The least luminous known galaxies have historically been those closest to our own galaxy, the Milky Way. Whether visually or with automated searches, resolved stars reveal the presence of nearby dwarf galaxies with surface brightness too low to be discovered by diffuse light alone. Even until recently, nearly all cataloged dwarfs resided within the Local Group (LG) of galaxies. In 1999, the LG contained 36 known members, of which 11 are Milky Way (MW) satellites. Four of these eleven MW dwarf galaxies are more than 10,000 times less luminous than the Milky Way itself. There is now a new class of “ultra-faint” dwarf companions to the Milky Way known to have even lower absolute magnitudes. The resolved stellar populations of these near-field cosmological laboratories have been used to derive their star formation and chemical evolution histories and to model their dark mass content in detail. The future will reveal whether we have yet seen the ultimate limit of galaxy formation. The possibilities remain that either the low luminosities of the ultra-faint dwarfs are an artifact of nature, rather than nurture, and/or the present survey data are not deep enough to reveal the very least luminous systems and a vast population of ultra-faint dwarfs lie just beyond our fingertips. Regardless, at least dozens of ultra-faint satellites will be discovered in the near future, with the possibility of hundreds or more.

17. The primary purpose of the passage is to

A. ccompare ultra-faint galaxies to galaxies with the normal range of luminosity
B. question whether current methods of data-gathering are sufficient
C. present evidence regarding a rare phenomenon
D. discuss the current status of research about a topic
E. outline the history of a unique stellar object



18. In the context in which it appears, “resolved” most nearly means

A. decided
B. settled
C. separated
D. solved
E. strengthened



19. The author of the passage mentions the history of the least luminous known galaxies primarily in order to

A. highlight the importance of the faint galaxies proximal to the Earth
B. suggest that, unlike other nearby galaxies, the Milky Way is particularly rich in faint galaxies
C. imply that this phenomenon is due in part to the limitations of current technology
D. indicate that the inherent brightness of the Milky Way is responsible for the difficulty in detecting faint galaxies
E. argue that the Milky Way spawns dozens of faint galaxies