In a six-year program started in July 2014, the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) will conduct novel cosmological observations using the BOSS spectrograph at Apache Point Observatory. These observations will be conducted simultaneously with the Time Domain Spectroscopic Survey (TDSS) designed for variability studies and the Spectroscopic Identification of eROSITA Sources (SPIDERS) program designed for studies of X-ray sources. In particular, eBOSS will measure with percent-level precision the distance-redshift relation with baryon acoustic oscillations (BAO) in the clustering of matter. eBOSS will use four different tracers of the underlying matter density field to vastly expand the volume covered by BOSS and map the large-scale-structures over the relatively unconstrained redshift range 0.6 < z < 2.2. Using more than 250,000 new, spectroscopically confirmed luminous red galaxies at a median redshift z = 0.72, we project that eBOSS will yield measurements of the angular diameter distance d A (z) to an accuracy of 1.2% and measurements of H(z) to 2.1% when combined with the z > 0.6 sample of BOSS galaxies. With ∼ 195, 000 new emission line galaxy redshifts, we expect BAO measurements of d A (z) to an accuracy of 3.1% and H(z) to 4.7% at an effective redshift of z = 0.87. A sample of more than 500,000 spectroscopically-confirmed quasars will provide the first BAO distance measurements over the redshift range 0.9 < z < 2.2, with expected precision of 2.8% and 4.2% on d A (z) and H(z), respectively. Finally, with 60,000 new quasars and reobservation of 60,000 BOSS quasars, we will obtain new Lyα forest measurements at redshifts z > 2.1; these new data will enhance the precision of d A (z) and H(z) at z > 2.1 by a factor of 1.44 relative to BOSS. Furthermore, eBOSS will provide improved tests of General Relativity on cosmological scales through redshift-space distortion (RSD) measurements, improved tests for non-Gaussianity in the primordial density field, and new constraints on the summed mass of all neutrino species. Here, we provide an overview of the cosmological goals, spectroscopic target sample, demonstration of spectral quality from early data, and projected cosmological constraints from eBOSS. eBOSS 3 confidence, where w is the ratio of pressure to energy density for dark energy. Thus, current observations are generally consistent with the simplest picture where dark energy is described completely by Einstein's cosmological constant (Λ).New precise observations can unravel the origin of the accelerating universe; specifically, to determine if cosmic acceleration is caused by deviations in General Relativity (GR) on large scales or by a new form of (dark) energy. It is possible to decouple scenarios of acceleration that require dark energy from those that require modifications to GR by independently probing both cosmic expansion history and the structure growth rate. Four primary observational techniques are generally accepted as the most powerful toward obtaining that goal (e.g. Albrech...

We study galaxy mergers using a high-resolution cosmological hydro/N-body simulation with star formation, and compare the measured merger timescales with theoretical predictions based on the Chandrasekhar formula. In contrast to Navarro et al., our numerical results indicate, that the commonly used equation for the merger timescale given by Lacey and Cole, systematically underestimates the merger timescales for minor mergers and overestimates those for major mergers. This behavior is partly explained by the poor performance of their expression for the Coulomb logarithm, ln(m pri /m sat ). The two alternative forms ln(1 + m pri /m sat ) and 1/2 ln[1 + (m pri /m sat ) 2 ] for the Coulomb logarithm can account for the mass dependence of merger timescale successfully, but both of them underestimate the merger time scale by a factor 2. Since ln(1 + m pri /m sat ) represents the mass dependence slightly better we adopt this expression for the Coulomb logarithm. Furthermore, we find that the dependence of the merger timescale on the circularity parameter ǫ is much weaker than the widely adopted power-law ǫ 0.78 , whereas 0.94ǫ 0.60 + 0.60 provides a good match to the data. Based on these findings, we present an accurate and convenient fitting formula for the merger timescale of galaxies in cold dark matter models.

Future weak lensing measurements of cosmic shear will reach such high accuracy that second order effects in weak lensing modeling, like the influence of baryons on structure formation, become important. We use a controlled set of high-resolution cosmological simulations to quantify this effect by comparing pure N-body dark matter runs with corresponding hydrodynamical simulations, carried out both in non-radiative, and in dissipative form with cooling and star formation. In both hydrodynamical simulations, the clustering of the gas is suppressed while that of dark matter is boosted at scales k > 1 hMpc −1. Despite this counterbalance between dark matter and gas, the clustering of the total matter is suppressed by up to 1 percent at 1 k 10 hMpc −1 , while for k ≈ 20 hMpc −1 it is boosted, up to 2 percent in the non-radiative run and 10 percent in the run with star formation. The stellar mass formed in the latter is highly biased relative to the dark matter in the pure N-body simulation. Using our power spectrum measurements to predict the effect of baryons on the weak lensing signal at 100 < l < 10000, we find that baryons may change the lensing power spectrum by less than 0.5 percent at l < 1000, but by 1 to 10 percent at 1000 < l < 10000. The size of the effect exceeds the predicted accuracy of future lensing power spectrum measurements and will likely be detected. Precise determinations of cosmological parameters with weak lensing, and studies of small-scale fluctuations and clustering, therefore rely on properly including baryonic physics.

Clusters, filaments, sheets and voids are the building blocks of the cosmic web. Forming dark matter halos respond to these different large-scale environments, and this in turn affects the properties of galaxies hosted by the halos. It is therefore important to understand the systematic correlations of halo properties with the morphology of the cosmic web, as this informs both about galaxy formation physics and possible systematics of weak lensing studies. In this study, we present and compare two distinct algorithms for finding cosmic filaments and sheets, a task which is far less well established than the identification of dark matter halos or voids. One method is based on the smoothed dark matter density field, the other uses the halo distributions directly. We apply both techniques to one high resolution N-body simulation and reconstruct the filamentary/sheet like network of the dark matter density field. We focus on investigating the properties of the dark matter halos inside these structures, in particular on the directions of their spins and the orientation of their shapes with respect to the directions of the filaments and sheets. We find that both the spin and the major axes of filament-halos with masses 10 13 h −1 M ⊙ are preferentially aligned with the direction of the filaments. The spins and major axes of halos in sheets tend to lie parallel to the sheets. There is an opposite mass dependence of the alignment strengths for the spin (negative) and major (positive) axes, i.e. with increasing halo mass the major axis tends to be more strongly aligned with the direction of the filament whereas the alignment between halo spin and filament becomes weaker with increasing halo mass. The alignment strengths as a function of distance to the most massive node halo indicate that there is a transit large scale environment impact: from the 2-D collapse phase of the filament to the 3-D collapse phase of the cluster/node halo at small separation. Overall, the two algorithms for filament/sheet identification investigated here agree well with each other. The method based on halos alone can be easily adapted for use with observational data sets. Subject headings: methods: data analysis -dark matter -large-scale structure of universe -galaxies: halos

Simulating the evolution of the local universe is important for studying galaxies and the intergalactic medium in a way free of cosmic variance. Here we present a method to reconstruct the initial linear density field from an input non-linear density field, employing the Hamiltonian Markov Chain Monte Carlo (HMC) algorithm combined with Particle Mesh (PM) dynamics. The HMC+PM method is applied to cosmological simulations, and the reconstructed linear density fields are then evolved to the present day with N -body simulations. The constrained simulations so obtained accurately reproduce both the amplitudes and phases of the input simulations at various z. Using a PM model with a grid cell size of 0.75 h −1 Mpc and 40 time-steps in the HMC can recover more than half of the phase information down to a scale k ∼ 0.85 hMpc −1 at high z and to k ∼ 3.4 hMpc −1 at z = 0, which represents a significant improvement over similar reconstruction models in the literature, and indicates that our model can reconstruct the formation histories of cosmic structures over a large dynamical range. Adopting PM models with higher spatial and temporal resolutions yields even better reconstructions, suggesting that our method is limited more by the availability of computer resource than by principle. Dynamic models of structure evolution adopted in many earlier investigations can induce non-Gaussianity in the reconstructed linear density field, which in turn can cause large systematic deviations in the predicted halo mass function. Such deviations are greatly reduced or absent in our reconstruction.Subject headings: dark matter -large-scale structure of the universe -galaxies: haloes -methods: statistical

The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a "non-interacting" warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array (VLA) HI 21-cm line maps. The visible material in the ring has an integrated luminosity ≤ 10 8 L ⊙ and its brightest part has a color R-I ∼ 0.9. All of these properties are consistent with the ring being a tidally-disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (∆V = 45 km s −1 ) to the giant spiral galaxy. This dwarf is seen at the tip of the HI warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered "non-interacting," but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies.

Massive spectroscopic redshift surveys open a promising window to accurately measure peculiar velocity at cosmological distances through redshift space distortion (RSD). In Paper I Zhang et al. [Phys. Rev. D 87, 063526 (2013)] of this series of work, we proposed decomposing peculiar velocity into three eigenmodes (v , v S , and v B ) in order to facilitate the RSD modeling and peculiar velocity reconstruction. In the current paper we measure the dark matter RSD-related statistics of the velocity eigenmodes through a set of N-body simulations. These statistics include the velocity power spectra, correlation functions, onepoint probability distribution functions, cumulants, and the damping functions describing the Finger of God effect. We have carried out a number of tests to quantify possible numerical artifacts in these measurements and have confirmed that these numerical artifacts are under control. Our major findings are as follows: (1) The power spectrum measurement shows that these velocity components have distinctly different spatial distribution and redshift evolution, consistent with predictions in Paper I. In particular, we measure the window functionWðk; zÞ.W describes the impact of nonlinear evolution on the v -density relation. We confirm that the approximationW ¼ 1 can induce a significant systematic error of Oð10%Þ in RSD cosmology. We demonstrate thatW can be accurately described by a simple fitting formula with one or two free parameters. (2) The correlation function measurement shows that the correlation length is Oð100Þ, Oð10Þ, and Oð1Þ Mpc for v , v S , and v B , respectively. These correlation lengths determine where we can treat the velocity fields as spatially uncorrelated. Hence, they are important properties in RSD modeling.(3) The velocity probability distribution functions and cumulants quantify non-Gaussianities of the velocity fields. We confirm speculation in Paper I that v is largely Gaussian, but with non-negligible non-Gaussianity. We confirm that v B is significantly non-Gaussian. We also measure the damping functions. Despite the observed non-Gaussianities, the damping functions and hence the Finger of God effect are all well approximated as Gaussian ones at scales of interest.

We study the dependence of the galaxy contents within halos on the halo formation time using two galaxy formation models, one being a semianalytic model utilizing the halo assembly history from a high resolution N-body simulation and the other being a smoothed particle hydrodynamics simulation including radiative cooling, star formation, and energy feedback from galactic winds. We confirm the finding by Gao et al. that at fixed mass, the clustering of halos depends on the halo formation time, especially for low-mass halos. This age dependence of halo clustering makes it desirable to study the correlation between the occupation of galaxies within halos and the halo age. We find that, in halos of fixed mass, the number of satellite galaxies has a strong dependence on halo age, with fewer satellites in older halos. The youngest one-third of the halos can have an order of magnitude more satellites than the oldest one-third. For central galaxies, in halos that form earlier, they tend to have more stars and thus appear to be more luminous, and the dependence of their luminosity on halo age is not as strong as that of stellar mass. The results can be understood through the star formation history in halos and the merging of satellites onto central galaxies. The age dependence of the galaxy contents within halos would constitute an important ingredient in a more accurate halo-based model of galaxy clustering.Comment: 4 pages, 2 figures, Accepted by ApJ Letters, emulateapj layout. Minor changes. Poisson errors added in Figure 1. We remove the last figure, which is available on http://bias.cosmo.fas.nyu.edu/galevolution/hod/f3.ep