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Register a new userCANDELS+3D-HST: compact SFGs at z~2-3, the progenitors of the first quiescent galaxies
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We analyze the star-forming and structural properties of 45 massive (log(M/Msun)>10) compact star-forming 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. Most compact SFGs lie either within the SFR-M main sequence (65%) or below (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 (tau) 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 tau models. While both models yield good SED fits, SAM SFHs better match the observed slope and zero point of the SFR-M main sequence. Some low-mass compact SFGs (log(M/Msun)=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.
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We combine high-resolution HST/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (log(M*) > 10) galaxies at redshifts z = 1.4 - 3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5 - 3. At z > 2 most cSFGs have specific star-formation rates (sSFR = 10^-9 yr^-1) half that of typical, massive SFGs at the same epoch, and host X-ray luminous AGN 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 by 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 scenarios of QG formation: an early (z > 2), fast-formation path of rapidly-quenched cSFGs that evolve into cQGs that later enlarge within the quiescent phase, and a slow, late-arrival (z < 2) path for SFGs to form QGs without passing through a compact state.
We examine the fraction of massive ($M_{*}>10^{10} M_{odot}$), compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at $zsim2$. 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 $6.2sigma$ 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.
[Abridged] Using public data from the NMBS and CANDELS surveys, we study the population of massive galaxies at z>3 to identify the potential progenitors of z~2 compact, massive, quiescent (CMQ) galaxies, furthering our understanding of the evolution of massive galaxies. Our work is enabled by high-resolution CANDELS images and accurate photometric redshifts, stellar masses and star formation rates (SFRs) from 37-band NMBS photometry. The total number of z>3 massive galaxies is consistent with the number of massive quiescent (MQ) galaxies at z~2, implying that the SFRs for all of these galaxies must be much lower by z~2. We discover 4 CMQ galaxies at z>3, pushing back the time for which such galaxies have been observed. However, the volume density for these galaxies is significantly less than that of galaxies at z<2 with similar masses, SFRs, and sizes, implying that additional CMQ galaxies must be created in the ~1 Gyr between z=3 and z=2. We find 5 star-forming galaxies at z~3 that are compact (Re<1.4 kpc) and have stellar mass M*>10^(10.6)Msun, likely to become members of the CMQ galaxy population at z~2. We evolve the stellar masses and SFRs of each individual z>3 galaxy adopting 5 different star formation histories (SFHs) and studying the resulting population of massive galaxies at z=2.3. We find that declining or truncated SFHs are necessary to match the observed number density of MQ galaxies at z~2, whereas a constant SFH results in a number density significantly smaller than observed. All of our assumed SFHs imply number densities of CMQ galaxies at z~2 that are consistent with the observed number density. Better agreement with the observed number density of CMQ galaxies at z~2 is obtained if merging is included in the analysis and better still if star formation quenching is assumed to shortly follow the merging event, as implied by recent models of formation of MQ galaxies.
We investigate the differences in the stellar population properties, the structure, and the environment between massive compact star-forming galaxies (cSFGs) with or without active galactic nucleus (AGN) at $2<z<3$ in the five 3D-HST/CANDELS fields. In a sample of 221 massive cSFGs, we constitute the most complete AGN census so far, identifying 66 AGNs by the X-ray detection, the mid-infrared color criterion, and/or the SED fitting, while the rest (155) are non-AGNs. Further dividing these cSFGs into two redshift bins, i.e., $2<z<2.5$ and $2.5 leq z<3$, we find that in each redshift bin the cSFGs with AGNs have similar distributions of the stellar mass, the specific star formation rate, and the ratio of $L_{rm IR}$ to $L_{rm UV}$ to those without AGNs. After having performed a two-dimensional surface brightness modeling for those cSFGs with X-ray-detected AGNs (37) to correct for the influence of the central point-like X-ray AGN on measuring the structural parameters of its host galaxy, we find that in each redshift bin the cSFGs with AGNs have comparable distributions of all concerned structural parameters, i.e., the Sersic index, the 20%-light radius, the Gini coefficient, and the concentration index, to those without AGNs. With a gradual consumption of available gas and dust, the structure of cSFGs, indicated by the above structural parameters, seem to be slightly more concentrated with decreasing redshift. At $2<z<3$, the similar environment between cSFGs with and without AGNs suggests that their AGN activities are potentially triggered by internal secular processes, such as gravitational instabilities or/and dynamical friction.
Based on a large sample of massive ($M_{*}geq 10^{10} M_{odot}$) compact galaxies at $1.0 < z < 3.0$ in five 3D-HST/CANDELS fields, we quantify the fractional abundance and comoving number density of massive compact galaxies as a function of redshift. The samples of compact quiescent galaxies (cQGs) and compact star-forming galaxies (cSFGs) are constructed by various selection criteria of compact galaxies in literatures, and the effect of compactness definition on abundance estimate is proved to be remarkable, particularly for the cQGs and cSFGs at high redshifts. Regardless of the compactness criteria adopted, their overall redshift evolutions of fractional abundance and number density are found to be rather similar. Large samples of the cQGs exhibit a sustaining increase in number density from $z sim 3$ to 2 and a plateau at $1<z<2$. For massive cSFGs, a plateau in the number density at $2<z<3$ can be found, as well as a continuous drop from $z sim 2$ to 1. The evolutionary trends of the cQG and cSFG abundances support the scenario that the cSFGs at $z geq 2$ may have been rapidly quenched into quiescent phase via violent dissipational processes such as major merger and disk instabilities. Rarity of the cSFGs at lower redshifts ($z < 1$) can be interpreted by the decrease of gas reservoirs in dark matter halos and the consequent low efficiency of gas-rich dissipation.
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