arXiv subjects

Vincent Desjacques

Publications and source records attributed to Vincent Desjacques.

100 records · Page 6Linked to original sources

arXiv2008

Baryon acoustic signature in the clustering of density maxima

We reexamine the two-point correlation of density maxima in Gaussian initial conditions. Spatial derivatives of the linear density correlation, which were ignored in the calculation of Bardeen, Bond, Kaiser & Szalay (1986), are included in our analysis. These functions exhibit large oscillations around the sound horizon scale for generic CDM power spectra. We derive the exact leading-order expression for the correlation of density peaks and demonstrate the contribution of those spatial derivatives. In particular, we show that these functions can modify significantly the baryon acoustic signature of density maxima relative to that of the linear density field. The effect depends upon the exact value of the peak height, the filter shape and size, and the small-scale behaviour of the transfer function. In the LambdaCDM cosmology, for maxima identified in the density field smoothed at mass scale M\approx 10^{12}-10^{14}M_sun/h and with linear threshold height ν=1.673/σ(M), the contrast of the BAO can be a few tens of percent larger than in the linear matter correlation. Overall, the BAO is amplified (damped) for threshold heights larger (less) than unity. Density maxima thus behave quite differently than linearly biased tracers of the density field, whose acoustic signature is a simple scaled version of the linear baryon acoustic oscillation. We also calculate the mean streaming of peak pairs in the quasi-linear regime. We show that the leading-order 2-point correlation and pairwise velocity of density peaks are consistent with a nonlinear, local biasing relation involving gradients of the density field. Biasing will be an important issue in ascertaining how much of the enhancement of the BAO in the primeval correlation of density maxima propagates into the late-time clustering of galaxies.

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arXiv2008

Statistical properties of the linear tidal shear

Large-scale structures originate from coherent motions induced by inhomogeneities in the primeval gravitational potential. Here, we investigate the two-point statistics of the second derivative of the potential, the tidal shear, under the assumption of Gaussianity. We derive an exact closed form expression for the angular averaged, two-point distribution of the shear components which is valid for an arbitrary Lagrangian separation. This result is used to write down the two-point statistics of the shear eigenvalues in compact form. Next, we examine the large-scale asymptotics of the correlation of the shear eigenvalues, and the alignment of the principal axes. The analytic results are in good agreement with measurements obtained from random realizations of the gravitational potential. Finally, we show that a number of two-point distributions of the shear eigenvalues are well approximated by Gaussian bivariates over a wide range of separation and smoothing scales. We speculate that the Gaussian approximation also holds for multiple point distributions of the shear eigenvalues. It is hoped that these results will be relevant for studies aimed at describing the properties of the (evolved) matter distribution in terms of the statistics of the primordial shear field.

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arXiv2008

Environmental dependence in the ellipsoidal collapse model

N-body simulations have demonstrated a correlation between the properties of haloes and their environment. In this paper, we assess whether the ellipsoidal collapse model can produce a similar dependence. First, we explore the statistical correlation that originates from Gaussian initial conditions. We derive analytic expressions for a number of joint statistics of the shear tensor and estimate the sensitivity of the local characteristics of the shear to the global geometry of the large scale environment. Next, we concentrate on the dynamical aspect of the environmental dependence using a simplified model that takes into account the interaction between a collapsing halo and its environment. We find that the tidal force exerted by the surrounding mass distribution causes haloes embedded in overdense regions to virialize earlier. An effective density threshold whose shape depends on the large scale density provides a good description of this environmental effect. We show that, using this approach, a correlation between formation redshift, large scale bias and environment density naturally arises. The strength of the effect is comparable, albeit smaller, to that seen in simulations. It is largest for low mass haloes and decreases as one goes to higher mass objects. Furthermore, haloes that formed early are substantially more clustered than those that assembled recently. On the other hand, our analytic model predicts a decrease in median formation redshift with increasing environment density, in disagreement with the trend detected in overdense regions. However, our results appear consistent with the behaviour inferred in relatively underdense regions. We argue that the ellipsoidal collapse model may apply in low density environments where nonlinear effects are negligible (abridged).

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arXiv2006

The Probability Distribution of the Lya transmitted flux from a sample of SDSS quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lya forest from a sample of 3492 quasars included in the SDSS DR3 data release. Our intention is to investigate the sensitivity of the Lya flux PDF as measured from low resolution and low signal-to-noise data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. The quasar continuum is described by the superposition of a power law and emission lines. We perform a power law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of 0.59 +/- 0.36 in the redshift range 2.5<z<3.5. Taking into account the variation in the continuum indices increases the mean flux by 3 and 7 per cent at z=3 and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lya flux PDF and power spectrum of high resolution data. Using our power law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of 10-15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than 10 per cent.

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arXiv2006

The morphology of cosmological reionization by means of Minkowski Functionals

The morphology of the total gas and the neutral hydrogen (H I) distributions during the cosmological epoch of reionization can be quantified with Minkowski Functionals (MFs) of isodensity surfaces. We compute the MFs from the output of a high-resolution numerical simulation which includes explicit treatment of transfer of UV ionizing radiation. "Galaxies" identified in the simulation using semi-analytic models of galaxy formation are assumed to be the sole sources of UV photons. The MFs of the total gas distribution are well described by the analytic expressions derived for lognormal random fields. The statistical properties of the diffuse H I depend on the gas distribution and on the way ionized regions propagate in the inter-galactic medium (IGM). The deviations of the MFs of the H I distribution from those of a lognormal random field are, therefore, caused by reionization. We use the MFs to discriminate between the various stages of reionization in the simulation. We suggest a simple model of reionization which reproduces the MFs derived from this simulation. Using random realizations of lognormal density fields, we also assess the ability of MFs to distinguish between different reionization scenarios. Our results are relevant to the analysis of future cosmological twenty-one centimeter maps.

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arXiv2005

Numerical action reconstruction of the dynamical history of dark matter haloes in N-body simulations

We test the ability of the numerical action method (NAM) to recover the individual orbit histories of mass tracers in an expanding universe in a region of radius 26Mpc/h, given the masses and redshift-space coordinates at the present epoch. The mass tracers are represented by dark matter haloes identified in a high resolution N-body simulation of the standard LCDM cosmology. Since previous tests of NAM at this scale have traced the underlying distribution of dark matter particles rather than extended haloes, our study offers an assessment of the accuracy of NAM in a scenario which more closely approximates the complex dynamics of actual galaxy haloes. We show that NAM can recover present-day halo distances with typical errors of less than 3 per cent, compared to 5 per cent errors assuming Hubble flow distances. The total halo mass and the linear bias were both found to be constained at the 50 per cent level. The accuracy of individual orbit reconstructions was limited by the inability of NAM, in some instances, to correctly model the positions of haloes at early times solely on the basis of the redshifts, angular positions, and masses of the haloes at the present epoch. Improvements in the quality of NAM reconstructions may be possible using the present-day three-dimensional halo velocities and distances to further constrain the dynamics. This velocity data is expected to become available for nearby galaxies in the coming generations of observations by SIM and GAIA.

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arXiv2005

The impact of galactic winds from LBGs on the Intergalactic Medium

An excess of sight-lines close to Lyman-break galaxies (LBGs) with little or no absorption in QSO absorption spectra has been reported and has been interpreted as the effect of galactic winds on the Intergalactic Medium. We use here numerical simulations to investigate the flux probability function close to plausible sites of LBGs. We show that the flux distribution near our LBGs in the simulation depends strongly on redshift, and is very sensitive to the averaging procedure. We show that a model without galactic winds and a model with a wind bubble size of 0.5Mpc/h (comoving) are equally consistent with the new determination of the conditional flux distribution by Adelberger et al. (2005). Models with the larger bubble sizes suggested by the previous observations of Adelberger et al. (2003) based on a much smaller sample at higher redshift are not consistent with the new data. We therefore argue that the volume filling factor of galactic winds driven by LBGs may be much smaller than previously thought, and that most of the metals responsible for the metal absorption associated with the low column density Lya forest are unlikely to have been ejected by LBGs.

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arXiv2005

Joint modeling of the probability distribution and power spectrum of the Lya forest: comparison with observations at z=3

We presents results of joint modeling of the probability distribution function (PDF) and the one-dimensional power spectrum (PS) of the Lya forest flux decrement. The sensitivity of these statistical measures to the shape and amplitude of the linear matter power spectrum is investigated using two variants of the LCDM cosmology. In the first model, the linear power spectrum has a scale-invariant spectral index, whereas in the second, it has a negative running index (RSI). We generate mock catalogs of QSO spectra, and compare their statistical properties to those of the observations at z=3. We perform a joint fit of the power spectrum and PDF. A scale-invariant model with σ_8=0.9 matches well the data if the mean IGM temperature is T\leq 15000K. For higher temperature, it tends to overestimate the flux power spectrum over scales k < 0.01s/km. The discrepancy is less severe when the PS alone is fitted. However, models matching the PS alone do not yield a good fit to the PDF. A joint analysis of the flux PS and PDF tightens the constraints on the model parameters and reduces systematic biases. The RSI model is consistent with the observed PS and PDF only if the temperature is T\geq 20000K. The best fit models reproduce the slope and normalisation of the column density distribution, irrespective of the shape and amplitude of the linear power spectrum. They are also consistent with the observed line-width distribution given the large uncertainties. Our joint analysis suggests that σ_8 is likely to be in the range 0.7 - 0.9 for a temperature 10000K<T<20000K and a reasonable reionization history.

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arXiv2004

Redshift distortions in one-dimensional power spectra

We present a model for one-dimensional (1D) matter power spectra in redshift space as estimated from data provided along individual lines of sight. We derive analytic expressions for these power spectra in the linear and nonlinear regimes, focusing on redshift distortions arising from peculiar velocities. In the linear regime, redshift distortions enhance the 1D power spectra only on small scales, and do not affect the power on large scales. This is in contrast to the effect of distortions on three-dimensional (3D) power spectra estimated from data in 3D space, where the enhancement is independent of scale. For CDM cosmologies, the 1D power spectra in redshift and real space are similar for wavenumbers $q<0.1h/Mpc$ where both have a spectral index close to unity, independent of the details of the 3D power spectrum. Nonlinear corrections drive the 1D power spectrum in redshift space into a nearly universal shape over scale $q<10h/Mpc$, and suppress the power on small scales as a result of the strong velocity shear and random motions. The redshift space, 1D power spectrum is mostly sensitive to the amplitude of the initial density perturbations. Our results are useful in particular for power spectra computed from the SDSS quasars sample.

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arXiv2004

Galactic winds and the Lya forest

We study the effect of galactic outflows on the Lya forest. The winds are modelled as fully ionised spherical bubbles centered around galactic haloes. The observed flux probability distribution and flux power spectrum limit the volume filling factor of bubbles to be less than 10%. We have compared the mean flux as a function of distance from haloes with the Adelberger etal (ASSP) measurement. A model with bubbles of constant radius of 1.5Mpc/h surrounding the most massive haloes matches the high transmissivity at separations <0.5Mpc/h, but the increase of the transmissivity at small scales is more gradual than observed. At separations <0.5Mpc/h, our estimate of the cosmic variance error is $ΔF\sim 0.3$, 30% higher than that of ASSP. The difficulty in matching the rise in the transmissivity at small separations is caused by residual absorption of neutral hydrogen lying physically outside the bubbles but having a redshift position similar to the haloes. The flux level is thus sensitive to the amplitude of the coherent velocity shear near haloes which increases with halo mass in the simulation. A model where LBGs are starbursts in small mass haloes matches the observations with smaller bubble size than one where more massive haloes host the LBGs. A starburst model with a bubble radius of 1Mpc/h and a volume filling factor of 2% is consistent with the ASSP measurements at the 1-1.5$σ$ level. If this model is correct the sharp rise of the transmissivity at small separation in the ASSP sample is due to cosmic variance and is expected to become more moderate for a larger sample.

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