By combining high-sensitivity LOFAR 150 MHz, uGMRT 400 MHz, GMRT 610 MHz, and JVLA 5 GHz data in the ELAIS-N1 field, we study the radio spectral properties of radio-detected star-forming galaxies (SFGs) at observer-frame frequencies of 150-5000 MHz. We select ~ 3,500 SFGs that have both LOFAR 150 MHz and GMRT 610 MHz detections by removing AGN from the two radio samples, and obtain a median spectral index of α610150 = −0.51±0.01 with a scatter of σ=0.2. Due to the relatively lower sensitivity of uGMRT 400 MHz data, we apply a flux cut of S610> 300 μJy and obtain the median spectral indices of α385150 = −0.42+0.03−0.02, α610385 = −0.44+0.03−0.04, and α610150 = −0.42+0.02−0.01 for the sample of 258 SFGs that have detections at these three radio frequencies. The JVLA 5GHz observations only cover the central 0.1deg2, where ~100 SFGs are selected, for which we obtain median α5000 610= −1.14+0.04−0.05, α5000 385=−1.08+0.01−0.02 and α5000 150=−0.87±0.01. Overall, the results show that the radio spectrum is flatter if we include a lower frequency dataset when measuring the radio spectral index at 150-5000 MHz. We study the correlations between radio spectral index and physical properties of radio-selected SFGs and find that, on average, the radio spectrum slightly steepens with increasing stellar mass. However, we only find that the radio spectrum flattens with increasing optical depth at V-band at ν < ∼1 GHz. We suggest that spectral ageing due to the energy loss of cosmic ray electrons and thermal free-free absorption could be among the most likely physical mechanisms that drive the two correlations respectively. Both of these could be the physical causes of why the radio spectrum is flatter at low frequency than at high frequency.
Radio spectral properties at 150-5000MHz of star-forming galaxies in the ELAIS-N1 field, Fangxia An (PMO, IDIA, UWC), M. Vaccari (IDIA, UCT), P. N. Best (Edinburgh), E. F. Ocran (KASI), C. H. Ishwara-Chandra (NCRA), A. R. Taylor (IDIA, UCT), S. K. Leslie (Leiden), H. J. A. Röttgering (Leiden), R. Kondapally (Edinburgh), P. Haske (Hertfordshire), J. D. Collier (IDIA, UCT), M. Bonato (INAF), submitted to MNRAS on 20 Feb 2023, arXiv:2303.06941