We present a new implementation of star formation in cosmological simulations, by considering star clusters as a unit of star formation. Cluster particles grow in mass over several million years at the rate determined by local gas properties, with high time resolution. The particle growth is terminated by its own energy and momentum feedback on the interstellar medium. We test this implementation for Milky Way-sized galaxies at high redshift, by comparing the properties of model clusters with observations of young star clusters. We find that the cluster initial mass function is best described by a Schechter function rather than a single power law. In agreement with observations, at low masses the logarithmic slope is α ≈ 1.8 − 2, while the cutoff at high mass scales with the star formation rate. A related trend is a positive correlation between the surface density of star formation rate and fraction of stars contained in massive clusters. Both trends indicate that the formation of massive star clusters is preferred during bursts of star formation. These bursts are often associated with major merger events. We also find that the median timescale for cluster formation ranges from 0.5 to 4 Myr and decreases systematically with increasing star formation efficiency. Local variations in the gas density and cluster accretion rate naturally lead to the scatter of the overall formation efficiency by an order of magnitude, even when the instantaneous efficiency is kept constant. Comparison of the formation timescale with the observed age spread of young star clusters provides an additional important constraint on the modeling of star formation and feedback schemes.

Globular cluster (GC) systems around galaxies of a vast mass range show remarkably simple scaling relations. The combined mass of all GCs is a constant fraction of the total galaxy mass and the mean metallicity and metallicity dispersion of the GC system scale up weakly with galaxy mass. The metallicity of massive, metal-poor ("blue") clusters increases with cluster mass, while that of metal-rich ("red") clusters does not. A significant age-metallicity relation emerges from analysis of resolved stellar populations in Galactic GCs and unresolved populations in nearby galaxies. Remarkably, all these trends can be explained by a simple merger-based model developed in previous work and updated here using recent observations of galaxy scaling relations at high redshift.We show that the increasing dispersion of GC metallicity distributions with galaxy mass is a robust prediction of the model. It arises from more massive galaxies having more mergers that combine satellite GC systems. The average metallicity also increases by 0.6 dex over 3 dex in halo mass. The models show a non-linear trend between the GC system mass and host galaxy mass which is consistent with the data. The model does not consider GC self-enrichment, yet predicts a correlation between cluster mass and metallicity for massive blue clusters. The age-metallicity relation is another robust prediction of the model. Half of all clusters are predicted to form within the redshift range 5 < z < 2.3, corresponding to ages of 10.8 − 12.5 Gyr, in halos of masses 10 11 − 10 12.5 M .

We performed a comprehensive study of catalytic activities of subnanometer Au clusters supported on TiO2(110) surface (Aun/TiO2, n = 1-4, 7, 16-20) by means of density functional theory (DFT) calculations and microkinetics analysis. The creditability of the chosen DFT/microkienetics methodologies was demonstrated by the very good agreement between predicted catalytic activities with experimental measurement (J. Am. Chem. Soc, 2004, 126, 5682-5483) for the Au1-4/TiO2 and Au7/TiO2 benchmark systems. For the first time, the size- and shape-dependent catalytic activities of the subnanometer Au clusters (Au16-Au20) on TiO2 supports were systematically investigated. We found that catalytic activities of the Aun/TiO2 systems increase with the size n up to Au18, for which the hollow-cage Au18 isomer exhibits highest activity for the CO oxidation, with a reaction rate ∼30 times higher than that of Au7/TiO2 system. In stark contrast, the pyramidal isomer of Au18 exhibits much lower activity comparable to the Au3-4/TiO2 systems. Moreover, we found that the hollow-cage Au18 is robust upon the soft-landing with an impact velocity of 200 m/s to the TiO2 substrate, and also exhibits thermal stability upon CO and O2 co-adsorption. The larger pyramidal Au19 and Au20 clusters (on the TiO2 support) display much lower reaction rates than the pyramidal Au18. Results of rate of reactions for unsupported (gas-phase) and supported Au clusters can be correlated by a contour plot that illustrates the dependence of the reaction rates on the CO and O2 adsorption energies. With the TiO2 support, however, the catalytic activities can be greatly enhanced due to the weaker adsorption of CO on the TiO2 support than on the Au clusters, thereby not only the ratio of O2/CO adsorption energy and the probability for the O2 to occupy the Ti sites are increased but also the requirement for meeting the critical line becomes weaker. The obtained contour plot not only can provide guidance for the theoretical investigation of catalytic activity on other metal cluster/support systems, but also assist experimental design of optimal metal cluster/support systems to achieve higher catalytic efficiency.

The mass distribution and chemical composition of globular cluster (GC) systems preserve fossil record of the early stages of galaxy formation. The observed distribution of GC colors within massive early-type galaxies in the ACS Virgo Cluster Survey (ACSVCS) reveals a multi-modal shape, which likely corresponds to a multimodal metallicity distribution. We present a simple model for the formation and disruption of GCs that aims to match the ACSVCS data. This model tests the hypothesis that GCs are formed during major mergers of gas-rich galaxies and inherit the metallicity of their hosts. To trace merger events, we use halo merger trees extracted from a large cosmological N-body simulation. We select 20 halos in the mass range of 2 × 10 12 to 7 × 10 13 M and match them to 19 Virgo galaxies with K-band luminosity between 3 × 10 10 and 3 × 10 11 L . To set the [Fe/H] abundances, we use an empirical galaxy mass-metallicity relation. We find that a minimal merger ratio of 1:3 best matches the observed cluster metallicity distribution. A characteristic bimodal shape appears because metal-rich GCs are produced by late mergers between massive halos, while metal-poor GCs are produced by collective merger activities of less massive hosts at early times. The model outcome is robust to alternative prescriptions for cluster formation rate throughout cosmic time, but a gradual evolution of the massmetallicity relation with redshift appears to be necessary to match the observed cluster metallicities. We also affirm the age-metallicity relation, predicted by an earlier model, in which metal-rich clusters are systematically several billion years younger than their metal-poor counterparts. Keywords: galaxies: formation -galaxies: star clusters -globular clusters: general 1. INTRODUCTION Globular cluster (GC) systems have been found in various types of galaxies. Because of their old age and compact structure, GCs are believed to carry information on galaxy assembly history at early times (e.g., Brodie & Strader 2006). In particular, the colors and metallicities of GC systems provide a unique record of the early star formation and chemical enrichment in their host galaxies. One of the remaining puzzles is the origin of the commonly seen bimodal distribution of GC colors, within galaxies ranging from spirals to giant ellipticals. The bimodality in color is indicative of bimodality in metallicity, which has been used to separate GCs into two subpopulations: metal-poor and metal-rich (Harris 2001;Peng et al. 2006;Harris et al. 2006).In general, GCs have systematically lower metallicity than the field stars of their host galaxy. Therefore, they must have formed earlier than the bulk of stars, at least from the chemical evolution point of view. Early galaxies were smaller and less metal-enriched than those of today. Motivated by this fact, we test a hypothesis that major mergers of gas-rich galaxies (which happened more frequently at high redshift, in the hierarchical galaxy formation framework) are predominantly responsible for...