[Paper Review] The Interplay between SF and AGN Activity, and its role in Galaxy Evolution

K. McAlpine, I. Prandoni+14 more|arXiv (Cornell University)|Dec 18, 2014

Galaxies: Formation, Evolution, Phenomena2 references3 citations

TL;DR

This paper proposes that the Square Kilometre Array (SKA) will enable unprecedented, high-resolution, and sensitive radio surveys to disentangle star formation and AGN activity in high-redshift galaxies (1 ≤ z ≤ 4), where both processes peak. By combining sub-arcsecond resolution and μJy-level sensitivity at high frequencies (e.g., 10 GHz), the SKA will resolve nuclear AGN cores and extended star-forming disks, revealing their co-evolution and feedback mechanisms, with key capabilities requiring baselines >1000 km for full scientific impact.

ABSTRACT

It has become apparent that active galactic nuclei (AGN) may have a significant impact on the growth and evolution of their host galaxies and vice versa but a detailed understanding of the interplay between these processes remains elusive. Deep radio surveys provide a powerful, obscuration-independent tool for measuring both star formation and AGN activity in high-redshift galaxies. Multiwavelength studies of deep radio fields show a composite population of star-forming galaxies and AGN, with the former dominating at the lowest flux densities (S$_{1.4\mathrm{GHz}}

Motivation & Objective

• To understand the co-evolution of star formation and AGN activity in high-redshift galaxies, particularly during the cosmic peak of both processes (1 ≤ z ≤ 4).
• To overcome the limitations of current surveys in resolving AGN cores and star-forming disks due to confusion and beam smearing, especially at high redshift.
• To quantify the role of AGN feedback—particularly 'quasar-mode' feedback—in regulating star formation and shaping galaxy evolution.
• To establish the technical requirements for the SKA, especially high angular resolution and sensitivity at high frequencies, to achieve morphological separation of AGN and star formation components.
• To demonstrate that high-frequency (e.g., 10 GHz) observations are essential for detecting thermal (free-free) emission from high-redshift star formation, providing a more direct tracer than synchrotron emission.

Proposed method

• Utilize deep, high-sensitivity radio surveys with the SKA at 1.4 GHz and 10 GHz to detect star formation and AGN activity down to flux densities of ~3 nJy (1σ), enabling detection of 50 M⊙ yr⁻¹ star formation out to z ~ 2.
• Apply high angular resolution (≤0.1′′) imaging, achievable only with baselines >1000 km, to resolve AGN cores from extended star-forming disks in individual high-redshift galaxies.
• Use spectral index measurements (Sν ∝ ν⁻¹) at high frequencies to distinguish between thermal (star formation) and non-thermal (AGN) emission components.
• Combine observations at multiple frequencies (e.g., 5 GHz with JVLA and e-MERLIN) to achieve sub-arcsecond resolution and morphological decomposition of AGN and SF contributions.
• Leverage the SKA’s sensitivity and resolution to detect low-luminosity AGN cores (10¹⁹ W Hz⁻¹) at z ~ 0.5, comparable to local faint Seyferts.
• Deploy a high-frequency (band 5) counterpart to the VLASS deep survey at 10 GHz to achieve ~0.6 μJy beam⁻¹ sensitivity over 1 square degree in ~1000 hours, enabling resolved spectral index studies.

Experimental results

Research questions

• RQ1 How can the SKA resolve the spatially distinct contributions of nuclear AGN activity and extended star formation in high-redshift galaxies (z ~ 1–4)?
• RQ2 What is the role of 'quasar-mode' AGN feedback in regulating star formation, and how can it be directly observed via high-resolution radio surveys?
• RQ3 To what extent do high-frequency (10 GHz) radio observations improve the detection of thermal (free-free) emission from high-redshift star formation compared to lower-frequency synchrotron-based tracers?
• RQ4 What are the critical technical requirements—especially angular resolution and baseline length—for the SKA to achieve morphological separation of AGN and star formation components in individual galaxies?
• RQ5 How will the SKA’s sensitivity and resolution enable the detection of low-luminosity AGN cores at z ~ 0.5, and what does this imply for the evolution of AGN activity?

Key findings

• The SKA will achieve sub-arcsecond resolution (≤0.1′′) at 1.4 GHz, enabling the separation of AGN cores from star-forming disks in high-redshift galaxies out to z ~ 2.
• A 1.4 GHz survey to 3 nJy (1σ) at ~0.03′′ resolution can detect resolved star formation at 50 M⊙ yr⁻¹ out to z ~ 2.
• A 10 GHz survey to 3 nJy (1σ) can detect resolved star formation at 50 M⊙ yr⁻¹ at 0.1′′ resolution or 100 M⊙ yr⁻¹ at 0.07′′ resolution, significantly improving morphological resolution.
• The SKA will detect low-luminosity AGN cores of 10¹⁹ W Hz⁻¹ at z ~ 0.5, comparable to those observed in local faint Seyfert galaxies.
• Only VLBI-like baselines (>1000 km) will securely pinpoint AGN cores in high-redshift sources, highlighting the need for long baselines in the full SKA design.
• A 50% SKA1 with baselines up to 100 km can achieve ~1.5′′ resolution at 1 GHz, but only longer baselines (>1000 km) will enable the required morphological decomposition of AGN and SF components.