What does ultraviolet light from hot stars in very distant galaxies reaches us in the form of?

No matter how far a photon of light travels, once it has been emitted at a certain frequency, the frequency remains constant.

The only process by which light can alter its frequency is through collisions with matter, in which case the light is absorbed and excites the matter (usually an electron in an atom). The photon that is released when the electron returns to its lower energy state is solely determined by the energy difference between the two electron states.

Therefore, unless a distant star interacts with matter, its ultraviolet light will remain unchanged when it reaches us; if it does, it will change into a light frequency that corresponds to the makeup of the matter it last interacted with.

Ultraviolet light from hot stars in very distant galaxies reaches us in the form of electromagnetic radiation.