Search for radio halos in starburst galaxies

Abstract

Context. Starburst galaxies are undergoing intense episodes of star formation. In these galaxies, gas is ejected into the surrounding environment through winds created by the effect of hot stars and supernova explosions. When interacting with the intergalactic medium, these winds can produce strong shocks capable of accelerating cosmic rays. The radiation from these cosmic rays mainly occurs in radio and gamma rays. The radio halo can be characterized using the scale height, which is an important parameter for understanding cosmic ray acceleration and transport. Aims. We searched for the presence of radio halos in a sample of edge-on starburst galaxies gathered from the MeerKAT 1.28 GHz Atlas of Southern Sources in the IRAS Revised Bright Galaxy Sample. The investigation of how the radio halos relate to the global properties of the galaxies can shed light on the understanding of the halo origin and the underlying cosmic ray population. Methods. We selected a sample of 25 galaxies with inclinations i > 80° from the original sample and modeled their disk and halo contributions. We determined the scale heights and the radio luminosity of the halos when detected. Results. We have detected and characterized 11 radio halos from a sample of 25 edge-on galaxies. Seven of them are reported here for the first time. The average radio scale height is ∼1 kpc. We found that the halo scale heights increase linearly with the radio diameters and this relation does not depend on the star formation rate. All galaxies in our sample follow the radio-infrared relation with a q parameter value of 2.5 ± 0.1. The halo luminosity linearly increases with the infrared luminosity and star formation rate. Conclusions. The dependence of the halo luminosity on the star formation rate and the infrared luminosity supports the hypothesis that the radio halos are the result of synchrotron radiation produced by relativistic electrons and points toward the fact that the star formation activity plays a crucial role in halo creation. The average scale height of 1 kpc implies a dynamical range of 4 Myr, several orders of magnitude greater than the synchrotron losses for electrons of 10 TeV. This suggests that some process must exist to reaccelerate cosmic rays in the halo if gamma-ray emission of a leptonic origin is detected from the halo. According to the relation between the radio and gamma-ray luminosities, we found that NGC 4666 is a potential gamma-ray source for future observations.