My apologies for the vague title, in the interests of avoiding spoilers.
In The Force Awakens, a large part of the plot revolves around
the part map that BB-8 had on a memory stick. The First Order have the rest of the map, and, as it turns out, so does R2-D2.
Is there any explanation as to why the latter has it, or how he obtained it? Or are there any upcoming comics or novelisations that may answer the question?
Also relevant: What made R2-D2 activate?
R2-D2 downloaded the map back in A New Hope, but didn't realise it at the time.
There's an article by Entertainment Weekly which recounts a post-screening Q&A with Abrams and co-writers Lawrence Kasdan and Michael Arndt. It explains how R2-D2 came to have part of the star map:
The story group’s thinking went back to the 1977 original movie, when R2-D2 accessed the Empire’s mainframe as the heroes searched for the captured Princess Leia. “We had the idea about R2 plugging into the information base of the Death Star, and that’s how he was able to get the full map and find where the Jedi temples are,” Arndt said.
Abrams says he chose to spell this out indirectly in the movie because he didn’t want the story to get bogged down in “how s–t happened 30 years ago.”
“But the idea was that in that scene where R2 plugged in, he downloaded the archives of the Empire, which was referenced by Kylo Ren,” Abrams said. Thirty-eight years later, in both our own and galactic time, that data becomes useful in The Force Awakens when a new droid approaches the dormant R2.
This doesn't explain how R2-D2's map has a missing piece – it seems unusual that the Empire's data would have such a specific hole – but apparently that's how it came to be.
I'm not sure there is yet any specific evidence to this, but I think it's an easy jump in logic to assume
Luke gave R2-D2 the majority of the map before he disappeared. They even show a vision in the movie of luke with R2-D2 sometime in the recent past, presumably before he vanished as C-3PO mentions that R2-D2 went into low power mode just after Luke vanished.
I believe it's much simpler. R2 has the map, he's Luke's astromech droid after all. Knowing what may come by force use (precognition) he foresaw Ben, now Kylo Ren, and his thirst to finish his grandfather's job. So he fragmented the map to keep his location hidden, but as another person had posted left breadcrumbs in case he was truly needed again.
We must remember R2 was new back in Episode 1. His age as a droid should be relevant as his data banks have never been purged. So him having the galactic map in full should be no surprise.
Now as for that map fragment, we see in Rey's flash that he touched R2. Subsequently leading a story arc on how that little R2 unit chose to go to low power and stayed that way until the right information was collected.
I saw someone mention on why the First Order didn't have this map. They have a fragmented map, something they scavenged from the corpse of the Empire. So its safe to say that during Empirical rule whole systems were negated and destroyed so as to no longer offer harbor to a runaway Jedi.
Both sides are rebuilding, trying to get themselves operating again to their old powers. So things like starmaps would be more for local starship captains than new orders right? You don't just know the known world once your in power, you ultimately find that information out from your dealings with others.
Though JJ had said R2's reveal was for cinematic effect, which is OK with me as it was a pretty important artifact for the majority of the film. Negating the drama would've put things too fast for this one movie, which is part of a trilogy, so really you can't fault them too much for that.
3¢ given
