arXiv subjects

Luigi Guzzo

Publications and source records attributed to Luigi Guzzo.

45 records · Page 3Linked to original sources

arXiv2001

The BMW X-ray Cluster Survey

We describe the main features of the BMW survey of serendipitous X-ray clusters, based on the still unexploited ROSAT-HRI archival observations. The sky coverage, surface density and first deep optical CCD images of the candidates indicate that this sample can represent an excellent complement to the existing PSPC deep cluster surveys and will provide us with a fully independent probe of the evolution of the cluster abundance, in addition to significantly increasing the number of clusters known at z>0.6.

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arXiv2000

The ROSAT-ESO Flux-Limited X-Ray (REFLEX) Galaxy Cluster Survey III: The Power Spectrum

We present a measure of the power spectrum on scales from 15 to 800 Mpc/h using the ROSAT-ESO Flux-Limited X-Ray(REFLEX) galaxy cluster catalogue. The REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of galaxies with the nominal flux limit S=3.0 10^{-12}erg/s/cm2 for the ROSAT energy band (0.1-2.4)keV. Several tests are performed showing no significant incompletenesses of the REFLEX clusters with X-ray luminosities brighter than 10^{43}erg/s up to scales of about 800 Mpc/h. They also indicate that cosmic variance might be more important than previous studies suggest. We regard this as a warning not to draw general cosmological conclusions from cluster samples with a size smaller than REFLEX. Power spectra, P(k), of comoving cluster number densities are estimated for flux- and volume-limited subsamples. The most important result is the detection of a broad maximum within the comoving wavenumber range 0.022<k<0.030 h/Mpc. The data suggest an increase of the power spectral amplitude with X-ray luminosity. Compared to optically selected cluster samples the REFLEX P(k)is flatter for wavenumbers k<0.05 h/Mpc thus shifting the maximum of P(k) to larger scales. The smooth maximum is not consistent with the narrow peak detected at k=0.05 h/Mpc using the Abell/ACO richness $\ge 0$ data. In the range 0.02<k<0.4 h/Mpc general agreement is found between the slope of the REFLEX P(k) and those obtained with optically selected galaxies. A semi-analytic description of the biased nonlinear power spectrum in redshift space gives the best agreement for low-density Cold Dark Matter models with or without a cosmological constant.

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arXiv1999

Large-Scale Structure at the Turn of the Millennium

I review the current status of studies of the large-scale structure of the Universe using redshift surveys of galaxies and clusters of galaxies. I first summarise the advances we have made in our knowledge of the cosmography of the z<0.2 Universe during the last 25 years, as well as the status of the major surveys in progress. The question of how the "a priori" selection of some classes of objects "biases" the mapping of the underlying mass density field is discussed in some detail. I then emphasise the advantages of using clusters of galaxies selected in the X-ray band as tracers of large--scale structure, summarising the most recent results of the REFLEX survey, which is under completion. The strong potential of using X-ray clusters to study the evolution of structure to large redshifts is underlined. I then summarise some of the most recent statistical results on the clustering of galaxies and clusters, using the two-point correlation function xi(s) and the power spectrum P(k). In particular, I concentrate on the increased information available on the detailed shape of these functions on large scales, lambda~100/h Mpc. I argue that significant evidence is accumulating from different observations that P(k) has a well-defined and possibly narrow peak around k~0.05 h/Mpc. I conclude with a glimpse into the future of large-scale structure surveys at high redshifts, describing the features of the VIRMOS deep survey, which will soon start collecting redshifts with the ESO VLT for ~150,000 galaxies at a typical depth of z=1.

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arXiv1998

EISily looking for distant galaxy clusters

We present a new algorithm to search for distant clusters of galaxies on catalogues deriving from imaging data, as those of the ESO Imaging Survey. We discuss the advantages of our technique relative to the ones developed before and present preliminary results of its application to the I--band data of the survey's patch A.

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arXiv1997

Is the Universe Homogeneous? (On Large Scales)

I critically discuss in a pedagogical and phenomenological way a few crucial tests challenging the claims by Pietronero and collaborators that there is no evidence from available galaxy catalogues that the Universe is actually homogeneous above a certain scale. In a series of papers, these authors assert that observations are consistent with a fractal distribution of objects extending to the limit of the present data. I show that while galaxies are indeed clustered in a scale-free way on small and intermediate scales, this behaviour does not continue indefinitely. Although the specific wavelength at which the galaxy distribution apparently turns to homogeneity is dangerously close to the size of the largest samples presently available, there are serious hints suggesting that this turnover is real and that its effects are detected in the behaviour of statistical estimators. The most recent claims of a continuing fractal hierarchy up to scales of several hundreds Mpc seem to be abscribable to the use of incomplete samples or to an improper treatment of otherwise high- quality data sets. The fractal perspective, nevertheless, can represent a fruitful way to look at the clustering properties of galaxies, when properly coupled to the traditional gravitational instability scenario. In the last part of this paper I therefore try to clarify, at a very simple level, some of the confusion existing on the actual scaling properties of the galaxy distribution. In this context, I shall discuss how these can provide hints on the evolution of the large-scale structure of the Universe, when one properly takes into account the effect of both initial conditions and nonlinear gravitational growth in producing the presently observed distribution.

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arXiv1997

Redshift-Space Distortions and the Real-Space Clustering of Different Galaxy Types

We study the distortions induced by peculiar velocities on the redshift-space correlation function of galaxies of different morphological types in the Pisces-Perseus redshift survey. Redshift-space distortions affect early- and late-type galaxies in different ways. In particular, at small separations, the dominant effect comes from virialized cluster cores, where ellipticals are the dominant population. The net result is that a meaningful comparison of the clustering strength of different morphological types can be performed only in real space, i.e., after projecting out the redshift distortions on the two-point correlation function xi(r_p,pi). A power-law fit to the projected function w_p(r_p) on scales smaller than 10/h Mpc gives r_o = 8.35_{-0.76}^{+0.75} /h Mpc, γ= 2.05_{-0.08}^{+0.10} for the early-type population, and r_o = 5.55_{-0.45}^{+0.40} /h Mpc, γ= 1.73_{-0.08}^{+0.07} for spirals and irregulars. These values are derived for a sample luminosity brighter than M_{Zw} = -19.5. We detect a 25% increase of r_o with luminosity for all types combined, from M_{Zw} = -19 to -20. In the framework of a simple stable-clustering model for the mean streaming of pairs, we estimate sigma_12(1), the one-dimensional pairwise velocity dispersion between 0 and 1 /h Mpc, to be 865^{+250}_{-165} km/s for early-type galaxies and 345^{+95}_{-65} km/s for late types. This latter value should be a fair estimate of the pairwise dispersion for ``field'' galaxies; it is stable with respect to the presence or absence of clusters in the sample, and is consistent with the values found for non-cluster galaxies and IRAS galaxies at similar separations.

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arXiv1996

Redshift Surveys and Large--Scale Structure

I review the present status of the mapping of the large-scale structure of the Universe through wide-angle redshift surveys. In the first part of the paper, I discuss the current state of the art, describing in some detail the recently completed ESO Slice Project (ESP). I then briefly recall some of the basic motivations for performing larger surveys. Then, in the final section, I present in detail three large redshift survey projects that in the years from now to the end of the Century are expected to satisfy this demand.

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arXiv1994

The ESO All-Southern-Sky Redshift Survey of ROSAT Clusters of Galaxies

We discuss the rationale and the present status of a large redshift survey aiming at measuring distances for all the clusters of galaxies detected by the ROSAT X--ray All--Sky Survey in the southern hemisphere, with flux larger than $\sim 2\times 10^{-12}$ erg s$^{-1}$ cm$^{-2}$. The survey is being performed using three ESO telescopes in parallel. The final sample will contain about 700 clusters. It will represent a unique database for the study of the spatial distribution of clusters, their X--ray luminosity function and their intrinsic physical properties. Serendipitous results obtained so far include two new gravitational arc systems in two clusters of galaxies. These findings, obtained on rather short exposures, confirm the advantage of searching for arcs in X--ray selected clusters.

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arXiv1994

Modelling the Power Spectrum of Density Fluctuations: A Phenomenological Approach

We show how, based on considerations on the observed form of the galaxy 2-point spatial correlation function xi(r), a very simplified -- yet surprisingly effective -- model for the linear density fluctuations power spectrum can be constructed. We first relate the observed large-scale shape of xi(r) to a power-law form for the power spectrum, P(k)\propto k^{-2.2}. For a plausible value of the bias parameter b = 1/sigma_8 ~ 1.8, one has (delta_rho / rho)_{rms} ~ 1 r ~ 3.5/h Mpc, suggesting that the change of slope observed in xi(r) around this scale marks the transition between the linear and nonlinear gravitational regimes. Under this working hypothesis, we use a simple analytical form to fit the large-scale correlations constraints together with the COBE CMB anisotropy measurement, thus constructing a simple phenomenological model for the linear power spectrum. Despite its simplicity, the model fits remarkably well directly estimated power spectra from different optical galaxy samples, and when evolved through an N-body simulation it provides a good match to the observed galaxy correlations. One of the most interesting features of the model is the small-scale one-dimensional velocity dispersion produced: sigma_{1d} = 450 Km s^{-1} at 0.5/h Mpc and sigma_{1d} = 350 Km s^{-1} for separations larger than ~ 2/h Mpc.

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