have been mapped with the IRAM
30m telescope, with resolutions HPBW=12'' and HPBW=21'',
within the inner 8 kpc of the disk (1''=82pc, assuming
Mpc). Complementary 
(1-0) observations of
the nucleus, spiral and interarm regions are used to study the nature
of H 
gas.
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.