Dark matter from dark energy-baryonic matter couplings

We present a scenario in which a scalar field dark energy is coupled to the trace of the energy momentum tensor of the baryonic matter fields. In the slow-roll regime, this interaction could give rise to the cosmological features of dark matter. We work out the cosmological background solutions and fit the parameters of the model using the Union 2 supernovae data set. Then, we develop the cosmological perturbations up to linear order, and we find that the perturbed variables have an acceptable behavior, in particular the density contrast of baryonic matter grows similar to that in the $\Lambda$CDM model for a suitable choice of the strength parameter of the coupling.Comment: 10 pages, 8 figures, in this version small typos are corrected and it matches the published version in Phys. Rev. D15, January 201

The Dynamics of Galaxy Pairs in a Cosmological Setting

We use the Millennium Simulation, and an abundance-matching framework, to investigate the dynamical behaviour of galaxy pairs embedded in a cosmological context. Our main galaxy-pair sample, selected to have separations under 250 kpc/h, consists of over 1.3 million pairs at redshift z = 0, with stellar masses greater than 10^9 Msun, probing mass ratios down to 1:1000. We use dark matter halo membership and energy to classify our galaxy pairs. In terms of halo membership, central-satellite pairs tend to be in isolation (in relation to external more massive galaxies), are energetically- bound to each other, and are also weakly-bound to a neighbouring massive galaxy. Satellite-satellite pairs, instead, inhabit regions in close proximity to a more massive galaxy, are energetically-unbound, and are often bound to that neighbour. We find that 60% of our paired galaxies are bound to both their companion and to a third external object. Moreover, only 9% of our pairs resemble the kind of systems described by idealised binary merger simulations in complete isolation. In sum, we demonstrate the importance of properly connecting galaxy pairs to the rest of the Universe.Comment: 25 pages, 14 figures, accepted by MNRA

Mapping galaxy encounters in numerical simulations: The spatial extent of induced star formation

We employ a suite of 75 simulations of galaxies in idealised major mergers (stellar mass ratio ~2.5:1), with a wide range of orbital parameters, to investigate the spatial extent of interaction-induced star formation. Although the total star formation in galaxy encounters is generally elevated relative to isolated galaxies, we find that this elevation is a combination of intense enhancements within the central kpc and moderately suppressed activity at large galacto-centric radii. The radial dependence of the star formation enhancement is stronger in the less massive galaxy than in the primary, and is also more pronounced in mergers of more closely aligned disc spin orientations. Conversely, these trends are almost entirely independent of the encounter's impact parameter and orbital eccentricity. Our predictions of the radial dependence of triggered star formation, and specifically the suppression of star formation beyond kph-scales, will be testable with the next generation of integral-field spectroscopic surveys.Comment: 12 pages, 8 figures, accepted by MNRA

Evaluación de los efectos genotóxicos del herbicida 2,4-D en Piaractus mesopotamicus a través del test de micronúcleos

The herbicide 2,4-D is currently one of the most used agrochemicals in agriculture. The fish are target of contamination, these when being in contact with toxics develop later alterations that can be studied, reason why they are used as models in the evaluation of aquatic ecosystems. There is scarce information about the effects of these pesticides in fish. Because of this, the objective of this work was to evaluate the possible impact of a contamination with 2,4-D, in sub-lethal concentrations in Piaractus mesopotamicus, through the frequency of micronuclei (MN) and alterations in the shape of the nuclei (NMA) in peripheral blood erythrocytes in chronic conditions (70 days). Two trials were conducted, one with the herbicide 2,4-D pure (P) and another with a commercial formulation (2,4-D bitter amine) (FC). Each experience was composed of five aquariums with two specimens in each, where different concentrations of the pesticide were administered (1 ppm, 1.8 ppm, 3.2 ppm, 5.6 ppm and 10 ppm), and in another with well water (control). A total of 4000 cells per individual were analyzed. Through the test MN and NMA the presence of diverse nuclear alterations was evidenced. The mentioned test for P and dilutions 1 ppm, 1.8 ppm, 3.2 ppm, 5.6 ppm of FC did not show significant differences with the control, while the concentration of 10 ppm of FC differed statistically from its control, this could be due to additional components in FC.Los herbicidas están siendo abundantemente utilizados haciendo que los mismos no solo lleguen al organismo específico sino también contaminen el medio ambiente donde son empleados. El herbicida 2,4-D es en la actualidad uno de los agroquímicos más utilizado en la agricultura. Los peces son blanco de la contaminación, éstos al estar en contacto con tóxicos desarrollan posteriormente alteraciones que pueden ser estudiadas, por lo cual son utilizados como modelos en la evaluacion de ecosistemas acuáticos. Existe escasa información acerca de los efectos de estos pesticidas en peces, siendo el actual trabajo el primero en evaluar efectos crónicos. El objetivo de la presente investigación fue evaluar el posible impacto de una contaminación con 2,4-D en concentraciones subletales en Piaractus mesopotamicus, a través de la frecuencia de micronúcleos (MN) y de las alteraciones en la forma de los núcleos (NMA) en eritrocitos de sangre periférica en condiciones crónicas (70 días). Se realizaron dos ensayos, uno con el herbicida 2,4-D puro (P) y otro con una formulación comercial (2,4-D amina Sumargo) (FC). Cada experiencia estuvo compuesta por cinco acuarios con dos ejemplares en cada uno, donde se administraron difererentes concentraciones del plaguicida (1 ppm, 1,8 ppm, 3,2 ppm, 5,6 ppm y 10 ppm), y en otra con agua de pozo (control). Se analizó un total de 4000 células por individuo. A través del test MN y NMA se evidenció la presencia de diversas alteraciones nucleares. El mencionado test para P y las diluciones 1 ppm, 1,8 ppm, 3,2 ppm, 5,6 ppm de FC no mostraron diferencias significativas con el control, mientras que la concentración de 10 ppm de FC se diferenció estadísticamente de su control. El presente trabajo aporta luz de los potenciales efectos nocivos del 2,4-D en el medio ambiente y posibilita nuevos estudios prospectivos y retrospectivos

Bridging the Gap: Observations and Theory of Star Formation Meet on Large and Small Scales

The drive to understand galaxy formation and evolution over the lifetime of the universe has justified vast space-based and ground-based telescope facilities, as well as the development of new technologies. The details of star formation, and the modes by which that activity couples to the broader galactic environment, occurs on small spatial scales. These scales can only be traced with great sophistication in the local universe, as witnessed by observations using the Spitzer Space Telescope, the Herschel Space Observatory, the Stratospheric Observatory for Infrared Astronomy (SOFIA), and the Atacama Large Millimeter Array (ALMA). A critical realization of the last decade, however, is that the large scales and small scales are strongly coupled, and cannot be treated in isolation. At the same time, the scales studied in the local universe are "sub-grid" for the purpose of cosmological simulations that make valiant efforts to include the physics of star formation and its feedback to the local environment. The fundamental uncertainties in how this sub-grid physics is incorporated into the larger picture are by far the greatest limitation in understanding galaxy formation and evolution. The main goal of cosmological simulations is to understand the formation and evolution of the universe over a wide range of scales going from the Hubble volume to sub-light-year scales within galaxies. However, due to computational limitations, the physics of star formation in galaxies and the effects this process has on the formation and evolution of galaxies are often greatly simplified. For instance, accounting for the effect of stellar feedback in cosmological simulations is a crucial factor in galaxy formation and evolution. Without stellar feedback in galaxies, the gas would rapidly cool and collapse, converting all available gas into stars within a dynamical time. This consequence is in sharp conflict with observations—there are vastly fewer stars in our universe than the models would predict. Until recently, numerical simulations have been unable to regulate star formation efficiently enough to reproduce observations, with many consequences: models with too many stars also predict they form at the wrong times in the universe’s history, that there are the wrong abundances of heavy elements, that galaxies look nothing like the Milky Way, and even that the observable properties of dark matter are (apparently) discordant with new precision-cosmology measurements. All of these problems may, in fact, be due to the fundamental problem of understanding how small and large scales interact