We explore the dependence of the incidence of moderate-luminosity (L 0.5−8 keV = 10 41.9−43.7 erg s −1 ) active galactic nuclei (AGNs) and the distribution of their accretion rates on host color at 0.5 < z < 2.5. Based on the deepest X-ray and UV-to-far-infrared data in the two The Great Observatories Origins Deep Survey (GOODS) fields, we identify 221 AGNs within a mass-complete parent galaxy sample down to M * > 10 10 M . We use extinction-corrected rest-frame U − V colors to divide both AGN hosts and non-AGN galaxies into red sequence (red), green valley (green), and blue cloud (blue) populations. We find that the fraction of galaxies hosting an AGN at fixed X-ray luminosity increases with stellar mass and redshift for all the three galaxy populations, independent of their colors. However, both the AGN fraction at fixed stellar mass and its evolution with redshift are clearly dependent on host colors. Most notably, red galaxies have the lowest AGN fraction (∼5%) at z ∼ 1 yet with most rapid evolution with redshift, increasing by a factor of ∼5 (24%) at z ∼ 2. Green galaxies exhibit the highest AGN fraction across all redshifts, which is most pronounced at z ∼ 2 with more than half of them hosting an AGN at M * > 10 10.6 M . Together with the high AGN fraction in red galaxies at z ∼ 2, this indicates that (X-ray) AGNs could be important in both transforming (quenching) star-forming galaxies into quiescent ones and subsequently maintaining their quiescence at high redshift. Furthermore, consistent with previous studies at lower redshifts, we show that the probability of hosting an AGN for the total galaxy population can be characterized by a universal Eddington ratio (as approximated byEdd ), which is independent on host mass. Yet consistent with their different AGN fractions, galaxies with different colors appear to also have different p(λ Edd ) with red galaxies exhibiting more rapid redshift evolution compared with that for green and blue galaxies. Evidence for a steeper power-law distribution of p(λ Edd ) in red galaxies (p(λ Edd ) ∼ λ −0.6 Edd ) is also presented, though larger samples are needed to confirm. These results suggest that the AGN accretion or the growth of supermassive black holes is related to their host properties, and may also influence their hosts in a different mode dependent on the host color.

To explore the evolutionary connection among red, green, and blue galaxy populations, based on a sample of massive (M * > 10 10 M ⊙ ) galaxies at 0.5 < z < 2.5 in five 3D-HST /CANDELS fields, we investigate the dust content, morphologies, structures, active galactic nucleus (AGN) fractions, and environments of these three galaxy populations. Green valley galaxies are found to have intermediate dust attenuation, and reside in the middle of the regions occupied by quiescent and star-forming galaxies in the U V J diagram. Compared with blue and red galaxy populations at z < 2, green galaxies have intermediate compactness and morphological parameters. The above findings seem to favor the scenario that green galaxies are at a transitional phase when star-forming galaxies are being quenched into quiescent status. The green galaxies at z < 2 show the highest AGN fraction, suggesting that AGN feedback may have played an important role in star formation quenching. For the massive galaxies at 2 < z < 2.5, both red and green galaxies are found to have a similarly higher AGN fraction than the blue ones, which implies that AGN feedback may help to keep quiescence of red galaxies at z > 2. A significant environmental difference is found between green and red galaxies at z < 1.5. Green and blue galaxies at z > 0.5 seem to have similar local density distributions, suggesting that environment quenching is not the major mechanism to cease star formation at z > 0.5. The fractions of three populations as functions of mass support a "downsizing" quenching picture that the bulk of star formation in more massive galaxies is completed earlier than that of lower-mass galaxies.

We present a study of the classification of z ∼ 1 extremely red objects (EROs), using a combination of Hubble Space Telescope (HST ) Advanced Camera for Surveys (ACS), Spitzer Infrared Array Camera (IRAC), and ground-based images of the COSMOS field. Our sample includes ∼ 5300 EROs with i − K s ≥ 2.45 (AB, equivalently I − K s = 4 in Vega) and K s ≤ 21.1 (AB). For EROs in our sample, we compute, using the ACS F814W images, their concentration, asymmetry, as well as their Gini coefficient and the second moment of the brightest 20% of their light. Using those morphology parameters and the Spitzer IRAC [3.6] − [8.0] color, the spectral energy distribution (SED) fitting method, we classify EROs into two classes: old galaxies (OGs) and young, dusty starburst galaxies (DGs). We found that the fraction of OGs and DGs in our sample is similar, about 48 percentages of EROs in our sample are OGs, and 52 percentages of them are DGs. To reduce the redundancy of these three different classification methods, we performed a principal component analysis on the measurements of EROs, and find that morphology parameters and SEDs are efficient in segregating OGs and DGs. The [3.6] − [8.0] color, which depends on reddening, redshift, and photometric accuracy, is difficult to separate EROs around the discriminating line between starburst and elliptical. We investigate the dependence of the fraction of EROs on their observational properties, and the results suggest that DGs become increasingly important at fainter magnitudes, redder colors, and higher redshifts. The clustering of the entire EROs, DGs, and OGs was estimated by calculating their correlation function, and we find that the clustering of EROs is much stronger than that of full K−limited samples of galaxies; the clustering amplitude of OGs is a factor of ∼ 2 larger than that of DGs.

We present a study on the physical properties of compact star-forming galaxies (cSFGs) with M * ≥ 10 10 M ⊙ and 2 ≤ z ≤ 3 in the COSMOS and GOODS-S fields. We find that massive cSFGs have a comoving number density of (1.0 ± 0.1) × 10 −4 Mpc −3 . The cSFGs are distributed at nearly the same locus on the main sequence as extended star-forming galaxies (eSFGs) and dominate the high-mass end. On the rest-frame U −V vs. V − J and U − B vs. M B diagrams, cSFGs are mainly distributed at the middle of eSFGs and compact quiescent galaxies (cQGs) in all colors, but are more inclined to "red sequence" than "green valley" galaxies. We also find that cSFGs have distributions similar to cQGs on the nonparametric morphology diagrams. The cQGs and cSFGs have larger Gini and smaller M 20 , while eSFGs have the reverse. About one-third of cSFGs show signatures of postmergers, and almost none of them can be recognized as disks. Moreover, those visually extended cSFGs all have lower Gini coefficients (Gini < 0.4), indicating that the Gini coefficient could be used to clean out noncompact galaxies in a sample of candidate cSFGs. The X-ray-detected counterparts are more frequent among cSFGs than that in eSFGs and cQGs, implying that cSFGs have previously experienced violent gas-rich interactions(such as major mergers or disk instabilities), which could trigger both star formation and black hole growth in an active phase.

The spectra of 413 star-forming (or H ii) regions in M33 (NGC 598) were observed by using the multifiber spectrograph of Hectospec at the 6.5-m Multiple Mirror Telescope (MMT). By using this homogeneous spectra sample, we measured the intensities of emission lines and some physical parameters, such as electron temperatures, electron densities, and metallicities. Oxygen abundances were derived via the direct method (when available) and two empirical strong-line methods, namely, O3N2 and N2. In the high-metallicity end, oxygen abundances derived from O3N2 calibration were higher than those derived from N2 index, indicating an inconsistency between O3N2 and N2 calibrations. We presented a detailed analysis of the spatial distribution of gas-phase oxygen abundances in M33 and confirmed the existence of the axisymmetric global metallicity distribution widely assumed in literature. Local variations were also observed and subsequently associated with spiral structures to provide evidence of radial migration driven by arms. Our O/H gradient fitted out to 1.1 R 25 resulted in slopes of −0.17 ± 0.03, −0.19 ± 0.01, and −0.16 ± 0.17 dex R −1 25 utilizing abundances from O3N2, N2 diagnostics, and direct method, respectively.