arXiv subjects
Publications and source records attributed to L. Guzzo.
The paper describes the goals and first results of the ROSAT ESO Flux-Limited X-ray (REFLEX) Cluster Survey, a redshift survey of the X-ray brightest galaxy clusters in the ROSAT All-Sky Survey in the southern hemisphere conducted in the frame of an ESO key programme.
Based on the ROSAT All-Sky Survey we have conducted a large redshift survey as an ESO key programme to identify and secure redshifts for the X-ray brightest clusters found in the southern hemisphere. We present first results for a highly controlled sample for a flux limit of 3 10-12 erg/s/cm+2 comprising 475 clusters (87% with redshifts). The logN-logS function of the sample shows an almost perfect Euclidian slope and a preliminary X-ray luminosity function is presented.
We present a detailed discussion of the redshift errors associated to the ESO Slice Project measurements. For a subsample of 742 galaxies with redshifts determined both from the absorption lines (V_{abs}) and from the emission lines (V_{emi}), we find an average difference ~ +100$ km/s. We find that a similar effect is present in another, deeper redshift survey, the Durham/Anglo-Australian Telescope faint galaxy redshift survey, while is absent in surveys at brighter magnitude limits. We have investigated in detail many possible sources of such a discrepancy, and we can exclude possible zero-point shifts or calibration problems. We have detected and measured systematic velocity differences produced by the different templates used in the cross-correlation. We conclude that such differences can in principle explain the effect, but in this case the non-trivial implication would be that the best-fitting template does not necessarily give the best velocity estimate. As we do not have any a priori reason to select a template different from the best-fitting one, we did not apply any correction to the ESO Slice Project velocities. However, as for a small number of galaxies the effect is so large that it is likely to have a physical explanation, we have also taken into account the possibility that the discrepancy can be partly real: in this case, it might help to understand the role of gas outflows in the process of galaxy evolution. In view of the future large spectroscopic surveys, we stress the importance of using different templates and making them publicly available, in order to assess the amplitude of systematic effects, and to allow a direct comparison of different catalogues.
The ESO Slice Project (ESP) is a galaxy redshift survey extending over about 23 square degrees, in a region near the South Galactic Pole. The survey is ~85% complete to the limiting magnitude b_J=19.4 and consists of 3342 galaxies with redshift determination. The ESP survey is intermediate between shallow, wide angle samples and very deep, one-dimensional pencil beams; the spanned volume is ~ 5 x 10^4 Mpc^3 at the sensitivity peak (z ~ 0.1). In this paper we present the description of the observations and of the data reduction, the ESP redshift catalogue and the analysis of the quality of the velocity determinations.
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.
We have re-examined the nature of the cluster galaxy luminosity function using the data from the Edinburgh-Durham Southern Galaxy Catalogue and the Edinburgh-Milano Redshift Survey. We derive a best fit luminosity function over the range M(bj)=-18 to -21, for a composite sample of 22 of the richer clusters that has M*=-20.16+/-0.02 and alpha=-1.22+/-0.04. The dominant error in these values results from the choice of background subtraction method. From extensive simulations we can show that when the LF is fitted over this narrow range, it is difficult to discriminate against bright values of M*in the single cluster fits, but that faint values provide a strong test of the universality of the luminosity function. We find that all the individual cluster data are well fit by a Schechter function with alpha fixed at -1.25, and that <=10% of these have fitted values of M* that disagree from the average at the 99% confidence level. We further show that fitting only a single parameter Schechter function to composite subsets of the data can give erroneous results for the derived M*. By considering two parameter fits, the results of Monte-Carlo simulations and direct two-sample chi-squared tests we conclude that there is only weak evidence for differences between the data when broken down into subsets based on physical properties (Bautz-Morgan class, richness, velocity dispersion): from our simulations, only the evidence for a difference between subsets based on velocity dispersion may in fact be significant. We find no evidence at all that a Schechter function is not a good model for the intrinsic cluster luminosity function over this absolute magnitude range. Models that invoke strong evolution of galaxy luminosity of all galaxies within clusters are inconsistent with our results.
(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.
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.
We analyse a set of new pencil-beam galaxy redshift data in three small regions around the South Galactic Pole (SGP) area. We investigate whether we can find any evidence of the quasi-periodic peaks discovered by Broadhurst et al. (1990) in the distribution of galaxies along the NGP-SGP directions. We use both a power spectrum analysis and a cross-correlation with a sliding comb-like window (the comb-template technique). Despite the data are less deep (~600 h^{-1} Mpc) and certainly not optimal for such an investigation, there is evidence of the same preferential ~130 h^{-1} Mpc scale in two fields displaced respectively 15 and 30 degrees West of the Broadhurst et al. original probe. Taken alone, however, this peak would not be statistically distinguishable from a noise fluctuation. Nevertheless, the statistical significance of the detection raises to 99% when one considers the CONDITIONAL probability of finding a peak around THE SAME scale measured by Broadhurst et al.
We report the discovery of two bright arcs in what turns out to be the brightest X--ray cluster in the ROSAT band ever observed, RXJ1347.5-1145. Its luminosity is $(6.2\pm0.6) \cdot10^{45}$erg s$^{-1}$ (in the range 0.1--2.4~keV). The arcs are most probably gravitationally lensed images of background galaxies. They were found serendipitously during our ongoing large--scale redshift survey of X--ray clusters detected by the ROSAT All Sky Survey. The arcs are almost opposite to each other with respect to the cluster centre, with a distance from it of about $35''$ ($=240 h^{-1}_{50}$ kpc), a radius that enables the probing of a rather large cluster volume. In this Letter we limit ourselves to the discussion of the general optical and X--ray features of this cluster and to the potential implications of the gravitational arcs. A more detailed discussion of the different mass estimates and of the cosmological implications for this exceptional object are left for future work based on more accurate optical and X--ray data, which are currently being collected.
We present the data from an extensive, moderately deep (b_J = 19.5) spectroscopic survey of $\sim 600$ galaxies within four regions of sky located near the South Galactic Pole. About 75% of the measured galaxies are in a 3deg x 1.5deg region dominated by the rich cluster Klemola 44 (Abell 4038). The other three smaller areas cover about 1 square degree each. Here we discuss in detail the observing and data reduction strategies, the completeness and errors on the measured redshifts. The data collected are being used for: (1) a study of the large--scale redshift distribution of the galaxies in each field, and (2) a thorough dynamical investigation of Klemola 44. Results from these analyses will be presented in forthcoming papers.
We present some new results from an ongoing investigation of the anisotropy of the two--point correlation function for optically selected galaxies. We have estimated xi(r_p,pi) from the Perseus-Pisces redshift survey, which is now virtually 100\% complete for all morphological types to m_B = 15.5. We detect strong distortions of the iso-correlation contours in xi(r_p,pi). These correspond to a global (`cluster' plus `field') pairwise velocity dispersion at 1 h^{-1} Mpc, sigma(1), which is about a factor of two higher than the canonical value \sim 350 km s^{-1} obtained from the CfA1 survey and recently from the 1.2 Jy IRAS redshift survey. However, the variance of this value is still rather strong even within the relatively large volumes surveyed (typically 90$^\circ$ $\times$ 45$^\circ$ $\times$ 100 h^{-1} Mpc). From the largest volume explorable (126 h^{-1} Mpc depth), the best bet for the `cosmic' value of sigma(1) seems to be between 600 and 700 km s^{-1}. However, for a hopefully robust determination of its value we will have to wait for the analyses of the next generation of larger {\tenbf optical} redshift surveys (such as, e.g., the now completed ESP or the forthcoming Sloan Digital Sky Survey).
We present the redshifts of the galaxies and galaxy clusters which form the Edinburgh--Milano (EM) cluster redshift survey. A total of 777 galaxy redshifts have been measured in 94 clusters extracted from the digitised Edinburgh Durham Cluster Catalogue. We also present the radial velocity dispersions for 37 clusters. Observational and data reduction techniques are discussed in detail, together with the strategy adopted to determine the mean redshift of a cluster and to identify and discard plausible phantom clusters. Some 10% of our clusters show heavy contamination, indicating that projection is a serious problem for optically selected rich clusters. The median velocity dispersion estimated for a sub--sample of richness R \geq 1 clusters is 742 \pm63 km/sec From a simple comparison with Omega=1 Cold Dark Matter models of structure formation, these results favour a biasing parameter b=1.6-2.0 and are inconsistent with a bias outside of the range b=1.3-2.5.
We present here the results of a statistical search for cluster alignments using the Edinburgh/Milano cluster redshift survey. This survey is a unique cluster database which has been objectively constructed to help minimise the systematic biases associated with previous optical cluster catalogues. We find some evidence for cluster alignments out to spatial separations of $<10\mpc$, however, it is not statistically significant. On larger scales, we find no evidence, statistically significant or not, for cluster alignments. These results are in most disagreement with the recent observations of West and Plionis; both of whom see significant cluster alignments out to $\simeq30\mpc$ and beyond in the Abell \& Lick catalogues of clusters. Our findings are consistent with other searches for cluster alignments that do not involve these catalogues.
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.
We use the void probability statistics to study the redshift-space galaxy distribution as described by a volume-limited subsample of the Perseus-Pisces survey. We compare the results with the same analysis realized on artificial samples, extracted from high-resolution N-body simulations by reproducing the observational biases of the real data set. Simulations are run for the Cold+HotDM model (CHDM) and for unbiased and biased (b=1.5) CDM models in a 50 Mpc/h box. We identify galaxies as residing in peaks of the evolved density field. We fragment overmerged structures into individual galaxies so as to reproduce both the correct luminosity function (after assuming M/ L values for the resulting galaxy groups) and the two-point correlation function. Our main result is that a void-probability function (VPF) from the standard CHDM model with fractions 60% cold, 30% hot, 10% barions, exceeds the observational VPF with a high confidence level. CDM models produce smaller VPF independent of the biasing parameter. We verify the robustness of this result against changing the observer position in the simulations and the galaxy identification in the evolved density field.
We report the serendipitous discovery of a giant grvitational arc in the Abell Supplementary cluster S295 during optical follow-up observations of clusters detected in the ROSAT All-Sky Survey. The cluster has a redshift z=0.3007 and a velocity dispersion of about 900 km/sec. This is the first giant arc newly found in a ROSAT Survey cluster, indicating the potential advantage of searching for giant arcs in X-ray selected clusters.
We discuss the effects of redshift--space distortions on the estimate of high order galaxy correlation functions. We use both the Perseus--Pisces redshift survey and the results of high--resolution N--body simulations to explore the consequences of working in redshift space on the detection of deviations from hierarchical clustering. Both for real and simulated data, significant deviations from hierarchical clustering seem to be present in real space. Their behaviour is coherent with that expected from an initially biased galaxy field, once the displacement from the sites where galaxies formed, due to the nonlinear gravitational evolution, is taken into account. The passage to redshift space has the net effect of filtering out the higher powers required to fit the distribution in real space. This magnifies the distortions operated by nonlinear evolution on the initial distribution, erasing the residual narrow scale range (2--5~$h^{-1}$Mpc) where deviations from hierarchical clustering are still detectable in real space. We conclude that such deviations can hardly be estimated using data in redshift space.