The problem with fast wide angle lenses is that a fast lens by definition has a large entrance pupil, and to illuminate the image plane the entrance pupil has to be visible across the field of view. So a combination of wide aperture and wide field of view is very difficult to achieve. In addition wide angle lenses for digital cameras are often retrofocus design, and aberrations become very hard to control the wider the aperture.

There are 24mm lenses available for the Canon mount that are f/1.4, namely the Canon 24mm f/1.4L (I and II) and the Samyang/Rokinion/Bower (same lens different name) 24mm f/1.4. The latter is probably a better choice than the Canon as it is much cheaper and being manual focus only doesn't really matter for astrophotography.

Sigma make 24mm f/1.8 and 20mm f/1.8 lenses for full frame, as well as an 18-35 f/1.8 zoom for APS-C. If you are shooting APS-C then the widest you can go sub-f/2.8 is with the Samyang/Rokinion/Bower 16mm f/2.0, same as above, manual focus only but should be fine for astrophotography.