1,091 research outputs found
The Influence of Interactions and Minor Mergers on the Structure of Galactic Disks: I.Observations and Disk Models
This paper is the first part in our series on the influence of tidal
interactions and minor mergers on the radial and vertical disk structure of
spiral galaxies. We report on the sample selection, our observations, and data
reduction. Surface photometry of the optical and near infrared data of a sample
of 110 highly-inclined/edge-on disk galaxies are presented. This sample
consists of two subsamples of 61 non-interacting galaxies (control sample) and
of 49 interacting galaxies/minor merging candidates. Additionally, 41 of these
galaxies were observed in the near infrared. We show that the distribution of
morphological types of both subsamples is almost indistinguishable, covering
the range between 0 <= T <= 9. An improved, 3-dimensional disk modelling- and
fitting procedure is described in order to analyze and to compare the disk
structure of our sample galaxies by using characteristic parameters. We find
that the vertical brightness profiles of galactic disks respond very sensitive
even to small deviations from the perfect edge-on orientation. Hence,
projection effects of slightly inclined disks may cause substantial changes in
the value of the disk scale height and must therefore be considered in the
subsequent study.Comment: LaTeX, 36 pages, 5 figures, complete series of papers incl. all
figures of higher quality is available at
http://aurora.as.arizona.edu/~schwarz
Properties of tidally-triggered vertical disk perturbations
We present a detailed analysis of the properties of warps and
tidally-triggered perturbations perpendicular to the plane of 47
interacting/merging edge-on spiral galaxies. The derived parameters are
compared with those obtained for a sample of 61 non-interacting edge-on
spirals. The entire optical (R-band) sample used for this study was presented
in two previous papers. We find that the scale height of disks in the
interacting/merging sample is characterized by perturbations on both large
(~disk cut-off radius) and short (~z0) scales, with amplitudes of the order of
280pc and 130pc on average, respectively. The size of these large (short)
-scale instabilities corresponds to 14% (6%) of the mean disk scale height.
This is a factor of 2 (1.5) larger than the value found for non-interacting
galaxies. A hallmark of nearly all tidally distorted disks is a scale height
that increases systematically with radial distance. The frequent occurrence and
the significantly larger size of these gradients indicate that disk asymmetries
on large scales are a common and persistent phenomenon, while local
disturbances and bending instabilities decline on shorter timescales. Nearly
all (93%) of the interacting/merging and 45% of the non-interacting galaxies
studied are noticeably warped. Warps of interacting/merging galaxies are ~2.5
times larger on average than those observed in the non-interacting sample, with
sizes of the order of 340pc and 140pc, respectively. This indicates that tidal
distortions do considerably contribute to the formation and size of warps.
However, they cannot entirely explain the frequent occurrence of warped disks.Comment: LaTeX, 35 pages, 6 figures, all figures and appendix of higher
quality available at http://aurora.as.arizona.edu/~schwarz
An empirical study of the tails of mutual fund size
The mutual fund industry manages about a quarter of the assets in the U.S.
stock market and thus plays an important role in the U.S. economy. The question
of how much control is concentrated in the hands of the largest players is best
quantitatively discussed in terms of the tail behavior of the mutual fund size
distribution. We study the distribution empirically and show that the tail is
much better described by a log-normal than a power law, indicating less
concentration than, for example, personal income. The results are highly
statistically significant and are consistent across fifteen years. This
contradicts a recent theory concerning the origin of the power law tails of the
trading volume distribution. Based on the analysis in a companion paper, the
log-normality is to be expected, and indicates that the distribution of mutual
funds remains perpetually out of equilibrium.Comment: 6 pages, 3 figure
The cause of universality in growth fluctuations
Phenomena as diverse as breeding bird populations, the size of U.S. firms,
money invested in mutual funds, the GDP of individual countries and the
scientific output of universities all show unusual but remarkably similar
growth fluctuations. The fluctuations display characteristic features,
including double exponential scaling in the body of the distribution and power
law scaling of the standard deviation as a function of size. To explain this we
propose a remarkably simple additive replication model: At each step each
individual is replaced by a new number of individuals drawn from the same
replication distribution. If the replication distribution is sufficiently heavy
tailed then the growth fluctuations are Levy distributed. We analyze the data
from bird populations, firms, and mutual funds and show that our predictions
match the data well, in several respects: Our theory results in a much better
collapse of the individual distributions onto a single curve and also correctly
predicts the scaling of the standard deviation with size. To illustrate how
this can emerge from a collective microscopic dynamics we propose a model based
on stochastic influence dynamics over a scale-free contact network and show
that it produces results similar to those observed. We also extend the model to
deal with correlations between individual elements. Our main conclusion is that
the universality of growth fluctuations is driven by the additivity of growth
processes and the action of the generalized central limit theorem.Comment: 18 pages, 4 figures, Supporting information provided with the source
files
Three-dimensional modelling of edge-on disk galaxies
We present detailed three-dimensional modelling of the stellar luminosity
distribution for the disks of 31 relatively nearby (<= 110 Mpc) edge-on spiral
galaxies. In contrast to most of the standard methods available in the
literature we take into account the full three-dimensional information of the
disk. We minimize the difference between the observed 2D-image and an image of
our 3D-disk model integrated along the line of sight. Thereby we specify the
inclination, the fitting function for the z-distribution of the disk, and the
best values for the structural parameters such as scalelength, scaleheight,
central surface brightness, and a disk cut-off radius. From a comparison of two
independently developed methods we conclude, that the discrepancies e.g. for
the scaleheights and scalelengths are of the order of ~10%. These differences
are not due to the individual method itself, but rather to the selected fitting
region, which masks the bulge component, the dust lane, or present foreground
stars. Other serious limitations are small but appreciable intrinsic deviations
of real disks compared to the simple input model. In this paper we describe the
methods and present contour plots as well as radial profiles for all galaxies
without previously published surface photometry. Resulting parameters are given
for the complete sample.Comment: LaTeX, 25 pages, 28 figures higher quality figures available at
http://www.astro.ruhr-uni-bochum.de/astro/publications/pub2000.htm
Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem
Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis
Afraid of the Dark? The Influence of Natural and Artificial Light at Night on the Behavioral Activity of a Nocturnal Gecko
Both natural and artificial light at night can strongly influence animal behavior. Nocturnal animals often alter activity dependent on lunar light levels, to increase prey capture, minimize detection by predators, or both. Trade-offs among these ecological effects are likely to have a strong influence on behavior and fitness. Here, we examined the influence of light at night on nocturnal geckos that are both predators and prey, and use both natural and anthropogenic habitats. We tested the influence of illumination on the relative abundance and behavioral activity of native geckos in natural woodlands and under laboratory conditions. We hypothesized that Australian native house geckos (Gehyra dubia) would avoid activity on nights with high moon brightness, to minimize exposure to predators, consistent with the predation risk hypothesis. Counter to our prediction, we found a positive relationship between house gecko activity and moon brightness, i.e., house geckos were more active on bright nights. This behavior may allow house geckos to better see their prey while also increasing the visibility of approaching predators. In the laboratory, house geckos had shorter latency times to emerge from a shelter under low light conditions compared to darkness equivalent to a new moon, a trend consistent with higher activity under brighter conditions in the field. Light at night, from both natural and artificial sources, clearly influences the behavior and activity of geckos, but perhaps not in the ways we expect. Reducing the risk of attack from predators in darkness, and increasing prey capture success using vision, may increase the benefits of activity in lit conditions, compared to total darkness
Life-history consequences of divergent selection on egg size in Drosophila melanogaster
Life histories are generally assumed to evolve via antagonistic pleiotropy (negative genetic correlations) among traits, and trade-offs between life-history traits are typically studied using either phenotypic manipulations or selection experiments. We investigated the trade-off between egg size and fecundity in Drosophila melanogaster by examining both the phenotypic and genetic relationships between these traits after artificial selection for large and small eggs, relative to female body size. Egg size responded strongly to selection in both directions, increasing in the large-egg selected lines and decreasing in the small-egg selected lines. Phenotypic correlations between egg size and fecundity in the large-egg selected lines were negative, but no relationship between these traits occurred in either the control or small-egg selected lines. There was no negative genetic correlation between egg size and fecundity. Total reproductive allocation decreased in the small-egg selected lines but did not increase in the large-egg lines. Our results have three implications. First, our selection procedure may have forced females selected for large eggs into a physiological trade-off not reflected in a negative genetic correlation between these traits. Second, the lack of a negative genetic correlation between egg size and number suggests that the phenotypic trade-off frequently observed between egg size and number in other organisms may not evolve over the short term via a direct genetic trade-off whereby increases in egg size are automatically accompanied by decreased fecundity. Finally, total reproductive allocation may not evolve independently of egg size as commonly assumed
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