CO emission traces a markedly asymmetric two spiral-arm structure stretching out from a molecular gas bar. The CO bar has a diameter D 90 and it is aligned with the stellar bar seen in I and K band images (PA= ). Arm I (II) springs off the western (eastern) side of the bar at . Arm II is split up in two armlets (at ), it disappears and finally shows up at a position angle close to the major axis northern crossing (PA= ). The splitting starts near the inner 3::1 resonance identified by Elmegreen et al (1992) in the enhanced optical images of M100. Molecular gas in the bar is strongly concentrated in a nuclear disk of r 30 and .
Arms I-II display different arm-interarm contrasts (on average, (CO) increases with radius from 2.5 (r 30 ) up to 6.5 (r 110 ), being higher for arm I than for arm II. Also arms I-II relate differently to other spiral arm tracers, underlying the asymmetry in the disk. Comparison between CO, H I and H maps show that there is no coherent sequence in the relative location of the star formation tracers along the spiral arms. Evidences of triggering of star formation along spiral arms are poor: R(H ) is only 2 R(CO) and systematic offsets between H and CO ridges (expected to lie downstream and upstream the spiral potential minimum, respectively, assuming trailing arms inside corotation) are hard to find and at places they are absent or even inverted.
CO reveals as the best tracer of gas kinematics in the inner disk. The CO rotation curve (V ) is steeper than the curves derived from the HI and H data. reaches in less than 1 kpc. The signature of the m=3 instability has been also identified in the CO derived velocity field. The magnitude and the sign of streaming motions, associated with the spiral arms and the bar, are consistent with the CO disk to be inside corotation ( ). A secondary wave compression develops in the eastern side where arm II is split.
Massive star formation (MSF) is inhibited along the gaseous bar, indicating that M100 is an evolved barred system (Friedli and Benz, 1995). Star formation rate (measured as the ratio SFR=F(H )/ ) is lower for the nuclear disk than for the disk itself. However we suspect the measurement of SFR to be subjected to major uncertainties: the X=N(H conversion factor might be 3 times lower than implicitly assumed and to vary within the disk (lower for the interarm than for the arms and nuclear region) . Moreover extinction affects F(H ) mostly in the nucleus where the classical Schmidt law breaks down paradoxically (SFR , ). MSF is set on at a distance kpc where N(H ) approaches the Toomre (1964) gravitational instability threshold ( ). Also for , the neutral gas is mostly in the H phase. HI is underabundant in the inner 6kpc and cannot be accounted for by photodissociation of H by HII regions.
Asymmetry in the observed molecular gas distribution and kinematics of M100 seems related with the three-arm structure studied by Elmegreen and collaborators. Although the m=2 spiral mode is still predominant in M100, other secondary modes seem at interplay reflecting the secular evolution of the disk. Compared to M51, M100 appears as an evolved barred spiral.
Figure 2: Overlaid on gray scale images of I and I . We plot the first-moment isovelocity contours derived from the 1-0 and 2-1 transitions of CO (figures (a)(top) and (b)(bottom), respectively). Isovelocities go from 1450kms to 1750kms by steps of . Thick contours stand for the systemic velocity . A close view shows the different strength and degree of symmetry of the streaming motions in the southern (I) and northern (II) arms. Along arm I, streaming motions can be followed more continuously and their amplitude is stronger than in arm II. The velocity field along arm II reflects again the asymmetry of the gaseous spiral response in M100: a secondary wave compression appears in the eastern side, coinciding with the splitting of arm II. This kinematical feature, present in the 2-1 and the 1-0 maps, assesses our view that we are detecting the signature of the m=3 mode.