We study dark matter halo density profiles in a high‐resolution N‐body simulation of a ΛCDM cosmology. Our statistical sample contains ∼5000 haloes in the range 1011–1014 h−1 M⊙, and the resolution allows a study of subhaloes inside host haloes. The profiles are parametrized by an NFW form with two parameters, an inner radius rs and a virial radius Rvir, and we define the halo concentration cvir≡Rvirrs. First, we find that, for a given halo mass, the redshift dependence of the median concentration is cvir∝(1+z)−1. This corresponds to rs(z)∼constant, and is contrary to earlier suspicions that cvir does not vary much with redshift. The implications are that high‐redshift galaxies are predicted to be more extended and dimmer than expected before. Secondly, we find that the scatter in halo profiles is large, with a 1σΔ(log cvir)=0.18 at a given mass, corresponding to a scatter in maximum rotation velocities of ΔVmaxVmax=0.12. We discuss implications for modelling the Tully–Fisher relation, which has a smaller reported intrinsic scatter. Thirdly, subhaloes and haloes in dense environments tend to be more concentrated than isolated haloes, and show a larger scatter. These results suggest that cvir is an essential parameter for the theory of galaxy modelling, and we briefly discuss implications for the universality of the Tully–Fisher relation, the formation of low surface brightness galaxies, and the origin of the Hubble sequence. We present an improved analytic treatment of halo formation that fits the measured relations between halo parameters and their redshift dependence, and can thus serve semi‐analytic studies of galaxy formation.

We address the origin of the robust bimodality observed in galaxy properties about a characteristic stellar mass ∼3 × 10 10 M . Less massive galaxies tend to be ungrouped blue star forming discs, while more massive galaxies are typically grouped red old-star spheroids. Colour-magnitude data show a gap between the red and blue sequences, extremely red luminous galaxies already at z ∼ 1, a truncation of today's blue sequence above L * , and massive starbursts at z ∼ 2-4. We propose that these features are driven by the thermal properties of the inflowing gas and their interplay with the clustering and feedback processes, all functions of the dark matter halo mass and associated with a similar characteristic scale. In haloes below a critical shock-heating mass M shock 10 12 M , discs are built by cold streams, not heated by a virial shock, yielding efficient early star formation. It is regulated by supernova feedback into a long sequence of bursts in blue galaxies constrained to a 'fundamental line'. Cold streams penetrating through hot media in M M shock haloes preferentially at z 2 lead to massive starbursts in L > L * galaxies. At z < 2, in M > M shock haloes hosting groups, the gas is heated by a virial shock, and being dilute it becomes vulnerable to feedback from energetic sources such as active galactic nuclei. This shuts off gas supply and prevents further star formation, leading by passive evolution to 'red-and-dead' massive spheroids starting at z ∼ 1. A minimum in feedback efficiency near M shock explains the observed minimum in M/L and the qualitative features of the star formation history. The cold flows provide a hint for solving the angular momentum problem. When these processes are incorporated in simulations they recover the main bimodality features and solve other open puzzles.

Massive galaxies in the young Universe, ten billion years ago, formed stars at surprising intensities. Although this is commonly attributed to violent mergers, the properties of many of these galaxies are incompatible with such events, showing gas-rich, clumpy, extended rotating disks not dominated by spheroids. Cosmological simulations and clustering theory are used to explore how these galaxies acquired their gas. Here we report that they are 'stream-fed galaxies', formed from steady, narrow, cold gas streams that penetrate the shock-heated media of massive dark matter haloes. A comparison with the observed abundance of star-forming galaxies implies that most of the input gas must rapidly convert to stars. One-third of the stream mass is in gas clumps leading to mergers of mass ratio greater than 1:10, and the rest is in smoother flows. With a merger duty cycle of 0.1, three-quarters of the galaxies forming stars at a given rate are fed by smooth streams. The rarer, submillimetre galaxies that form stars even more intensely are largely merger-induced starbursts. Unlike destructive mergers, the streams are likely to keep the rotating disk configuration intact, although turbulent and broken into giant star-forming clumps that merge into a central spheroid. This stream-driven scenario for the formation of discs and spheroids is an alternative to the merger picture.

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The Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) is designed to document the first third of galactic evolution, over the approximate redshift (z) range 8-1.5. It will image >250,000 distant galaxies using three separate cameras on the Hubble Space Telescope, from the mid-ultraviolet to the near-infrared, and will find and measure Type Ia supernovae at z > 1.5 to test their accuracy as standardizable candles for cosmology. Five premier multi-wavelength sky regions are selected, each with extensive ancillary data. The use of five widely separated fields mitigates cosmic variance and yields statistically robust and complete samples of galaxies down to a stellar mass of 10 9 M to z ≈ 2, reaching the knee of the ultraviolet luminosity function of galaxies to z ≈ 8. The survey covers approximately 800 arcmin 2 and is divided into two parts. The CANDELS/Deep survey (5σ point-source limit H = 27.7 mag) covers ∼125 arcmin 2 within Great Observatories Origins Deep Survey (GOODS)-N and GOODS-S. The CANDELS/Wide survey includes GOODS and three additional fields (Extended Groth Strip, COSMOS, and Ultra-deep Survey) and covers the full area to a 5σ pointsource limit of H 27.0 mag. Together with the Hubble Ultra Deep Fields, the strategy creates a three-tiered "wedding-cake" approach that has proven efficient for extragalactic surveys. Data from the survey are nonproprietary and are useful for a wide variety of science investigations. In this paper, we describe the basic motivations for the survey, the CANDELS team science goals and the resulting observational requirements, the field selection and geometry, and the observing design. The Hubble data processing and products are described in a companion paper.

We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the ΛCDM cosmology. For each halo at z = 0, we identify its merger-history tree, and determine concentration parameters c vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a c , defined when the log mass accretion rate d log M/d log a falls below a critical value S. We find that late forming galaxies tend to be less concentrated, such that c vir "observed" at any epoch a o is strongly correlated with a c via c vir = c 1 a o /a c . Scatter about this relation is mostly due to measurement errors in c vir and a c , implying that the actual spread in c vir for halos of a given mass can be mostly attributed to scatter in a c . We demonstrate that this relation can also be used to predict the mass and redshift dependence of c vir , and the scatter about the median c vir (M, z), using accretion histories derived from the Extended Press-Schechter (EPS) formalism, after adjusting for a constant offset between the formation times as predicted by EPS and as measured in the simulations; this new ingredient can thus be easily incorporated into semi-analytic models of galaxy formation. The correlation found between halo concentration and mass accretion rate suggests a physical interpretation: for high mass infall rates the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant and the halo concentration just grows as R vir . Because of the direct connection between halo concentration and velocity rotation curves, and because of probable connections between halo mass assembly history and star formation history, the tight correlation between these properties provides an essential new ingredient for galaxy formation modeling.

We study the angular-momentum profiles of a statistical sample of halos drawn from a high-resolution N -body simulation of the ΛCDM cosmology. We find that the cumulative mass distribution of specific angular momentum j in a halo of mass M v is well fit by a universal function, M (< j) = M v µj/(j 0 + j). This profile is defined by one shape parameter (µ or j 0 ) in addition to the global spin parameter λ. It follows a power-law M (< j) ∝ j over most of the mass, and flattens at large j, with the flattening more pronounced for small values of µ (or large j 0 at a fixed λ). Compared to a uniform sphere in solid-body rotation, most halos have a higher fraction of their mass in the low-and high-j tails of the distribution. High-λ halos tend to have high µ values, corresponding to a narrower, more uniform j distribution. The spatial distribution of angular momentum in halos tends to be cylindrical and is well-aligned within each halo for ∼ 80% of the halos. The more misaligned halos tend to have low-µ values. When averaged over spherical shells encompassing mass M , the halo j profiles are fit by j(M ) ∝ M s with s = 1.3 ± 0.3. We investigate two ideas for the origin of this profile. The first is based on a revised version of linear tidal-torque theory combined with extended Press-Schechter mass accretion, and the second focuses on j transport in minor mergers.Finally, we briefly explore implications of the M (< j) profile on the formation of galactic disks assuming that j is conserved during an adiabatic baryonic infall. The implied gas density profile deviates from an exponential disk, with a higher density at small radii and a tail extending to large radii. The steep central density profiles may imply disk scale lengths that are smaller than observed. This is reminiscent of the "angular-momentum problem" seen in hydrodynamic simulations, even though we have assumed perfect j conservation. A possible solution is to associate the central excesses with bulge components and the outer regions with extended gaseous disks.

We investigate the conditions for the existence of an expanding virial shock in the gas falling within a spherical dark-matter halo. The shock relies on pressure support by the shock-heated gas behind it. When the radiative cooling is efficient compared to the infall rate the post-shock gas becomes unstable; it collapses inwards and cannot support the shock. We find for a monoatomic gas that the shock is stable when the post-shock pressure and density obey gamma effective>10/7, with gamma effective begin the time depended equivalent to the adiabatic index. We express the effective gamma in terms of r, u and rho at the shock to obtain a simple condition for shock stability. This result is confirmed by hydrodynamical simulations, using an accurate spheri-symmetric Lagrangian code. When the stability analysis is applied in cosmology, we find that a virial shock does not develop in most haloes that form before z ~ 2, and it never forms in haloes less massive than a few 10^11 solar masses. In such haloes the infalling gas is never heated to the virial temperature, and it does not need to cool radiatively before it drops into a disc. Instead, the gas collapses at T ~ 10^4K directly into the disc. This should have nontrivial effects on the star-formation rate and on the gas removal by supernova-driven winds. Instead of radiating soft x rays, this gas would emit lyman alpha thus helping explain the low flux of soft x-ray background and the lyman alpha emitters observed at high redshift.Comment: 17 pages, 13 eps figs, latex, mn2e, accepted by MNRAS - minor additions to the discussio