The Evolution of Galaxy Mergers and Morphology at z < 1.2 in the Extended Groth Strip
Le Floc'h, E.; Papovich, C.; Cooper, M.; Faber, S. M.; Lotz, Jennifer M.; Weiner, B. J.; Huang, J.; Koo, D. C.; Conselice, C. J.; Hopkins, A. M.; Willmer, C. N. A.; Lin, Lihwai; Gwyn, S.; Newman, J.; Coil, A.; Davis, M.; Guhathakurta, P.; Primack, J.; Noeske, K.; Metevier, A.; Rieke, G.
United States, Germany, Canada, France, United Kingdom, Australia
Abstract
We present the quantitative rest-frame B morphological evolution and galaxy merger fraction at 0.2 < z < 1.2 as observed by the All-Wavelength Extended Groth Strip International Survey (AEGIS). We use the Gini coefficient and M20 to identify major mergers and classify galaxy morphology for a volume-limited sample of 3009 galaxies brighter than 0.4L*B, assuming pure luminosity evolution. We find that the merger fraction remains roughly constant at 10% +/- 2% for 0.2 < z < 1.2. The fraction of E/S0/Sa galaxies increases from 21% +/- 3% at z ~ 1.1 to 44% +/- 9% at z ~ 0.3, while the fraction of Sb-Ir galaxies decreases from 64% +/- 6% at z ~ 1.1 to 47% +/- 9% at z ~ 0.3. The majority of z < 1.2 Spitzer MIPS 24 μm sources with L(IR) > 1011 L⊙ are disk galaxies, and only ~15% are classified as major merger candidates. Edge-on and dusty disk galaxies (Sb-Ir) are almost a third of the red sequence at z ~ 1.1, while E/S0/Sa make up over 90% of the red sequence at z ~ 0.3. Approximately 2% of our full sample are red mergers. We conclude (1) the merger rate does not evolve strongly between 0.2 < z < 1.2; (2) the decrease in the volume-averaged star formation rate density since z ~ 1 is a result of declining star formation in disk galaxies rather than a disappearing population of major mergers; (3) the build-up of the red sequence at z < 1 can be explained by a doubling in the number of spheroidal galaxies since z ~ 1.2.