The star formation history and chemical evolution of star forming galaxies in the nearby universe

We have determined the O/H and N/O of a sample of 122751 SFGs from the DR7 of the SDSS. For all these galaxies we have also determined their morphology and their SFH using the code STARLIGHT. The comparison of the chemical abundance with the SFH allows us to describe the chemical evolution in the nearby universe (z < 0.25) in a manner which is consistent with the formation of their stellar populations and morphologies. A 45% of the SFGs in our sample show an excess of abundance in nitrogen relative to their metallicity. We also find this excess to be accompanied by a deficiency of oxygen, which suggests that this could be the result of effective starburst winds. However, we find no difference in the mode of star formation of the nitrogen rich and nitrogen poor SFGs. Our analysis suggests they all form their stars through a succession of bursts of star formation extended over a few Gyr period. What produces the chemical differences between these galaxies seems therefore to be the intensity of the bursts: the galaxies with an excess of nitrogen are those that are presently experiencing more intense bursts, or have experienced more intense bursts in their past. We also find evidence relating the chemical evolution process to the formation of the galaxies: the galaxies with an excess of nitrogen are more massive, have more massive bulges and earlier morphologies than those showing no excess. As a possible explanation we propose that the lost of metals consistent with starburst winds took place during the formation of the galaxies, when their potential wells were still building up, and consequently were weaker than today, making starburst winds more efficient and independent of the final mass of the galaxies. In good agreement with this interpretation, we also find evidence consistent with downsizing, according to which the more massive SFGs formed before the less massive ones.Comment: 69 pages, 27 figures, accepted for publication in Ap

Riesgo de inundación en la subcuenca del río La Antigua, Veracr z, México

El objetivo del presente trabajo fue aplicar el modelo de simulación hidráulica HEC-RAS para delimitar las zonas con riesgo a inundación en la subcuenca del río La Antigua, Veracruz, México, utilizando la metodología del Centro Nacional de Prevención de Desastres (Cenapred), que considera la vulnerabilidad y el peligro de un sistema ante la ocurrencia de un fenómeno extremo perturbador. El modelo HEC-RAS usa ecuaciones diferenciales determinísticas que simulan el comportamiento y la dinámica de los niveles de agua en secciones transversales del cauce; su aplicación bajo condiciones de flujo no permanente y régimen mixto permitió definir las áreas de inundación alcanzadas durante una avenida extraordinaria. Para la simulación hidráulica se construyeron hidrogramas unitarios sintéticos con datos de precipitación registrados durante un evento hidrometeorológico extremo; la geometría del cauce se obtuvo con la extensión HEC-GeoRAS, herramienta que extrajo y procesó información georreferenciada del Modelo Digital de Elevación (MDE) LiDAR tipo terreno y los coeficientes de rugosidad (h de Manning) se estimaron de acuerdo con las características físicas del canal. Para calibrar los datos estimados por el modelo se utilizó información hidrométrica (niveles registrados) de la estación 28003 Cardel. La aplicación de tres estadísticas de prueba: R, RMSE y DMA demostraron un alto ajuste entre datos observados versus simulados, respaldando la aplicabilidad del modelo HEC-RAS para simular, predecir y delimitar zonas de riesgo a inundación

What makes a galaxy radio-loud?

We compare the Spectral Energy Distribution (SED) of radio-loud and radio-quiet AGNs in three different samples observed with SDSS: radio-loud AGNs (RLAGNs), Low Luminosity AGNs (LLAGNs) and AGNs in isolated galaxies (IG-AGNs). All these galaxies have similar optical spectral characteristics. The median SED of the RLAGNs is consistent with the characteristic SED of quasars, while that of the LLAGNs and IG-AGNs are consistent with the SED of LINERs, with a lower luminosity in the IG-AGNs than in the LLAGNs. We infer the masses of the black holes (BHs) from the bulge masses. These increase from the IG-AGNs to the LLAGNs and are highest for the RLAGNs. All these AGNs show accretion rates near or slightly below 10% of the Eddington limit, the differences in luminosity being solely due to different BH masses. Our results suggests there are two types of AGNs, radio quiet and radio loud, differing only by the mass of their bulges or BHs.Comment: 3 pages, 3 figures; to appear in Proceedings of IAU Symposium No. 284, The Spectral Energy Distribution of Galaxies (SED2011), Preston, UK, 5-9 sep. 201

A widespread riboswitch candidate that controls bacterial genes involved in molybdenum cofactor and tungsten cofactor metabolism

We have identified a highly conserved RNA motif located upstream of genes encoding molybdate transporters, molybdenum cofactor (Moco) biosynthesis enzymes, and proteins that utilize Moco as a coenzyme. Bioinformatics searches have identified 176 representatives in γ-Proteobacteria, δ-Proteobacteria, Clostridia, Actinobacteria, Deinococcus-Thermus species and DNAs from environmental samples. Using genetic assays, we demonstrate that a Moco RNA in Escherichia coli associated with the Moco biosynthetic operon controls gene expression in response to Moco production. In addition, we provide evidence indicating that this conserved RNA discriminates against closely related analogues of Moco. These results, together with extensive phylogenetic conservation and typical gene control structures near some examples, indicate that representatives of this structured RNA represent a novel class of riboswitches that sense Moco. Furthermore, we identify variants of this RNA that are likely to be triggered by the related tungsten cofactor (Tuco), which carries tungsten in place of molybdenum as the metal constituent