Far-infrared - radio correlation and magnetic field strength in star-forming early-type galaxies

Abstract

A tight far-infrared - radio correlation similar to that in star-forming late-type galaxies is also indicated in star-forming blue early-type galaxies, in which the nuclear optical-line emissions are primarily due to star-forming activities without a significant contribution from active galactic nuclei. The average value of far-infrared to 1.4 GHz radio flux-ratio commonly represented as the ${}^{\prime }{q}^{\prime }$ parameter is estimated to be $2.35\pm 0.05$ with a scatter of 0.16 dex. The average star formation rate estimated using 1.4 GHz radio continuum is $\sim 6$ M${}_{\odot }$ yr${}^{-1}$ in good agreement with those estimated using far-infrared and H$\alpha$ luminosities. The radio emission is detected mainly from central region which could be associated with the star-forming activities, most likely triggered by recent tidal interactions. The average thermal contribution to total radio flux is estimated to be $\sim 12$ per cent. The average value of the magnetic field strengths in the star-forming early-type galaxies is estimated to be 12${}_{-4}^{+11}$ $\mu$G. These magnetic fields are very likely generated via fast amplification in small-scale turbulent dynamos acting in the star-bursting regions.

Revised version, 17 pages, 2 figures, 2 tables, accepted in MNRAS (doi.org/10.1093/mnras/sty740)

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