arXiv subjects

Enrique Gaztanaga

Publications and source records attributed to Enrique Gaztanaga.

At least 127 records · Page 7Linked to original sources

arXiv2003

Correlation between galaxies and QSO in the SDSS:na signal from gravitational lensing magnification?

We report a detection of galaxy-QSO cross-correlation w_GQ in the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR) over 0.2-30 arc-minute scales. We cross-correlate galaxy samples of different mean depths r'=19-22 (z_G =0.15-0.35) with the main QSO population (i'<19.1) at mean z_Q=1.6. We find significant positive correlation in all cases except for the faintest QSOs, as expected if the signal were due to weak lensing magnification. The amplitude of the signal on arc-minute scales is about 20%. This is a few times larger than currently expected from weak lensing in LCDM but confirms, at a higher significance, previous measurements by several groups. When compared to the galaxy-galaxy correlation w_GG, a weak lensing interpretation indicates a strong and steep non-linear amplitude for the underlaying matter fluctuations: sigma=400 on scales of 0.2 Mpc/h, in contradiction with non-linear modeling of LCDM fluctuations. We also detect a normalized skewness (galaxy-galaxy-QSO correlation) of S_3=21 +/-6 at z=0.15 (S_3= 14 +/- 4 at z=0.35), which several sigma low, as compared to LCDM expectations. These observational trends can be reconciled with lensing in a flat Lambda universe with sigma_8=1, provided the linear spectrum is steeper (n = 1) than in the LCDM model on these small (cluster) scales. Under this interpretation, the galaxy distribution traces the matter variance with an amplitude that is 100 times smaller: ie galaxies are anti-bias with b=0.1 on small scales, increasing to b=1 at 10 Mpc/h.

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arXiv2002

Large scale structures in the Early SDSS: Comparison of the North and South Galactic strips

We compare the large scale galaxy clustering between the North and South SDSS early data release (EDR) and also with the clustering in the APM Galaxy Survey. The three samples are independent and cover an area of 150, 230 and 4300 square degrees respectively. We combine SDSS data in different ways to approach the APM selection. Given the good photometric calibration of the SDSS data and the very good match of its North and South number counts, we combine them in a single sample. The joint clustering is compared with equivalent subsamples in the APM. The final sampling errors are small enough to provide an independent test for some of the results in the APM. We find evidence for an inflection in the shape of the 2-point function in the SDSS which is very similar to what is found in the APM. This feature has been interpreted as evidence for non-linear gravitational growth. By studying higher order correlations, we can also confirm good agreement with the hypothesis of Gaussian initial conditions (and small biasing) for the structure traced by the large scale SDSS galaxy distribution.

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arXiv2001

Constraints on the Accuracy of Photometric Redshifts Derived from BLAST and Herschel/SPIRE Sub-mm Surveys

More than 150 galaxies have been detected in blank-field millimetre and sub-millimetre surveys. However the redshift distribution of sub-mm galaxies remains uncertain due to the difficulty in identifying their optical-IR counterparts, and subsequently obtaining their spectroscopic emission-line redshifts. In this paper we discuss results from a Monte-Carlo analysis of the accuracy with which one can determine redshifts from photometric measurements at sub-millimetre-FIR wavelengths. The analysis takes into account the dispersion in colours introduced by including galaxies with a distribution of SEDs, and by including photometric and absolute calibration errors associated with real observations. We present examples of the probability distribution of redshifts for individual galaxies detected in the future BLAST and Herschel/SPIRE surveys. We show that the combination of BLAST and 850um observations constrain the photometric redshifts with sufficient accuracy to pursue a program of spectroscopic follow-up with the 100m GBT.

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arXiv2001

The Real and Redshift Space Density Distribution Function for Large-Scale Structure in the Spherical Collapse Approximation

We use the spherical collapse (SC) approximation to derive expressions for the smoothed redshift-space probability distribution function (PDF), as well as the $p$-order hierarchical amplitudes $S_p$, in both real and redshift space. We compare our results with numerical simulations, focusing on the $Ω=1$ standard CDM model, where redshift distortions are strongest. We find good agreement between the SC predictions and the numerical PDF in real space even for $σ_L \simgt 1$, where $σ_L$ is the linearly-evolved rms fluctuation on the smoothing scale. In redshift space, reasonable agreement is possible only for $σ_L \simlt 0.4$. Numerical simulations also yield a simple empirical relation between the real-space PDF and redshift-space PDF: we find that for $σ\simlt 1$, the redshift space PDF, P[δ_z], is, to a good approximation, a simple rescaling of the real space PDF, P[δ], i.e., P[δ/σ] d[δ/σ] = P[δ_z/σ_z] d[δ_z/σ_z], where $σ$ and σ_z are the real-space and redshift-space rms fluctuations, respectively. This result applies well beyond the validity of linear perturbation theory, and it is a good fit for both the standard CDM model and the Lambda-CDM model. It breaks down for SCDM at $σ\approx 1$, but provides a good fit to the Λ-CDM models for $σ$ as large as 0.8.

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arXiv2001

The transition to nonlinearity and new constraints on biasing

We present two new dynamical tests of the biasing hypothesis. The first is based on the amplitude and the shape of the galaxy-galaxy correlation function, $ξ_g(r)$, where $r$ is the separation of the galaxy pair. The second test uses the mean relative peculiar velocity for galaxy pairs, $\vs(r)$. This quantity is a measure of the rate of growth of clustering and it is related to the two-point correlation function for the matter density fluctuations, $ξ(r)$. Under the assumption that galaxies trace the mass ($ξ_g = ξ$), the expected relative velocity can be calculated directly from the observed galaxy clustering. The above assumption can be tested by confronting the expected $\vs$ with direct measurements from velocity-distance surveys. Both our methods are checked against N-body experiments and then compared with the $ξ_g(r)$ and $\vs$ estimated from the {\sc APM} galaxy survey and the Mark III catalogue, respectively. Our results suggest that cosmological density parameter is low, $Ω_m \approx 0.3$, and that the {\sc APM} galaxies trace the mass at separations $r \ga 5 \Mlu$, where $h$ is the Hubble constant in units of 100 km s$^{-1}$Mpc. The present results agree with earlier studies, based on comparing higher order correlations in the {\sc APM} with weakly non-linear perturbation theory. Both approaches constrain the linear bias factor to be within 20% of unity. If the existence of the feature we identified in the {\sc APM} $ξ_g(r)$ -- the inflection point near $ξ_g = 1$ -- is confirmed by more accurate surveys, we may have discovered gravity's smoking gun: the long awaited ``shoulder'' in $ξ$, generated by gravitational dynamics and predicted by Gott and Rees 25 years ago.

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arXiv2001

Clustering in Deep (Submillimeter) Surveys

Hughes & Gaztanaga (2001, see article in these proceedings) have presented realistic simulations to address key issues confronting existing and forthcoming submm surveys. An important aspect illustrated by the simulations is the effect induced on the counts by the sampling variance of the large-scale galaxy clustering. We find factors of up to 2-4 variation (from the mean) in the extracted counts from deep surveys identical in area (6 sqr arcmin) to the SCUBA surveys of the Hubble Deep Fields (HDF). Here we present a recipe to model the expected degree of clustering as a function of sample area and redshift.

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arXiv2001

On the APM power spectrum and the CMB anisotropy: Evidence for a phase transition during inflation?

Adams et al. (1997b) have noted that according to our current understanding of the unification of fundamental interactions, there should have been phase transitions associated with spontaneous symmetry breaking {\em during} the inflationary era. This may have resulted in the breaking of scale-invariance of the primordial density perturbation for brief periods. A possible such feature was identified in the power spectrum of galaxy clustering in the APM survey at the scale $k \sim 0.1 h$ Mpc^{-1} and it was shown that the secondary acoustic peaks in the power spectrum of the CMB anisotropy should consequently be suppressed. We demonstrate that this prediction is confirmed by the recent Boomerang and Maxima observations, which favour a step-like spectral feature in the range $k \sim (0.06-0.6)h$ Mpc^{-1}, independently of the similar previous indication from the APM data. Such a spectral break enables an excellent fit to both APM and CMB data with a baryon density consistent with the BBN value. It also allows the possibility of a matter-dominated universe with zero cosmological constant, which we show can now account for even the evolution of the abundance of rich clusters.

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arXiv2000

Non-Linear gravitational growth of large scale structures inside and outside standard Cosmology

We reconsider the problem of gravitational structure formation inside and outside General Relativity (GR), both in the weakly and strongly non-linear regime. We show how these regimes can be explored observationally through clustering of high order cumulants and through the epoch of formation, abundance and clustering of collapse structures, using Press-Schechter formalism and its extensions. We address the question of how different are these predictions when using a non-standard theory of Gravity. We study examples of cosmologies that do not necessarily obey Einstein's field equations: scalar-tensor theories (STT), such as Brans-Dicke (BD), parametrized with $ω$, a non-standard parametrisation of the Hubble law, $H^2= a^{-3(1+ε)}$, or a non-standard cosmic equation of state $p=γρ$, where $γ$ can be chosen irrespective of the cosmological parameters ($Ω_M$ and $Ω_Λ$). We present some preliminary bounds on $γ$, $ω$ and $ε$ from observations of the skewness and kurtosis in the APM Galaxy Survey. This test is independent of the overall normalization of rms fluctuations. We also show how abundances and formation times change under these assumptions. Upcoming data on non-linear growth will place strong constraints on such variations from the standard paradigm.

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arXiv2000

Clusters of Galaxies at High Redshift: The LMT/GTM Perspective

The lack of reliable cluster samples at intermediate and high redshift has motivated many optical/infrared and X-ray cluster searches. The motivation of such searches is well justified: the abundance of massive collapsed regions gives a direct constraint on Omega(matter). Alternatively, the Sunyaev-Zeldovich effect (SZE) offers a robust method for the detection of clusters which is, almost, independent of redshift. In this paper we comment on the possibility of mapping the SZE at high resolution in the millimeter regime using the Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT/GTM) and the bolometer array BOLOCAM. The construction of the LMT/GTM facility is underway atop Cerro La Negra (latitude=18deg 59min; height=4600 m) in Mexico.

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arXiv2000

Simulated Submillimetre Galaxy Surveys

Current submillimetre surveys are hindered in their ability to reveal detailed information on the epoch of galaxy formation and the evolutionary history of a high-redshift starburst galaxy population. The difficulties are due to the small primary apertures (D < 15 m) of existing submillimetre telescopes and the limited sensitivities of their first generation of bolometer cameras. This situation is changing rapidly due to a variety of powerful new ground-based, airborne and satellite FIR to millimetre wavelength facilities. Improving our understanding of the luminosity and clustering evolution provides the motivation for conducting cosmological submillimetre and millimetre surveys. It is therefore important that we quantify the limitations of the future surveys and the significance of the results that can be drawn from them. In this paper we present simulated surveys which are made as realistic as possible in order to address some key issues confronting existing and forthcoming surveys. We discuss the results from simulations with a range of wavelengths (200 um - 1.1 mm), spatial resolutions (6 arcsec - 27 arcsec) and flux densities (0.01-310 mJy). We address how the measured source-counts could be affected by resolution and confusion, by the survey sensitivity and noise, and by the sampling variance due to clustering and shot-noise.

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arXiv2000

Large Scale Structure in the weakly non-linear regime

Is gravitational growth responsible for the observed large scale structure in the universe? Do we need non-gaussian initial conditions or non-gravitational physics to explain the large scale features traced by galaxy surveys? I will briefly revise the basic ideas of non-linear perturbation theory (PT) as a tool to understand structure formation, in particular through the study of higher order statistics, like the skewness and the 3-point function. Contrary to what happens with the second order statistics (the variance or power-spectrum), this test of gravitational instability is independent of the overall amplitude of fluctuations and of cosmic evolution, so that it does not require comparing the clustering at different redshifts. Predictions from weakly non-linear PT have been compared with observations to place constraints on our assumptions about structure formation, the initial conditions and how galaxies trace the mass.

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arXiv1999

Inverting the Angular Correlation Function

The two point angular correlation function is an excellent measure of structure in the universe. To extract from it the three dimensional power spectrum, one must invert Limber's Equation. Here we perform this inversion using a Bayesian prior constraining the smoothness of the power spectrum. Among other virtues, this technique allows for the possibility that the estimates of the angular correlation function are correlated from bin to bin. The output of this technique are estimators for the binned power spectrum and a full covariance matrix. Angular correlations mix small and large scales but after the inversion, small scale data can be trivially eliminated, thereby allowing for realistic constraints on theories of large scale structure. We analyze the APM catalogue as an example, comparing our results with previous results. As a byproduct of these tests, we find -- in rough agreement with previous work -- that APM places stringent constraints on Cold Dark Matter inspired models, with the shape parameter constrained to be $0.25\pm 0.04$ (using data with wavenumber $k \le 0.1 h{\rm Mpc}^{-1}$). This range of allowed values use the full power spectrum covariance matrix, but assumes negligible covariance in the off-diagonal angular correlation error matrix, which is estimated with a large angular resolution of 0.5degrees (in the range 0.5 and 20 degrees).

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arXiv1999

Information content in Gaussian noise: optimal compression rates

We approach the theoretical problem of compressing a signal dominated by Gaussian noise. We present expressions for the compression ratio which can be reached, under the light of Shannon's noiseless coding theorem, for a linearly quantized stochastic Gaussian signal (noise). The compression ratio decreases logarithmically with the amplitude of the frequency spectrum $P(f)$ of the noise. Entropy values and compression rates are shown to depend on the shape of this power spectrum, given different normalizations. The cases of white noise (w.n.), $f^{n_p}$ power-law noise ---including $1/f$ noise---, (w.n.$+1/f$) noise, and piecewise (w.n.+$1/f |$ w.n.$+1/f^2$) noise are discussed, while quantitative behaviours and useful approximations are provided.

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arXiv1999

Predictions for the clustering properties of the Lyman-alpha Forest - I. One-point statistics

We present predictions for the one-point probability distribution and cumulants of the transmitted QSO flux in the high redshift Lyman-$α$ Forest. We make use of the correlation between the Lyman-$α$ optical depth and the underlying matter density predicted by gravitational instability theory and seen in numerical hydrodynamic simulations. We have modelled the growth of matter fluctuations using the non-linear shear-free dynamics, an approximation which reproduces well the results of perturbation theory for the cumulants in the linear and weakly non-linear clustering regime. As high matter overdensities tend to saturate in spectra, the statistics of the flux distribution are dominated by weakly non-linear overdensities. As a result, our analytic approach can produce accurate predictions, when tested against N-body simulation results, even when the underlying matter field has rms fluctuations larger than unity. Our treatment can be applied to either Gaussian or non-Gaussian initial conditions. Here we concentrate on the former case, but also include a study of a specific non-Gaussian model. We discuss how the methods and predictions we present can be used as a tool to study the generic clustering properties of the \lya forest at high-redshift. With such an approach, rather than concentrating on simulating specific cosmological models, we may be in the position to directly test our assumptions for the Gaussian nature of the initial conditions, and the gravitational instability origin of structure itself. In a separate paper we present results for two-point statistics.

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arXiv1998

Biased-estimations of the Variance and Skewness

Nonlinear combinations of direct observables are often used to estimate quantities of theoretical interest. Without sufficient caution, this could lead to biased estimations. An example of great interest is the skewness $S_3$ of the galaxy distribution, defined as the ratio of the third moment $\xibar_3$ and the variance squared $\xibar_2^2$. Suppose one is given unbiased estimators for $\xibar_3$ and $\xibar_2^2$ respectively, taking a ratio of the two does not necessarily result in an unbiased estimator of $S_3$. Exactly such an estimation-bias affects most existing measurements of $S_3$. Furthermore, common estimators for $\xibar_3$ and $\xibar_2$ suffer also from this kind of estimation-bias themselves: for $\xibar_2$, it is equivalent to what is commonly known as the integral constraint. We present a unifying treatment allowing all these estimation-biases to be calculated analytically. They are in general negative, and decrease in significance as the survey volume increases, for a given smoothing scale. We present a re-analysis of some existing measurements of the variance and skewness and show that most of the well-known systematic discrepancies between surveys with similar selection criteria, but different sizes, can be attributed to the volume-dependent estimation-biases. This affects the inference of the galaxy-bias(es) from these surveys. Our methodology can be adapted to measurements of analogous quantities in quasar spectra and weak-lensing maps. We suggest methods to reduce the above estimation-biases, and point out other examples in LSS studies which might suffer from the same type of a nonlinear-estimation-bias.

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arXiv1998

Cosmological Perturbation Theory and the Spherical Collapse Model - II. Non-Gaussian initial conditions

In Part I of this series, we introduced the Spherical Collapse (SC) approximation in Lagrangian space as a way of estimating the cumulants $ξ_J$ of density fluctuations in cosmological Perturbation Theory (PT). Within this approximation, the dynamics is decoupled from the statistics of the initial conditions, so we are able to present here the cumulants for generic Non-Gaussian initial conditions, which can be estimated to arbitrary order including the smoothing effects. The SC model turns out to recover the exact leading-order non-linear contributions up to terms involving non-local integrals of the $J$-point functions. We argue that for the hierarchical ratios $S_J$, these non-local terms are sub-dominant and tend to compensate each other. The resulting predictions show a non-trivial time evolution that can be used to discriminate between models of structure formation. We compare these analytic results to Non-Gaussian N-body simulations, which turn out to be in very good agreement up to scales where $σ\simlt 1$.

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arXiv1997

The Space Density of Galaxy Peaks and the Linear Matter Power Spectrum

One way of recovering information about the initial conditions of the Universe is by measuring features of the cosmological density field which are preserved during gravitational evolution and galaxy formation. In this paper we study the total number density of peaks in a (galaxy) point distribution smoothed with a filter, evaluating its usefulness as a means of inferring the shape of the initial (matter) power spectrum. We find that in numerical simulations which start from Gaussian initial conditions, the peak density follows well that predicted by the theory of Gaussian density fields, even on scales where the clustering is mildly non-linear. For smaller filter scales, $r \simlt 4-6 \hmpc$, we see evidence of merging as the peak density decreases with time. On larger scales, the peak density is independent of time. One might also expect it to be fairly robust with respect to variations in biasing, i.e. the way galaxies trace mass fluctuations. We find that this is the case when we apply various biasing prescriptions to the matter distribution in simulations. If the initial conditions are Gaussian, it is possible to use the peak density measured from the evolved field to reconstruct the shape of the initial power spectrum. We describe a stable method for doing this and apply it to several biased and unbiased non-linear simulations. We are able to recover the slope of the linear matter power spectrum on scales $k \simlt 0.4 \hmpc^{-1}$. The reconstruction has the advantage of being independent of the cosmological parameters ($Ω$, $Λ$, $H_0$) and of the clustering normalisation ($σ_8$). The peak density and reconstructed power spectrum slope therefore promise to be powerful discriminators between popular cosmological scenarios.

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arXiv1994

High-order galaxy correlation functions in the APM Galaxy Survey

We estimate J-point galaxy averaged correlation functions $\wbar_J(θ)$ for $J=2,...,9$, in a sample of the APM Galaxy Survey with more than $1.3 \times 10^6$ galaxies and a depth $ \calD \sim 400 \Mpc$. The hierarchical amplitudes $s_J=\wbar_J/\wbar_2^{J-1}$ are roughly constant, up to $J=9$, between $0.5 \Mpc$ and $2 \Mpc$ and decrease slowly for larger scales. At scales larger than $7 \Mpc$ we find strong similarities between the statistical properties of the galaxy fluctuations and the theoretical properties of matter fluctuations evolving under the influence of gravity in an expanding universe on assumption that the initial fluctuations are small and Gaussian. This is most easily explained if at large scales there is no significant biasing between matter and galaxy fluctuations. The comparison of the skewness in the CfA and SSRS catalogues with comparable sub-samples of the APM indicates that the volume of a ``fair sample'' has to be much larger that the one in the combined CfA/SSRS catalogues.

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