A cloud-cloud collision in Sgr B2? 3D simulations meet SiO observations
  We compare the properties of shocked gas in Sgr B2 with maps obtained from 3D
simulations of a collision between two fractal clouds. In agreement with
$^{13}$CO(1-0) observations, our simulations show that a cloud-cloud collision
produces a region with a highly turbulent density substructure with an average
$N_{\rm H2}\gtrsim 5\times10^{22}\,\rm cm^{-2}$. Similarly, our numerical
multi-channel shock study shows that colliding clouds are efficient at
producing internal shocks with velocities of $5-50\,\rm km\,s^{-1}$ and Mach
numbers of $\sim4-40$, which are needed to explain the $\sim 10^{-9}$ SiO
abundances inferred from our SiO(2-1) IRAM observations of Sgr B2. Overall, we
find that both the density structure and the shocked gas morphology in Sgr B2
are consistent with a $\lesssim 0.5\,\rm Myr$-old cloud-cloud collision.
High-velocity shocks are produced during the early stages of the collision and
can ignite star formation, while moderate- and low-velocity shocks are
important over longer time-scales and can explain the extended SiO emission in
Sgr B2.
