WISDOM Project -- IX Giant Molecular Clouds in the Lenticular Galaxy
  NGC4429: Effects of Shear and Tidal Forces on Clouds
  We present high spatial resolution (12pc) Atacama Large
Millimeter/sub-millimeter Array CO(J=3-2) observations of the nearby lenticular
galaxy NGC4429. We identify 217 giant molecular clouds within the 450pc radius
molecular gas disc. The clouds generally have smaller sizes and masses but
higher surface densities and observed linewidths than those of Milky Way disc
clouds. An unusually steep size - line width relation and large cloud internal
velocity gradients (0.05 - 0.91 km s^-1 pc^-1) and observed Virial parameters
(alpha_obs,vir = 4.0) are found, that appear due to internal rotation driven by
the background galactic gravitational potential. Removing this rotation, an
internal Virial equilibrium appears to be established between the
self-gravitational (Usg) and turbulent kinetic (Eturb) energies of each cloud,
i.e. alpha_sg,vir=Usg/Eturb = 1.3. However, to properly account for both self
and external gravity (shear and tidal forces), we formulate a modified Virial
theorem and define an effective Virial parameter alpha_eff,vir = alpha_sg,vir +
Usg/Eext (and associated effective velocity dispersion). The NGC4429 clouds
then appear to be in a critical state in which the self-gravitational energy
and the contribution of external gravity to the cloud's energy budget (Eext)
are approximately equal, i.e. Eext/Usg~1. As such, alpha_eff,vir = 2.2 and most
clouds are not virialised but remain marginally gravitationally bound. We show
this is consistent with the clouds having sizes similar to their tidal radii
and being generally radially elongated. External gravity is thus as important
as self-gravity to regulate the clouds of NGC4429.
