Radio-to-submillimetre spectral energy distributions of NGC 1365

Abstract

ABSTRACT We analyse the radio-to-submillimetre spectral energy distribution (SED) for the central pseudo-bulge of NGC 1365 using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array. This analysis shows that free–free emission dominates the continuum emission at 50–120 GHz and produces about 75 per cent of the 103 GHz continuum emission. However, the fraction of 103 GHz continuum emission originating from free–free emission varies significantly among different subregions in the pseudo-bulge, particularly for an outflow from the active galactic nuclei (AGN) on the eastern pseudo-bulge where the synchrotron emission produces half of the 103 GHz continuum emission. Free–free emission also dominates at 103 GHz within the central 400 pc diameter region, but this emission is associated with the AGN rather than star formation. The star formation rate (SFR) within the pseudo-bulge derived from the ALMA free–free emission is 8.9 ± 1.1 M⊙ yr−1. This is comparable to the SFR from the mid-infrared emission but higher than the SFR from the extinction-corrected H α line emission, mainly because the pseudo-bulge is heavily dust obscured. The 1.5 GHz emission yields a comparable SFR for the pseudo-bulge but may have lower SFRs within subregions of the pseudo-bulge because of the diffusion outside of these regions of the electrons producing the synchrotron radiation. We propose that applying a correction factor of 75 per cent to the 80–110 GHz continuum emission could provide valuable estimates of the free–free emission without performing any SED decomposition, which could derive extinction-free SFRs within 20 per cent accuracy.