The artifact research tree refers to artifacts that are currently under your control. Artifacts are similar to planet modifiers (reduced gravity well, penal colonies etc...), but they affect your entire empire. Their effects vary. One gives all your ships +12%(?) damage. Another reduces the cost for researching.
They are found by using the 'Explore' improvement on one of your colonies. Most places will not have one on them, as there are 9 artifacts in a game max, and smaller maps will only have a fractions of them.
When one is found, every player receives a notification that 'another empire has uncovered an artifact', and it will tell you which planet. Planets with discovered artifacts also have indicators next to them in the empire map. If you colonize a planet with a discovered artifact, you will have to explore the planet to find it and gain its effect.
