“Ghost has picked up a broadcast fro the Sunbreakers. Find the source.”
Time to start clearing out the Vex that have overrun this area. You have a number of Goblins and Hobgoblins to deal with. There are a few Harpies in the area as well. Take your time and fight, starting from the entrance. This gives you a lot of cover when the fight starts.
Approach the Sunbreaker beacon and Deploy Ghost to have him interface with it to learn more about where the Sunbreakers have gone. Apparently you need to head deeper in because they pulled back through a Vex Gate.