Constraining the Minimum Mass of High-Redshift Galaxies and Their Contribution to the Ionization State of the IGM

We model the latest HST WFPC3/IR observations of > 100 galaxies at redshifts z=7-8 in terms of a hierarchical galaxy formation model with starburst activity. Our model provides a distribution of UV luminosities per dark matter halo of a given mass and a natural explanation for the fraction of halos hosting galaxies. The observed luminosity function is best fit with a minimum halo mass per galaxy of 10^{9.4+0.3-0.9} Msun, corresponding to a virial temperature of 10^{4.9+0.2-0.7} K. Extrapolating to faint, undetected galaxies, the total production rate of ionizing radiation depends critically on this minimum mass. Future measurements with JWST should determine whether the entire galaxy population can comfortably account for the UV background required to keep the intergalactic medium ionized.Comment: 9 pages, 6 figures, submitted to ApJ, comments welcom

Extreme Galaxies During Reionization: Testing ISM and Disk Models

We test the ability of equilibrium galactic disk and one-zone interstellar medium models to describe the physical and emission properties of quasar hosts, submillimeter galaxies, and Lyman-alpha emitters at z>~6. The size, line widths, star formation rates, black hole accretion rates, gas masses and temperatures, and the relationships between these properties are all well-described by our model, and we provide approximate fitting formulae for comparison with future observations. However, comparing our carbon line predictions to observations reveals differences between the ISM at low and high redshifts. Our underestimate of the [CII] line emission indicates either higher star formation efficiencies in high-redshift molecular clouds or less depletion of metals into dust at fixed metallicity. Further, our over-prediction of the CO(6-5)/CO(1-0) ratio suggests that molecular clouds in real high-redshift galaxies have a lower turbulent Mach number and more subthermal CO(6-5) emission than expected owing either to sizes smaller than the local Jeans mass or to a pressure support mechanism other than turbulence.Comment: Accepted in MNRAS; 19 pages; 10 figures; 4 table

Verifying the Identity of High-Redshift Massive Galaxies Through the Clustering of Lower Mass Galaxies Around Them

Massive high-redshift galaxies form in over-dense regions where the probability of forming other galaxies is also strongly enhanced. Given an observed flux of a galaxy, the inferred mass of its host halo tends to be larger as its inferred redshift increases. As the mass and redshift of a galaxy halo increase, the expected clustering of other galaxies around it gets stronger. It is therefore possible to verify the high-redshift identity of a galaxy (prior to an unambiguous spectral identification) from the clustering of other galaxies around it. We illustrate this method for the massive galaxy suggested by Mobasher et al. (2005) to be at redshift z~6.5. If this galaxy were to exist at z~6.5, there should have been a mean of ~10 galaxies larger than a hundredth of its mass and having z-band magnitudes less than ~25 detected as i-dropouts in the HUDF. We calculate an approximate probability distribution for neighbor galaxies and determine that there is less than a ~0.3% chance of detecting no massive neighbor galaxies. The lack of other massive z~6.5 galaxies in the HUDF image argues that the Mobasher et al. (2005) galaxy is instead a low redshift interloper. We generalize our results to other galaxy masses and redshifts.Comment: 7 pages, 5 figures. Typos correcte

Formation in-situ of metallic nanoclusters of Mn interlayered into bentonite previously pillared with Al

Hasta el momento, el crecimiento de agregados polinucleares en el espacio interlaminar de diferentes aluminosilicatos por reacción en el estado sólido requiere de tiempos prolongados de contacto (alrededor de 4 meses). En consecuencia, en el presente trabajo se ha evaluado una metodología novedosa para llevar a cabo tal síntesis, específicamente para el crecimiento in-situ de MnS en el espacio interlaminar de una bentonita que involucra un tiempo aproximado de 10 horas de reacción. Sin embargo, los resultados han evidenciado que hay varias condiciones que se deben controlar por cuanto afectan la estabilidad de los clusters; entre estos factores están el tratamiento químico, térmico, el tipo de atmósfera empleado en la calcinación y la densidad de los pilares de Al

In-situ growing of MnS and FeS nanoclusters at the interlayer of Al-pillared bentonite

Introduction Since formation of metal nanoclusters is thermodynamically unstable and difficult to control, in this work it has been explored the in-situ growing of either MnS or FeS nanoclusters in the interlayer space of a bentonite by means of a pretty short process taking only around 12 h. The interlayered polynuclear sulfidized metal clusters were prepared by cationic exchange of either Mn 2+ or Fe 2+ on the bentonite previously interlayered/pillared with aluminium under different conditions. These metal sulfidized nanomaterials have attracted substantial interests due to their unique optical and electrical properties and wide variety of potential applications in electroluminescence 1 and nonlinear optical devices 2. Since the main physical and optical properties of such metal sulfides primarily depend on their shape and size, the immobilization of metal sulfide nanoparticles in a spatially confined environment is a way to control the photo-physical and photo-chemical properties which result in very interesting strategy of morphological control

Understanding the Observed Evolution of the Galaxy Luminosity Function from z=6-10 in the Context of Hierarchical Structure Formation

Recent observations of the Lyman-break galaxy (LBG) luminosity function (LF) from z~6-10 show a steep decline in abundance with increasing redshift. However, the LF is a convolution of the mass function of dark matter halos (HMF)--which also declines sharply over this redshift range--and the galaxy-formation physics that maps halo mass to galaxy luminosity. We consider the strong observed evolution in the LF from z~6-10 in this context and determine whether it can be explained solely by the behavior of the HMF. From z~6-8, we find a residual change in the physics of galaxy formation corresponding to a ~0.5 dex increase in the average luminosity of a halo of fixed mass. On the other hand, our analysis of recent LF measurements at z~10 shows that the paucity of detected galaxies is consistent with almost no change in the average luminosity at fixed halo mass from z~8. The LF slope also constrains the variation about this mean such that the luminosity of galaxies hosted by halos of the same mass are all within about an order-of-magnitude of each other. We show that these results are well-described by a simple model of galaxy formation in which cold-flow accretion is balanced by star formation and momentum-driven outflows. If galaxy formation proceeds in halos with masses down to 10^8 Msun, then such a model predicts that LBGs at z~10 should be able to maintain an ionized intergalactic medium as long as the ratio of the clumping factor to the ionizing escape fraction is C/f_esc < 10.Comment: 15 pages, 2 figures; results unchanged; accepted by JCA

The Flatness and Sudden Evolution of the Intergalactic Ionizing Background

The ionizing background of cosmic hydrogen is an important probe of the sources and absorbers of ionizing radiation in the post-reionization universe. Previous studies show that the ionization rate should be very sensitive to changes in the source population: as the emissivity rises, absorbers shrink in size, increasing the ionizing mean free path and, hence, the ionizing background. By contrast, observations of the ionizing background find a very flat evolution from z~2-5, before falling precipitously at z~6. We resolve this puzzling discrepancy by pointing out that, at z~2-5, optically thick absorbers are associated with the same collapsed halos that host ionizing sources. Thus, an increasing abundance of galaxies is compensated for by a corresponding increase in the absorber population, which moderates the instability in the ionizing background. However, by z~5-6, gas outside of halos dominates the absorption, the coupling between sources and absorbers is lost, and the ionizing background evolves rapidly. Our halo based model reproduces observations of the ionizing background, its flatness and sudden decline, as well as the redshift evolution of the ionizing mean free path. Our work suggests that, through much of their history, both star formation and photoelectric opacity in the universe track halo growth.Comment: 16 pages, 7 figures, submitted to MNRAS, revised methods leading to same but more convincing conclusion

Lattice dynamics of mixed semiconductors (Be,Zn)Se from first-principles calculations

Vibration properties of Zn(1-x)Be(x)Se, a mixed II-VI semiconductor haracterized by a high contrast in elastic properties of its pure constituents, ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation and frozen phonons. The calculations within the local density approximation has been done with the Siesta method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure endsystems were moreover done also by all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes early detected in Raman spectra in the intermediate region (20 to 80%) of ZnBe concentration. This was early discussed on the basis of a percolation phenomenon, i.e., the result of the formation of wall-to-wall --Be--Se-- chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of Be--Se bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence on the short-range order.Comment: 11 pages, 10 figures, to be published in Phys. Rev.