The PdBI Arcsecond Whirlpool Survey (PAWS): Multi-phase cold gas
  kinematic of M51
  The kinematic complexity and the favorable position of M51 on the sky make
this galaxy an ideal target to test different theories of spiral arm dynamics.
Taking advantage of the new high resolution PdBI Arcsecond Whirlpool Survey
(PAWS) data, we undertake a detailed kinematic study of M51 to characterize and
quantify the origin and nature of the non-circular motions. Using a tilted-ring
analysis supported by several other archival datasets we update the estimation
of M51's position angle (PA=(173 +/- 3) deg) and inclination (i=(22 +/- 5)
deg). Harmonic decomposition of the high resolution (40 pc) CO velocity field
shows the first kinematic evidence of an m=3 wave in the inner disk of M51 with
a corotation at R(CR,m=3)=1.1 +/- 0.1 kpc and a pattern speed of Omega_p(m=3) =
140 km/(s kpc). This mode seems to be excited by the nuclear bar, while the
beat frequencies generated by the coupling between the m=3 mode and the main
spiral structure confirm its density-wave nature. We observe also a signature
of an m=1 mode that is likely responsible for the lopsidedness of M51 at small
and large radii. We provide a simple method to estimate the radial variation of
the amplitude of the spiral perturbation (Vsp) attributed to the different
modes. The main spiral arm structure has <Vsp>=50-70 km/s, while the streaming
velocity associated with the m=1 and m=3 modes is, in general, 2 times lower.
Our joint analysis of HI and CO velocity fields at low and high spatial
resolution reveals that the atomic and molecular gas phases respond differently
to the spiral perturbation due to their different vertical distribution and
emission morphology.
