A direct consequence of hierarchical galaxy formation is the existence of dual supermassive black holes, which may be preferentially triggered as active galactic nuclei (AGNs) during galaxy mergers. Despite decades of searching, however, dual AGNs are extremely rare, and most have been discovered serendipitously. Using the all-sky WISE survey, we identified a population of over 100 morphologically identified interacting galaxies or mergers that display red mid-infrared colors often associated in extragalactic sources with powerful AGNs. The vast majority of these advanced mergers are optically classified as star-forming galaxies, which suggests that they may represent an obscured population of AGNs that cannot be found through optical studies. In this work, we present Chandra/ACIS observations and near-infrared spectra with the Large Binocular Telescope of six advanced mergers with projected pair separations less than similar to 10 kpc. The combined X-ray, near-infrared, and mid-infrared properties of these mergers provide confirmation that four out of the six mergers host at least one AGN, with four of the mergers possibly hosting dual AGNs with projected separations less than similar to 10 kpc, despite showing no firm evidence for AGNs based on optical spectroscopic studies. Our results demonstrate that (1) optical studies miss a significant fraction of single and dual AGNs in advanced mergers, and (2) mid-infrared pre-selection is extremely effective in identifying dual AGN candidates in late-stage mergers. Our multi-wavelength observations suggest that the buried AGNs in these mergers are highly absorbed, with intrinsic column densities in excess of similar to N-H > 10(24) cm(-2), consistent with hydrodynamic simulations.

To Understand how galaxies were assembled across the cosmic time remains one of the most outstanding questions in astronomy. The core of this question is how today's Hubble Sequence, namely the differentiation of galaxy morphology and its correlation to galaxy physical properties, is formed. In this thesis, we investigate the origin of the Hubble Sequence through galaxies at z~2, an epoch when the cosmic star formation activity reaches its peak and the properties of galaxies undergo dramatic transitions. Galaxies at z~2 have two important features that are distinct from nearby galaxies: much higher frequency of clumpy morphology in star-forming systems, and much compacter size. To understand the nature of the two features requires investigations on the sub-structure of galaxies in a multi-wavelength way. In this thesis, we study samples of galaxies that are selected from GOODS and HUDF, where ultra-deep and high-resolution optical and near-infrared images allow us to study the stellar populations of the sub-structures of galaxies at the rest-frame optical bands for the first time, to answer two questions: (1) the nature of kiloparsec-scale clumps in star-forming galaxies at z$\sim$2 and (2) the existence of color gradient and stellar population gradient in passively evolving galaxies at z~2, which may provide clues to the mechanisms of dramatic size evolution of this type of galaxies. We further design a set of color selection criteria to search for dusty star-forming galaxies and passively evolving galaxies at z~3 to explore the question: when today's Hubble Sequence has begun to appear.

We report the detection of morphology-dependent stellar age in massive quenched galaxies (QGs) at z similar to 1.2. The sense of the dependence is that compact QGs are 0.5-2 Gyr older than normal-sized ones. The evidence comes from three different age indicators-D(n)4000, H-delta, and fits to spectral synthesis models-applied to their stacked optical spectra. All age indicators consistently show that the stellar populations of compact QGs are older than those of their normal-sized counterparts. We detect weak [O II] emission in a fraction of QGs, and the strength of the line, when present, is similar between the two samples; however, compact galaxies exhibit a. significantly lower frequency of [O II] emission than normal ones. Fractions of both samples are individually detected in 7Ms Chandra X-ray images (luminosities similar to 10(40) - 10(41) erg s(-1)). The 7Ms stacks of nondetected galaxies show similarly low luminosities in the soft band only, consistent with a hot gas origin for the X-ray emission. While both [O II] emitters and nonemitters are also X-ray sources among normal galaxies, no compact galaxy with [O II] emission is an X-ray source, arguing against an active galactic nucleus (AGN) powering the line in compact galaxies. We interpret the [O II] properties as further evidence that compact galaxies are older and further along in. the process of quenching star formation and suppressing gas accretion. Finally, we argue that the older age of compact QGs is evidence of progenitor bias: compact QGs simply reflect the smaller sizes of galaxies at their earlier quenching epoch, with stellar density most likely having nothing directly to do with cessation of star formation.

We present spectra of five ultra-diffuse galaxies (UDGs) in the vicinity of the Coma cluster obtained with the Multi-object Double Spectrograph on the Large Binocular Telescope. We confirm four of these as members of the cluster, quintupling the number of spectroscopically confirmed systems. Like the previously confirmed large (projected half-light radius > 4.6 kpc) UDG, DF44, the systems we targeted all have projected half-light radii > 2.9 kpc. As such, we spectroscopically confirm a population of physically large UDGs in the Coma cluster. The remaining UDG is located in the field, about 45 Mpc behind the cluster. We observe Balmer and Ca II H and K absorption lines in all of our UDG spectra. By comparing the stacked UDG spectrum against stellar population synthesis models, we conclude that, on average, these UDGs are composed of metal-poor stars ([Fe/H] less than or similar to -1.5). We also discover the first UDG with [O II] and [O III] emission lines within a clustered environment, demonstrating that not all cluster UDGs are devoid of gas and sources of ionizing radiation.

Searching for active galactic nuclei (AGNs) in dwarf galaxies is important for our understanding of the seed black holes that formed in the early universe. Here, we test infrared selection methods for AGN activity at low galaxy masses. Our parent sample consists of similar to 18,000 nearby dwarf galaxies (M-* < 3 x 10(9) M-circle dot, z < 0.055) in the Sloan Digital Sky Survey with significant detections in the first three bands of the AllWISE data release from the Wide-field Infrared Survey Explorer (WISE). First, we demonstrate that the majority of optically selected AGNs in dwarf galaxies are not selected as AGNs using WISE infrared color diagnostics and that the infrared emission is dominated by the host galaxies. We then investigate the infrared properties of optically selected star-forming dwarf galaxies, finding that the galaxies with the reddest infrared colors are the most compact, with blue optical colors, young stellar ages, and large specific star formation rates. These results indicate that great care must be taken when selecting AGNs in dwarf galaxies using infrared colors, as star-forming dwarf galaxies are capable of heating dust in such a way that mimics the infrared colors of more luminous AGNs. In particular, a simple W1 - W2 color cut alone should not be used to select AGNs in dwarf galaxies. With these complications in mind, we present a sample of 41 dwarf galaxies that fall in the. WISE infrared color space typically occupied by more luminous AGNs and that are worthy of follow-up observations.

The correlation between active galactic nuclei (AGNs) and environment provides important clues to AGN fuelling and the relationship of black hole growth to galaxy evolution. In this paper, we analyse the fraction of galaxies in clusters hosting AGN as a function of redshift and cluster richness for X-ray-detected AGN associated with clusters of galaxies in Dark Energy Survey (DES) Science Verification data. The present sample includes 33 AGNs with LX > 1043 erg s(-1) in non-central, host galaxies with luminosity greater than 0.5L(*) from a total sample of 432 clusters in the redshift range of 0.1< z <0.95. Analysis of the present sample reveals that the AGN fraction in red-sequence cluster members has a strong positive correlation with redshift such that the AGN fraction increases by a factor of similar to 8 from low to high redshift, and the fraction of cluster galaxies hosting AGN at high redshifts is greater than the low-redshift fraction at 3.6 sigma. In particular, the AGN fraction increases steeply at the highest redshifts in our sample at z > 0.7. This result is in good agreement with previous work and parallels the increase in star formation in cluster galaxies over the same redshift range. However, the AGN fraction in clusters is observed to have no significant correlation with cluster mass. Future analyses with DES Year 1 through Year 3 data will be able to clarify whether AGN activity is correlated to cluster mass and will tightly constrain the relationship between cluster AGN populations and redshift.

We report new ALMA observations of the CO(3-2) line emission from the 2.1 +/- 0.3*10(10)M(circle dot). molecular gas reservoir in the central galaxy of the Phoenix cluster. The cold molecular gas is fueling a vigorous starburst at a rate of 500-800M(circle dot)yr(-1) and powerful black hole activity in the forms of both intense quasar radiation and radio jets. The radio jets have inflated huge bubbles filled with relativistic plasma into the hot, X-ray atmospheres surrounding the host galaxy. The ALMA observations show that extended filaments of molecular gas, each 10-20 kpc long with a mass of several billion solar masses, are located along the peripheries of the radio bubbles. The smooth velocity gradients and narrow line widths along each filament reveal massive, ordered molecular gas flows around each bubble, which are inconsistent with gravitational free-fall. The molecular clouds have been lifted directly by the radio bubbles, or formed via thermal instabilities induced in low-entropy gas lifted in the updraft of the bubbles. These new data provide compelling evidence for close coupling between the radio bubbles and the cold gas, which is essential to explain the self-regulation of feedback. The very feedback mechanism that heats hot atmospheres and suppresses star formation may also paradoxically stimulate production of the cold gas required to sustain feedback in massive galaxies.

We present a robust measurement of the rest-frame UV luminosity function (LF) and its evolution during the peak epoch of cosmic star formation at 1 < z < 3. We use our deep near-ultraviolet imaging from WFC3/UVIS on the Hubble Space Telescope and existing Advanced Camera for Surveys (ACS)/WFC and WFC3/IR imaging of three lensing galaxy clusters, Abell 2744 and MACS J0717 from the Hubble Frontier Field survey and Abell 1689. Combining deep UV imaging and high magnification from strong gravitational lensing, we use photometric redshifts to identify 780 ultra-faint galaxies with M-UV < -12.5 AB mag at 1 < z < 3. From these samples, we identified five new, faint, multiply imaged systems in A1689. We run a Monte Carlo simulation to estimate the completeness correction and effective volume for each cluster using the latest published lensing models. We compute the rest-frame UV LF and find the best-fit faint-end slopes of alpha = -1.56 +/- 0.04, alpha = -1.72 +/- 0.04, and alpha = -1.94 +/- 0.06 at 1.0 < z < 1.6, 1.6 < z < 2.2, and 2.2 < z < 3.0, respectively. Our results demonstrate that the UV LF becomes steeper from z similar to 1.3 to z similar to 2.6 with no sign of a turnover down to MUV = -14 AB mag. We further derive the UV LFs using the Lyman break "dropout" selection and confirm the robustness of our conclusions against different selection methodologies. Because the sample sizes are so large and extend to such faint luminosities, the statistical uncertainties are quite small, and systematic uncertainties (due to the assumed size distribution, for example) likely dominate. If we restrict our analysis to galaxies and volumes above >50% completeness in order to minimize these systematics, we still find that the faint-end slope is steep and getting steeper with redshift, though with slightly shallower (less negative) values (alpha = -1.55 +/- 0.06, -1.69 +/- 0.07, and -1.79 +/- 0.08 for z similar to 1.3, 1.9, and 2.6, respectively). Finally, we conclude that the faint star-forming galaxies with UV magnitudes of -18.5 < M-UV < -12.5 covered in this study produce the majority (55%-60%) of the unobscured UV luminosity density at 1 < z < 3.

We present a (CO)-C-12 (2-1) mosaic map of the spiral galaxy NGC 6946 by combining data from the Submillimeter Array and the IRAM 30m telescope. We identify 390 giant molecular clouds (GMCs) from the nucleus to 4.5 kpc in the disk. GMCs in the inner 1 kpc are generally more luminous and turbulent, some of which have luminosities > 10(6)K. km. s(-1) pc(2) and velocity dispersions > 10. km s(-1). Large-scale bar-driven dynamics likely regulate GMC properties in the nuclear region. Similar to the MilkyWay and other disk galaxies, GMC mass function of NGC 6946 has a shallower slope (index > -2) in the inner region, and a steeper slope (index < -2) in the outer region. This difference in mass spectra may be indicative of different cloud formation pathways: gravitational instabilities might play a major role in the nuclear region, while cloud coalescence might be dominant in the outer disk. Finally, the NGC 6946 clouds are similar to those inM33 in terms of statistical properties, but they are generally less luminous and turbulent than the M51 clouds.

The determination of galaxy redshifts in the James Webb Space Telescope's (JWST) blank-field surveys will mostly rely on photometric estimates, based on the data provided by JWST's Near-Infrared Camera (NIRCam) at 0.6-5.0 mu m and Mid Infrared Instrument (MIRI) at lambda > 5.0 mm. In this work we analyze the impact of choosing different combinations of NIRCam and MIRI broadband filters (F070W to F770W), as well as having ancillary data at lambda < 0.6 mu m, on the derived photometric redshifts (z(phot)) of a total of 5921 real and simulated galaxies, with known input redshifts z = 0-10. We found that observations at lambda < 0.6 mm are necessary to control the contamination of high-z samples by low-z interlopers. Adding MIRI (F560W and F770W) photometry to the NIRCam data mitigates the absence of ancillary observations at l < 0.6 mm and improves the redshift estimation. At z = 7-10, accurate zphot can be obtained with the NIRCam broadbands alone when S/N >= 10, but the z(phot) quality significantly degrades atb S/N <= 5. Adding MIRI photometry with 1 mag brighter depth than the NIRCam depth allows for a redshift recovery of 83%-99%, depending on spectral energy distribution type, and its effect is particularly noteworthy for galaxies with nebular emission. The vast majority of NIRCam galaxies with [F150W] = 29. AB mag at z =7-10 will be detected with MIRI at [F560W, F770W] < 28 mag if these sources are at least mildly evolved or have spectra with emission lines boosting the mid-infrared fluxes.