arXiv subjects

S. Maurogordato

Publications and source records attributed to S. Maurogordato.

102 records · Page 6Linked to original sources

arXiv1998

Is the Universe a fractal? Results from the SSRS2

We perform a fractal analysis of the Southern Sky Redshift Survey 2, following the methods prescribed by Pietronero and collaborators, to check their claim that the galaxy distribution is fractal at all scales, and we discuss and explain the reasons of some controversial points, through tests on both galaxy samples and simulations. We confirm that the amplitude of the two-point correlation function does not depend on the sample depth, but increases with luminosity. We find that there is no contradiction between the results of standard and non-standard statistical analysis; moreover, such results are consistent with theoretical predictions derived from standard CDM models of galaxy formation, and with the hypothesis of homogeneity at large scale ($\sim 100$ \h). However, for our SSRS2 volume-limited subsamples we show that the first zero-point of the autocorrelation function $ξ(s)$ increases linearly with the sample depth, and that its value is comparable to the radius of the maximum sphere which can be completely included in the sampled volume; this implies that the true zero-crossing point of $ξ(s)$ has not been reached. We conclude that the apparent fractal behavior is due to a combination of a luminosity-dependent correlation amplitude and the recovering of power at larger scales in deeper samples.

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arXiv1998

The ESO Slice Project [ESP] galaxy redshift survey: V. Evidence for a D=3 sample dimensionality

The issue of the approximate isotropy and homogeneity of the observable universe is one of the major topics in modern Cosmology: the common use of the Friedmann-Robertson-Walker [FWR] metric relies on these assumptions. Therefore, results conflicting with the ``canonical'' picture would be of the utmost importance. In a number of recent papers it has been suggested that strong evidence of a fractal distribution with dimension D~2 exists in several samples, including Abell clusters [ACO] and galaxies from the ESO Slice Project redshift survey [ESP].Here we report the results of an independent analysis of the radial density run,N(<R)~R^D, of the ESP and ACO data. For the ESP data the situation is such that the explored volume, albeit reasonably deep, is still influenced by the presence of large structures. Moreover, the depth of the ESP survey (z<0.2) is such to cause noticeable effects according to different choices of k-corrections, and this adds some additional uncertainty in the results. However, we find that for a variety of volume limited samples the dimensionality of the ESP sample is D~3, and the value $D = 2$ is always excluded at the level of at least five (bootstrap) standard deviations. The only way in which we reproduce D~2 is by both unphysically ignoring the galaxy k-correction and using Euclidean rather than FRW cosmological distances. In the cluster case the problems related to the choice of metrics and k-correction are much lessened, and we find that ACO clusters have D_{ACO} = 3.07 +- 0.18 and D_{ACO} = 2.93 +- 0.15 for richness class R \geq 1 and R \geq 0, respectively. Therefore D=2 is excluded with high significance also for the cluster data.

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arXiv1998

Properties of Very Luminous Galaxies

Recent analysis of the SSRS2 data based on cell-counts and two-point correlation function has shown that very luminous galaxies are much more strongly clustered than fainter galaxies. In fact, the amplitude of the correlation function of very luminous galaxies ($L > L^*$) asymptotically approaches that of $R \ge 0$ clusters. In this paper we investigate the properties of the most luminous galaxies, with blue absolute magnitude $M_B \le -21$. We find that: 1) the population mix is comparable to that in other ranges of absolute magnitudes; 2) only a small fraction are located in bona fide clusters; 3) the bright galaxy-cluster cross-correlation function is significantly higher on large scales than that measured for fainter galaxies; 4) the correlation length of galaxies brighter than \MB $ \sim -20.0$, expressed as a function of the mean interparticle distance, appears to follow the universal dimensionless correlation function found for clusters and radio galaxies; 5) a large fraction of the bright galaxies are in interacting pairs, others show evidence for tidal distortions, while some appear to be surrounded by faint satellite galaxies. We conclude that very luminous optical galaxies differ from the normal population of galaxies both in the clustering and other respects. We speculate that this population is highly biased tracers of mass, being associated to dark halos with masses more comparable to clusters than typical loose groups.

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arXiv1997

The ESO Slice Project (ESP) galaxy redshift survey: II. The luminosity function and mean galaxy density

(Abridged) The ESO Slice Project (ESP) is a galaxy redshift survey we have completed as an ESO Key-Project over ~23 square degrees, in a region near the South Galactic Pole. The survey is nearly complete to the limiting magnitude b_J=19.4 and consists of 3342 galaxies with reliable redshift determination. The ESP survey is intermediate between shallow, wide angle samples and very deep, one-dimensional pencil beams: spanning a volume of ~ 5 x 10^4 Mpc^3 at the sensitivity peak (z ~ 0.1), it provides an accurate determination of the "local" luminosity function and the mean galaxy density. We find that, although a Schechter function is an acceptable representation of the luminosity function over the entire range of magnitudes (M < -12.4), our data suggest the presence of a steepening of the luminosity function for M > -17. The amplitude and the alpha and M^* parameters of our luminosity function are in good agreement with those of the AUTOFIB redshift survey (Ellis et al. 1996). Viceversa, our amplitude is significantly higher, by a factor ~ 1.6 at M ~ M^*, than that found for both the Stromlo-APM (Loveday et al. 1992) and the Las Campanas (Lin et al. 1996) redshift surveys. Also the faint end slope of our luminosity function is significantly steeper than that found in these two surveys. Large over- and under- densities are clearly seen in our data. In particular, we find evidence for a "local" underdensity (for D < 140 Mpc) and a significant overdensity at z ~ 0.1. When these radial density variations are taken into account, our derived luminosity function reproduces very well the observed counts for b_J < 19.4, including the steeper than Euclidean slope for b_J < 17.

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arXiv1997

The ESO Slice Project (ESP) galaxy redshift survey: I. Description and First Results

The ESO Slice Project (ESP) is a galaxy redshift survey we have recently completed as an ESO Key-Project. The ESP covers 23.3 square degrees in a region close to the South Galactic Pole. The survey is nearly complete (85%) to the limiting magnitude b_J=19.4 and consists of 3342 galaxies with reliable redshift determination. In this paper, the first in a series that will present the results of the ESP survey, we describe the main characteristics of the survey and briefly discuss the properties of the galaxy sample. From a preliminary spectral analysis of a large sub-sample of 2550 galaxies we find that the fraction of actively star-forming galaxies increases from a few percent for the brightest galaxies up to about 40% for the galaxies fainter than M= -16.5. The most outstanding feature in the ESP redshift distribution is a very significant peak at z ~ 0.1. The detection of similar peaks, at the same distance, in other surveys in the same region of the sky, suggests the presence of a large bidimensional structure perpendicular to the line of sight. The minimum size of this structure would be of the order of 100 x 50 Mpc, comparable with the size of the Great Wall.

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arXiv1996

Five contributions to the 3rd ESO-VLT Workshop ``Galaxy Scaling Relations''

In the first contribution the basic problems posed by the existence of the Fundamental Plane (FP), and its relations with the Virial Theorem are discussed. Various possibilities are presented that can produce the observed uniform departure from homology and/or from a constant stellar mass-to-light ratio. The role of orbital anisotropy and its relation with the FP thickness are also discussed. In the second contribution the observational evidence for the existence of a FP for galaxy clusters, analogous to that of ellipticals, is presented and discussed. In the third contribution the dynamical properties of $R^{1/m}$ spherical galaxies with variable orbital anisotropy are discussed. The stability of the models against radial--orbit instability is studied, and the consequences on the FP tilt and thickness are discussed. An innovative tool for the construction of Evolutionary Synthesis models of Stellar Populations is described in the fourth contribution. As an illustrative example, the evolution of the stellar mass-to-light ratios has been computed and the implications on the FP of elliptical galaxies are briefly discussed. Finally, in the fifth contribution, with the aid of the first large sample of X-ray emitting early-type galaxies with known kinematics, the effect of rotation on the X-ray emission, particularly with regard to the X-ray underluminosity of flat systems is discussed. The role of structural parameters as the ellipticity, the isophotal shape parameter $a_4/a$, and the inner slope of the surface brightness profile recently measured by HST are also investigated.

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arXiv1996

Biasing in the Galaxy Distribution

We investigate the variation of galaxy clustering with luminosity using the recently completed SSRS2 sample. Clustering measurements based on the two-point correlation function and the variance of counts in cells reveal the existence of a strong dependence of the clustering amplitude on luminosity for galaxies brighter than L*, while no significant variation is detected for fainter galaxies. We derive a relative bias versus magnitude relation which can be compared with theoretical predictions. Existing models of galaxy formation cannot adequately reproduce the simultaneous steep rise of biasing at high luminosities and the plateau at the low-luminosity end. Improved modeling of the halo-galaxy relation and larger samples including low luminosity galaxies are required to draw more definitive conclusions.

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arXiv1994

Preliminary Results from the ESO Slice Project (ESP)

We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accomplishing as an ESO Key-Project over about 30 square degrees in a region near the South Galactic Pole. The limiting magnitude is b_J = 19.4. Observations have been almost completed and about 90% of the data obtained so far has been reduced providing about 3000 galaxy redshifts. We present some preliminary results concerning the large scale galaxy distribution and their luminosity function.

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arXiv1994

High-Order Correlations of Rich Galaxy Clusters

We analyse the two--dimensional all--sky distribution of rich Abell and ACO galaxy clusters by using counts in cells and measuring the high--order area--averaged angular correlation functions. Confirming previous results, we find a well defined hierarchical relation between the two and three--point correlation functions, remarkably constant with scale. In the angular range $2^\circ \le θ\le 4^\circ$, the southern sample, limited at $b_{II} \le -40^\circ$ and including both Abell and ACO clusters, shows a remarkable hierarchical behavior up to the 6th order, while northern Abell clusters give positive correlations in the same range only up to the 4th order. The inferred deprojected values of the 3--D coefficients $S_J$, where $S_J = \barξ_J / {\barξ_2}^{J-1}$, are similar to those measured for the galaxy distribution, and consistent with theoretical predictions. These results are confirmed to the 4th order by our analysis of a 3--D sample of Abell and ACO clusters. Assuming that selection effects and / or the absence of a cluster fair sample are the reason of the difference between the two galactic hemispheres, and between Abell and ACO clusters, our results indicate that the statistical properties of the cluster distribution originate from the underlying galaxy distribution and show that the biasing between clusters and galaxies is non--linear.

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arXiv1993

Preliminary Results from the ESO Slice Project (ESP)

We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accomplishing as an ESO Key--Project over about 40 square degrees in a region near the South Galactic Pole. The limiting magnitude is $b_J = 19.4$. Up to now $\sim 85\%$ of the observations has been completed and $\sim 65\%$ of the data has been reduced providing $\sim 2000$ galaxy redshifts. We present some preliminary results concerning the large scale galaxy distribution, the luminosity function and the properties of galaxies with spectra showing prominent emission lines.

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arXiv1993

The Fundamental Plane of Galaxy Clusters

Velocity dispersion $σ$, radius $R$ and luminosity $L$ of elliptical galaxies are known to be related, leaving only two degrees of freedom and defining the so-called ``fundamental plane". In this {\em Letter} we present observational evidence that rich galaxy clusters exhibit a similar behaviour. Assuming a relation $L \propto R^ασ^{2 β}$, the best-fit values of $α$ and $β$ are very close to those defined by galaxies. The dispersion of this relation is lower than 10 percent, i.e. significantly smaller than the dispersion observed in the $L-σ$ and $L-R$ relations. We briefly suggest some possible implications on the spread of formation times of objects and on peculiar velocities of galaxy clusters.

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arXiv1992

The Spatial Distribution of Nearby Galaxy Clusters in the Northern and Southern Galactic Hemispheres

We compare the spatial distributions of galaxy clusters in the northern and southern galactic hemispheres, and the Abell and ACO clusters distributions. We perform a statistical (correlation and cluster) analysis of a sample of Abell and ACO galaxy clusters in the southern galactic hemisphere. We compare these results with a symmetric sample at northern galactic latitude taken from Postman et al. (1992). For the northern sample, we substantially confirm the results of Postman et al. We find that the two-point spatial correlation function of northern and southern clusters is comparable, with mean correlation length 19.6 Mpc and slope -1.8 positive up to about 45 Mpc. Percolation properties are remarkably similar in the northern and southern cluster samples. We give also a catalog of superclusters. In the south galactic hemisphere the main feature is a very rich, extended supercluster in the Horologium region at a redshift 0.06, near to a large void.

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