The star formation history and evolution of the circumnuclear region of M100

Context. Star-forming nuclear rings in barred galaxies are common in nearby spirals, and their detailed study can lead to important insights into the evolution of galaxies, their bars and their central regions. We present integral field spectroscopic observations obtained with SAURON of the bar and circumnuclear region of the barred spiral galaxy M100, complemented by new Spitzer Space Telescope imaging of the region. Aims. We use these data to enhance our understanding of the formation, evolution and current properties of the bar and ring. Methods. We derive the kinematics of the gas and the stars and quantify circular and non-circular motions using kinemetry. We analyse this in conjunction with the optical and infrared morphology, and our previously published dynamical modelling. By comparing line indices to simple stellar population models we estimate the ages and metallicities of the stellar populations present within the region, especially in and around the ring. Results. The stellar and gaseous velocity fields are remarkably similar, and we confirm that the velocity fields show strong evidence for non-circular motions due to the bar and the associated density wave. These are strongest just outside the nuclear ring, where our kinemetric analysis indicates inflow across the spiral armlets and into the ring region. The line strength maps all indicate the presence of a younger population within this ring, but detailed modelling of the line strengths shows that in addition to this young population, old stars are present. These old stars must have been formed in an event of massive star formation which produced the bulk of the mass, and which ended some 3 Gyr ago, a constraint set by the age of the stars in the bar and the nucleus. Our best-fitting model is one in which the current star formation is but the latest of a series of relatively short bursts of star formation which have occurred for the last 500 Myr or so. A clear bipolar azimuthal age gradient is seen within the ring, with the youngest stars occurring near where the bar dust lanes connect with the ring. Conclusions. Our kinematic and morphological results all confirm the picture in which the nuclear ring in M100, considered typical, is fed by gas flowing in from the disc under the action of the bar, is slowed down near a pair of resonances, and forms significant amounts of massive stars. Detailed stellar population modelling shows how the underlying bulge and disc were put in place a number of Gyr ago, and that the nuclear ring has been forming stars since about 500 Myr ago in a stable succession of bursts. This confirms that nuclear rings of this kind can form under the influence of a resonant structure set up by a bar, and proves that they are stable features of a galaxy rather than one-off starburst events.