  The Large Magellanic Cloud (LMC) is the closest and most studied example of
an irregular galaxy. Among its principal defining morphological features, its
off-centred bar and single spiral arm stand out, defining a whole family of
galaxies known as the Magellanic spirals (Sm). These structures are thought to
be triggered by tidal interactions and possibly maintained via gas accretion.
However, it is still unknown whether they are long-lived stable structures. In
this work, by combining photometry that reaches down to the oldest main
sequence turn-off in the colour-magnitude diagrams (CMD, up to a distance of
$\sim$4.4 kpc from the LMC centre) from the SMASH survey and CMD fitting
techniques, we find compelling evidence supporting the long-term stability of
the LMC spiral arm, dating the origin of this structure to more than 2~Gyr ago.
The evidence suggests that the close encounter between the LMC and the Small
Magellanic Cloud (SMC) that produced the gaseous Magellanic Stream and its
Leading Arm (LA) also triggered the formation of the LMC's spiral arm. Given
the mass difference between the Clouds and the notable consequences of this
interaction, we can speculate that this should have been one of their closest
encounters. These results set important constraints on the timing of LMC-SMC
collisions, as well as on the physics behind star formation induced by tidal
encounters.
