Annu. Rev. Astron. Astrophys. 1989. 27: 235-277
Copyright © 1989 by . All rights reserved

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6. SUMMARY: SECULAR EVOLUTION BY ACCRETION

The observations reviewed in Sections 3 - 5 suggest that galaxy structure may be altered significantly by the accretion of gas and small companions. In the past, galaxies showing obvious effects of mergers were regarded as peculiar. Now we know that accretion happens often enough in a typical galaxy that even modest events can add up to a significant effect. However, this is secular evolution and not like the more violent mergers that may completely destroy a disk and convert two spirals into one elliptical. The present results are independent of the debate about what fraction of ellipticals formed in this way. Accreted material continues to trickle in long after dissipational collapse or merger formation is complete.

The amount of material added and its effects on the structure of a typical galaxy are unknown. However, typical masses of dust and gas (107 - 108 Msun ; Section 4.1) are not much less than the mass of stars in a core. Substantial changes in core properties could result; this may increase the scatter in core parameter relations (cf. Lauer 1988b). Also, enough material can be accreted to make disks that would change a galaxy's apparent morphological type (Section 4.4). And many shell galaxies have blue colors and early-type spectra implying recent bursts of star formation (Carter et al. 1988).

These results form part of a gradually changing picture of the formation and evolution of elliptical galaxies. Traditionally, galaxies were thought to form on a gravitational collapse time scale, with little subsequent evolution. Recent results suggest a picture in which formation is more gradual. As in disk galaxies, where gas infall and disk building are still going on today (Gunn 1982), accretion and the rearranging of mass in ellipticals may be a significant evolutionary process that is far from over (Schweizer 1983, 1986, Schweizer & Seitzer 1988).

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