Near-infrared spectroscopy of the circumnuclear star formation regions in M100: evidence for sequential triggering

We present low-resolution (R similar to 450) K-band spectroscopy for 16 of the 43 circumnuclear star-forming knots in M100 identified by Ryder & Knapen. We compare our measurements of equivalent widths for the Br gamma emission line and CO 2.29-mum absorption band in each knot with the predictions of starburst models from the literature, and derive ages and burst parameters for the knots. The majority of these knots are best explained by the result of short, localized bursts of star formation between 8 and 10 Myr ago. By examining both radial and azimuthal trends in the age distribution, we present a case for sequential triggering of star formation, most likely resulting from the action of a large-scale shock. In an appendix, we draw attention to the fact that the growth in the CO spectroscopic index with decreasing temperature in supergiant stars is not as regular as is commonly assumed.