NEWLY QUENCHED GALAXIES AS THE CAUSE FOR THE APPARENT EVOLUTION IN AVERAGE SIZE OF THE POPULATION
- Institute for Astronomy, Swiss Federal Institute of Technology (ETH Zurich), CH-8093 Zurich (Switzerland)
- Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35121 Padova (Italy)
- Spitzer Science Center, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125 (United States)
- Laboratoire d'Astrophysique de Marseille, 38 rue Frederic Joliot Curie, F-13388 Marseille (France)
- California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125 (United States)
- Department of Physics and Astronomy, University of California, Riverside, CA 92508 (United States)
- Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr, Honolulu, HI 96822 (United States)
We use the large COSMOS sample of galaxies to study in an internally self-consistent way the change in the number densities of quenched early-type galaxies (Q-ETGs) of a given size over the redshift interval 0.2 < z < 1 in order to study the claimed size evolution of these galaxies. In a stellar mass bin at 10{sup 10.5} < M{sub galaxy} < 10{sup 11} M{sub Sun }, we see no change in the number density of compact Q-ETGs over this redshift range, while in a higher mass bin at >10{sup 11} M{sub Sun }, where we would expect merging to be more significant, we find a small decrease, by {approx}30%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition to the size function of larger and more diffuse Q-ETGs. At all masses, compact Q-ETGs become systematically redder toward later epochs, with a (U - V) color difference which is consistent with a passive evolution of their stellar populations, indicating that they are a stable population that does not appreciably evolve in size. We find furthermore, at all epochs, that the larger Q-ETGs (at least in the lower mass bin) have average rest-frame colors that are systematically bluer than those of the more compact Q-ETGs, suggesting that the former are indeed younger than the latter. The idea that new, large, Q-ETGs are responsible for the observed growth in the median size of the population at a given mass is also supported by analysis of the sizes and number of the star-forming galaxies that are expected to be the progenitors of the new Q-ETGs over the same period. In the low mass bin, the new Q-ETGs appear to have {approx}30% smaller half-light radii than their star-forming progenitors. This is likely due to the fading of their disks after they cease star formation. Comparison with higher redshifts shows that the median size of newly quenched galaxies roughly scales, at constant mass, as (1 + z){sup -1}. We conclude that the dominant cause of the size evolution seen in the Q-ETG population is that the average sizes and thus stellar densities of individual Q-ETGs roughly scale with the average density of the universe at the time when they were quenched, and that subsequent size changes in individual objects, through merging or other processes, are of secondary importance, especially at masses below 10{sup 11} M{sub Sun}.
- OSTI ID:
- 22130991
- Journal Information:
- Astrophysical Journal, Vol. 773, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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