Determination of resonance locations in NGC 613 from morphological arguments

In this paper, we present BVRI imaging data of NGC 613. We use these data to determine the corotation radius of the bar, using the photometric phase crossing method. This method uses the phase angle of the spiral structure in several wavebands, and looks for a crossing between the blue (B) light and the redder wavebands (e.g. R or I). For NGC 613, we find two phase crossings, an outer phase crossing at 136 ± 8 arcsec and an inner phase crossing at 16 ± 8 arcsec. We argue that the outer phase crossing is due to the bar corotation radius, and from the bar length of R_bar = 90.0 ± 4.0 arcsec we go on to calculate a relative bar pattern speed of R = 1.5 ± 0.1, which is consistent with the results of previous methods described in the literature. For a better understanding of the inner phase crossing, we have created structure maps in all four wavebands and a B - R colour map. All of our structure maps and our colour map highlight a nuclear ring of star formation at a radius of ~4 arcsec, which had also been observed recently using Atacama Large Millimeter Array. Furthermore, the radius of our inner phase crossing appears to be consistent with the size of a nuclear disc of star formation that has been recently detected and described in the literature. We therefore suggest that the phase crossing method can be used to detect the size of nuclear star formation regions as well as the location of corotation resonances in spiral galaxies.