18,144 research outputs found
The Shared Reward Dilemma
One of the most direct human mechanisms of promoting cooperation is rewarding
it. We study the effect of sharing a reward among cooperators in the most
stringent form of social dilemma, namely the Prisoner's Dilemma. Specifically,
for a group of players that collect payoffs by playing a pairwise Prisoner's
Dilemma game with their partners, we consider an external entity that
distributes a fixed reward equally among all cooperators. Thus, individuals
confront a new dilemma: on the one hand, they may be inclined to choose the
shared reward despite the possibility of being exploited by defectors; on the
other hand, if too many players do that, cooperators will obtain a poor reward
and defectors will outperform them. By appropriately tuning the amount to be
shared a vast variety of scenarios arises, including traditional ones in the
study of cooperation as well as more complex situations where unexpected
behavior can occur. We provide a complete classification of the equilibria of
the -player game as well as of its evolutionary dynamics.Comment: Major rewriting, new appendix, new figure
Analysis of the acoustic cut-off frequency and HIPs in six Kepler stars with stochastically excited pulsations
Gravito-acoustic modes in the Sun and other stars propagate in resonant
cavities with a frequency below a given limit known as the cut-off frequency.
At higher frequencies, waves are no longer trapped in the stellar interior and
become traveller waves. In this article we study six pulsating solar-like stars
at different evolutionary stages observed by the NASA Kepler mission. These
high signal-to-noise targets show a peak structure that extends at very high
frequencies and are good candidates for studying the transition region between
the modes and the interference peaks or pseudo-modes. Following the same
methodology successfully applied on Sun-as-a-star measurements, we uncover the
existence of pseudo-modes in these stars with one or two dominant interference
patterns depending on the evolutionary stage of the star. We also infer their
cut-off frequency as the midpoint between the last eigenmode and the first peak
of the interference patterns. By using ray theory we show that, while the
period of one of the interference pattern is very close to half the large
separation the other, one depends on the time phase of mixed waves, thus
carrying additional information on the stellar structure and evolution.Comment: Accepted for publication in A&A. 14 pages, 28 figure
Powering AGNs with super-critical black holes
We propose a novel mechanism for powering the central engines of Active
Galactic Nuclei through super-critical (type II) black hole collapse. In this
picture, ~ of material collapsing at relativistic speeds can
trigger a gravitational shock, which can eject a large percentage of the
collapsing matter at relativistic speeds, leaving behind a "light" black hole.
In the presence of a poloidal magnetic field, the plasma collimates along two
jets, and the associated electron synchrotron radiation can easily account for
the observed radio luminosities, sizes and durations of AGN jets. For Lorentz
factors of order 100 and magnetic fields of a few hundred , synchrotron
electrons can shine for yrs, producing jets of sizes of order 100 kpc.
This mechanism may also be relevant for Gamma Ray Bursts and, in the absence of
magnetic field, supernova explosions.Comment: 4 pages, 1 figur
Rewarding cooperation in social dilemmas
One of the most direct human mechanisms of promoting cooperation is rewarding it. We study the effect of sharing a reward among cooperators in the most stringent form of social dilemma. Thus, individuals confront a new dilemma: on the one hand, they may be inclined to choose the shared reward despite the possibility of being exploited by defectors; on the other hand, if too many players do that, cooperators will obtain a poor reward and defectors will outperform them. By appropriately tuning the amount to be shared we can cast a vast variety of scenarios, including traditional ones in the study of cooperation as well as more complex situations where unexpected behavior can occur. We provide a complete classification of the equilibria of the nplayer game as well as of the evolutionary dynamics. Beyond, we extend our analysis to a general class of public good games where competition among individuals with the same strategy exists.
F stars, metallicity, and the ages of red galaxies at z > 1
We explore whether the rest-frame near-UV spectral region, observable in
high-redshift galaxies via optical spectroscopy, contains sufficient
information to allow the degeneracy between age and metallicity to be lifted.
We do this by testing the ability of evolutionary synthesis models to reclaim
the correct metallicity when fitted to the near-UV spectra of F stars of known
(sub-solar and super-solar) metallicity. F stars are of particular interest
because the rest-frame near-UV spectra of the oldest known elliptical galaxies
at z > 1 appear to be dominated by F stars near to the main-sequence turnoff.
We find that, in the case of the F stars, where the HST ultraviolet spectra
have high signal:noise, model-fitting with metallicity allowed to vary as a
free parameter is rather successful at deriving the correct metallicity. As a
result, the estimated turnoff ages of these stars yielded by the model fitting
are well constrained. Encouraged by this we have fitted these same variable-
metallicity models to the deep, optical spectra of the z \simeq 1.5 mJy radio
galaxies 53W091 and 53W069 obtained with the Keck telescope. While the
age-metallicity degeneracy is not so easily lifted for these galaxies, we find
that even when metallicity is allowed as a free parameter, the best estimates
of their ages are still \geq 3 Gyr, with ages younger than 2 Gyr now strongly
excluded. Furthermore, we find that a search of the entire parameter space of
metallicity and star formation history using MOPED (Heavens et al., 2000) leads
to the same conclusion. Our results therefore continue to argue strongly
against an Einstein-de Sitter universe, and favour a lambda-dominated universe
in which star formation in at least these particular elliptical galaxies was
completed somewhere in the redshift range z = 3 - 5.Comment: 10 pages, LaTeX, uses MNRAS style file, incorporates 14 postscript
figures, submitted to MNRAS. Changes include: inclusion of single stellar
atmosphere model fits; more rigorous calculation of confidence regions; some
re-structurin
Stellar indices and kinematics in Seyfert 1 nuclei
We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst
galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges
centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the
Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs)
of these features and the Fe52 and Fe53 spectral indices. We found that the
strength of the CaT in type 1 Seyfert galaxies with prominent central point
sources, is larger than what would be expected from the observed strength of
the blue indices. This could be explained by the presence of red supergiants in
the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these
galaxies could also be diluted by the strong FeII multiplets that can be seen
in their spectra.
We have also measured the stellar and gas velocity dispersions of the
galaxies in the sample. The stellar velocity dispersions were measured using
both, the Mgb and CaT stellar features. The velocity dispersion of the gas in
the narrow line region (NLR) was measured using the strong emission lines
[OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity
dispersions and find that both magnitudes are correlated in Seyfert galaxies.
Most of the Seyfert 1 we observe have stellar velocity dispersion somehow
greater than that of the gas in the NLR.Comment: To appear in MNRAS, 18 pages, 9 figure
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