Dichromatic number and forced subdivisions

We investigate bounds on the dichromatic number of digraphs which avoid a fixed digraph as a topological minor. For a digraph F, denote by mader_χ→(F) the smallest integer k such that every k-dichromatic digraph contains a subdivision of F. As our first main result, we prove that if F is an orientation of a cycle then mader_χ→(F)=v(F). This settles a conjecture of Aboulker, Cohen, Havet, Lochet, Moura and Thomassé. We also extend this result to the more general class of orientations of cactus graphs, and to bioriented forests. Our second main result is that mader_χ→(F)=4 for every tournament F of order 4. This is an extension of the classical result by Dirac that 4-chromatic graphs contain a K₄-subdivision to directed graphs.