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We investigate the relationship between active galactic nucleus (AGN) activity and host galaxy properties using a sample of massive galaxies at z ∼ 2 in the Chandra Deep Field-South (CDFS).A sample of 268 galaxies with M∗ >1010.5Mꙩ at 1.4< z< 3 are selected from Hubble Space Telescope wide field camera 3 (WFC3) H-band observations in CDFS taken as part of the cosmic assembly near-infrared deep extragalactic legacy survey (CANDELS) survey. We find that a large fraction (22.0±2.5 per cent) are detected in the 4 Ms Chandra/Advanced CCD Image Spectrometer observations in the field, implying a high AGN content in these massive galaxies. To investigate further the relationship between these AGN and their hosts, we create four subsamples, based on their star formation rates (star-forming versus quiescent) and galaxy size (compact versus extended), following Barro et al. and perform X-ray spectral fitting. We find a clear effect whereby the AGN in compact galaxies – be they star forming or quiescent – show significantly higher luminosities and levels of obscuration than the AGN in extended galaxies. These results provide clear evidence for two modes of black hole growth in massive galaxies at high redshift. The dominant growth mode is a luminous, obscured phase which occurs overwhelmingly in compact galaxies while another lower luminosity, unobscured phase is predominantly seen in extended galaxies. Both modes could produce AGN feedback, with violent transformative feedback in the former and a gentler ‘maintenance mode’ produced by the latter. [ABSTRACT FROM AUTHOR]

Nearby galaxies come in two flavors: red quiescent galaxies (QGs) with old stellar populations, and blue young star-forming galaxies (SFGs). This color bimodality seems to be already in place at z = 2 − 3, presenting also strong correlations with size and morphology. Surprisingly, massive QGs at higher redshifts are ~5 times smaller than local, equal mass analogs. In contrast, most of the massive SFGs at these redshifts are still relatively large disks. The strong bimodality in both SFR and sizes indicates that some SFGs must experience strong structural transformations accompanied by a rapid truncation of the star-formation to match the observed properties of QGs. Using high-resolution HST/WFC3 F160W imaging from the CANDELS survey in GOODS-S and UDS, along with multi-wavelength ancillary data, we analyze stellar masses, SFRs and sizes of a sample of massive (M* > 1010M⊙) galaxies at z = 1.4 − 3.0 to identify a population of compact SFGs with similar structural properties as compact QGs at z~2. We also find that the number density of QGs increases rapidly since z = 3. Among these, the number of compact QGs builds up first, and only at z < 1.8 we do start finding a sizable number of extended QGs. This suggests that the bulk of these galaxies are assembled at late times by both continuous migration (quenching) of non-compact SFGs and size growth of cQGs. As a result of this growth, the population of cQGs disappears by z~1. Simultaneously, we identify a population of compact SFGs (cSFGs) whose number density decreases steadily with time since z = 3.0, being almost completely absent at z < 1.4. The number of cSFGs makes up less than 20% of all massive SFGs, but they present similar number densities as cQGs down to z~2, suggesting an evolutionary link between the two populations. [ABSTRACT FROM PUBLISHER]