The inner ring of NGC 4736: Star formation on a resonant pattern

We revise the explanation for the expanding H II regions ring'' of NGC 4736. From the analysis of long-slit spectra, we identify structures within the emission region previously reported in the literature as a single ring (former parameters: Rapproximate to47, FWHM=21). The H II ring is located at 35 (1.12 kpc) galactocentric distance, and its thickness is 10. The line width at the ring location is typical of H II regions, whereas a higher velocity dispersion (chaotic motions) is measured just outside. The location of the H II ring and of the outer stellar (300) ring are defined by the resonances obtained from the stellar rotation curve. After modeling the surface brightness profile into the different structural components, we conclude that the massive triaxial bulge (r(e)=10, (1-b/a)(r e)approximate to0.25) produces the galactic resonance pattern. We also find evidence for a fossil starburst nucleus of about 150 pc size. The bulge and the inner bar drive disk gas motion, causing inward movements outside the H II ring and outward just inside, thereby accumulating material to trigger star formation on the ring. In the central part the bar drives the gas toward the center, which explains the substantial amount of gas in the nucleus in spite of the presence of a fossil starburst. The peculiar motions reported in the literature in reference to the ionized gas of the H II ring can be understood as infalling gas encountering the shock waves generated by the starburst knots on the H II ring and being raised above the galaxy disk. The scenario of star formation propagating from the nucleus outward used to explain the apparent expanding motion of the Halpha ring is not fully supported, in light of a comparison of the location of the Halpha ring with that of the FUV ring. The FUV ring peaks at about 45-48, which might point to an inward-propagating star formation scenario.