[Paper Review] Hosts and environments: a (large-scale) radio history of AGN and star-forming galaxies

M. Magliocchetti|arXiv (Cornell University)|Jun 30, 2022

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy426 references36 citations

TL;DR

This comprehensive review synthesizes the current state of knowledge on extragalactic radio sources—active galactic nuclei (AGN) and star-forming galaxies—focusing on their hosts, large-scale environments, and cosmological evolution. It integrates multi-wavelength data and clustering analyses to assess how radio activity correlates with galaxy mass, morphology, accretion mode, and environmental density across cosmic time, highlighting unresolved tensions in the literature and identifying key open questions for future surveys like the SKA.

ABSTRACT

Despite their relative sparseness, during the recent years it has become more and more clear that extragalactic radio sources (both AGN and star-forming galaxies) constitute an extremely interesting mix of populations, not only because of their intrinsic value, but also for their fundamental role in shaping our Universe the way we see it today. Indeed, radio-active AGN are now thought to be the main players involved in the evolution of massive galaxies and clusters. At the same time, thanks to the possibility of being observed up to very high redshifts, radio galaxies can also provide crucial information on both the star-formation history of our Universe and on its Large-Scale Structure properties and their evolution. In the light of present and forthcoming facilities such as LOFAR, MeerKAT and SKA that will probe the radio sky to unprecedented depths and widths, this review aims at providing the current state of the art on our knowledge of extragalactic radio sources in connection with their hosts, large-scale environments and cosmological context.

Motivation & Objective

• To synthesize the current understanding of radio-emitting AGN and star-forming galaxies in relation to their host galaxies and large-scale environments.
• To resolve inconsistencies in the literature regarding the environmental dependence of radio AGN, particularly concerning FRI/FRII types, HERG/LERG populations, and radio luminosity.
• To assess the cosmological evolution of the IR-radio correlation in star-forming galaxies and its environmental modulation.
• To identify critical open questions in radio AGN physics and environmental co-evolution that future deep radio surveys must address.
• To provide a foundation for interpreting data from upcoming facilities such as LOFAR, MeerKAT, and the SKA by unifying host and environmental properties of radio sources.

Proposed method

• Systematic review and synthesis of continuum radio surveys, optical spectroscopy, and multi-wavelength data to classify radio sources by morphology (FRI/FRII), accretion mode (HERG/LERG), and luminosity.
• Application of clustering cross-correlation functions to quantify the spatial clustering of radio sources with respect to galaxies and large-scale structures across redshifts up to z ~ 1.7.
• Use of direct environmental pinpointing via cross-matching radio sources with known clusters, protoclusters, and overdensities from optical, X-ray, and infrared surveys.
• Analysis of the IR-radio correlation in star-forming galaxies across redshifts, using SED-fitting and luminosity measurements to assess its evolution and environmental dependence.
• Statistical comparison of environmental overdensity metrics (e.g., local galaxy density, cluster membership) for different radio AGN subtypes and luminosity bins.
• Critical evaluation of conflicting results in the literature regarding environmental dependence, including redshift-, luminosity-, and morphology-dependent trends.

Experimental results

Research questions

• RQ1 How do the host galaxy properties (mass, morphology, star-formation activity) of radio AGN vary with radio luminosity, FRI/FRII type, and accretion mode (HERG/LERG)?
• RQ2 Is there a consistent environmental preference (e.g., overdense vs. underdense) for FRI vs. FRII radio AGN across cosmic time, and does this correlate with their accretion mode?
• RQ3 Does the IR-radio correlation in star-forming galaxies evolve with redshift, and is it modulated by environmental density, particularly in high-redshift protoclusters?
• RQ4 Why is there a persistent discrepancy in the literature regarding the environmental dependence of radio AGN—does it stem from redshift range, luminosity binning, or sample selection?
• RQ5 Do radio-loud quasars and radio-AGN cluster differently in large-scale structure, and what does this imply for their fuelling mechanisms and evolution?

Key findings

• FRII radio galaxies are found in overdense environments at high redshift (e.g., z = 1.7 protocluster around a FRII), challenging the assumption that FRIIs are only in low-density regions.
• There is no consensus on whether higher-luminosity radio AGN reside in denser environments; conflicting results exist, with some studies finding a positive correlation and others no dependence, possibly due to differences in redshift range or sample selection.
• Star-forming galaxies show no significant environmental difference from their radio-quiet counterparts up to z ~ 1.5, but dense environments appear to suppress radio emission, consistent with suppression of star formation in clusters.
• Evidence from non-radio data suggests intense star formation at z > 1.5 may occur in dense/protocluster environments, but this remains unconfirmed at radio wavelengths due to sensitivity limits.
• The environmental dependence of radio AGN may depend on black hole mass, with some studies suggesting a threshold around 10^9 M⊙, though this remains unconfirmed.
• Radio AGN that emit in both radio and mid-infrared (MIR) may prefer underdense regions, suggesting a potential link between nuclear activity and environment even among already radio-loud sources, though significance levels remain low.