THE CENTRAL REGION IN M100: OBSERVATIONS AND MODELING

摘要

We present new high-resolution observations of the central region in the late-type spiral galaxy M100 (NGC 4321) supplemented by three-dimensional 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 strength, 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," forming an inner barlike region. The galaxy exhibits a mild circumnuclear starburst 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 color index image. We interpret the twists in K isophotes and the swinging of spiral arms through ～ 360° inside the corotation radius as being indicative of the presence of a double inner Lindblad resonance (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 verified using nonlinear orbit analysis by determining the spatial extent of the family of orbits oriented along the minor axis of the bar. We have reproduced the basic morphology of the region inside corotation, including (1) the ～60" bar; (2) the large-scale trailing shocks representing the offset dust lanes in the bar; (3) two symmetric K peaks corresponding to gas compression maxima which lie at the caustic formed by the interaction of a pair of trailing and leading shocks in the vicinity of the inner ILR, both peaks being sites of SF; and (4) two additional zones of SF corresponding to gas compression maxima 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 further that the twisting of K isophotes in the neighborhood of the resonance region requires a population of red stars which are dynamically young and follow gas rather than stellar orbits, i.e., red supergiants. At the same time, a substantial contribution from OB stars to the K light is expected within the inner kiloparsec and especially in the symmetric K peaks. We also conclude that NGC 4321 hosts a single stellar bar which fuels the starburst activity within the circumnuclear "ring" by channeling gas there at the median rate of ～0.1-1 solar mass yr~(-1).