The phrase "luminous galaxies" refers to galaxies that are very bright and shiny. They emit a lot of light and can be easily seen in the night sky. Full definition
This artist's concept depicts the most luminous galaxy known in the universe. (space.com)
In previous observations we could only see the most luminous galaxies in the distant past, and now we can look at more normal galaxies. (sciencedaily.com)
This animation is an actual plot of the 2500 most luminous galaxies within 100 million light years from us. (atlasoftheuniverse.com)