The Radio-FIR Correlation in the Context of Deep Radio Source Counts
Abstract
Increasingly deep, confusion-limited radio surveys have pushed direct radio source-count measurements down to tens of μJy at 1.4 GHz. Confusion-noise P(D) analyses extend the statistical counts to below 1 μJy. Radio source counts have allowed for constraints on the radio-derived star formation rate density (SFRD) history through models of the backwards evolution of the local radio luminosity function, using the radio-FIR correlation, q ∝ log(LFIR/L1.4), to convert radio luminosities to FIR luminosities and hence star-formation rates. Recent deep radio source counts from MeerKAT suggest a potential tension in the SFRD history between radio and UV/IR measurements at 1≲ z≲ 2. This corresponds to a ≳ 3σ discrepancy between the predicted and measured source counts near the star forming galaxy source count S2n(S) peak of ∼30 μJy under both a pure luminosity (PLE) and combined luminosity and density evolution (LADE). We consider what the requirement of agreement between radio source counts and the observed UV/IR SFRD indicates about the redshift evolution of the radio-FIR correlation and its intrinsic scatter out to z=3. We introduce a radio-luminosity based parameterization to qFIR(z) based on changing thermal radio fractions alone that agrees with the observed stellar-mass dependent qFIR(z) better than a non-evolving or decreasing qFIR(z). Despite this, we find that a decreasing qFIR(z) at fixed radio luminosity provides better agreement between source counts and the observed SFRD, while a qFIR(z) that breaks down due to cosmic ray losses requires an intrinsic scatter up to σq≈ 0.3 dex.
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BibTeX
@misc{surti2026radiofir,
title = {{The Radio-FIR Correlation in the Context of Deep Radio Source Counts}},
author = {Tirth D. Surti and Vikram Ravi and Allison Matthews and Viviana A. Rosero},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.02717},
eprint = {2606.02717},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.02717},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Radio-FIR Correlation in the Context of Deep Radio Source Counts
AU - Tirth D. Surti
AU - Vikram Ravi
AU - Allison Matthews
AU - Viviana A. Rosero
PY - 2026
DA - 2026/09/29
PB - arXiv
DO - 10.48550/arxiv.2606.02717
UR - https://arxiv.org/abs/2606.02717
AB - Increasingly deep, confusion-limited radio surveys have pushed direct radio source-count measurements down to tens of μJy at 1.4 GHz. Confusion-noise P(D) analyses extend the statistical counts to below 1 μJy. Radio source counts have allowed for constraints on the radio-derived star formation rate density (SFRD) history through models of the backwards evolution of the local radio luminosity function, using the radio-FIR correlation, q ∝ log(L_FIR/L_1.4), to convert radio luminosities to FIR luminosities and hence star-formation rates. Recent deep radio source counts from MeerKAT suggest a potential tension in the SFRD history between radio and UV/IR measurements at 1≲ z≲ 2. This corresponds to a ≳ 3σ discrepancy between the predicted and measured source counts near the star forming galaxy source count S^2n(S) peak of ∼30 μJy under both a pure luminosity (PLE) and combined luminosity and density evolution (LADE). We consider what the requirement of agreement between radio source counts and the observed UV/IR SFRD indicates about the redshift evolution of the radio-FIR correlation and its intrinsic scatter out to z=3. We introduce a radio-luminosity based parameterization to q_FIR(z) based on changing thermal radio fractions alone that agrees with the observed stellar-mass dependent q_FIR(z) better than a non-evolving or decreasing q_FIR(z). Despite this, we find that a decreasing q_FIR(z) at fixed radio luminosity provides better agreement between source counts and the observed SFRD, while a q_FIR(z) that breaks down due to cosmic ray losses requires an intrinsic scatter up to σ_q≈ 0.3 dex.
ER -