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

**Authors:** J. Delhaize, V. Smolcic, I. Delvecchio, M. Novak, M. Sargent, N. Baran, B. Magnelli, G. Zamorani, E. Schinnerer, E. J. Murphy, M. Aravena, S. Berta, M. Bondi, P. Capak, P. Ciliegi, F. Civano, O. Ilbert, A. Karim, C. Laigle, O. Le Fevre  
**arXiv:** [1703.09723](https://arxiv.org/abs/1703.09723)  
**Date:** 2017-03-28  
**Summary page:** https://www.summarizepaper.com/en/paper/1703.09723v3/

## Key points

- Investigated the infrared-radio correlation (IRRC) across a wide redshift range of $0<z<6$
- Used 3GHz observations from the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory within the 2deg$^{2}$ COSMOS field
- Utilized doubly-censored survival analysis to distinguish between star-forming objects and those with an active galactic nucleus (AGN)
- Found that the infrared-to-1.4GHz radio luminosity ratio ($q_{\rm TIR}$) decreases with increasing redshift among star-forming galaxies
- Identified deviations in AGN with moderate-to-high radiative luminosity from the trend observed in star-forming galaxies
- Highlighted potential unidentified AGN contributions solely in the radio regime impacting observed trends
- Challenged conventional assumptions about radio spectral energy distributions (SEDs) in star-forming galaxies, suggesting a more intricate SED shape
- Introduced a novel redshift-dependent relationship between rest-frame 1.4GHz radio luminosity and star formation rate

## Summary

We investigate the infrared-radio correlation (IRRC) across a wide redshift range of $0<z<6$, utilizing cutting-edge 3GHz observations from the Karl G. Jansky Very Large Array (VLA) and infrared data from the Herschel Space Observatory within the 2deg$^{2}$ COSMOS field. By distinguishing between star-forming objects and those with an active galactic nucleus (AGN), we use a doubly-censored survival analysis to account for non-detections in either domain. Our findings show that the infrared-to-1.4GHz radio luminosity ratio ($q_{\rm TIR}$) decreases with increasing redshift among star-forming galaxies: $q_{\rm TIR}(z)=(2.88\pm0.03)(1+z)^{-0.19\pm0.01}$. AGN with moderate-to-high radiative luminosity deviate from this trend, displaying a lower normalization and steeper decline with redshift. We also acknowledge potential unidentified AGN contributions solely in the radio regime, which may impact the observed steepening trend of $q_{\rm TIR}(z)$ among star-forming populations. Our analysis highlights a rising fractional contribution of free-free emission from star-forming galaxies to the observed 3GHz flux, potentially affecting derived evolutionary patterns. Surprisingly, our results challenge conventional assumptions based on M82 for typical radio spectral energy distributions (SEDs) in star-forming galaxies, suggesting a more intricate SED shape and emphasizing potential imperfections in radio $K$ corrections governing the redshift trend of $q_{\rm TIR}$. Additionally, we introduce a novel redshift-dependent relationship between rest-frame 1.4GHz radio luminosity and star formation rate, incorporating the derived redshift trend into our calculations. This comprehensive investigation sheds light on evolving correlations between infrared and radio emissions in different galaxy populations across varying cosmic epochs, offering valuable insights into their underlying physical mechanisms and evolutionary pathways.