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The relationship between galaxy properties and environment is a widely discussed topic within astrophysics. Here, we use galaxy samples from hydrodynamical resimulations to examine this relationship. We use the overdensity (δ1)within a h-1Mpc sphere around a galaxy to evaluate its environment. Then, the relations between galaxy properties, such as specific star formation rate(sSFR), fraction of star-forming galaxies, g –r color, and δ1 are examined within three galactic samples formed from galaxies within large clusters, those in the vicinity of large clusters, and those in the field. We find tight environmental correlations for these galaxy properties. In brief, galaxies in denser environments tend to be redder and are more likely to be quenched; this is consistent with observations. We find that although the sSFR decreases with δ1, this is mainly because galaxies with higher stellar mass reside in an environment with higher overdensity. At fixed overdensity, a galaxy’s color is also independent of whether it lives within a cluster or within the field, but the relative fractions of the two samples varies dramatically with overdensity, and this drives an apparent evolution.

Cosmological simulations are the key tool for investigating the different processes involved in the formation of the universe from small initial density perturbations to galaxies and clusters of galaxies observed today. The identification and analysis of bound objects, halos, is one of the most important steps in drawing useful physical information from simulations. In the advent of larger and larger simulations, a reliable and parallel halo finder, able to cope with the ever-increasing data files, is a must. In this work we present the freely available MPI parallel halo finder AHF. We provide a description of the algorithm and the strategy followed to handle large simulation data. We also describe the parameters a user may choose in order to influence the process of halo finding, as well as pointing out which parameters are crucial to ensure untainted results from the parallel approach. Furthermore, we demonstrate the ability of AHF to scale to high resolution simulations., Comment: 18 pages, 18 figures. Accepted for publication in ApJS

The statistics of high speed satellite galaxies, as reported in the recent literature, can be a powerful diagnosis of the depth of the potential well of the host halo, and hence discriminate between competing gravitational theories. Naively one expects that high speed satellites are more common in Modified Newtonian Dynamics (MOND) than in cold dark matter (CDM) since an isolated MONDian system has an infinite potential well, while CDM halos have finite potential wells. In this \textit{Letter} we report on an initial test of this hypothesis in the context of the first generation of cosmological simulations utilising a rigorous MONDian Poisson solver. We find that such high speed encounters are approximately a factor of four more common in MOND than in the concordance $\Lambda$CDM model of cosmic structure formation., Comment: 4 pages. 2 figures. Accepted for publication in ApJL

We contend that a single power law halo mass distribution is appropriate for direct matching to the stellar masses of observed Local Group dwarf galaxies, allowing the determination of the slope of the stellar mass-halo mass relation for low mass galaxies. Errors in halo masses are well defined as the Poisson noise of simulated local group realisations, which we determine using constrained local universe simulations (CLUES). For the stellar mass range 10$^7$, Comment: ApJL Accepted version. Comments to cbabrook@gmail.com

The Astrophysical Journal 777.1 (2013): 51 reproduced by permission of the AAS, We present the results of a set of numerical simulations aimed at studying reionization at the galactic scale. We use a high-resolution realization of the formation of the Milky Way (MW)-M31 system to simulate the reionization of the Local Group. The reionization calculation was performed with the post-processing radiative transfer code ATON and the underlying cosmological simulation was performed as part of the CLUES project (http://www.clues-project.org). We vary the source models to bracket the range of source properties used in the literature. We investigate the structure and propagation of the galactic ionization fronts by a visual examination of our reionization maps. Within the progenitors, we find that reionization is patchy and proceeds locally inside-out. The process becomes patchier with decreasing source photon output. It is generally dominated by one major H II region and one to four additional isolated smaller bubbles, which eventually overlap. Higher emissivity results in faster and earlier local reionization. In all models, the reionization of the MW and M31 are similar in duration, i.e., between 203 Myr and 22 Myr depending on the source model, placing their z reion between 8.4 and 13.7. In all models except the most extreme, the MW and M31 progenitors reionize internally, ignoring each other despite being relatively close to each other, even during the epoch of reionization. Only in the case of strong supernova feedback suppressing star formation in halos less massive than 109 M ⊙, and using our highest emissivity, do we find that the MW is reionized by M31, A.K. is supported by the Spanish Ministerio de Ciencia e Innovación (MICINN) in Spain through the Ramón y Cajal program as well as the grants CSD2009-00064 and CAM S2009/ESP-1496 and the Ministerio de Economía y Competitividad (MINECO) through grant AYA2012-31101. He further thanks Peter Thomas for angels who burn their wings. G.Y. acknowledges support from MICINN under research grants AYA2009-13875-C03-02, FPA2009-08958

Astrophysical Journal Letters 766.2 (2013) L15 reproduced by permission of the AAS, In the standard model of cosmology, structure emerges out of a non-rotational flow and the angular momentum of collapsing halos is induced by tidal torques. The growth of angular momentum in the linear and quasi-linear phases is associated with a shear, curl-free, flow and it is well described within the linear framework of tidal torque theory (TTT). However, TTT ceases to be applicable as halos approach turnaround when their ambient flow field becomes rotational. Subsequently, halos become embedded in a vortical flow field and the growth of their angular momentum is affected by the vorticity of their ambient velocity field. Using a cosmological simulation, we have examined the importance of the curl of the velocity field in determining halo spin, finding a significant alignment between the two: the vorticity tends to be perpendicular to the axis of the fastest collapse of the velocity shear tensor (e 1). This is independent of halo masses and cosmic web environment. Our results agree with previous findings on the tendency of halo spin to be perpendicular to e 1, and of the spin of (simulated) halos and (observed) galaxies to be aligned with the large-scale structure. It follows that angular momentum growth proceeds in two distinct phases. First, the angular momentum emerges out of a shear, curl-free, potential flow, as described by TTT. In the second phase, in which halos approach virialization, the angular momentum emerges out of a vortical flow and halo spin becomes partially aligned with the vorticity of the ambient flow field.