  Using detailed 3D hydrodynamic simulations we study the nature of the
Galactic supernova remnant (SNR) CTB 109 (G109.1-1.0), which is well-known for
its semicircular shape and a bright diffuse X-ray emission feature inside the
SNR. Our model has been designed to explain the observed morphology, with a
special emphasis on the bright emission feature inside the SNR. Moreover, we
determine the age of the remnant and compare our findings with X-ray
observations. With CTB 109 we test a new method of detailed numerical
simulations of diffuse young objects, using realistic initial conditions
derived directly from observations. We performed numerical 3D simulations with
the RAMSES code. The initial density structure has been directly taken from
$^{12}$CO emission data, adding an additional dense cloud, which, when it is
shocked, causes the bright emission feature. From parameter studies we obtained
the position $(\ell , b)=(109.1545^\circ , -1.0078^\circ)$ for an elliptical
cloud with $n_\text{cloud}=25~\text{cm}^{-3}$ based on the preshock density
from Chandra data and a maximum diameter of 4.54 pc, whose encounter with the
supernova (SN) shock wave generates the bright X-ray emission inside the SNR.
The calculated age of the remnant is about 11,000 yr according to our
simulations. In addition, we can also determine the most probable site of the
SN explosion. Hydrodynamic simulations can reproduce the morphology and the
observed size of the SNR CTB 109 remarkably well. Moreover, the simulations
show that it is very plausible that the bright X-ray emission inside the SNR is
the result of an elliptical dense cloud shocked by the SN explosion wave. We
show that numerical simulations using observational data for an initial model
can produce meaningful results.
