Tracing High Redshift Starformation in the Current and Next Generation of Radio Surveys

TL;DR

This paper reviews methods to distinguish star formation from active galactic nuclei in deep radio surveys and discusses implications for understanding high-redshift star formation history.

Contribution

It proposes radio-based diagnostics for separating AGN and star formation in faint radio sources and analyzes their effectiveness in current and future surveys.

Findings

Deep radio surveys reveal a high fraction of star-forming galaxies at high redshift.

The IR-radio relation generally holds at z>1 but shows mild deviations depending on IR wavebands.

Star-forming galaxies, especially LIRGs and ULIRGs, dominate the star formation budget from z=1 to 3.

Abstract

The current deepest radio surveys detect hundreds of sources per square degree below 0.1mJy. There is a growing consensus that a large fraction of these sources are dominated by star formation although the exact proportion has been debated in the literature. However, the low luminosity of these galaxies at most other wavelengths makes determining the nature of individual sources difficult. If future, deeper surveys performed with the next generation of radio instrumentation are to reap high scientific reward we need to develop reliable methods of distinguishing between radio emission powered by active galactic nuclei (AGN) and that powered by star formation. In particular, we believe that such discriminations should be based on purely radio, or relative to radio, diagnostics. These diagnostics include radio morphology, radio spectral index, polarisation, variability, radio luminosity and flux density ratios with non-radio wavelengths e.g. with different parts of the infrared (IR) regime. We discuss the advantages and limitations of these various diagnostics methods with current and future surveys. However, weeding AGN out of deep radio surveys can already provide several insights into the star formation at high redshift. As well as reproducing the well known rise with redshift in the comoving star formation rate density, we also see evidence for the continued dominance of LIRGs and ULIRGs to the total star forming budget across redshifts 1-3. Additionally, while we see that the IR-radio relation for star forming galaxies does hold to high redshifts (z>1) there is a mild deviation depending on the IR waveband used and the range of IR SEDs found. We will discuss the possible reasons behind this change in properties.