Galactic Ins and Outs

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Science  22 Jul 2011:
Vol. 333, Issue 6041, pp. 387
DOI: 10.1126/science.333.6041.387-b

The study of stellar populations in present-day galaxies provides clues to the formation history of those galaxies. NGC 3115, also known as the Spindle, is a nearby disk galaxy without a spiral structure (i.e., a lenticular galaxy), for which Arnold et al. obtained high-quality images and detailed spectra with the Subaru, Keck, and Magellan telescopes. The data provide global information on the velocities and chemical abundances of the stars in that galaxy and show that its inner part spins faster than its outer part. Comparison of the results with those expected from theoretical models of galaxy formation suggests that NGC 3115 may not have formed in one go: Its inner part is likely the result of the violent merging of two large galaxies, whereas its outer part may have formed little by little as smaller galaxies joined in at later times. The fact that the stars in the outer part are also less chemically enriched than those in the inner part supports this hypothesis. The inner part of the galaxy would have hosted its own star formation, enriching the interstellar medium with newly synthesized elements, whereas the accreted stars in the outer part of the galaxy would have formed in much smaller galaxies, with much less chemically enriched interstellar media.

Astrophys. J. 736, L26 (2011).

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