The VLA-COSMOS 3~GHz Large Project: AGN and host-galaxy properties out to z$\lesssim$6 (1703.09720v2)

Abstract: We explore the multiwavelength properties of AGN host galaxies for different classes of radio-selected AGN out to z$\lesssim$6 via a multiwavelength analysis of about 7700 radio sources in the COSMOS field. The sources were selected with the Very Large Array (VLA) at 3 GHz (10 cm) within the VLA-COSMOS 3 GHz Large Project, and cross-matched with multiwavelength ancillary data. This is the largest sample of high-redshift (z$\lesssim$6) radio sources with exquisite photometric coverage and redshift measurements available. We constructed a sample of moderate-to-high radiative luminosity AGN (HLAGN) via spectral energy distribution (SED) decomposition combined with standard X-ray and mid-infrared diagnostics. Within the remainder of the sample we further identified low-to-moderate radiative luminosity AGN (MLAGN) via excess in radio emission relative to the star formation rates in their host galaxies. We show that AGN power in HLAGN occurs predominantly in radiative form, while MLAGN display a substantial mechanical AGN luminosity component. We found significant differences in the host properties of the two AGN classes, as a function of redshift. At z$<$1.5, MLAGN appear to reside in significantly more massive and less star-forming galaxies compared to HLAGN. At z$>$1.5, we observed a reversal in the behaviour of the stellar mass distributions with the HLAGN populating the higher stellar mass tail. We interpret this finding as a possible hint of the downsizing of galaxies hosting HLAGN, with the most massive galaxies triggering AGN activity earlier than less massive galaxies, and then fading to MLAGN at lower redshifts. Our conclusion is that HLAGN and MLAGN samples trace two distinct galaxy and AGN populations in a wide range of redshifts, possibly resembling the radio AGN types often referred to as radiative- and jet-mode (or high- and low-excitation), respectively.

Overview of AGN and Host-Galaxy Properties at High Redshift

The paper titled "The VLA-COSMOS 3 GHz Large Project: AGN and Host-Galaxy Properties out to z$\lesssim$6" investigates the multifaceted characteristics of active galactic nuclei (AGN) host galaxies by utilizing the prolific dataset from the VLA-COSMOS 3 GHz survey. This work focuses on understanding the properties of AGN across a broad redshift range, extending up to z$\lesssim$6, by analyzing roughly 7,700 radio sources identified in the COSMOS field. The research leverages the exceptional photometric and redshift data available for these radio sources, selected with the Very Large Array (VLA) at 3 GHz, to conduct a detailed examination of AGN activities and their host galaxies.

The authors employ a spectral energy distribution (SED) decomposition approach to delineate between moderate-to-high radiative luminosity AGN (HLAGN) and low-to-moderate radiative luminosity AGN (MLAGN). The classification is refined using standard X-ray and mid-infrared diagnostics, enhancing the detection capabilities for distinct AGN populations. Notably, HLAGN exhibit predominantly radiative AGN power while MLAGN demonstrate a substantial mechanical luminosity component. A novel aspect of this work is identifying the radio-excess metric against star formation rates that aids in distinguishing MLAGN, underlining the mechanical feedback potentially linked to radio jets.

Key Findings and Numerical Highlights

1. The paper successfully identifies 1,604 HLAGN and 1,333 MLAGN utilizing a multiwavelength approach. The characterization hinges on different proxies such as X-ray, MIR, and SED features besides radio excess for the latter.
2. The analysis reveals stark differences between HLAGN and MLAGN with regards to host-galaxy properties - notably in stellar mass and star formation rate (SFR) distributions. At z$<$1.5, MLAGN are found in more massive, less star-forming galaxies as opposed to HLAGN, while at z$>$1.5, the distribution changes, with HLAGN appearing in the higher stellar mass tail.
3. The use of radio-excess relative to SFR highlights underlying AGN power mechanics, providing deeper insights into AGN feedback mechanisms at play in different AGN classes.
4. The observation of a "downsizing" trend is notable, suggesting the more massive galaxies host HLAGN at higher redshifts, which transition to MLAGN as the redshift decreases.

Implications and Future Directions

The findings underline a complex interplay between AGN activity and host-galaxy evolution, suggesting two distinct AGN pathways that potentially mirror different epochs or stages of galactic life cycles tied to SMBH accretion modes. Practically, the work cautions on considering radio emission intricacies when inferring AGN feedback processes, painting a more dynamic picture of galaxy evolution driven by AGN activities.

Theoretically, the contrasts in AGN properties over cosmic time point towards differential accretion mechanisms being responsible for the observed radio AGN dichotomy, tied inherently to varying cosmic gas supply conditions. This suggests avenues for further cosmological simulations that can incorporate such nuanced AGN feedback mechanisms more accurately.

Future developments warrant an integrated analysis across larger datasets and deeper investigations into molecular outflows and their feedback efficiency in host galaxies under AGN activity. Also, refining classifications with advanced diagnostics and considering next-generation radio arrays would be crucial in exploring this domain.

This work within the VLA-COSMOS framework extends our comprehension of the AGN and their host-galaxy dynamics across epochs, contributing substantially to the cosmic evolution narrative and setting a benchmark for future high-redshift radio-AGN studies.