BERKELEY-ILLINOIS-MARYLAND ARRAY OBSERVATIONS OF MOLECULAR SPIRAL STRUCTURE IN M100 (NGC-4321)

We present BIMA observations of CO J=1-0 emission in the grand-design spiral galaxy M100 (NGC 4321) in the Virgo cluster. Two 100″ (FWHM) fields have been mapped at resolutions of 7.7″×6.2″ and 4.6″×3.6″ and combined with a mosaicking procedure. These fields cover the nuclear region, the large-scale bar seen in the near-IR, and the main arm which emerges from the north-west end of the bar and swings around to the south. Molecular spiral structure is clearly detected, with several concentrations of CO emission along the main arm, each representing several 107 M⊙ of molecular gas, with properties similar to the Giant Molecular Associations in M51. Many of the GMAs are unresolved, however, and both their CO-based and virial masses may be overestimated. Bright emission is seen from the central regions, and loosely wrapped spiral structure is seen in the region of the large-scale bar, suggesting a relatively weak bar potential. A map of the central regions at 3.6″×3.1″ has also been made. The central regions and the bar will be discussed more fully in a subsequent paper. Radial streaming motions near the minor axis on the west side of the galaxy are consistent with the flow of gas through a density wave compression. Despite the grand-design appearance of M100, the streaming is of much lower amplitude than in M51. Where the arm crosses the major axis on the south side, little or no tangential streaming is observed. This location may be near the corotation radius of the potential, as other analyses have indicated. The CO arm north and west of the nucleus shows a good correspondence with dust lanes and the centroid of the arm is offset upstream from the centroid of the Hα arm. This CO-Hα offset is evidence for triggering of star formation by the density wave, in agreement with the analysis of arm and interarm Hα luminosities and gas densities by Knapen et al. [A&A (submitted) (1995a)]. However, the southern CO arm segment shows a better correspondence with star-forming regions than with dust lanes, as is the case in a region of M83 [Lord & Kenney, ApJ, 381, 130 (1991)]. Possible reasons for this behavior are discussed. An arm-interarm contrast of molecular surface density can be roughly derived from the amplitude of the radial streaming motions, and is found to be 2-3, in approximate agreement with the contrast in CO brightness inferred from a limited number of single-dish pointings by Knapen et al. [A&A (submitted) (1995a)]. This agreement suggests that the assumed conversion factor between CO luminosity and molecular mass, X, does not vary significant from arm to interarm regions. Mechanisms of GMA growth in the spiral arms are considered, with results similar to those for M51 [Rand, ApJ, 410, 68 (1993b)]. Using two criteria for the onset of gravitational instabilities, it is found that the arm gas is somewhat above the threshold for instabilities, while the interarm gas is approximately at the threshold, as long as X is not too far from the Galactic value. Collisional growth of GMAs while the gas is in the arms is also feasible. © 1995 American Astronomical Society.