Astronomy quiz question: Milky Way capture of dwarf galaxies

Astronomy 210 Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Evidence that small galaxies were captured by the growing Milky Way is/are:
(A) different ages of globular clusters.
(B) metal-rich absorption lines in the sun.
(C) dark matter in the halo.
(D) stars orbiting the central supermassive black hole.

Correct answer: (A)

According to the monolithic collapse model, the Milky Way evolved from a spherical shape to its current disk shape, leaving behind the globular clusters out in its halo. Thus globular clusters should all have the identical (old) ages (as determined from the lack of metal absorption lines, and their H-R diagram turn-off points), while the disk should be comprised of mixed age (old and newer) stars. However, the presence of globular clusters of different ages indicates that they may have been inherited from smaller dwarf galaxies.

Section 70160
Exam code: quiz06n4Rd
(A) : 18 students
(B) : 4 students
(C) : 0 students
(D) : 3 students

Success level: 75% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.57

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