THE CENTRAL REGION IN M100 - OBSERVATIONS AND MODELING

We present new high-resolution observations of the central region in t he late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimen sional numerical modeling of stellar and gas dynamics, including star formation (SF). Near-infrared imaging has revealed a small bulge of 4' ' effective diameter; a 60'' radial length stellar bar of moderate str ength, previously inferred from optical and 21 cm observations; and an ovally shaped, ringlike structure in the plane of the disk between 10 ''-22'' from the center, whose major axis makes a large angle with the bar. The K isophotes become progressively elongated and skewed to the position angle of the bar both outside and inside the ''ring,'' formi ng an inner barlike region. The galaxy exhibits a mild circumnuclear s tarburst concentrated in the inner part of the K ''ring.'' This SF is prominent in Ha and the U, B, and V bands, forming an incomplete ring. In addition, two strong local maxima of K emission have been observed to lie remarkably symmetrically with respect to the galactic nucleus and equidistant from it at 7.''5, slightly leading the stellar bar. CO molecular emission is peaked in the dust lanes seen in the I - K colo r index image. We interpret the twists in K isophotes and the swinging of spiral arms through similar to 360 degrees inside the corotation r adius as being indicative of the presence of a double inner Lindblad r esonance (ILR) and test this hypothesis by modeling the gas flow in a self-consistent gas+stars disk embedded in a halo, with an overall NGC 4321-like mass distribution in the system. Both ILRs have been verifi ed using nonlinear orbit analysis by determining the spatial extent of the family of orbits oriented along the minor axis of the bar. We hav e reproduced the basic morphology of the region inside corotation, inc luding (1) the similar to 60 degrees bar; (2) the large-scale trailing shocks representing the offset dust lanes in the bar; (3) two symmetr ic K peaks corresponding to gas compression maxima which lie at the ca ustic formed by the interaction of a pair of trailing and leading shoc ks in the vicinity of the inner ILR, both peaks being sites of SF; and (4) two additional zones of SF corresponding to gas compression maxim a at the bar's minor axis, where the large-scale shocks start to curl and which are referred in the literature as ''twin peaks.'' We argue f urther that the twisting of K isophotes in the neighborhood of the res onance region requires a population of red stars which are dynamically young and follow gas rather than stellar orbits, i.e., red supergiant s. At the same time, a substantial contribution from OB stars to the K light is expected within the inner kiloparsec and especially in the s ymmetric K peaks. We also conclude that NGC 4321 hosts a single stella r bar which fuels the starburst activity within the circumnuclear ''ri ng'' by channeling gas there at the median rate of similar to 0.1-1 Mo yr(-1).