19,734 research outputs found
PAIRWISE VELOCITIES OF GALAXIES IN THE CFA AND SSRS2 REDSHIFT SURVEYS
(compressed version) We combine the CfA Redshift Survey (CfA2) and the
Southern Sky Redshift Survey (SSRS2) to estimate the pairwise velocity
dispersion of galaxies \sig12 on a scale of \sim 1 \hmpc. Both surveys are
complete to an apparent magnitude limit . Our sample includes 12,812
galaxies distributed in a volume 1.8 \times 10^6 \hmpc3. We conclude: 1) The
pairwise velocity dispersion of galaxies in the combined CfA2+SSRS2 redshift
survey is \sig12=540 \kms \pm 180 \kms. Both the estimate and the variance of
\sig12 significantly exceed the canonical values \sig12=340 \pm40 measured
by Davis \& Peebles (1983) using CfA1. 2) We derive the uncertainty in \sig12
from the variation among subsamples with volumes on the order of \hmpc3. This variation is nearly an order of magnitude larger than the
formal error, 36 \kms, derived using least-squares fits to the CfA2+SSRS2
correlation function. This variation among samples is consistent with the
conclusions of Mo \etal (1993) for a number of smaller surveys and with the
analysis of CfA1 by Zurek \etal (1994). 3) When we remove Abell clusters with
from our sample, the pairwise velocity dispersion of the remaining
galaxies drops to 295 \pm 99 \kms. Thus the dominant source of variance in
\sig12 is the shot noise contributed by dense virialized systems. 4) The
distribution of pairwise velocities is consistent with an isotropic exponential
with velocity dispersion independent of scale.Comment: 61 pages uuencoded, compressed postscript in 5 pieces. Also available
in one piece at http://www.dao.nrc.ca/DAO/SCIENCE/science.htm
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 () asymptotically
approaches that of clusters. In this paper we investigate the
properties of the most luminous galaxies, with blue absolute magnitude . 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 ,
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.Comment: 29 pages (6 figures) + 2 tables; paper with all figures and images
available at http://boas5.bo.astro.it/~cappi/papers.html; The Astronomical
Journal, in pres
The Power Spectrum of Galaxies in the Nearby Universe
We compute the power spectrum of galaxy density fluctuations in a recently
completed redshift survey of optically-selected galaxies in the southern
hemisphere (SSRS2). The amplitude and shape of the SSRS2 power spectrum are
consistent with results of the Center for Astrophysics redshift survey of the
northern hemisphere (CfA2), including the abrupt change of slope on a scale of
30-50Mpc/h; these results are reproducible for independent volumes of space and
variations are consistent with the errors estimated from mock surveys. Taken
together, the SSRS2 and CfA2 form a complete sample of 14,383 galaxies which
covers one-third of the sky. The power spectrum of this larger sample continues
to rise on scales up to ~ 200Mpc/h, with weak evidence for flattening on the
largest scales. The SSRS2+CfA2 power spectrum and the power spectrum
constraints implied by COBE are well-matched by an Omega*h ~ 0.2,
Omega+lambda_0=1 CDM model with minimal biasing of optically-selected galaxies.Comment: Accepted for publication in The Astrophysical Journal Letters, Sept.
23, 1994. 10 pages uuencoded compressed postscript, including two figures.
JHU-9410200
2D pattern evolution constrained by complex network dynamics
Complex networks have established themselves along the last years as being
particularly suitable and flexible for representing and modeling several
complex natural and human-made systems. At the same time in which the
structural intricacies of such networks are being revealed and understood,
efforts have also been directed at investigating how such connectivity
properties define and constrain the dynamics of systems unfolding on such
structures. However, lesser attention has been focused on hybrid systems,
\textit{i.e.} involving more than one type of network and/or dynamics. Because
several real systems present such an organization (\textit{e.g.} the dynamics
of a disease coexisting with the dynamics of the immune system), it becomes
important to address such hybrid systems. The current paper investigates a
specific system involving a diffusive (linear and non-linear) dynamics taking
place in a regular network while interacting with a complex network of
defensive agents following Erd\"os-R\'enyi and Barab\'asi-Albert graph models,
whose nodes can be displaced spatially. More specifically, the complex network
is expected to control, and if possible to extinguish, the diffusion of some
given unwanted process (\textit{e.g.} fire, oil spilling, pest dissemination,
and virus or bacteria reproduction during an infection). Two types of pattern
evolution are considered: Fick and Gray-Scott. The nodes of the defensive
network then interact with the diffusing patterns and communicate between
themselves in order to control the spreading. The main findings include the
identification of higher efficiency for the Barab\'asi-Albert control networks.Comment: 18 pages, 32 figures. A working manuscript, comments are welcome
Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence
We present a spectroscopic analysis of main sequence (MS) stars in the young
globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our
photometric survey on Magellanic Clouds clusters has revealed that NGC1818,
similarly to the other young objects with age 600 Myrs, displays not only an
extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2
Gyrs), but also a split MS. The most straightforward interpretation of the
double MS is the presence of two stellar populations: a sequence of
slowly-rotating stars lying on the blue-MS and a sequence of fast rotators,
with rotation close to the breaking speed, defining a red-MS. We report the
first direct spectroscopic measurements of projected rotational velocities
vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of
line profiles includes non-LTE effects, required for correctly deriving v sini
values. Our results suggest that: (i) the mean rotation for blue- and red-MS
stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91
km/s), respectively; (ii) eMSTO stars have different vsini, which are generally
lower than those inferred for red-MS stars, and (iii) as expected, Be stars
display the highest vsini values. This analyis supports the idea that distinct
rotational velocities play an important role in the appearence of multiple
stellar populations in the color-magnitude diagrams of young clusters, and
poses new constraints to the current scenarios.Comment: 16 pages, 1 table, 9 figures. Accepted for publication in AJ
(11/07/2018
Labels for non-individuals
Quasi-set theory is a first order theory without identity, which allows us to
cope with non-individuals in a sense. A weaker equivalence relation called
``indistinguishability'' is an extension of identity in the sense that if
is identical to then and are indistinguishable, although the
reciprocal is not always valid. The interesting point is that quasi-set theory
provides us a useful mathematical background for dealing with collections of
indistinguishable elementary quantum particles. In the present paper, however,
we show that even in quasi-set theory it is possible to label objects that are
considered as non-individuals. We intend to prove that individuality has
nothing to do with any labelling process at all, as suggested by some authors.
We discuss the physical interpretation of our results.Comment: 11 pages, no figure
New Techniques for Relating Dynamically Close Galaxy Pairs to Merger and Accretion Rates : Application to the SSRS2 Redshift Survey
We introduce two new pair statistics, which relate close galaxy pairs to the
merger and accretion rates. We demonstrate the importance of correcting these
(and other) pair statistics for selection effects related to sample depth and
completeness. In particular, we highlight the severe bias that can result from
the use of a flux-limited survey. The first statistic, denoted N_c, gives the
number of companions per galaxy, within a specified range in absolute
magnitude. N_c is directly related to the galaxy merger rate. The second
statistic, called L_c, gives the total luminosity in companions, per galaxy.
This quantity can be used to investigate the mass accretion rate. Both N_c and
L_c are related to the galaxy correlation function and luminosity function in a
straightforward manner. We outline techniques which account for various
selection effects, and demonstrate the success of this approach using Monte
Carlo simulations. If one assumes that clustering is independent of luminosity
(which is appropriate for reasonable ranges in luminosity), then these
statistics may be applied to flux-limited surveys.
These techniques are applied to a sample of 5426 galaxies in the SSRS2
redshift survey. Using close dynamical pairs, we find N_c(-21<M_B<-18) =
0.0226+/-0.0052 and L_c(-21<M_B<-18) = 0.0216+/-0.0055 10^{10} h^2 L_sun at
z=0.015. These are the first secure estimates of low-z close pair statistics.
If N_c remains fixed with redshift, simple assumptions imply that ~ 6.6% of
present day galaxies with -21<M_B<-18 have undergone mergers since z=1. When
applied to redshift surveys of more distant galaxies, these techniques will
yield the first robust estimates of evolution in the galaxy merger and
accretion rates. [Abridged]Comment: 26 pages (including 10 postscript figures) plus 3 gif figures.
Accepted for publication in ApJ. Paper (including full resolution images)
also available at http://www.astro.utoronto.ca/~patton/ssrs2, along with
associated pair classification experiment (clickable version of Figure 5
Prominent effect of soil network heterogeneity on microbial invasion
Using a network representation for real soil samples and mathematical models for microbial spread, we show that the structural heterogeneity of the soil habitat may have a very significant influence on the size of microbial invasions of the soil pore space. In particular, neglecting the soil structural heterogeneity may lead to a substantial underestimation of microbial invasion. Such effects are explained in terms of a crucial interplay between heterogeneity in microbial spread and heterogeneity in the topology of soil networks. The main influence of network topology on invasion is linked to the existence of long channels in soil networks that may act as bridges for transmission of microorganisms between distant parts of soil
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