Resolving Circumnuclear Star Formation in M100 with Image Stabilisation at UKIRT

Near-infrared (JHK) images of the nucleus of the barred spiral galaxy M100 (NGC 4321) have been obtained using a high-bandwidth tip-tilt secondary and fast guider system on the 3.8m United Kingdom Infrared Telescope (UKIRT). The resulting images, with a resolution of 0: 00 34 at K, reveal for the rst time a host of compact \knots", which appear to be the result of a recent burst (or bursts) of star formation. Connrmation of this comes from K-band spectroscopy of these knots with CGS4 on UKIRT, which shows Br emission and CO absorption features. A comparison with starburst evolutionary models suggests ages for these knots of 15{25 Myr, and a stellar population dominated by late-type supergiants. 1 The Upgraded UKIRT In an eeort to deliver image quality which is as close as possible to the diiraction limit (FWHM = 0: 00 11 at =2.2 m), the 3.8m United Kingdom Infrared Telescope (UKIRT) has been upgraded with the addition of several new features (Hawarden et al. 1998), including: A high-bandwidth tip-tilt secondary mirror driven by a Fast Guider sampling at rates up to 100 Hz, supplied by the Max Planck Institut f ur Astronomie (MPIA), Heidelberg. Low-order active control of the primary mirror, calibrated with monthly wavefront sensing measurements , and active focus control. A set of 16 louvred apertures in the dome itself to assist natural dome ushing by wind, or by a forced ventilation system. Primary mirror cooling, and dome oor insulation (both yet to be implemented). Together, these upgrades have resulted in a median seeing of 0: 00 45 at K. As part of an ongoing eeort to understand the relationship between circumnuclear star formation, nuclear activity, and gas ows within the central regions of barred spiral galaxies, we have been carrying out high-resolution observations in the optical, near-infrared, and radio regimes of a sample of nearby barred spirals. We have assembled a fairly extensive dataset (UBVRIK and H, Hubble Space Telescope V-band, and CO molecular emission) for M100 (NGC 4321), the brightest spiral galaxy in the Virgo cluster (Knapen et al. 1995a, b). EEorts to obtain images in the K-band with a resolution approaching that of the HST optical