Spiral structure of molecular clouds in response to bar forcing: a particle simulation.

The evolution of an ensemble of molecular clouds is studied by simulation using a collisional model coupled with the cloud dynamics in a barred galaxy. A density wave of spiral shape is generated, a persistent two-armed and open structure. The spirals obtained in the simulations are well representative of barred spiral structures observed in SB galaxies. It is shown that giant molecular clouds can form in the spiral arms by coalescence. The formation time of giant clouds is on the order of their lifetime, 40 Myr. Due to the gravitational torques exerted by the bar on the spiral arms, the gas evolves to a ring structure at Outer Lindblad Resonance in a time scale on the order of 20 rotations. With a low angular velocity of the bar, angular momentum is transferred inwards and particles gather in an elongated orbit near the center.