We combine high-resolution Hubble Space Telescope/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (M > 10 10 M ) galaxies at redshifts z = 1.4-3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5-3. At z 2, cSFGs present SFR = 100-200 M yr −1 , yet their specific star formation rates (sSFR ∼ 10 −9 yr −1 ) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30 times (∼30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 10 8 yr). The cSFGs are continuously being formed at z = 2-3 and fade to cQGs down to z ∼ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early (z 2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival (z 2) path in which larger SFGs form extended QGs without passing through a compact state.

We present new JHK photometry on the MKO-NIR system and JHK spectroscopy for a large sample of L and T dwarfs. Photometry has been obtained for 71 dwarfs and spectroscopy for 56. The sample comprises newly identified very red objects from the Sloan Digital Sky Survey (SDSS) and known dwarfs from the SDSS and the Two Micron All Sky Survey (2MASS). Spectral classification has been carried out using four previously defined indices (from Geballe et al. 2002, G02) that measure the strengths of the near infrared water and methane bands. We identify 9 new L8-9.5 dwarfs and 14 new T dwarfs from SDSS, including the latest yet found by SDSS, the T7 dwarf SDSS J175805.46+463311.9. We classify 2MASS J04151954−0935066 as T9, the latest and coolest dwarf found to date.We combine the new results with our previously published data to produce a sample of 59 L dwarfs and 42 T dwarfs with imaging data on a single photometric system -2and with uniform spectroscopic classification. We compare the near-infrared colors and absolute magnitudes of brown dwarfs near the L-T transition with predictions made by models of the distribution and evolution of photospheric condensates. There is some scatter in the G02 spectral indices for L dwarfs, suggesting that these indices are probing different levels of the atmosphere and are affected by the location of the condensate cloud layer. The near-infrared colors of the L dwarfs also show scatter within a given spectral type, which is likely due to variations in the altitudes, spatial distributions and thicknesses of the clouds. We have identified a small group of late L dwarfs that are relatively blue for their spectral type and that have enhanced FeH, H 2 O and K I absorption, possibly due to an unusually small amount of condensates.The scatter seen in the H − K color for late T dwarfs can be reproduced by models with a range in surface gravity. The variation is probably due to the effect on the K-band flux of pressure-induced H 2 opacity. The correlation of H − K color with gravity is supported by the observed strengths of the J-band K I doublet. Gravity is closely related to mass for field T dwarfs with ages > 10 8 yrs and the gravities implied by the H − K colors indicate that the T dwarfs in our sample have masses in the range 15 -75 M Jupiter . One of the SDSS dwarfs, SDSS J111010.01+011613.1, is possibly a very low mass object, with log g ∼ 4.2 -4.5 and mass ∼ 10 -15 M Jupiter .

We use HST/WFC3 imaging from the CANDELS Multicycle Treasury Survey, in conjunction with the Sloan Digital Sky Survey, to explore the evolution of galactic structure for galaxies with stellar masses > 3 × 10 10 M ⊙ from z = 2.2 to the present epoch, a time span of 10 Gyr. We explore the relationship between rest-frame optical color, stellar mass, star formation activity and galaxy structure. We confirm the dramatic increase from z = 2.2 to the present day in the number density of non-star-forming galaxies above 3 × 10 10 M ⊙ reported by others. We further find that the vast majority of these quiescent systems have concentrated light profiles, as parameterized by the Sérsic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, Sérsic index, stellar mass, inferred velocity dispersion, and stellar surface density. Quiescence correlates poorly with stellar mass at all z < 2.2. Quiescence correlates well with Sérsic index at all redshifts. Quiescence correlates well with 'velocity dispersion' and stellar surface density at z > 1.3, and somewhat less well at lower redshifts. Yet, there is significant scatter between quiescence and galaxy structure: while the vast majority of quiescent galaxies have prominent bulges, many of them have significant disks, and a number of bulge-dominated galaxies have significant star formation. Noting the rarity of quiescent galaxies without prominent bulges, we argue that a prominent bulge (and perhaps, by association, a supermassive black hole) is an important condition for quenching star formation on galactic scales over the last 10 Gyr, in qualitative agreement with the AGN feedback paradigm.

We study the evolution of the number density, as a function of the size, of passive early-type galaxies with a wide range of stellar masses (10 10 M ⊙ < M * 10 11.5 M ⊙ ) from z ∼ 3 to z ∼ 1, exploiting the unique dataset available in the GOODS-South field, including the recently obtained WFC3 images as a part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). In particular, we select a sample of ∼107 massive (M * > 10 10 M ⊙ ), passive (SSF R < 10 −2 Gyr −1 ) and morphologically spheroidal galaxies at 1.2 < z < 3, taking advantage of the panchromatic dataset available for GOODS, including VLT, CFHT, Spitzer, Chandra and HST ACS+WFC3 data. We find that at 1 < z < 3 the passively evolving early-type galaxies are the reddest and most massive objects in the Universe, and we prove that a correlation between mass, morphology, color and star-formation activity is already in place at that epoch. We measure a significant evolution in the mass-size relation of passive early-type galaxies (ETGs) from z ∼ 3 to z ∼ 1, with galaxies growing on average by a factor of 2 in size in a 3 Gyr timescale only. We witness also an increase in the number density of passive ETGs of 50 times over the same time interval. We find that the first ETGs to form at z 2 are all compact or ultra-compact, while normal sized ETGs (meaning ETGs with sizes comparable to those of local counterparts of the same mass) are the most common ETGs only at z < 1. The increase of the average size of ETGs at 0 < z < 1 is primarily driven by the appearance of new large ETGs rather than by the size increase of individual galaxies.

We present an original phenomenological model to describe the evolution of galaxy number counts, morphologies, and spectral energy distributions across a wide range of redshifts (0.2 < z < 15) and stellar masses [log(M/M ) ≥ 6]. Our model follows observed mass and luminosity functions of both star-forming and quiescent galaxies, and reproduces the redshift evolution of colors, sizes, star-formation and chemical properties of the observed galaxy population. Unlike other existing approaches, our model includes a self-consistent treatment of stellar and photoionized gas emission and dust attenuation based on the BEAGLE tool. The mock galaxy catalogs generated with our new model can be used to simulate and optimize extragalactic surveys with future facilities such as the James Webb Space Telescope (JWST), and to enable critical assessments of analysis procedures, interpretation tools, and measurement systematics for both photometric and spectroscopic data. As a first application of this work, we make predictions for the upcoming JWST Advanced Deep Extragalactic Survey (JADES), a joint program of the JWST/NIRCam and NIRSpec Guaranteed Time Observations teams. We show that JADES will detect, with NIRCam imaging, thousands of galaxies at z 6, and tens at z 10 at m AB 30 (5σ) within the 236 arcmin 2 of the survey. The JADES data will enable accurate constraints on the evolution of the UV luminosity function at z > 8, and resolve the current debate about the rate of evolution of galaxies at z 8. Ready to use mock catalogs and software to generate new realizations are publicly available as the JAdes extraGalactic Ultradeep Artificial Realizations (JAGUAR) package.

We analyze the star-forming and structural properties of 45 massive (log(M/M ⊙ )> 10) compact starforming galaxies (SFGs) at 2 < z < 3 to explore whether they are progenitors of compact quiescent galaxies at z ∼ 2. The optical/NIR and far-IR Spitzer/Herschel colors indicate that most compact SFGs are heavily obscured. Nearly half (47%) host an X-ray bright AGN. In contrast, only about 10% of other massive galaxies at that time host AGNs. Compact SFGs have centrally-concentrated light profiles and spheroidal morphologies similar to quiescent galaxies, and are thus strikingly different from other SFGs, which typically are disk-like and sometimes clumpy or irregular. Most compact SFGs lie either within the SFR-mass main sequence (65%) or below it (30%), on the expected evolutionary path towards quiescent galaxies. These results show conclusively that galaxies become more compact before they lose their gas and dust, quenching star formation. Using extensive HST photometry from CANDELS and grism spectroscopy from the 3D-HST survey, we model their stellar populations with either exponentially declining (τ ) star formation histories (SFHs) or physically-motivated SFHs drawn from semi-analytic models (SAMs). SAMs predict longer formation timescales and older ages ∼2 Gyr, which are nearly twice as old as the estimates of the τ models. While both models yield good SED fits, SAM SFHs better match the observed slope and zero point of the SFR-mass main sequence. Contrary to expectations, some low-mass compact SFGs (log(M/M ⊙ )= 10 − 10.6) have younger ages but lower sSFRs than that of more massive galaxies, suggesting that the low-mass galaxies reach the red sequence faster. If the progenitors of compact SFGs are extended SFGs, state-of-the-art SAMs show that mergers and disk instabilities are both able to shrink galaxies, but disk instabilities are more frequent (60% versus 40%) and form more concentrated galaxies. We confirm this result via high-resolution hydrodynamic simulations.

We examine the fraction of massive (M * > 10 10 M ⊙ ), compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at z ∼ 2. These cSFGs are likely the direct progenitors of the compact quiescent galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that 39.2 +3.9 −3.6 % (65/166) of cSFGs at 1.4 < z < 3.0 host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the 6.2σ level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive, quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.

We use GOODS and CANDELS images to identify progenitors of massive (M > 10 10 M ⊙ ) compact "early-type" galaxies (ETGs) at z ∼ 1.6. Since merging and accretion increase the size of the stellar component of galaxies, if the progenitors are among known star-forming galaxies, these must be compact themselves. We select candidate progenitors among compact Lyman-break galaxies at z ∼ 3 based on their mass, SFR and central stellar density and find that these account for a large fraction of, and possibly all, compact ETGs at z ∼ 1.6. We find that the average far-UV SED of the candidates is redder than that of the non-candidates, but the optical and mid-IR SED are the same, implying that the redder UV of the candidates is inconsistent with larger dust obscuration, and consistent with more evolved (aging) star-formation. This is in line with other evidence that compactness is a sensitive predictor of passivity among high-redshift massive galaxies. We also find that the light distribution of both the compact ETGs and their candidate progenitors does not show any extended "halos" surrounding the compact "core", both in individual images and in stacks. We argue that this is generally inconsistent with the morphology of merger remnants, even if gas-rich, as predicted by N-body simulations. This suggests that the compact ETGs formed via highly dissipative, mostly gaseous accretion of units whose stellar components are very small and undetected in the HST images, with their stellar mass assembling in-situ, and that they have not experienced any major merging until the epoch of observations at z ∼ 1.6.