolutionary states: [Ne V] sources tend to be in a pre-quenching stage, while CIV sources are more AGN-dominated and likely quenched by AGN activity, consistent with blowout-regime expectations. The results underscore the critical role of radio data in disentangling AGN and host contributions, especially when IR/X-ray data are incomplete, and provide a framework for extending obscured AGN census with upcoming large-area spectroscopic surveys.

Abstract

Understanding black hole-galaxy co-evolution and the role of AGN feedback requires complete AGN samples, including heavily obscured systems. In this work, we present the first UV line-selected ([Nev]3426 and CIV1549) sample of obscured AGN with full X-ray-to-radio coverage, assembled by combining data from the Chandra COSMOS Legacy survey, the COSMOS2020 catalogue, IR photometry from XID+, and radio observations from the VLA and MIGHTEE surveys. Using CIGALE to perform spectral energy distribution (SED) fitting, we analyse 184 obscured AGN at 0.6 < z < 1.2 and 1.5 < z < 3.1, enabling detailed measurements of AGN and host galaxy properties, and direct comparison with SIMBA hydrodynamical simulations. We find that X-ray and radio data are essential for accurate SED fits, with the radio band proving critical when X-ray detections are missing or in cases of poor IR coverage. Comparisons with matched non-active galaxies and simulations suggest that the [NeV]-selected sources are in a pre-quenching stage, while the CIV-selected ones are likely quenched by AGN activity. Our results indicate that [NeV] and CIV selections target galaxies in a transient phase of their co-evolution, characterised by intense, obscured accretion, and pave the way for future extensions with upcoming large area high-z spectroscopic surveys.

Figures (17)

• Figure 1: Redshift distribution of the iv C iv- (blue) and [Ne v]- (red) selected AGN. The redshift distribution is primarily determined by the requirement that the [Ne v]$3426\AA$ and C iv$1549\AA$ emission lines fall within the wavelength coverage of VIMOS.
• Figure 2: Obscuration as a function of the intrinsic $2-10\,$keV luminosity for X-ray detected sources in the iv C iv (blue) and [Ne v] samples (red). The grey lines are the associated $90\%$ uncertainties. The triangles denote upper limits on the amount of obscuration due to the difficulties in properly constraining it in the case of spectra with a low number of photon counts.
• Figure 3: Comparison of the stellar mass (left) and AGN torus optical depth (right) obtained from SED fitting for the real and mock observations. While we can robustly estimate the host-galaxy stellar mass, with the available data, we are not able to put proper constraints on the AGN optical depth.
• Figure 4: Comparison of the positions of the [Ne v] sample in the $SFR-M_*$ plane. The squares represent the [Ne v] sources detected in the radio band (either MeerKAT or VLA), while the circles represent those without radio detection. The colour code denotes the $1-1000\,\mu$m AGN fraction. The grey solid line is the schreiber15 MS at the mean redshift of the sources, the grey dashed lines its $1\,\sigma$ dispersion. The source with $\log{(M_*\,/\,\rm{M_{\odot}})}=11.04$ and $\log{(SFR_*\,/\,\rm{M_{\odot}\,yr^{-1}})}<-3$ is zCOSMOS ID 813366, for which we were not able to properly constrain the SFR (and the AGN properties) due to its complete lack of detections in the X-ray, radio, and IR bands. The source is therefore excluded from the rest of this work.
• Figure 5: Comparison of the positions of the iv C iv sample in the$SFR-M_*$ plane. The squares represent the iv C iv sources detected in the radio band (either MeerKAT or VLA), while the circles represent those without radio detection. The colour code denotes the $1-1000\,\mu$m AGN fraction. The grey solid line is the schreiber15 MS at the mean redshift of the sources, the grey dashed lines its $1\,\sigma$ dispersion.