Relative hyperbolicity, thickness, and the hierarchically hyperbolic boundary

We study the boundaries of relatively hyperbolic HHGs. Using the simplicial structure on the hierarchically hyperbolic boundary, we characterize both relative hyperbolicity and being thick of order 1 among HHGs. In the case of relatively hyperbolic HHGs, we show that the Bowditch boundary of the group is the quotient of the HHS boundary obtained by collapsing the limit sets of the peripheral subgroups to a point. In establishing this, we give a construction that allows one to modify an HHG structure by including a collection of hyperbolically embedded subgroups into the HHG structure.