arXiv subjects

Cristiano Porciani

Publications and source records attributed to Cristiano Porciani.

At least 91 records · Page 5Linked to original sources

arXiv2006

Luminosity and redshift dependent quasar clustering

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 0.8 1.3. 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.

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arXiv2005

The origin of intergalactic metals around Lyman-break galaxies

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<z<12 can quantitatively explain the high cross-correlation between CIV systems and LBGs observed at z=3. The reason is twofold. First, both LBGs and high-z dwarfs are biased tracer of the mass distribution and form from exceptionally high density fluctuations which are strongly clustered. Second, the action of gravity tends to increase the spatial association between metal bubbles and LBGs. Our analysis shows that, in order to match the abundance of CIV systems observed at z=3, the metal bubbles generated by high-z dwarfs must have comoving sizes of ~100 kpc. We conclude that the observed galaxy-CIV spatial association needs not to be generated by late ``superwinds'' from LBGs.

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arXiv2005

Fluorescent Ly-alpha emission from the high-redshift intergalactic medium

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).

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arXiv2004

Cosmic Evolution of Quasar Clustering: Implications for the Host Haloes

We present detailed clustering measurements from the 2dF QSO Redshift Survey (2QZ) in the redshift range 0.8 10^{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.

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arXiv2004

Galaxy Clustering and Dark Energy

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-$α$ 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.

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arXiv2003

Cosmic and Galactic Neutrino Backgrounds from Thermonuclear Sources

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.

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arXiv2003

The Halo Occupation Number and Spatial Distribution of 2dF Galaxies

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)

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arXiv2002

Testing Tidal-Torque Theory: I. Spin Amplitude and Direction

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.

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arXiv2002

Testing Tidal-Torque Theory: II. Alignment of Inertia and Shear and the Characteristics of Proto-haloes

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.

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arXiv2001

The Clustering Properties of Lyman-Break Galaxies at Redshift z~3

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.

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arXiv2000

Correlations of Cosmic Tidal Fields

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.

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arXiv2000

A universal angular momentum profile for galactic halos

[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(<j) = Mv μj/(j_0+j). This profile is defined by one shape parameter (μor j_0) in addition to the global spin parameter λ. It follows a power-law over most of the mass, and flattens at large j, with the flattening more pronounced for small values of μ. 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. The spatial distribution of angular momentum in halos tends to be cylindrical and is well-aligned within each halo for ~80% of the halos. 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.

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arXiv2000

On the Association of Gamma-ray Bursts with Massive Stars: Implications for Number Counts and Lensing Statistics

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).

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arXiv2000

Two Ways of Biasing Galaxy Formation

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.

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arXiv1999

Gravitational Lensing of Distant Supernovae in Cold Dark Matter Universes

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.

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arXiv1998

How the dark matter halos cluster in Lagrangian space

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.

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arXiv1998

On the spatial distribution of dark matter halos

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.

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arXiv1998

Excursion set approach to the clustering of dark matter haloes in Lagrangian space

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.

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