33,048 research outputs found
Stellar archeology of the nearby LINER galaxies NGC 4579 and NGC 4736
Stellar archeology of nearby LINER galaxies may reveal if there is a stellar
young population that may be responsible for the LINER phenomenon. We show
results for the classical LINER galaxies NGC 4579 and NGC 4736 and find no
evidence of such populations.Comment: 2 pages, 2 figures, to be published in the Proceedings of the IAU
Symposium no. 26
Mapping low and high density clouds in astrophysical nebulae by imaging forbidden line emission
Emission line ratios have been essential for determining physical parameters
such as gas temperature and density in astrophysical gaseous nebulae. With the
advent of panoramic spectroscopic devices, images of regions with emission
lines related to these physical parameters can, in principle, also be produced.
We show that, with observations from modern instruments, it is possible to
transform images taken from density sensitive forbidden lines into images of
emission from high and low-density clouds by applying a transformation matrix.
In order to achieve this, images of the pairs of density sensitive lines as
well as the adjacent continuum have to be observed and combined. We have
computed the critical densities for a series of pairs of lines in the infrared,
optical, ultraviolet and X-rays bands, and calculated the pair line intensity
ratios in the high and low-density limit using a 4 and 5 level atom
approximation. In order to illustrate the method we applied it to GMOS-IFU data
of two galactic nuclei. We conclude that this method provides new information
of astrophysical interest, especially for mapping low and high-density clouds;
for this reason we call it "the ld/hd imaging method".Comment: 7 pages, 4 figures, accepted for publication on MNRA
Simulations of a mortality plateau in the sexual Penna model for biological ageing
The Penna model is a strategy to simulate the genetic dynamics of
age-structured populations, in which the individuals genomes are represented by
bit-strings. It provides a simple metaphor for the evolutionary process in
terms of the mutation accumulation theory. In its original version, an
individual dies due to inherited diseases when its current number of
accumulated mutations, n, reaches a threshold value, T. Since the number of
accumulated diseases increases with age, the probability to die is zero for
very young ages (n = T). Here, instead
of using a step function to determine the genetic death age, we test several
other functions that may or may not slightly increase the death probability at
young ages (n < T), but that decreases this probability at old ones. Our
purpose is to study the oldest old effect, that is, a plateau in the mortality
curves at advanced ages. Imposing certain conditions, it has been possible to
obtain a clear plateau using the Penna model. However, a more realistic one
appears when a modified version, that keeps the population size fixed without
fluctuations, is used. We also find a relation between the birth rate, the
age-structure of the population and the death probability.Comment: submitted to Phys. Rev.
Scaling laws of human interaction activity
Even though people in our contemporary, technological society are depending
on communication, our understanding of the underlying laws of human
communicational behavior continues to be poorly understood. Here we investigate
the communication patterns in two social Internet communities in search of
statistical laws in human interaction activity. This research reveals that
human communication networks dynamically follow scaling laws that may also
explain the observed trends in economic growth. Specifically, we identify a
generalized version of Gibrat's law of social activity expressed as a scaling
law between the fluctuations in the number of messages sent by members and
their level of activity. Gibrat's law has been essential in understanding
economic growth patterns, yet without an underlying general principle for its
origin. We attribute this scaling law to long-term correlation patterns in
human activity, which surprisingly span from days to the entire period of the
available data of more than one year. Further, we provide a mathematical
framework that relates the generalized version of Gibrat's law to the long-term
correlated dynamics, which suggests that the same underlying mechanism could be
the source of Gibrat's law in economics, ranging from large firms, research and
development expenditures, gross domestic product of countries, to city
population growth. These findings are also of importance for designing
communication networks and for the understanding of the dynamics of social
systems in which communication plays a role, such as economic markets and
political systems.Comment: 20+7 pages, 4+2 figure
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