Molecular gas in the barred spiral M 100 .1. The IRAM 30m map

We present a study on the distribution, kinematics and evolution of the molecular gas disk in the barred SBbc spiral M100(NGC4321). The J=2-1 and J=1-0 lines of (CO)-C-12 have been mapped with the IRAM 30m telescope, with resolutions HPBW=12 '' and HPBW=21 '', within the inner 8kpc of the disk (1 ''=82pc, assuming D-M100=17.1Mpc). Complementary (CO)-C-13(1-0) observations of the nucleus, spiral and interarm regions are used to study the nature of H-2 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 similar to 90 '' and it is aligned with the stellar bar seen in I and K band images (PA=110 degrees). Arm I (II) springs off the western (eastern) side of the bar at r similar to 50 ''. Arm II is split up in two armlets (at r similar to 40-50 ''), it disappears and finally shows up at a position angle close to the major axis northern crossing (PA=330 degrees). 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(nucl) similar to 30 '' and M(H-2)(nucl)=2.8 10(9)M.. Arms I-II display different arm-interarm contrasts (on average, R-alpha/(i alpha)(CO) increases with radius from 2.5 (r similar to 30 '') up to 6.5 (r similar to 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, HI 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-alpha/(i alpha)(H-alpha) is only similar to 2 R-alpha/(i alpha)(CO) and systematic offsets between H-alpha 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-rot) is steeper than the curves derived from the H I and H-alpha data. V-rot reaches 200km s(-1) in less than 1kpc. 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 (r(cor)=110 ''). 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 Bent, 1995). Star formation rate (measured as the ratio SFR=F(H-alpha)/I-CO) 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-2)/I-CO 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-alpha) mostly in the nucleus where the classical Schmidt law breaks down paradoxically (SFRND similar to N-gas(alpha), alpha similar to 1). MSF is set on at a distance r(c) similar to 12kpc where N(H-2) approaches the Toomre (1964) gravitational instability threshold (N-c similar to 7M.) Also for r<r(c), the neutral gas is mostly in the H-2 phase. H I is underabundant in the inner 6kpc and cannot be accounted for by photodissociation of H-2 by HII regions. Asymmetry in the observed molecular gas distribution and kinematics of M 100 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.