Dissecting the spiral galaxy M 83: mid-infrared emission and comparison with other tracers of star formation

We present a detailed mid-infrared study of the nearby, face-on spiral galaxy M 83 based on ISOCAM data. M 83 is a unique case study, since a wide variety of MIR broad-band filters as well as spectra, covering the wavelength range of 4 to 18 μ m, were observed and are presented here. Emission maxima trace the nuclear and bulge area, star-formation regions at the end of the bar, as well as the inner spiral arms. The fainter outer spiral arms and interarm regions are also evident in the MIR map. Spectral imaging of the central $3\arcmin \times 3\arcmin$ ($4\,{\rm kpc}\times 4\,{\rm kpc}$) field allows us to investigate five regions of different environments. The various MIR components (very small grains, polycyclic aromatic hydrocarbon (PAH) molecules, ionic lines) are analyzed for different regions throughout the galaxy. In the total λ 4 μ m to 18 μ m wavelength range, the PAHs dominate the luminosity, contributing between 60% in the nuclear and bulge regions and 90% in the less active, interarm regions. Throughout the galaxy, the underlying continuum emission from the small grains is always a smaller contribution in the total MIR wavelength regime, peaking in the nuclear and bulge components. The implications of using broad-band filters only to characterize the mid-infrared emission of galaxies, a commonly used ISOCAM observation mode, are discussed. We present the first quantitative analysis of new H α and λ 6 cm VLA+Effelsberg radio continuum maps of M 83. The distribution of the MIR emission is compared with that of the CO, HI, R band, H α and λ 6 cm radio. A striking correlation is found between the intensities in the two mid-infrared filter bands and the λ 6 cm radio continuum. To explain the tight mid-infrared-radio correlation we propose the anchoring of magnetic field lines in the photoionized shells of gas clouds.