2022MNRAS.514.4343S

Comprehending the radio-infrared (IR) relations of the faint extragalactic radio sources is important for using radio emission as a tracer of star formation in high redshift (z) star-forming galaxies (SFGs). Using deep uGMRT observations of the ELAIS-N1 field in the 0.3-0.5 GHz range, we study the statistical properties of the radio-IR relations and the variation of the 'q-parameter' up to z = 2 after broadly classifying the faint sources as SFGs and AGN. We find the dust temperature (T_dust) to increase with z. This gives rise to q_24 µm_, measured at 24 µm, to increase with z as the peak of IR emission shifts towards shorter wavelengths, resulting in the largest scatter among different measures of q-parameters. q_70 µm_ measured at 70 µm, and q_TIR using total-IR (TIR) emission are largely unaffected by T_dust. We observe strong, non-linear correlations between the radio luminosities at 0.4 and 1.4 GHz with 70 µm luminosity and TIR luminosity(L_TIR). To assess the possible role of the radio-continuum spectrum in making the relations non-linear, for the first time we study them at high z using integrated radio luminosity (L_RC) in the range 0.1-2 GHz. In SFGs, the L_RC-L_TIR relation remains non-linear with a slope of 1.07 ± 0.02, has a factor of 2 lower scatter compared to monochromatic radio luminosities, and q^RC_TIR decreases with z as q^RC_TIR = (2.27 ±0.03) (1+z)^-0.12 ±0.03^. A redshift variation of q is a natural consequence of non-linearity. We suggest that a redshift evolution of magnetic field strengths and/or cosmic ray acceleration efficiency in high-z SFGs could give rise to non-linear radio-IR relations.