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We use two cosmological simulations of structure formation to study the conditions under which dark matter haloes emerge from the linear density field. Our analysis focuses on matching sites of halo collapse to local density maxima, or "peaks", in the initial conditions of the simulations and provides a crucial test of the central ansatz of the peaks formalism. By identifying peaks on a variety of smoothed, linearly extrapolated density fields we demonstrate that as many as ~70% of well-resolved dark matter haloes form preferentially near peaks whose characteristic masses are similar to that of the halo, with more massive haloes showing a stronger tendency to reside near peaks initially. We identify a small but significant fraction of haloes that appear to evolve from peaks of substantially lower mass than that of the halo itself. We refer to these as "peakless haloes" for convenience. By contrasting directly the properties of these objects with the bulk of the proto-halo population we find two clear differences: 1) their initial shapes are significantly flatter and more elongated than the predominantly triaxial majority, and 2) they are, on average, more strongly compressed by tidal forces associated with their surrounding large scale structure. Using the two-point correlation function we show that peakless haloes tend to emerge from highly clustered regions of the initial density field implying that, at fixed mass, the accretion geometry and mass accretion histories of haloes in highly clustered environments differ significantly from those in the field. This may have important implications for understanding the origin of the halo assembly bias, of galaxy properties in dense environments and how environment affects the morphological transformation of galaxies near groups and rich galaxy clusters., Comment: 13 pages, 11 figures, published in MNRAS

We present a new three-dimensional radiative transfer (RT) code, RADAMESH, based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. RADAMESH uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a "cell-by-cell" basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a "classical" Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, RADAMESH is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (HI, HII, HeI, HeII, HeIII, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT Code Comparison Project and a new set of tests aimed at substantiating the new characteristics of RADAMESH. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during Reionization produces a large increase of the predicted gas temperature within the whole HII region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Stroemgren sphere test) with respect to the widely used "on-the-spot" approximation., Comment: 19 pages, 24 figures; accepted for publication in MNRAS, version with high-resolution figures is avalaible at http://www.ast.cam.ac.uk/~cantal/Papers/CP10.pdf

We study structure formation in the presence of primordial non-Gaussianity of the local type with parameters f_NL and g_NL. We show that the distribution of dark-matter halos is naturally described by a multivariate bias scheme where the halo overdensity depends not only on the underlying matter density fluctuation delta, but also on the Gaussian part of the primordial gravitational potential phi. This corresponds to a non-local bias scheme in terms of delta only. We derive the coefficients of the bias expansion as a function of the halo mass by applying the peak-background split to common parametrizations for the halo mass function in the non-Gaussian scenario. We then compute the halo power spectrum and halo-matter cross spectrum in the framework of Eulerian perturbation theory up to third order. Comparing our results against N-body simulations, we find that our model accurately describes the numerical data for wavenumbers k < 0.1-0.3 h/Mpc depending on redshift and halo mass. In our multivariate approach, perturbations in the halo counts trace phi on large scales and this explains why the halo and matter power spectra show different asymptotic trends for k -> 0. This strongly scale-dependent bias originates from terms at leading order in our expansion. This is different from what happens using the standard univariate local bias where the scale-dependent terms come from badly behaved higher-order corrections. On the other hand, our biasing scheme reduces to the usual local bias on smaller scales where |phi| is typically much smaller than the density perturbations. We finally discuss the halo bispectrum in the context of multivariate biasing and show that, due to its strong scale and shape dependence, it is a powerful tool for the detection of primordial non-Gaussianity from future galaxy surveys., Comment: 26 pages, 16 figures. Minor modifications, version accepted by Phys. Rev. D

New statistical properties of dark matter halos in Lagrangian space are presented. Tracing back the dark matter particles constituting bound halos resolved in a series of N-body simulations, we measure quantitatively the correlations of the proto-halo's inertia tensors with the local tidal tensors and investigate how the correlation strength depends on the proto-halo's sphericity, local density and filtering scale. It is shown that the majority of the proto-halos exhibit strong correlations between the two tensors provided that the tidal field is smoothed on the proto-halo's mass scale. The correlation strength is found to increase as the proto-halo's sphericity increases, as the proto-halo's mass increases, and as the local density becomes close to the critical value, delta_{ec}. It is also found that those peculiar proto-halos which exhibit exceptionally weak correlations between the two tensors tend to acquire higher specific angular momentum in Eulerian space, which is consistent with the linear tidal torque theory. In the light of our results, it is intriguing to speculate a hypothesis that the low surface brightness galaxies observed at present epoch correspond to the peculiar proto-halos with extreme low-sphericity whose inertia tensors are weakly correlated with the local tidal tensors., Comment: ApJ in press, accepted version, 20 pages, 8 figures, discussion on LSBGs improved, particle distribution of dark halos in Lagrangian space shown

Galaxies on the largest scales of the Universe are observed to be embedded in the filamentary cosmic web which is shaped by the nonlinear tidal field. As an efficient tool to quantitatively describe the statistics of this cosmic web, we present the anisotropic two-point correlation functions of the nonlinear traceless tidal field in the principal-axis frame, which are measured using numerical data from an N-body simulation. We show that both of the nonlinear density and traceless tidal fields are more strongly correlated along the directions perpendicular to the eigenvectors associated with the largest eigenvalues of the local tidal field. The correlation length scale of the traceless tidal field is found to be ~20 Mpc/h, which is much larger than that of the density field ~5 Mpc/h. We also provide analytic fitting formulae for the anisotropic correlation functions of the traceless tidal field, which turn out to be in excellent agreement with the numerical results. We expect that our numerical results and analytic formula are useful to disentangle cosmological information from the filamentary network of the large-scale structures., Comment: ApJ in press, accepted version, minor changes, discussion improved

We perform a series of high-resolution N-body simulations of cosmological structure formation starting from Gaussian and non-Gaussian initial conditions. We adopt the best-fitting cosmological parameters of WMAP (3rd- and 5th-year) and we consider non-Gaussianity of the local type parameterised by 8 different values of the non-linearity parameter F_NL. Building upon previous work based on the Gaussian case, we show that, expressed in terms of suitable variables, the mass function of friends-of-friends haloes is approximately universal (i.e. independent of redshift, cosmology, and matter transfer function) to good precision (nearly 10 per cent) also in non-Gaussian scenarios. We provide fitting formulae for the high-mass end (M>10^13 M_sol/h) of the universal mass function in terms of F_NL, and we also present a non-universal fit in terms of both F_NL and z to be used for applications requiring higher accuracy. In the Gaussian case, we extend our fit to a wider range of halo masses (M>2.4 x 10^10 M_sol/h) and we also provide a consistent fit of the linear halo bias. We show that, for realistic values of F_NL, the matter power-spectrum in non-Gaussian cosmologies departs from the Gaussian one by up to 2 per cent on the scales where the baryonic- oscillation features are imprinted on the 2-point statistics. We confirm the strong k-dependence of the halo bias on large scales (k<0.05 h Mpc^-1) which was already detected in previous studies. However, we find that commonly used parameterisations based on the peak-background split do not provide an accurate description of our simulations which present extra dependencies on the wavenumber, the non-linearity parameter and, possibly, the clustering strength. We provide an accurate fit of the simulation data that can be used as a benchmark for future determinations of F_NL with galaxy surveys., Comment: Comparisons with theoretical models and new results added

We explore a possible origin for the puzzling anti-correlation between the formation epoch of galactic dark-matter haloes and their environment density. This correlation has been revealed from cosmological N-body simulations and is in conflict with the Extended Press-Schechter model of halo clustering. Using similar simulations, we first quantify the straightforward association of an early formation epoch with a reduced mass growth rate at late times. We then find that a primary driver of suppressed growth, by accretion and mergers, is tidal effects dominated by a neighbouring massive halo. The tidal effects range from a slowdown of the assembly of haloes due to the shear along the large-scale filaments that feed the massive halo to actual mass loss in haloes that pass through the massive halo. Using the restricted three-body problem, we show that haloes are prone to tidal mass loss within 1.5 virial radii of a larger halo. Our results suggest that the dependence of formation epoch on environment density is a secondary effect induced by the enhanced density of haloes in filaments near massive haloes where the tides are strong. Our measures of assembly rate are particularly correlated with the tidal field at high redshifts z~1., Comment: 16 pages, 13 figures, MNRAS accepted. significantly expanded discussion of the connection of tides, shear and assembly. Version with high-resolution figures available at http://www.exp-astro.phys.ethz.ch/hahn/pub/HPDC09.pdf

Despite a history that dates back at least a quarter of a century studies of voids in the large--scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void--finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi--analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than $m_B = -20 $ is found to be smaller than about -0.8 by all our void finding algorithms., Comment: 14 pages, version accepted by MNRAS

We present a new method to directly map the neutral-hydrogen distribution during the reionization epoch and to constrain the emission properties of the highest-redshift quasars (QSOs). As a tracer of HI, we propose to use the Ly-alpha radiation produced by quasar ionization fronts (I-fronts) that expand in the partially ionized intergalactic medium (IGM) before reionization is complete. These Ly-alpha photons are mainly generated by collisional excitations of hydrogen atoms in the boundary of the rapidly expanding HII region. The observable signal is produced by the part of the I-front that lies behind the QSO with respect to the observer. Combining two radiative transfer models (one for the QSO ionizing radiation and one for the Ly-alpha photons), we estimate the expected Ly-alpha spectral shape and surface brightness (SB) for a large number of configurations where we varied both the properties of the ionizing QSO and of the surrounding medium. We find that the expected signal is observable as a single (broad) emission line with a characteristic width of 100-200 km/s. The expected SB produced at redshift z~6.5 within a fully neutral region (at mean density) by a typical QSO I-front lies in the range $10^{-21}-10^{-20}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ and decreases proportionally to $(1+z)^2$ for a given QSO age. QSOs with harder spectra may produce a significantly brighter emission at early phases. The signal may cover up to a few hundred square arcmin on the sky and should be already detectable with current facilities by means of moderate/high resolution spectroscopy. The detection of this Ly-alpha emission can shed new light on the reionization history, the age and the emission properties of the highest-redshift QSOs. (abridged), Comment: 12 pages, 9 figures, accepted for publication in ApJ

We use a series of N-body simulations of the LCDM cosmology to investigate the redshift evolution since z=1 of the properties and alignment with the large-scale structure of haloes in clusters, filaments, sheets and voids. We find that: (i) Once a rescaling of the halo mass with M*, the mass scale collapsing at redshift z, is performed, there is no further significant redshift dependence in the halo properties; (ii) The environment influences halo shape and formation time at all investigated redshifts for haloes with masses MM* in filaments spin more rapidly than similar mass haloes in clusters; haloes in voids have the lowest median spin parameters; c) Haloes with MM* perpendicular to it. For masses M>M*, the major axis of haloes in filaments and sheets is strongly aligned with the filament or the sheet. Such halo-LSS alignments may be of importance in weak lensing analyses of cosmic shear. A question that is opened by our study is why, in the 0 < z < 1 redshift regime that we have investigated, the mass scale M* sets roughly the threshold below which the LSS-environment either begins to affect, or reverses, the properties of dark matter haloes., Comment: 12 pages, 12 figures. Version accepted for publication in MNRAS. Version with high-resolution figures available at http://www.exp-astro.phys.ethz.ch/hahn/pub/HCPD07.pdf

The incidence of strong MgII systems in gamma-ray burst (GRB) spectra is a few times higher than in quasar (QSO) spectra. We investigate several possible explanations for this effect, including: dust obscuration bias, clustering of the absorbers, different beam sizes of the sources, multiband magnification bias of GRBs, association of the absorbers with the GRB event or the circumburst environment. We find that: i) the incidence rate of MgII systems in QSO spectra could be underestimated by a factor 1.3-2 due to dust obscuration; ii) the equivalent-width distribution of the MgII absorbers along GRBs is consistent with that observed along QSOs thus suggesting that the absorbers are more extended than the beam sizes of the sources; iii) on average, GRB afterglows showing more than one MgII system are a factor of 1.7 brighter than the others, suggesting a lensing origin of the observed discrepancy; iv) gravitational lensing (in different forms, from galaxy lensing to microlensing) can bias high the counts of MgII systems along GRBs if the luminosity functions of the prompt gamma-ray emission and of the optical afterglows have a mean faint-end slope approaching -5/3 -- -2; v) some of the absorbers can be associated with the circumburst environment or produced by supernova remnants unrelated to the GRB event itself but lying in the same star-forming region. With the possible exception of magnification bias, it is unlikely that one of these effects on its own can fully account for the observed counts. However, the combined action of some of them can substantially reduce the statistical significance of the discrepancy., Comment: 8 pages, 3 figures, ApJ in press, comments welcome

We report the results of a survey for fluorescent Ly-alpha emission carried out in the field surrounding the z=3.1 quasar QSO0420-388 using the FORS2 instrument on the VLT. We first review the properties expected for fluorescent Ly-alpha emitters, compared with those of other non-fluorescent Ly-alpha emitters. Our observational search detected 13 Ly-alpha sources sparsely sampling a volume of ~14000 comoving Mpc^3 around the quasar. The properties of these in terms of i) the line equivalent width, ii) the line profile and iii) the value of the surface brightness related to the distance from the quasar, all suggest that several of these may be plausibly fluorescent. Moreover, their number is in good agreement with the expectation from theoretical models. One of the best candidates for fluorescence is sufficiently far behind QSO0420-388 that it would imply that the quasar has been active for (at least) ~60 Myrs. Further studies on such objects will give information about proto-galactic clouds and on the radiative history (and beaming) of the high-redshift quasars., Comment: 10 pages, 4 figures.Update to match the version published on ApJ 657, 135, 2007 March 1

Brightness-temperature fluctuations in the redshifted 21-cm background from the cosmic dark ages are generated by irregularities in the gas-density distribution and can then be used to determine the statistical properties of density fluctuations in the early Universe. We first derive the most general expansion of brightness-temperature fluctuations up to second order in terms of all the possible sources of spatial fluctuations. We then focus on the three-point statistics and compute the angular bispectrum of brightness-temperature fluctuations generated prior to the epoch of hydrogen reionization. For simplicity, we neglect redshift-space distortions. We find that low-frequency radio experiments with arcmin angular resolution can easily detect non-Gaussianity produced by non-linear gravity with high signal-to-noise ratio. The bispectrum thus provides a unique test of the gravitational instability scenario for structure formation, and can be used to measure the cosmological parameters. Detecting the signature of primordial non-Gaussianity produced during or right after an inflationary period is more challenging but still possible. An ideal experiment limited by cosmic variance only and with an angular resolution of a few arcsec has the potential to detect primordial non-Gaussianity with a non-linearity parameter of f_NL ~ 1. Additional sources of error as weak lensing and an imperfect foreground subtraction could severely hamper the detection of primordial non-Gaussianity which will benefit from the use of optimal estimators combined with tomographic techniques., Comment: 15 pages, 4 figures, revised version accepted for publication in ApJ (contains an improved discussion of gas temperature fluctuations)

Using a series of high-resolution N-body simulations of the concordance cosmology we investigate how the formation histories, shapes and angular momenta of dark-matter haloes depend on environment. We first present a classification scheme that allows to distinguish between haloes in clusters, filaments, sheets and voids in the large-scale distribution of matter. This method is based on a local-stability criterion for the orbits of test particles and closely relates to the Zel'dovich approximation. Applying this scheme to our simulations we then find that: i) Mass assembly histories and formation redshifts strongly depend on environment for haloes of mass M

We present detailed clustering measurements for a flux limited sample of 14,000 quasars extracted from the 2dF QSO Redshift Survey (2QZ) in the redshift range 0.81.3. On the other hand, a number of tests based on information theory and Bayesian statistics show only marginal evidence for luminosity dependent clustering. Anyway, the quality of the data is not good enough to accurately quantify how quasar biasing depends on luminosity. We critically discuss the limitations of our dataset and show that a much larger sample is needed to rule out current models for luminosity segregation. Studying the evolution of the clustering amplitude with redshift, we detect an increase of the quasar correlation length with lookback time at the 99.3 per cent confidence level. Adopting the concordance cosmological model, we discuss the evolution of quasar biasing with cosmic epoch and show that quasars are typically hosted by dark matter haloes with mass ~10^{13} M_sol., Comment: MNRAS in press, 12 pages, 5 figures

Theoretical and observational arguments suggest that the intergalactic medium (IGM) might have been polluted with metals produced by early star formation. In this scenario, Lyman-break galaxies (LBGs) at redshift z=3 are likely to be surrounded by old metal bubbles, relics of an era when the characteristic mass of galaxies was small and gas retainment more difficult. We find that pregalactic enrichment of the IGM from 10^8-10^{10} M_sun dwarf galaxies at 6

We combine a high-resolution hydro-simulation of the LambdaCDM cosmology with two radiative transfer schemes (for continuum and line radiation) to predict the properties, spectra and spatial distribution of fluorescent Ly-alpha emission at z~3. We focus on line radiation produced by recombinations in the dense intergalactic medium ionized by UV photons. In particular, we consider both a uniform background and the case where gas clouds are illuminated by a nearby quasar. We find that the emission from optically thick regions is substantially less than predicted from the widely used static, plane-parallel model. The effects induced by a realistic velocity field and by the complex geometric structure of the emitting regions are discussed in detail. We make predictions for the expected brightness and size distributions of the fluorescent sources.Our results account for recent null detections and can be used to plan new observational campaigns both in the field (to measure the intensity of the diffuse UV background) and in the proximity of bright quasars (to understand the origin of high colum-density absorbers)., Comment: 15 pages, 12 figures, accepted for publication in ApJ

We present detailed clustering measurements from the 2dF QSO Redshift Survey (2QZ) in the redshift range 0.810^{12} M_sun and that the mean mass of their host haloes is of the order of 10^{13} M_sun. The observed clustering is consistent with assuming that the locally observed correlation between black-hole mass and host-galaxy circular velocity is still valid at z>1. From the fraction of haloes which contain active quasars, we infer that the characteristic quasar lifetime is t_Q ~ a few x 10^7 yr at z~1 and approaches 10^8 yr at higher redshifts., Comment: 23 pages,11 figures, accepted for publication in MNRAS

We estimate energy spectra and fluxes at the Earth's surface of the cosmic and Galactic neutrino backgrounds produced by thermonuclear reactions in stars. The extra-galactic component is obtained by combining the most recent estimates of the cosmic star formation history and the stellar initial mass function with accurate theoretical predictions of the neutrino yields all over the thermonuclear lifetime of stars of different masses. Models of the structure and evolution of the Milky Way are used to derive maps of the expected flux generated by Galactic sources as a function of sky direction. The predicted neutrino backgrounds depend only slightly on model parameters. In the relevant 50 keV-10 MeV window, the total flux of cosmic neutrinos ranges between 20 and 65 particles per square cm per s. Neutrinos reaching the Earth today have been typically emitted at redshift z~2. Their energy spectrum peaks at E~0.1-0.3 MeV. The energy and entropy densities of the cosmic background are negligible with respect to the thermal contribution of relic neutrinos originated in the early universe. In every sky direction, the cosmic background is outnumbered by the Galactic one, whose integrated flux amounts to 300-1000 particles per square cm per s. The emission from stars in the Galactic disk contributes more than 95 per cent of the signal., Comment: 30 pages, 8 figures, submitted to Astroparticle Physics, version with higher quality figures available at http://www.exp-astro.phys.ethz.ch/porciani/neutrino.pdf

We use the clustering results obtained by Madgwick et al. (2003) for a sample of 96,791 2dF galaxies with redshift 0.01 < z < 0.15 to study the distribution of late-type and early-type galaxies within dark matter haloes of different mass. The adopted method relies on the connection between the distribution of sources within haloes and their clustering properties by focussing on the issue of the halo occupation function i.e. the probability distribution of the number of galaxies brighter than some luminosity threshold hosted by a virialized halo of given mass. Within this framework, the distribution of galaxies within haloes is shown to determine galaxy-galaxy clustering on small scales, being responsible for the observed power-law behaviour at separations r < 3 Mpc. For a more extended analysis, we refer the reader to Magliocchetti & Porciani (2003), Comment: 4 pages, 2 figures, proceedings of the conference on Multi-Wavelength Cosmology (Mykonos, June 17-20, 2003)

We study the evolution of galaxy clustering in various cosmological models with quintessence. We investigate how the analytical predictions vary with change of dark energy equation of state $w_X$. Comparing these predictions against available data we discuss to what extent the problems of galaxy biasing can be modelled. This will be key in constraining the dark energy equation of state with future galaxy surveys. We use a compilation of various surveys to study the number density and amplitude of galaxy clustering from observations of the local universe at $z \sim 0$ to that of the Lyman break galaxies and Ly-$\alpha$ emitters at $z \sim 4.9$. We find that there is a degeneracy between the dark energy equation of state and the way galaxies populate dark matter haloes; objects are more biased in models with more negative values of dark energy equation of state $w_X$. We conclude that, while future all sky CMB observations will determine cosmological parameters with unprecedented precision, and cross correlation of weak lensing experiments and galaxy surveys will provide a cleaner and accurate picture of bias associated with collapsed objects, the rate of growth of large scale structure in such surveys can potentially constrain the equation of state of dark energy and the potential of the scalar field associated with quintessence. In particular, we show that the abundance and spatial distribution of galaxy clusters at intermediate redshifts strongly depend on the dark energy equation of state. When accurate measurement of galaxy clustering at high-redshit becomes possible, it will provide constraints on dark energy that are independent and complementary to type Ia supernova studies., Comment: 12 pages, 7 figures. Slightly modified version published in MNRAS

We present a new measure of the angular two-point correlation function of Lyman-break galaxies (LBGs) at z~3, obtained from the variance of galaxy counts in 2-dimensional cells. By avoiding binning of the angular separations, this method is significantly less affected by shot noise than traditional measures, and allows for a more accurate determination of the correlation function. We used a sample of about 1,000 galaxies with R<=25.5 extracted from the survey by Steidel and collaborators, and found the following results. At scales in the range 30<~theta<~100 arcsec, the angular correlation function w(theta) can be accurately described as a power law with slope beta=0.50 +0.25-0.50(1 sigma random)-0.10(systematic), shallower than the measure presented by Giavalisco et al. However, the spatial correlation length, derived by Limber deprojection, is in very good agreement with the previous measures, confirming the strong spatial clustering of these sources. We discuss in detail the effects of both random and systematic errors, in particular of the so called ``integral constraint'' bias, to which we set a lower limit using numerical simulations. This suggests that the current samples do not yet provide a ``fair representation'' of the large-scale distribution of LBGs at z~3. An intriguing result of our analysis is that at angular separations smaller than theta<~30 arcsec the correlation function seems to depart from the power-law fitted at larger scales and become smaller. This feature is detected at the ~90 per cent confidence level and, if real, it can provide information on the number density and spatial distribution of LBGs within their host halos as well as the size and the mass of the halos., Comment: 67 pages, 16 figures, accepted for publication in the Astrophysical Journal

We investigate the cross-talk between the two key components of tidal-torque theory, the inertia (I) and shear (T) tensors, using a cosmological N-body simulation with thousands of well-resolved haloes. We find that the principal axes of I and T are strongly aligned, even though I characterizes the proto-halo locally while T is determined by the large-scale structure. Thus, the resultant galactic spin, which plays a key role in galaxy formation, is only a residual due to ~10 per cent deviations from perfect alignment of T and I. The T-I correlation induces a weak tendency for the proto-halo spin to be perpendicular to the major axes of T and I, but this correlation is erased by non-linear effects at late times, making the observed spins poor indicators of the initial shear field. However, the T-I correlation implies that the shear tensor can be used for identifying the positions and boundaries of proto-haloes in cosmological initial conditions -- a missing piece in galaxy formation theory. The typical configuration is of a prolate proto-halo lying perpendicular to a large-scale high-density ridge, with the surrounding voids inducing compression along the major and intermediate inertia axes of the proto-halo. This leads to a transient sub-halo filament along the large-scale ridge, whose sub-clumps then flow along the filament and merge into the final halo. The centres of proto-haloes tend to lie in ~1 sigma over-density regions, but their association with linear density maxima smoothed on galactic scales is vague: only ~40 per cent of the proto-haloes contain peaks within them. Several other characteristics distinguish proto-haloes from density peaks, e.g., they tend to compress along two principal axes while many peaks compress along three axes., Comment: 15 pages, 10 figures. Revised version accepted for publication in MNRAS, minor changes. Version with higher quality figures and colour figures 3 and 4 are available at http://www.ast.cam.ac.uk/~porciani/ttt2/

We evaluate the success of linear tidal-torque theory (TTT) in predicting galactic-halo spin using a cosmological N-body simulation with thousands of well-resolved haloes. The proto-haloes are identified by tracing today's haloes back to the initial conditions. The TTT predictions for the proto-haloes match, on average, the spin amplitudes of today's virialized haloes if linear growth is assumed until ~t_0/3, or 55-70 per cent of the halo effective turn-around time. This makes it a useful qualitative tool for understanding certain average properties of galaxies, such as total spin and angular-momentum distribution within haloes, but with a random scatter of the order of the signal itself. Non-linear changes in spin direction cause a mean error of ~50 degrees in the TTT prediction at t_0, such that the linear spatial correlations of spins on scales >~ 1 Mpc/h are significantly weakened by non-linear effects. This questions the usefulness of TTT for predicting intrinsic alignments in the context of gravitational lensing. We find that the standard approximations made in TTT, including a second order expansion of the Zel'dovich potential and a smoothing of the tidal field, provide close-to-optimal results., Comment: 15 pages, 12 figures, revised version accepted for publication in MNRAS, contains a wider discussion of spin-spin correlations including data at redshift 1

We study correlations amongst tidal fields originated by the large-scale distribution of matter in the Universe. The two-point tidal correlation is described as a rank-4 tensor, whose elements can be written in terms of four fundamental scalar functions ranging, with respect the spatial separation, from purely transversal to purely longitudinal correlations. Tidal fields, both on galaxy and cluster scales, reveal to be correlated over distances larger than the mass-density correlation lenght, though traceless tidal fields show anti-correlation between diagonal terms along orthogonal directions. The cross-correlation between mass and tidal field is also analyzed. These results are relevant for galaxy formation and the interpretation of large-scale weak lensing phenomena., Comment: 6 pgs, 4 figs, using amssym.sty and mn.sty - MNRAS (in press)

[Abridged] We study the angular-momentum profiles of a statistical sample of halos drawn from a high-resolution N-body simulation of the LCDM cosmology. We find that the cumulative mass distribution of specific angular momentum, j, in a halo of mass Mv is well fit by a universal function, M(

Recent evidence appears to link gamma-ray bursts (GRBs) to star-forming regions in galaxies at cosmological distances. If short-lived massive stars are the progenitors of GRBs, the rate of events per unit cosmological volume should be an unbiased tracer (i.e. unaffected by dust obscuration and surface brightness limits) of the cosmic history of star formation. Here we use realistic estimates for the evolution of the stellar birthrate in galaxies to model the number counts, redshift distribution, and time-delay factors of GRBs. We present luminosity function fits to the BATSE log N-log P relation for different redshift distributions of the bursts. Our results imply about 1-2 GRBs every one million Type II supernovae, and a characteristic `isotropic-equivalent' burst luminosity in the range 3-20 x 1e51 ergs/s (for H_0=65 km/s/Mpc). We compute the rate of multiple imaging of background GRBs due to foreground mass condensations in a Lambda-dominated cold dark matter cosmology, assuming that dark halos approximate singular isothermal spheres on galaxy scales and Navarro-Frenk-White profiles on group/cluster scales, and are distributed in mass according to the Press-Schechter model. We show that the expected sensivity increase of Swift relative to BATSE could result in a few strongly lensed individual bursts detected down to a photon flux of 0.1 phot/s/cm^2 in a 3-year survey. Because of the partial sky coverage, however, it is unlikely that the Swift satellite will observe recurrent events (lensed pairs)., Comment: emulateapj, 13 pages, 7 figures, revised version accepted for publication in the ApJ

We calculate the galaxy bispectrum in both real and redshift space adopting the most common prescriptions for local Eulerian biasing and Lagrangian evolving-bias model. We show that the two biasing schemes make measurably different predictions for these clustering statistics. The Eulerian prescription implies that the galaxy distribution depends only on the present-day local mass distribution, while its Lagrangian counterpart relates the current galaxy distribution to the mass distribution at an earlier epoch when galaxies first formed. Detailed measurement of the galaxy bispectrum (of its reduced amplitude) can help establish whether galaxy positions are determined by the current mass distribution or an earlier mass distribution., Comment: 6 pages, added one figure and revised; MNRAS Pink Pages (in press), Latex file uses mn.sty

We investigate the clustering of dark matter halos in Lagrangian space in terms of their two-point correlation function, spanning more than 4 orders of magnitudes of halo masses. Analyzing a set of collisionless scale-free 128^3-particle N-body simulations with spectral indices n = -2,-1, we measure the first two Lagrangian bias parameters b_1 and b_2 relating halo and mass correlations. We find that the Mo & White leading-order formula for b_1 describes the clustering of halos with mass M ~ M_* quite accurately, where M_* indicates the characteristic non-linear mass. Smaller halos turn out to be less clustered in Lagrangian space than predicted by Mo & White. Our findings are consistent with the recent results of Jing for the clustering of halo populations in Eulerian space, demonstrating that the discrepancies between the N-body and analytical Mo & White prediction for the bias exist already in Lagrangian space. This shows that a more refined theoretical algorithm for selecting halos in the initial conditions needs to be developed. Finally, we present a very accurate fitting formula for the linear halo bias factor b_1 in Lagrangian space., Comment: 4 pages, Latex (included emulateapj.sty and psfig.tex), 2 figures. Submitted to ApJ Letters

Ongoing searches for supernovae (SNe) at cosmological distances have recently started to provide large numbers of events with measured redshifts and apparent brightnesses. Compared to quasars or galaxies, Type Ia SNe represent a population of sources with well-known intrinsic properties, and could be used to detect gravitational lensing even in the absence of multiple or highly distorted images. We investigate the lensing effect of background SNe due to mass condensations in three popular cold dark matter cosmologies (LambdaCDM, OCDM, SCDM), and compute lensing frequencies, rates of SN explosions, and distributions of arrival time differences and image separations. If dark halos approximate singular isothermal spheres on galaxy scales and NFW profiles on group/cluster scales, and are distributed in mass according to the Press-Schechter theory, then about one every 12 SNe at z~1 will be magnified by Delta m>0.1 mag (SCDM). The detection rate of SN Ia with magnification Delta m>0.3 is estimated to be of order a few events per year per square degree at maximum B-light and I<25, a hundred time smaller than the total rate expected at these magnitude levels. In the field, events magnified by more than 0.75 mag are 7 times less frequent: about one fifth of them gives rise to observable multiple images. Because of the flat K-correction and wide luminosity function, Type II SNe dominate the number counts at I>25 and have the largest fraction of lensed objects. The optimal survey sensitivity for Type Ia's magnified by Delta m>0.75 mag is I=23. Magnification bias increases their incidence by a factor of 50 in samples with I<22, dropping to a factor of 3.5 at 24 mag. At faint magnitudes the enhancement is larger for SN II., Comment: LaTeX, 34 pages, 11 figures, revised version accepted for publication in the ApJ

We study the spatial distribution of dark matter halos in the Universe in terms of their number density contrast, related to the underlying dark matter fluctuation via a non-local and non-linear bias random field. The description of the matter dynamics is simplified by adopting the `truncated' Zel'dovich approximation to obtain both analytical results and simulated maps. The halo number density field in our maps and its probability distribution reproduce with excellent accuracy those of halos in a high-resolution N-body simulation with the same initial conditions. Our non-linear and non-local bias prescription matches the N-body halo distribution better than any Eulerian linear and local bias., Comment: 4 pages, LaTeX (uses emulateapj; included psfig.tex), 3 figures, 1 table. Shortened version, matching the size requirements of ApJ Letters. Accepted for publication

We present a stochastic approach to the spatial clustering of dark matter haloes in Lagrangian space. Our formalism is based on a local formulation of the `excursion set' approach by Bond et al., which automatically accounts for the `cloud-in-cloud' problem in the identification of bound systems. Our method allows to calculate correlation functions of haloes in Lagrangian space using either a multi-dimensional Fokker-Planck equation with suitable boundary conditions or an array of Langevin equations with spatially correlated random forces. We compare the results of our method with theoretical predictions for the halo auto-correlation function considered in the literature and find good agreement with the results recently obtained within a treatment of halo clustering in terms of `counting fields' by Catelan et al.. The possible effect of spatial correlations on numerical simulations of halo merger trees is finally discussed., Comment: LaTeX, 19 pages, 3 figures. Submitted to MNRAS

We study the evolution of the mass autocorrelation function by describing the growth of density fluctuations through the Zel'dovich approximation. The results are directly compared with the predictions of the scaling hypothesis for clustering evolution extracted from numerical simulations (Hamilton et al. 1991), as implemented by Jain, Mo & White (1995). We find very good agreement between the correlations on mildly non-linear scales and on completely linear scales. In between these regimes, we note that the density fields evolved through the Zel'dovich approximation show more non-linear features than predicted by the scaling ansatz which is, however, forced to match the linear evolution on scales larger than the simulation box. In any case, the scaling ansatz by Baugh & Gaztanaga (1996), calibrated against large box simulations agrees better with ZA predictions on large scales, keeping good accuracy also on intermediate scales. We show that mode-coupling is able to move the first zero crossing of xi(r) as time goes on. A detailed fit of the time dependence of this shifting is given for a CDM model. The evolution of the cross correlation of the density fluctuation field evaluated at two different times is also studied. The possible implications of the results for the analysis of the observed correlation function of high redshift galaxies are discussed., Comment: revised version accepted for publication in MNRAS. The comparison with the scaling ansatz of Baugh & Gaztanaga (1996), one figure and some comments have been added. LaTex, 14 pages, 8 figures