A self-similar aperiodic set of 19 Wang tiles

We define a Wang tile set $\mathcal{U}$ of cardinality 19 and show that the set ${\mathrm{\Omega }}_{\mathcal{U}}$ of all valid Wang tilings ${\mathbb{Z}}^2\to \mathcal{U}$ is self-similar, aperiodic and is a minimal subshift of ${\mathcal{U}}^{{\mathbb{Z}}^2}$. Thus $\mathcal{U}$ is the second smallest self-similar aperiodic Wang tile set known after Ammann’s set of 16 Wang tiles. The proof is based on the unique composition property. We prove the existence of an expansive, primitive and recognizable $2$-dimensional morphism $\omega :{\mathrm{\Omega }}_{\mathcal{U}}\to {\mathrm{\Omega }}_{\mathcal{U}}$ that is onto up to a shift. The proof of recognizability is done in two steps using at each step the same criteria (the existence of marker tiles) for proving the existence of a recognizable one-dimensional substitution that sends each tile either on a single tile or on a domino of two tiles.