GMRT radio continuum study of Wolf–Rayet galaxies – I. NGC 4214 and NGC 4449

Abstract

We report low-frequency observations of Wolf–Rayet galaxies, NGC 4214 and NGC 4449 at 610, 325 and 150 MHz, using the Giant Meterwave Radio Telescope (GMRT). We detect diffuse extended emission from NGC 4214 at and NGC 4449. NGC 4449 is observed to be five times more radio luminous than NGC 4214, indicating vigorous star formation. We estimate synchrotron spectral index after separating the thermal free–free emission and obtain αnt = −0.63 ± 0.04 (S|$\propto \nu ^{\alpha _{\rm nt}}$|⁠) for NGC 4214 and −0.49 ± 0.02 for NGC 4449. About 22 per cent of the total radio emission from NGC 4214 and ∼9 per cent from NGC 4449 at 610 MHz is thermal in origin. We also study the spectra of two compact star-forming regions in NGC 4214 from 325 MHz to 15 GHz and obtain αnt = −0.32 ± 0.02 for NGC 4214-I and αnt = −0.94 ± 0.12 for NGC 4214-II. The luminosities of these star-forming regions (∼1019 WHz−1) appear to be similar to those in circumnuclear rings in normal disc galaxies observed with similar linear resolution. We detect the supernova remnant SNR J1228+441 in NGC 4449 and estimate the spectral index of the emission between 325 and 610 MHz to be −1.8 in the epoch 2008–2009. The galaxies follow the radio–FIR correlation slopes suggesting that star formation in Wolf–Rayet galaxies, which are low-metallicity systems, are similar to that of normal disc galaxies.