35 research outputs found
Topological Non--connectivity Threshold in long-range spin systems
We demonstrate the existence of a topological disconnection threshold,
recently found in Ref. \cite{JSP}, for generic anisotropic Heisenberg
models interacting with an inter--particle potential when
(here is the distance among spins). We also show that if
is greater than the embedding dimension then the ratio between
the disconnected energy region and the total energy region goes to zero when
the number of spins becomes very large. On the other hand, numerical
simulations in for the long-range case support the
conclusion that such a ratio remains finite for large values. The
disconnection threshold can thus be thought as a distinctive property of
anisotropic long-range interacting systems.Comment: submitted to PR
The Dynamics of Poor Systems of Galaxies
We assemble and observe a sample of poor galaxy systems that is suitable for
testing N-body simulations of hierarchical clustering (Navarro, Frenk, & White
1997; NFW) and other dynamical halo models (e.g., Hernquist 1990). We (1)
determine the parameters of the density profile rho(r) and the velocity
dispersion profile sigma(R), (2) separate emission-line galaxies from
absorption-line galaxies, examining the model parameters and as a function of
spectroscopic type, and (3) for the best-behaved subsample, constrain the
velocity anisotropy parameter, beta, which determines the shapes of the galaxy
orbits.
The NFW universal profile and the Hernquist (1990) model both provide good
descriptions of the spatial data. In most cases an isothermal sphere is ruled
out. Systems with declining sigma(R) are well-matched by theoretical profiles
in which the star-forming galaxies have predominantly radial orbits (beta > 0);
many of these galaxies are probably falling in for the first time. There is
significant evidence for spatial segregation of the spectroscopic classes
regardless of sigma(R).Comment: 36 pages, 20 figures, and 5 tables. To appear in the Astrophysical
Journa
Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a
complete, distance-limited (6000 km/s) and magnitude-limited
(B14) sample of 7000 optical galaxies. The sample covers 2/3 (8.27
sr) of the sky () and appears to have a good completeness in
redshift (98%). We select the sample on the basis of homogenized corrected
total blue magnitudes in order to minimize systematic effects in galaxy
sampling.
We identify the groups in this sample by means of both the hierarchical and
the percolation {\it friends of friends} methods. The resulting catalogs of
loose groups appear to be similar and are among the largest catalogs of groups
presently available. Most of the NOG galaxies (60%) are found to be
members of galaxy pairs (580 pairs for a total of 15% of objects)
or groups with at least three members (500 groups for a total of
45% of objects). About 40% of galaxies are left ungrouped (field
galaxies). We illustrate the main features of the NOG galaxy distribution.
Compared to previous optical and IRAS galaxy samples, the NOG provides a denser
sampling of the galaxy distribution in the nearby universe. Given its large sky
coverage, the identification of groups, and its high-density sampling, the NOG
is suited for the analysis of the galaxy density field of the nearby universe,
especially on small scales.Comment: 47 pages including 6 figures. Accepted for publication in Ap
The redshift-space two-point correlation functions of galaxies and groups in the Nearby Optical Galaxy sample
We use the two-point correlation function in redshift space, , to
study the clustering of the galaxies and groups of the Nearby Optical Galaxy
(NOG) sample, which is a nearly all-sky, complete, magnitude-limited sample of
7000 bright and nearby optical galaxies. The correlation function of
galaxies is well described by a power law, , with
slope and Mpc (on scales Mpc), in agreement with previous results of several redshift surveys of
optical galaxies. We confirm the existence of morphological segregation between
early- and late-type galaxies and, in particular, we find a gradual decreasing
of the strength of clustering from the S0 galaxies to the late-type spirals, on
intermediate scales. Furthermore, luminous galaxies turn out to be more
clustered than dim galaxies. The luminosity segregation, which is significant
for both early- and late-type objects, starts to become appreciable only for
galaxies brighter than () and is
independent on scale. The NOG group correlation functions are characterized by
-values ranging from Mpc (for groups with at least three
members) to Mpc (for groups with at least five members). The
degree of group clustering depends on the physical properties of groups.
Specifically, groups with greater velocity dispersions, sizes and masses tend
to be more clustered than those with lower values of these quantities.Comment: Astrophysical Journal, in press, 72 pages, 16 eps figure
Clustering of loose groups and galaxies from the Perseus--Pisces Survey
We investigate the clustering properties of loose groups in the
Perseus--Pisces redshift Survey (PPS). Previous analyses based on CfA and SSRS
surveys led to apparently contradictory results. We investigate the source of
such discrepancies, finding satisfactory explanations for them. Furthermore, we
find a definite signal of group clustering, whose amplitude exceeds the
amplitude of galaxy clustering (,
for the most significant case; distances are
measured in \hMpc). Groups are identified with the adaptive
Friends--Of--Friends (FOF) algorithms HG (Huchra \& Geller 1982) and NW
(Nolthenius \& White 1987), systematically varying all search parameters.
Correlation strenght is especially sensitive to the sky--link (increasing
for stricter normalization ), and to the (depth \mlim of the) galaxy
data. It is only moderately dependent on the galaxy luminosity function
, while it is almost insensitive to the redshift--link (both to
the normalization and to the scaling recipes HG or NW).Comment: 28 pages (LaTeX aasms4 style) + 5 Postscript figures ; ApJ submitted
on May 4th, 1996; group catalogs available upon request
([email protected]
Loose Groups of Galaxies in the Las Campanas Redshift Survey
A ``friends-of-friends'' percolation algorithm has been used to extract a
catalogue of dn/n = 80 density enhancements (groups) from the six slices of the
Las Campanas Redshift Survey (LCRS). The full catalogue contains 1495 groups
and includes 35% of the LCRS galaxy sample. A clean sample of 394 groups has
been derived by culling groups from the full sample which either are too close
to a slice edge, have a crossing time greater than a Hubble time, have a
corrected velocity dispersion of zero, or contain a 55-arcsec ``orphan'' (a
galaxy with a mock redshift which was excluded from the original LCRS redshift
catalogue due to its proximity to another galaxy -- i.e., within 55 arcsec).
Median properties derived from the clean sample include: line-of-sight velocity
dispersion sigma_los = 164km/s, crossing time t_cr = 0.10/H_0, harmonic radius
R_h = 0.58/h Mpc, pairwise separation R_p = 0.64/h Mpc, virial mass M_vir =
(1.90x10^13)/h M_sun, total group R-band luminosity L_tot = (1.30x10^11)/h^2
L_sun, and R-band mass-to-light ratio M/L = 171h M_sun/L_sun; the median number
of observed members in a group is 3.Comment: 32 pages of text, 27 figures, 7 tables. Figures 1, 4, 6, 7, and 8 are
in gif format. Tables 1 and 3 are in plain ASCII format (in paper source) and
are also available at http://www-sdss.fnal.gov:8000/~dtucker/LCLG . Accepted
for publication in the September 2000 issue of ApJ
Galaxy groups in the 2dF galaxy redshift survey: Large Scale Structure with Groups
We use the 2dF Galaxy Group Catalogue constructed by Merch\'an & Zandivarez
to study the large scale structure of the Universe traced by galaxy groups. We
concentrate on the computation of the power spectrum and the two point
correlation function. The resulting group power spectrum shows a similar shape
to the galaxy power spectrum obtained from the 2dF Galaxy Redshift Survey by
Percival et al., but with a higher amplitude quantified by a relative bias in
redshift space of . The group two point correlation function
for the total sample is well described by a power law with correlation length
s_0=8.9 \pm 0.3 \mpc and slope on scales s < 20 \mpc.
In order to study the dependence of the clustering properties on group mass we
split the catalogue in four subsamples defined by different ranges of group
virial masses. These computations allow a fair estimate of the relation
described by the correlation length and the mean intergroup separation
for galaxy systems of low mass. We also extend our study to the redshift
space distortions of galaxy groups, where we find that the anisotropies in the
clustering pattern of the 2dF group catalogue are consistent with gravitational
instability, with a flattening of the redshift-space correlation function
contours in the direction of the line of sight.Comment: 11 pages, 9 figures, resubmitted to MNRAS after revisio
The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined UZC and SSRS2
redshift surveys to construct a catalog of 1168 groups of galaxies; 411 of
these groups have 5 or more members within the redshift survey. The group
catalog covers 4.69 steradians and all groups exceed a number density contrast
threshold of 80. We demonstrate that the groups catalog is homogeneous across
the two underlying redshift surveys; the catalog of groups and their members
thus provides a basis for other statistical studies of the large-scale
distribution of groups and their physical properties. The median physical
properties of the groups are similar to those for groups derived from
independent surveys including the ESO Key Programme and the Las Campanas
Redshift Survey. We include tables of groups and their members.Comment: Accepted for publication on A
(So) Big Data and the transformation of the city
The exponential increase in the availability of large-scale mobility data has fueled the vision of smart cities that will transform our lives. The truth is that we have just scratched the surface of the research challenges that should be tackled in order to make this vision a reality. Consequently, there is an increasing interest among different research communities (ranging from civil engineering to computer science) and industrial stakeholders in building knowledge discovery pipelines over such data sources. At the same time, this widespread data availability also raises privacy issues that must be considered by both industrial and academic stakeholders. In this paper, we provide a wide perspective on the role that big data have in reshaping cities. The paper covers the main aspects of urban data analytics, focusing on privacy issues, algorithms, applications and services, and georeferenced data from social media. In discussing these aspects, we leverage, as concrete examples and case studies of urban data science tools, the results obtained in the âCity of Citizensâ thematic area of the Horizon 2020 SoBigData initiative, which includes a virtual research environment with mobility datasets and urban analytics methods developed by several institutions around Europe. We conclude the paper outlining the main research challenges that urban data science has yet to address in order to help make the smart city vision a reality
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray Properties of Clusters Detected as Extended X-ray Sources
In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination and galactic latitude and comprises sources with
a count rate counts s and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...Comment: 61 pages; ApJS in press; fixed bug in table file; also available at
(better image quality) http://www.xray.mpe.mpg.de/theorie/NORAS