A map of the Milky Way Galaxy
NGC 5139 was a globular cluster in the Beta Quadrant. It was located over twenty thousand light years from the galactic core.
The location of NGC 5139 was shown on Federation star charts in astrometrics aboard the USS Voyager during the later years of its seven-year journey in the Delta Quadrant. (VOY: "Inside Man", et al., display graphic)
In an alternate timeline, the star chart depicting Voyager's flight path home was also displayed in Captain Harry Kim's ready room aboard the USS Rhode Island in 2404. (VOY: "Endgame")
The graphic is shown in legible detail in Geoffrey Mandel's Star Trek: Star Charts (pp. 76-77).
According to Star Trek: Star Charts (p. 81) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), NGC 5139 was a quasar.