Exploring the Evolutionary Paths of the Most Massive Galaxies since z ~ 2

We use Spitzer MIPS data from the FIDEL Legacy Project in the extended Groth strip to analyze the stellar mass assembly of massive (M > 10¹¹ M_☉) galaxies at z < 2 as a function of structural parameters. We find 24 μm emission for more than 85% of the massive galaxies morphologically classified as disks, and for more than 57% of the massive systems morphologically classified as spheroids at any redshift, with about 8% of sources harboring a bright X-ray- and/or infrared-emitting AGN. More noticeably, ~60% of all compact massive galaxies at z = 1–2 are detected at 24 μm, even when rest-frame optical colors reveal that they are dead and evolving passively. For spheroid-like galaxies at a given stellar mass, the sizes of MIPS nondetections are smaller by a factor of ~1.2 in comparison with IR-bright sources. We find that disklike massive galaxies present specific SFRs ranging from 0.04 to 0.2 Gyr⁻¹ at z < 1 (SFRs ranging from 1 to 10 M_☉ yr⁻¹), typically a factor of 3-6 higher than massive spheroid-like objects in the same redshift range. At z > 1, and more pronouncedly at z > 1.3, the median specific SFRs of the disks and spheroids detected by MIPS are very similar, ranging from 0.1 to 1 Gyr⁻¹ (SFR = 10–200 M_☉ yr⁻¹). We estimate that massive spheroid-like galaxies may have doubled (at the most) their stellar mass from star-forming events at z < 2: less than 20% mass increase at 1.7 < z < 2.0, up to 40% more at 1.1 < z < 1.7, and less than 20% additional increase at z < 1. Disklike galaxies may have tripled (at the most) their stellar mass at z < 2 from star formation alone: up to ~40% mass increase at 1.7 < z < 2.0, and less than 180% additional increase below z = 1.7 occurred at a steady rate.