Radio Loud AGNs are Mergers

We measure the merger fraction of Type 2 radio-loud and radio-quiet active galactic nuclei at z>1 using new samples. The objects have HST images taken with WFC3 in the IR channel. These samples are compared to the 3CR sample of radio galaxies at z>1 and to a sample of non-active galaxies. We also consider lower redshift radio galaxies with HST observations and previous generation instruments (NICMOS and WFPC2). The full sample spans an unprecedented range in both redshift and AGN luminosity. We perform statistical tests to determine whether the different samples are differently associated with mergers. We find that all (92%) radio-loud galaxies at z>1 are associated with recent or ongoing merger events. Among the radio-loud population there is no evidence for any dependence of the merger fraction on either redshift or AGN power. For the matched radio-quiet samples, only 38% are merging systems. The merger fraction for the sample of non-active galaxies at z>1 is indistinguishable from radio-quiet objects. This is strong evidence that mergers are the triggering mechanism for the radio-loud AGN phenomenon and the launching of relativistic jets from supermassive black holes. We speculate that major BH-BH mergers play a major role in spinning up the central supermassive black holes in these objects.Comment: 16 pages, 6 figures, accepted for publication in the Ap

Role of Galaxy Mergers in Cosmic Star Formation History

We present a morphology study of intermediate-redshift (0.2<z<1.2) luminous infrared galaxies (LIRGs) and general field galaxies in the GOODS fields using a revised asymmetry measurement method optimized for deep fields. By taking careful account of the importance of the underlying sky-background structures, our new method does not suffer from systematic bias and offers small uncertainties. By redshifting local LIRGs and low-redshift GOODS galaxies to different higher redshifts, we have found that the redshift dependence of the galaxy asymmetry due to surface-brightness dimming is a function of the asymmetry itself, with larger corrections for more asymmetric objects. By applying redshift-, IR-luminosity- and optical-brightness-dependent asymmetry corrections, we have found that intermediate-redshift LIRGs generally show highly asymmetric morphologies, with implied merger fractions ~50% up to z=1.2, although they are slightly more symmetric than local LIRGs. For general field galaxies, we find an almost constant relatively high merger fraction (20-30%). The B-band LFs of galaxy mergers are derived at different redshifts up to z=1.2 and confirm the weak evolution of the merger fraction after breaking the luminosity-density degeneracy. The IR luminosity functions (LFs) of galaxy mergers are also derived, indicating a larger merger fraction at higher IR luminosity. The integral of the merger IR LFs indicates a dramatic evolution of the merger-induced IR energy density [(1+z)^(5-6)}], and that galaxy mergers start to dominate the cosmic IR energy density at z>~1.Comment: Accepted for publication in ApJ, 25 pages, 23 figures (2 colors). The high-resolution pdf is at http://cztsy.as.arizona.edu/~yong/Research/SHI_MERGER.pd

The Globular Cluster Luminosity Function and Specific Frequency in Dwarf Elliptical Galaxies

The globular cluster luminosity function, specific globular cluster frequency, S_N, specific globular cluster mass, T_MP, and globular cluster mass fraction in dwarf elliptical galaxies are explored using the full 69 galaxy sample of the HST WFPC2 Dwarf Elliptical Galaxy Snapshot Survey. The GCLFs of the dEs are well-represented with a t_5 function with a peak at M_{V,Z}^0(dE,HST) = -7.3 +/- 0.1. This is ~0.3 magnitudes fainter than the GCLF peaks in giant spiral and elliptical galaxies, but the results are consistent within the uncertainties. The bright-end slope of the luminosity distribution has a power-law form with slope alpha = -1.9 +/- 0.1. The trend of increasing S_N or T_MP with decreasing host galaxy luminosity is confirmed. The mean value for T_MP in dE,N galaxies is about a factor of two higher than the mean value for non-nucleated galaxies and the distributions of T_MP in dE,N and dE,noN galaxies are statistically different. These data are combined with results from the literature for a wide range of galaxy types and environments. At low host galaxy masses the distribution of T_MP for dE,noN and dI galaxies are similar. This supports the idea that one pathway for forming dE,noN galaxies is by the stripping of dIs. The formation of nuclei and the larger values of T_MP in dE,N galaxies may be due to higher star formation rates and star cluster formation efficiencies due to interactions in galaxy cluster environments.Comment: 53 pages, 13 figures, 12 tables, accepted by the Astrophysical Journa

Photometric properties of reionization-epoch galaxies in the Simba simulations

We study the photometric properties and sizes of the reionization-epoch galaxies in high-resolution Simba cosmological hydrodynamical simulations with box sizes of $[25,50]~h^{-1}$Mpc. Assuming various attenuation laws, we compute photometry by extincting each star particle's spectrum using the line-of-sight gas metal column density. The predicted ultraviolet luminosity function (UVLF) generally agrees with observations at $z=6$, owing to a partial cancellation between the high metallicities of the simulated galaxies and lower dust-to-metal ratios. The simulated $z=8$ UVLF is low compared to observations, likely owing to excessive dust extinction. Simba predicts UV continuum slopes ($\beta$) in agreement with the $z=6$ observations, with the best agreement obtained using a Calzetti extinction law. Interestingly, the gas-phase mass-metallicity relation in Simba is higher at $z\sim 6$ than at $z\sim 2$, suggesting that rapid early enrichment (and dust growth) might be necessary to match the observed $\beta$. We find that $\beta$ is more sensitive to the dust extinction law than the UVLF. By generating mock James Webb Space Telescope (JWST) images and analysing in a manner similar to observations, we show that Simba's galaxy size-luminosity relation well reproduces the current $z=6$ Hubble observations. Unlike observations at lower redshifts, Simba predicts similar rest-UV and rest-optical sizes of $z=6$ galaxies, owing to weak age gradients and dust extinction in star-forming regions counteract each other to weaken the color gradients within galaxies. These predictions will be testable with JWST.Comment: 15 pages, first revisio