Cool gas and massive stars: The nuclear ring in M100

The SAURON integral-field spectrograph was used to observe the central area of the barred spiral galaxy M100 (NGC 4321). M100 contains a nuclear ring of star formation, fueled by gas channeled inward by the galaxy's bar. We present maps of emission-line strengths, absorption-line strength indices, and the gas velocity dispersion across the field. The H beta emission is strongest in the ring, along two curved bar dust lanes and at the ends of the bar. The Mg b absorption-line strength shows a younger population of stars within the ring than in the surrounding area. The gas velocity dispersion is notably smaller than elsewhere in the field both in the ring and along the leading edge of the dust lanes. Low gas dispersion is correlated spatially with the Hb emission. We thus see stars being formed from cold (low-dispersion) gas that is being channeled inward along the dust lanes under the influence of a large bar and accumulated into a ring near the location of the inner Lindblad resonances. This lends further strong support to the interpretation of nuclear rings in barred galaxies as resonance phenomena.