Small but Young

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Science  27 Feb 2009:
Vol. 323, Issue 5918, pp. 1149
DOI: 10.1126/science.323.5918.1149a

Recent studies have shown that in the early universe, massive elliptical galaxies tended to be much smaller and denser than their present-day analogs, implying that most grew in size as they evolved through cosmic time. The mechanism by which they grew is not very well understood, but some theoretical models, involving collisions between galaxies, predict that a fraction of small dense galaxies could have survived intact up to the present universe. Using the Sloan Digital Sky Survey—a systematic map of a quarter of the sky, containing millions of galaxies—Trujillo et al. have found that small-sized high-mass galaxies, similar to those found in the early universe, are presently very scarce. Moreover, there are indications that their stellar populations are very young, contrary to what would be expected if they were relics of the early universe. If these galaxies are truly young and just recently formed, then explaining how massive galaxies swelled seems to demand a mechanism that acted in all massive elliptical galaxies and left none unaffected. Unfortunately, the available data are not sufficient to distinguish between different scenarios. — MJC

Astrophys. J. 692, L118 (2009).

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