VLA-COSMOS 3GHz Large Project: The infrared-radio correlation of star-forming galaxies and AGN to $z\lesssim6$

Abstract: We examine the behaviour of the infrared-radio correlation (IRRC) over the range $0<z<6$ using new, highly sensitive 3GHz observations with the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory in the 2deg$^{2}$ COSMOS field. We distinguish between objects where emission is believed to arise solely from star-formation, and those where an active galactic nucleus (AGN) is thought to be present. We account for non-detections in the radio or in the infrared using a doubly-censored survival analysis. We find that the IRRC of star-forming galaxies, quantified by the infrared-to-1.4GHz radio luminosity ratio ($q_{\rm TIR}$), decreases with increasing redshift: $q_{\rm TIR}(z)=(2.88\pm0.03)(1+z)^{-0.19\pm0.01}$. Moderate-to-high radiative luminosity AGN do not follow the same $q_{\rm TIR}$$(z)$ trend, having a lower normalisation and steeper decrease with redshift. We cannot rule out the possibility that unidentified AGN contributions only to the radio regime may be steepening the observed $q_{\rm TIR}(z)$ trend of the star-forming population. An increasing fractional contribution to the observed 3GHz flux by free-free emission of star-forming galaxies may also affect the derived evolution. However, we find that the standard (M82-based) assumption of the typical radio spectral energy distribution (SED) for star-forming galaxies is inconsistent with our results. This suggests a more complex shape of the typical radio SED for star-forming galaxies, and that imperfect $K$ corrections in the radio may govern the derived redshift trend of $q_{\rm TIR}$. Lastly, we present a redshift-dependent relation between rest-frame 1.4GHz radio luminosity and star formation rate taking the derived redshift trend into account.

An Analysis of the Infrared-Radio Correlation in Star-forming Galaxies and AGN

The paper "VLA-COSMOS 3 GHz Large Project: The infrared-radio correlation of star-forming galaxies and AGN to $z\apprle6$" by Delhaize et al. presents a comprehensive study of the infrared-radio correlation (IRRC) across a wide range of redshifts. Utilizing the Karl G. Jansky Very Large Array (VLA) at 3 GHz combined with data from the Herschel Space Observatory, the study analyzes the behavior of the IRRC for star-forming galaxies as well as active galactic nuclei (AGN) up to a redshift of approximately 6.

Methodology and Data

The research utilizes a highly sensitive 3 GHz survey conducted over the COSMOS field, leveraging data from the VLA-COSMOS 3 GHz Large Project, and supplements this with infrared observations from Herschel. The new dataset provides a robust basis for exploring the correlation between total infrared and 1.4 GHz radio luminosities. A sophisticated separation of galaxy populations into AGN and star-forming galaxies allows the study to examine the IRRC without significant contamination from AGN-dominated sources. A doubly-censored survival analysis is employed to account for non-detections, which enhances the reliability of the inferred correlations and trends.

Findings

The study finds that the IRRC, expressed through the parameter $q_{\rm TIR}$, decreases with increasing redshift for star-forming galaxies. Specifically, the IRRC trend is fit by $q_{\rm TIR}(z) = (2.88 \pm 0.03)(1+z)^{-0.19 \pm 0.01}$. This finding suggests a mild but significant evolution of the IRRC, aligning with several literature results, yet indicating a departure from the non-evolving scenario previously considered for many star-forming galaxies.

Interestingly, the evolution in AGN displays distinct characteristics from that of star-forming galaxies, with AGN showing a lower normalization and a steeper decrease in $q_{\rm TIR}$ with redshift. This suggests that the processes governing the radio and infrared emissions in AGN differ substantially when compared to star-forming galaxies, potentially due to underlying AGN activity influencing the radio domain.

Implications

This redshift-dependent behavior of the IRRC has profound implications for understanding the physical processes within galaxies over cosmic time. It suggests that evolutionary changes in galactic magnetic fields, star formation rates, and potentially the fraction of merger-driven starbursts could play a role in the observed trend. The paper also speculates on the possibility of unidentified AGN contributions influencing the IRRC slope.

Future Perspectives

To fully comprehend the IRRC's cosmic evolution, further studies need to refine the understanding of the radio spectral energy distribution of star-forming galaxies. Improved K-corrections and a deeper investigation into the free-free versus synchrotron emission contributions at different redshifts are crucial. As large-scale surveys from upcoming radio facilities like the Square Kilometre Array come online, they promise improvements in sensitivity and resolution, allowing for precise measurements of the IRRC across even broader redshift ranges and lower luminosities.

In conclusion, the study by Delhaize et al. highlights the intricate dynamics underlying the IRRC in distant galaxies, encouraging a reevaluation of standard models and offering a foundation for future astronomical inquiries into the evolving universe.