A Characteristic Mass Scale in the Mass-Metallicity Relation of Galaxies

We study the shape of the gas-phase mass-metallicity relation (MZR) of a combined sample of present-day dwarf and high-mass star-forming galaxies using IZI, a Bayesian formalism for measuring chemical abundances presented in Blanc et al. 2015. We observe a characteristic stellar mass scale at $M_* \simeq 10^{9.5}$M$_{\odot}$, above which the ISM undergoes a sharp increase in its level of chemical enrichment. In the $10^{6}-10^{9.5}$M$_{\odot}$ range the MZR follows a shallow power-law ($Z\propto M^{\alpha}_*$) with slope $\alpha=0.14\pm0.08$. At approaching $M_* \simeq 10^{9.5}$M$_{\odot}$ the MZR steepens significantly, showing a slope of $\alpha=0.37\pm0.08$ in the $10^{9.5}-10^{10.5}$M$_{\odot}$ range, and a flattening towards a constant metallicity at higher stellar masses. This behavior is qualitatively different from results in the literature that show a single power-law MZR towards the low mass end. We thoroughly explore systematic uncertainties in our measurement, and show that the shape of the MZR is not induced by sample selection, aperture effects, a changing N/O abundance, the adopted methodology used to construct the MZR, secondary dependencies on star formation activity, nor diffuse ionized gas (DIG) contamination, but rather on differences in the method used to measure abundances. High resolution hydrodynamical simulations can qualitatively reproduce our result, and suggest a transition in the ability of galaxies to retain their metals for stellar masses above this threshold. The MZR characteristic mass scale also coincides with a transition in the scale height and clumpiness of cold gas disks, and a typical gas fraction below which the efficiency of star formation feedback for driving outflows is expected to decrease sharply.Comment: 24 pages, 11 figures, 4 tables, accepted for publication in Ap

Ab initio study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties

We present a first-principles study of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. We have found metallicity for the combined system caused by n-doping of the silicon slab representing the surface by the SWNT. We confirm this metallicity for nanotubes of different diameters and chiral angles, and find the effect to be independent of the orientation of the nanotubes on the surface. We also present adsorption energetics and configurations which show semiconducting SWNTs farther apart from the surface and transferring less charge, in comparison with metallic SWNTs of similar diameter.Comment: Replaces old (Jan 2006) version; more supporting material. 11 pages, 8 figures, 7 table

Chlorophyll fluorescence analysis and cadmiumcopper bioaccumulation in Ulva rigida (C. Agardh)

The objective of the present study was to determine the usefulness of the alga Ulva rigida (C. Agardh) in monitoring dissolved cadmium and copper in seawater, as well as the physiological stress caused by both metals. Discs from the alga (d = 19 mm) were cultured in different Cd and Cu growth media during a 12-day period. The concentrations established were 0.1, 0.3, 0.9, 2.7 mg metal 1⁻¹, as well as the corresponding controls. Chlorophyll fluorescence readings were performed on days 1, 2, 5, 9 and 12. Discs were frozen for later AAS analysis (Perkin-Elmer). Treatments with higher Cd concentration (0.3, 0.9, 2.7 mg l⁻¹) presented clear signs of disturbing photosystem II, since optimum quantum yields (Fv/Fm) were lower than the control responses. Similarly, Cu treatments diminished the alga's photosynthetic efficiency. Cu and Cd internal levels in U. rigida increased successively during the experiments, which also were proportional to the treatments. These levels were transformed into contamination factors (CF), calculated by dividing each concentration by its respective control and adjusting with Michaelis- Menten kinetics.Mediante este estudio se pretende determinar la utilidad del alga Ulva rigida (C. Agardh) para monitorizar Cd y Cu disueltos en agua de mar, así como el estrés fisiológico causado por ambos metales. Para ello, discos de dicha alga (d = 19 mm) se colocaron en diferentes soluciones de Cd y Cu, durante un periodo de 12 días, siendo las concentraciones seleccionadas: 0,1; 0,3; 0,9; 2,7 mg metal l⁻¹ de solución, así como los respectivos controles. Las lecturas de fluorescencia de clorofila se efectuaron los días 1, 2, 5, 9 y se analizó el contenido metálico de los discos mediante espectrofotometría de absorción atómica con llama (AAS). Los tratamientos de Cu y los de mayor concentración de Cd (0,3; 0,9; 2,7 mg l⁻¹) presentaron claras señales de perturbación del fotosistema II, con reducciones claras del rendimiento cuántico óptimo (Fv/Fm). De igual forma, los tratamientos de Cu redujeron la eficiencia fotosintética del alga. Los niveles corporales de Cd y Cu en U. rigida registraron incrementos sucesivos durante el periodo experimental y fueron proporcionales a los tratamientos. Las cinéticas de carga fueron modelizadas mediante ajuste a Michaelis-Menten.Instituto Español de Oceanografí