Changes in the diet of hake associated with El Niño 1997?1998 in the northern Humboldt Current ecosystem

International audienceHake (Merluccius gayi peruanus) predation plays an important role in the dynamics of the Humboldt Current ecosystem (HCE). Changes in the hake trophic habits associated with physical variability are expected to impact prey populations and to propagate through the food web. Time series (1995?2002) of (a) stomach contents of hake, (b) biomass estimations of fish prey species of hake, and (c) depth of the 15°C isotherm was analysed with the aim of exploring the impacts of El Niño 1997?1998 on the diet of hake. Biomass estimations of fish prey species were used to indicate resource availability, and depth of the 15°C isotherm to represent variability associated with the ENSO cycle in the physical environment of hake. The richness of prey species increased during the months when 15°C isotherm reached its deepest position, supporting the hypothesis of increased biodiversity (tropicalization) of the HCE during El Niño events. An increased variability in stomach fullness of hake was detected after 1999 which could indicate high heterogeneity in the food supply as a consequence of impacts of the warm event in the biotic community structure of the HCE, a physiological impairment of hake or an effect of the abrupt reduction in the mean total length of hake, postulated as a compensatory response to fishery pressure. Hake can be characterized as an opportunist predator according to the observed changes in its diet during 1995?2002. Overall, the diet of hake in the northern HCE exhibited transitory (e.g. increased richness of prey species in the stomach contents) and medium term (e.g. increased variability in feeding activity) responses associated with El Niño 1997?1998, which should be incorporated both in population dynamics and food web analyses

The Fundamental Plane of Gamma-ray Globular Clusters

We have investigated the properties of a group of $\gamma$-ray emitting globular clusters (GCs) which have recently been uncovered in our Galaxy. By correlating the observed $\gamma$-ray luminosities $L_{\gamma}$ with various cluster properties, we probe the origin of the high energy photons from these GCs. We report $L_{\gamma}$ is positively correlated with the encounter rate $\Gamma_{c}$ and the metalicity $\left[{\rm Fe/H}\right]$ which place an intimate link between the gamma-ray emission and the millisecond pulsar population. We also find a tendency that $L_{\gamma}$ increase with the energy densities of the soft photon at the cluster location. Furthermore, the two-dimensional regression analysis suggests that $L_{\gamma}$, soft photon densities, and $\Gamma_{c}$/$\left[{\rm Fe/H}\right]$ possibly span fundamental planes which potentially provide better predictions for the $\gamma$-ray properties of GCs.Comment: 17 pages, 4 figures, 3 tables, published in Ap

Cosmic axion thermalization

Axions differ from the other cold dark matter candidates in that they form a degenerate Bose gas. It is shown that their huge quantum degeneracy and large correlation length cause cold dark matter axions to thermalize through gravitational self-interactions when the photon temperature reaches approximately 500 eV. When they thermalize, the axions form a Bose-Einstein condensate. Their thermalization occurs in a regime, herein called the `condensed regime', where the Boltzmann equation is not valid because the energy dispersion of the particles is smaller than their interaction rate. We derive analytical expressions for the thermalization rate of particles in the condensed regime, and check the validity of these expressions by numerical simulation of a toy model. We revisit axion cosmology in light of axion Bose-Einstein condensation. It is shown that axions are indistinguishable from ordinary cold dark matter on all scales of observational interest, except when they thermalize or rethermalize. The rethermalization of axions that are about to fall in a galactic potential well causes them to acquire net overall rotation as they go to the lowest energy state consistent with the total angular momentum they acquired by tidal torquing. This phenomenon explains the occurrence of caustic rings of dark matter in galactic halos. We find that photons may reach thermal contact with axions and investigate the implications of this possibility for the measurements of cosmological parameters.Comment: 38 pages, 1 figur

Characterization of Dicer-deficient murine embryonic stem cells

Dicer is an RNase III-family nuclease that initiates RNA interference (RNAi) and related phenomena by generation of the small RNAs that determine the specificity of these gene silencing pathways. We have previously shown that Dicer is essential for mammalian development, with Dicer-deficient mice dying at embryonic day 7.5 with a lack of detectable multipotent stem cells. To permit a more detailed investigation of the biological roles of Dicer, we have generated embryonic stem cell lines in which their single Dicer gene can be conditionally inactivated. As expected, Dicer loss compromises maturation of microRNAs and leads to a defect in gene silencing triggered by long dsRNAs. However, the absence of Dicer does not affect the ability of small interfering RNAs to repress gene expression. Of interest, Dicer loss does compromise the proliferation of ES cells, possibly rationalizing the phenotype previously observed in Dicer-null animals. Dicer loss also affects the abundance of transcripts from mammalian centromeres but does so without a pronounced affect on histone modification status at pericentric repeats or methylation of centromeric DNA. These studies provide a conditional model of RNAi deficiency in mammals that will permit the dissection of the biological roles of the RNAi machinery in cultured mammalian cells

Observing two dark accelerators around the Galactic Centre with Fermi Large Area Telescope

We report the results from a detailed $\gamma-$ray investigation in the field of two "dark accelerators", HESS J1745-303 and HESS J1741-302, with $6.9$ years of data obtained by the Fermi Large Area Telescope. For HESS J1745-303, we found that its MeV-GeV emission is mainly originated from the "Region A" of the TeV feature. Its $\gamma-$ray spectrum can be modeled with a single power-law with a photon index of $\Gamma\sim2.5$ from few hundreds MeV to TeV. Moreover, an elongated feature, which extends from "Region A" toward northwest for $\sim1.3^{\circ}$, is discovered for the first time. The orientation of this feature is similar to that of a large scale atomic/molecular gas distribution. For HESS J1741-302, our analysis does not yield any MeV-GeV counterpart for this unidentified TeV source. On the other hand, we have detected a new point source, Fermi J1740.1-3013, serendipitously. Its spectrum is apparently curved which resembles that of a $\gamma-$ray pulsar. This makes it possibly associated with PSR B1737-20 or PSR J1739-3023.Comment: 11 pages, 7 figures, 2 tables, accepted for publication in MNRA

Further investigation on chaos of real digital filters

This Letter displays, via the numerical simulation of a real digital filter, that a finite-state machine may behave in a near-chaotic way even when its corresponding infinite-state machine does not exhibit chaotic behavior

Persistence in Cluster--Cluster Aggregation

Persistence is considered in diffusion--limited cluster--cluster aggregation, in one dimension and when the diffusion coefficient of a cluster depends on its size $s$ as $D(s) \sim s^\gamma$. The empty and filled site persistences are defined as the probabilities, that a site has been either empty or covered by a cluster all the time whereas the cluster persistence gives the probability of a cluster to remain intact. The filled site one is nonuniversal. The empty site and cluster persistences are found to be universal, as supported by analytical arguments and simulations. The empty site case decays algebraically with the exponent $\theta_E = 2/(2 - \gamma)$. The cluster persistence is related to the small $s$ behavior of the cluster size distribution and behaves also algebraically for $0 \le \gamma < 2$ while for $\gamma < 0$ the behavior is stretched exponential. In the scaling limit $t \to \infty$ and $K(t) \to \infty$ with $t/K(t)$ fixed the distribution of intervals of size $k$ between persistent regions scales as $n(k;t) = K^{-2} f(k/K)$, where $K(t) \sim t^\theta$ is the average interval size and $f(y) = e^{-y}$. For finite $t$ the scaling is poor for $k \ll t^z$, due to the insufficient separation of the two length scales: the distances between clusters, $t^z$, and that between persistent regions, $t^\theta$. For the size distribution of persistent regions the time and size dependences separate, the latter being independent of the diffusion exponent $\gamma$ but depending on the initial cluster size distribution.Comment: 14 pages, 12 figures, RevTeX, submitted to Phys. Rev.

A sandpile model with tokamak-like enhanced confinement phenomenology

Confinement phenomenology characteristic of magnetically confined plasmas emerges naturally from a simple sandpile algorithm when the parameter controlling redistribution scalelength is varied. Close analogues are found for enhanced confinement, edge pedestals, and edge localised modes (ELMs), and for the qualitative correlations between them. These results suggest that tokamak observations of avalanching transport are deeply linked to the existence of enhanced confinement and ELMs.Comment: Manuscript is revtex (latex) 1 file, 7 postscript figures Revised version is final version accepted for publication in PRL Revisions are mino

Life at high Deborah number

In many biological systems, microorganisms swim through complex polymeric fluids, and usually deform the medium at a rate faster than the inverse fluid relaxation time. We address the basic properties of such life at high Deborah number analytically by considering the small-amplitude swimming of a body in an arbitrary complex fluid. Using asymptotic analysis and differential geometry, we show that for a given swimming gait, the time-averaged leading-order swimming kinematics of the body can be expressed as an integral equation on the solution to a series of simpler Newtonian problems. We then use our results to demonstrate that Purcell's scallop theorem, which states that time-reversible body motion cannot be used for locomotion in a Newtonian fluid, breaks down in polymeric fluid environments

A search for VHE counterparts of Galactic Fermi bright sources and MeV to TeV spectral characterization

Very high-energy (VHE; E>100 GeV) gamma-rays have been detected from a wide range of astronomical objects, such as pulsar wind nebulae (PWNe), supernova remnants (SNRs), giant molecular clouds, gamma-ray binaries, the Galactic Center, active galactic nuclei (AGN), radio galaxies, starburst galaxies, and possibly star-forming regions as well. At lower energies, observations using the Large Area Telescope (LAT) onboard Fermi provide a rich set of data which can be used to study the behavior of cosmic accelerators in the MeV to TeV energy bands. In particular, the improved angular resolution of current telescopes in both bands compared to previous instruments significantly reduces source confusion and facilitates the identification of associated counterparts at lower energies. In this paper, a comprehensive search for VHE gamma-ray sources which are spatially coincident with Galactic Fermi/LAT bright sources is performed, and the available MeV to TeV spectra of coincident sources are compared. It is found that bright LAT GeV sources are correlated with TeV sources, in contrast to previous studies using EGRET data. Moreover, a single spectral component seems unable to describe the MeV to TeV spectra of many coincident GeV/TeV sources. It has been suggested that gamma-ray pulsars may be accompanied by VHE gamma-ray emitting nebulae, a hypothesis that can be tested with VHE observations of these pulsars.Comment: Astronomy and Astrophysics, in press, 17 pages, 12 figures, 5 table