898 research outputs found
The Local Galaxy Density and the Arm Class of Spiral Galaxies
We have examined the effect of the environmental density on the arm
classification of an extensive sample of spiral galaxies included in the Nearby
Galaxy Catalog (Tully, 1988a). We have also explored the dependence of the arm
class of a galaxy on other factors, such as its blue absolute magnitude and its
disk-to-total mass ratio, inferred in the literature either from the gradient
of a good galaxy rotation curve or from a photometric mass decomposition
method. We have found that the arm class is strongly related to the absolute
magnitude in the mid-type spirals (in the sense that grand design galaxies are,
on average, more luminous than flocculent objects), whilst this relation is
considerably weaker in the early and late types. In general the influence of
the local density on the arm structure appears to be much weaker than that of
the absolute magnitude. The local density acts essentially in strengthening the
arm class--absolute magnitude relation for the mid types, whereas no
environmental density effects are observed in the early and late types. Using
the most recent estimates of the disk-to-total mass ratio, we do not confirm
this ratio to be a significant factor which affects the arm class;
nevertheless, owing to poor statistics and large uncertanties, the issue
remains open. Neither a local density effect nor an unambiguous bar effect on
the disk-to-total mass ratio is detectable; the latter finding may challenge
some theoretical viewpoints on the formation of bar structures.Comment: 15 pages, Latex, SISSA 102/93/A openbib.sty and 4 POSTSCRIPT figures
appende
Galaxies in group and field environments: a comparison of optical-NIR luminosities and colors
We compare properties of galaxies in loose groups with those in field
environment by analyzing the Nearby Optical Galaxy (NOG) catalog of galaxy
systems. We consider as group galaxies, objects belonging to systems with at
least five members identified by means of the "friends of friends method", and,
as field galaxies, all galaxies with no companions. We analyze both a
magnitude--limited sample of 959 and 2035 galaxies (groups vs. field galaxies,
respectively, B<14 mag, and 2000<cz<6000 km/s) and a volume-limited sample (M_B
<-19.01 mag, 2000<cz<4000 km/s 369 group and 548 field galaxies). For all these
galaxies, blue corrected magnitudes and morphological types are available. The
cross-correlation of NOG with the 2MASS second release allow us to assign K
magnitudes and obtain B-K colors for about half of the galaxies in our samples.
We analyze luminosity and color segregation-effects in relation with the
morphological segregation. For both B and K bands, we find that group galaxies
are, on average, more luminous than field galaxies and this effect is not
entirely a consequence of the morphological segregation. After taking into
account the morphological segregation, the luminosity difference between group
and field galaxies is about 10%. When considering only very early-type galaxies
(T<-2) the difference is larger than 30%. We also find that group galaxies are
redder than field galaxies, Delta(B-K) about 0.4 mag. However, after taking
into account the morphological segregation, we find a smaller B-K difference,
poorly significant (only at the c.l. of about 80%).Comment: 11 pages, 10 eps figures, A&A in pres
The Mass Function of Nearby Galaxy Clusters
We present the distribution of virial masses for nearby galaxy clusters, as
obtained from a data-set of 75 clusters, each having at least 20 galaxy members
with measured redshifts within 1 Abell radius. After having accounted for
problems of incompleteness of the data-set, we fitted a power-law to the
cluster mass distribution.Comment: 10 pages (2 figures not included, available upon request), LATEX,
Ref.SISSA 54/93/
Evidence of a new low field cross-over in the vortex critical velocity of type-II superconducting thin films
We measure current-voltage characteristics as function of magnetic field and
temperature in Nb strips of different thickness and width. The instability
voltage of the flux flow state related to the vortex critical velocity v* is
studied and compared with the Larkin-Ovchinnikov theory. Beside the usual
power-law dependence v* ~ B^-1/2, in the low field range a new cross-over
field, Bcr1, is observed below which v* decreases by further lowering the
external magnetic field B. We ascribe this unexpected cross-over to vortex
channeling due to a fan-like penetration of the applied magnetic field as
confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct
evidence of a general feature in type-II superconducting films at low fields,
that is a channel-like vortex motion induced by the inhomogeneous magnetic
state caused by the relatively strong pinning
Structures in Galaxy Clusters
The analysis of the presence of substructures in 16 well-sampled clusters of
galaxies suggests a stimulating hypothesis: Clusters could be classified as
unimodal or bimodal, on the basis of to the sub-clump distribution in the {\em
3-D} space of positions and velocities. The dynamic study of these clusters
shows that their fundamental characteristics, in particular the virial masses,
are not severely biased by the presence of subclustering if the system
considered is bound.Comment: (16 pages in LATEX, 4 tables in LATEX are at the end of the file, the
figures not included are available upon request), REF SISSA 158/93/
A new apparatus for deep patterning of beam sensitive targets by means of high-energy ion beam
The paper reports on a high precision equipment designed to modify over
3-dimensions (3D) by means of high-energy gold ions the local properties of
thin and thick films. A target-moving system aimed at creating patterns across
the volume is driven by an x-y writing protocol that allows one to modify beam
sensitive samples over micrometer-size regions of whatever shape. The apparatus
has a mechanical resolution of 15 nm. The issue of the local fluence
measurement has been particularly addressed. The setup has been checked by
means of different geometries patterned on beam sensitive sheets as well as on
superconducting materials. In the last case the 3D modification consists of
amorphous nanostructures. The nanostructures create zones with different
dissipative properties with respect to the virgin regions. The main analysis
method consists of magneto-optical imaging that provides local information on
the electrodynamics of the modified zones. Features typical of non-linear
current flow hint at which pattern geometry is more functional to applications
in the framework of nanostructures across superconducting films.Comment: 7 page
Gaze Behavior in One-Handed Catching and Its Relation with Interceptive Performance: What the Eyes Can't Tell
In ball sports, it is usually acknowledged that expert athletes track the ball more accurately than novices. However, there is also evidence that keeping the eyes on the ball is not always necessary for interception. Here we aimed at gaining new insights on the extent to which ocular pursuit performance is related to catching performance. To this end, we analyzed eye and head movements of nine subjects catching a ball projected by an actuated launching apparatus. Four different ball flight durations and two different ball arrival heights were tested and the quality of ocular pursuit was characterized by means of several timing and accuracy parameters. Catching performance differed across subjects and depended on ball flight characteristics. All subjects showed a similar sequence of eye movement events and a similar modulation of the timing of these events in relation to the characteristics of the ball trajectory. On a trial-by-trial basis there was a significant relationship only between pursuit duration and catching performance, confirming that keeping the eyes on the ball longer increases catching success probability. Ocular pursuit parameters values and their dependence on flight conditions as well as the eye and head contributions to gaze shift differed across subjects. However, the observed average individual ocular behavior and the eye-head coordination patterns were not directly related to the individual catching performance. These results suggest that several oculomotor strategies may be used to gather information on ball motion, and that factors unrelated to eye movements may underlie the observed differences in interceptive performance
The Local Galaxy Density and the Bars of Spiral Galaxies
Using a variety of parameters of local galaxy density, we have examined the
effects of the environmental density on the presence of bar structures in
spiral galaxies of various morphological types. For an extensive sample of
nearby galaxies, listed in the ``Nearby Galaxies Catalogue'' (Tully, 1988a), we
have found that the spirals characterized by a high local density tend to be
barred if they are early-type and early-type if they are barred (at the
significance level of 3 sigma), confirming some earlier suggestion of low
statistical significance. This fact, which is observed substantially in
low-luminosity spirals, indicates that galaxy interactions can stimulate the
formation of bars, primarily in early-type, low-luminosity spirals. This is in
partial, qualitative agreement with the most recent relevant N-body
simulations. On the other hand, no significant density segregation is observed
between pure S-shaped (S(s)) spirals and spirals with inner rings (S(r)), which
are often associated with bars. {\it Subject headings:} galaxies: general ---
galaxies: structure --- galaxies: interactions --- galaxies: clusteringComment: 23 pages, LaTeX, SISSA Ref. 178/92/
Optical Luminosities and Mass-to-Light Ratios of Nearby Galaxy Clusters
We analyze a sample of 105 clusters having virial mass homogeneously estimated and for which galaxy magnitudes are available with a well defined high degree of completeness. In particular, we consider a subsample of 89 clusters with B_j band galaxy magnitudes taken from the COSMOS/UKST Southern Sky Object Catalogue. We compute cluster luminosities L_{B_j} within several clustercentric distances and within the virialization radius R_{vir}. In particular, we use the luminosity function and background counts estimated by Lumsden et al. (1997) on the Edinburgh/Durham Southern Galaxy Catalogue. We analyze the effect of several uncertainties connected to photometric data, fore/background removal, and extrapolation below the completeness limit of the photometry, in order to assess the robustness of our cluster luminosity estimates. We draw our results on the relations between luminosity and dynamical quantities from the COSMOS sample by considering mass and luminosities determined within the virialization radius. We find a very good correlation between cluster luminosity, L_{B_j}, and galaxy velocity dispersion, sigma_v, with L_{B_j} proportional to sigma_v^{2.1--2.3}. Our estimate of typical value for the mass-to-light ratio is M/L_{B_j} about 250 (in solar units). We do not find any correlation of M/L_{B_j} with cluster morphologies, i.e. Rood--Sastry and Bautz--Morgan types, and only a weak significant correlation with cluster richness. We find that mass has a slight, but significant, tendency to increase faster than the luminosity does, M proportional to L_{B_j}^{1.2--1.3}. We verify the robustness of this relation against a number of possible systematics
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