We study the radio spectral properties of 2094 star-forming galaxies
(SFGs) by combining our early science data from the MeerKAT
International GHz Tiered Extragalactic Exploration (MIGHTEE) survey with
VLA, GMRT radio data, and rich ancillary data in the COSMOS field.
These SFGs are selected at VLA 3 GHz, and their flux densities from
MeerKAT 1.3 GHz and GMRT 325 MHz imaging data are extracted using the
‘superdeblending’ technique. The median radio spectral index is α3GHz1.3GHz=−0.80±0.01
without significant variation across the rest-frame frequencies
∼1.3–10 GHz, indicating radio spectra dominated by synchrotron
radiation. On average, the radio spectrum at observer-frame 1.3–3 GHz
slightly steepens with increasing stellar mass with a linear fitted
slope of β = −0.08 ± 0.01, which could be explained by age-related
synchrotron losses. Due to the sensitivity of GMRT 325 MHz data, we
apply a further flux density cut at 3 GHz (S3GHz≥50μJy)
and obtain a sample of 166 SFGs with measured flux densities at
325 MHz, 1.3 GHz, and 3 GHz. On average, the radio spectrum of SFGs
flattens at low frequency with the median spectral indices of α1.3GHz325MHz=−0.59+0.02−0.03 and α3.0GHz1.3GHz=−0.74+0.01−0.02.
At low frequency, our stacking analyses show that the radio spectrum
also slightly steepens with increasing stellar mass. By comparing the
far-infrared-radio correlations of SFGs based on different radio
spectral indices, we find that adopting α3GHz1.3GHz for k-corrections will significantly underestimate the infrared-to-radio luminosity ratio (qIR)
for >17 per cent of the SFGs with measured flux density at the three
radio frequencies in our sample, because their radio spectra are
significantly flatter at low frequency (0.33–1.3 GHz).