Multifrequency radio-continuum observations of NGC 1569: evidence for a convective wind

Abstract

We present high-sensitivity radio-continuum observations with the Very Large Array (VLA) and Ryle Telescope at 1.5, 4.9, 8.4 and 15.4 GHz of the dwarf irregular galaxy NGC 1569. The radio data show an extended, irregularly shaped halo with filamentary structure around the galaxy. The spectral index maps reveal an unusually patchy distribution with regions of flat spectral index extending into the halo. The data allow us to perform a spatially resolved spectral-fitting analysis of the continuum emission from which we derive maps of the thermal and synchrotron emission. The thermal radio emission is concentrated towards the brightest H ii region west of the super star clusters A and B, whereas the distribution of the synchrotron emission peaks in a bar-like structure in the disc extending between the two clusters. The total flux density of the thermal radio emission allows us to derive the integrated synchrotron spectrum and we confirm the break in the spectrum that was found by Israel & de Bruyn. We discuss various possibilities that could produce such a break and conclude that the only mechanism able to fit the radio data and remain consistent with data at other wavelengths is a convective wind allowing cosmic ray electrons to escape from the halo.