In this paper, we use stacking analysis to trace the mass growth, color evolution, and structural evolution of present-day massive galaxies ( M M log 11.5 * =  ( ) ) out to z=5. We utilize the exceptional depth and area of the latest UltraVISTA data release, combined with the depth and unparalleled seeing of CANDELS to gather a large, mass-selected sample of galaxies in the NIR (rest-frame optical to UV). Progenitors of present-day massive galaxies are identified via an evolving cumulative number density selection, which accounts for the effects of merging to correct for the systematic biases introduced using a fixed cumulative number density selection, and find progenitors grow in stellar mass by 1.5 dex » since z=5. Using stacking, we analyze the structural parameters of the progenitors and find that most of the stellar mass content in the central regions was in place by z 2 , and while galaxies continue to assemble mass at all radii, the outskirts experience the largest fractional increase in stellar mass. However, we find evidence of significant stellar mass build-up at r 3 kpc < beyond z 4 > probing an era of significant mass assembly in the interiors of present-day massive galaxies. We also compare mass assembly from progenitors in this study to the EAGLE simulation and find qualitatively similar assembly with z at r 3 kpc < . We identify z 1.5 as a distinct epoch in the evolution of massive galaxies where progenitors transitioned from growing in mass and size primarily through in situ star formation in disks to a period of efficient growth in r e consistent with the minor merger scenario.

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We investigate the median flattening of galaxies at 0.2 < z < 4.0 in all five CANDELS/3D-HST fields via the apparent axis ratio q. We separate the sample into bins of redshift, stellar-mass, sérsic index, size, and UVJ determined star-forming state to discover the most important drivers of the median q (q med ). Quiescent galaxies at z < 1 and M * > 10 11 M are rounder than those at lower masses, consistent with the hypothesis that they have grown significantly through dry merging. The massive quiescent galaxies at higher redshift become flatter, and are as flat as star forming massive galaxies at 2.5 < z < 3.5, consistent with formation through direct transformations or wet mergers. We find that in quiescent galaxies, correlations with q med and M * , z and r e are driven by the evolution in the sérsic index (n), consistent with the growing accumulation of minor mergers at lower redshift. Interestingly, n does not drive these trends fully in star-forming galaxies. Instead, the strongest predictor of q in star-forming galaxies is the effective radius, where larger galaxies are flatter. Our findings suggest that q med is tracing bulge-to-total (B/T ) galaxy ratio which would explain why smaller/more massive star-forming galaxies are rounder than their extended/less massive analogues, although it is unclear why Sersic index correlates more weakly with flattening for star forming galaxies than for quiescent galaxies

Measuring stellar velocity dispersions of quiescent galaxies beyond z ∼ 2 is observationally challenging. Such measurements require near-infrared spectra with a continuum detection of at least moderate signal-to-noise, often necessitating long integrations. In this paper, we present deep X-Shooter spectroscopy of one of only two known gravitationally-lensed massive quiescent galaxies at z > 2. This galaxy is quadruply imaged, with the brightest images magnified by a factor of ∼ 5. The total exposure time of our data is 9.8 hours on-source; however the magnification, and the slit placement encompassing 2 images, provides a total equivalent exposure time of 215 hours. From this deep spectrum we measure a redshift (z spec = 2.756 ± 0.001), making this one of the highest redshift quiescent galaxies that is spectroscopically confirmed. We simultaneously fit both the spectroscopic and photometric data to determine stellar population parameters and conclude this galaxy is relatively young (560 +100 −80 Myr), intermediate-mass (log M * /M = 10.59 +0.04 −0.05 ), consistent with low dust content (A V = 0.20 +0.26 −0.20 ), and has quenched only relatively recently. This recent quenching is confirmed by strong Balmer absorption, particularly Hδ (Hδ A = 6.66 +0.96−0.92 ). Remarkably, this proves that at least some intermediate-mass galaxies have already quenched as early as z ∼ 2.8. Additionally, we have measured a velocity dispersion (σ = 187 ± 43 km/s), making this the highest-redshift quiescent galaxy with a dispersion measurement. We confirm that this galaxy falls on the same mass fundamental plane (MFP) as galaxies at z=2.2, consistent with little to no evolution in the MFP up to z=2.8. Overall this galaxy is proof of existence of intermediate-mass quenched galaxies in the distant universe, and that lensing is a powerful tool for determining their properties with improved accuracy.

Using observed stellar mass functions out to z=5, we measure the main progenitor stellar mass growth of descendant galaxies with masses of M M log 11.5, 11.0, 10.5, 10.0 * =  at z 0.1 using an evolving cumulative number density selection. From these mass growth histories, we are able to measure the time at which half the total stellar mass of the descendant galaxy was assembled, t a , which in order of decreasing mass corresponds to redshifts of z 1.28, 0.92, 0.60 a = , and 0.51. We compare this to the median light-weighted stellar age t * (z 2.08, 1.49, 0.82 * = and 0.37) of a sample of low-redshift SDSS galaxies (from the literature) and find the timescales are consistent with more massive galaxies forming a higher fraction of their stars ex situ compared to lower-mass descendants. We find that both t * and t a strongly correlate with mass, which is in contrast to what is found in the EAGLE hydrodynamical simulation that shows a flat relationship between t a and M * . However, the semi-analytic model (SAM) of Henriques et al. is consistent with the observations in both t a and t * with M * , showing that the most recent SAMs are better able to decouple the evolution of the baryons from the dark matter in lower-mass galaxies.