A Population of Compact Elliptical Galaxies Detected with the Virtual Observatory

Compact elliptical galaxies are characterized by small sizes and high stellar densities. They are thought to form through tidal stripping of massive progenitors. However, only a handful of them were known, preventing us from understanding the role played by this mechanism in galaxy evolution. We present a population of 21 compact elliptical galaxies gathered with the Virtual Observatory. Follow-up spectroscopy and data mining, using high-resolution images and large databases, show that all the galaxies exhibit old metal-rich stellar populations different from those of dwarf elliptical galaxies of similar masses but similar to those of more massive early-type galaxies, supporting the tidal stripping scenario. Their internal properties are reproduced by numerical simulations, which result in compact dynamically hot remnants resembling the galaxies in our sample.Comment: 26 pages, 5 figures, 2 tables. Science in press, published in Science Express on 1/Oct/2009. Full resolution figures in the supplementary online material are available from the Science Magazine web-sit

Ab-initio calculation of all-optical time-resolved calorimetry of nanosized systems: Evidence of nanosecond-decoupling of electron and phonon temperatures

The thermal dynamics induced by ultrashort laser pulses in nanoscale systems, i.e. all-optical time-resolved nanocalorimetry is theoretically investigated from 300 to 1.5 K. We report ab-initio calculations describing the temperature dependence of the electron-phonon interactions for Cu nanodisks supported on Si. The electrons and phonons temperatures are found to decouple on the ns time scale at 10 K, which is two orders of magnitude in excess with respect to that found for standard low-temperature transport experiments. By accounting for the physics behind our results we suggest an alternative route for overhauling the present knowledge of the electron-phonon decoupling mechanism in nanoscale systems by replacing the mK temperature requirements of conventional experiments with experiments in the time-domain.Comment: 5 pages, 3 figures. Accepted on Physical Review B

Ward identities for disordered metals and superconductors

This article revisits Ward identities for disordered interacting normal metals and superconductors. It offers a simple derivation based on gauge invariance and recasts the identities in a new form that allows easy analysis of the quasiparticle charge conservation (as e.g. in a normal metal) or non-conservation (as e.g. in a d-wave superconductor).Comment: Discussion of decoherence at T=0 remove

The early days of the Sculptor dwarf spheroidal galaxy

We present the high resolution spectroscopic study of five -3.9<=[Fe/H]<=-2.5 stars in the Local Group dwarf spheroidal, Sculptor, thereby doubling the number of stars with comparable observations in this metallicity range. We carry out a detailed analysis of the chemical abundances of alpha, iron peak, light and heavy elements, and draw comparisons with the Milky Way halo and the ultra faint dwarf stellar populations. We show that the bulk of the Sculptor metal-poor stars follows the same trends in abundance ratios versus metallicity as the Milky Way stars. This suggests similar early conditions of star formation and a high degree of homogeneity of the interstellar medium. We find an outlier to this main regime, which seems to miss the products of the most massive of the TypeII supernovae. In addition to its value to help refining galaxy formation models, this star provides clues to the production of cobalt and zinc. Two of our sample stars have low odd-to-even barium isotope abundance ratios, suggestive of a fair proportion of s-process; we discuss the implication for the nucleosynthetic origin of the neutron capture elements.Comment: Replacement after language editio

Stellar metallicity gradients of Local Group dwarf galaxies

Aims. We explore correlations between the strength of metallicity gradients in Local Group dwarf galaxies and their stellar mass, star formation history timescales, and environment. Methods. We performed a homogeneous analysis of literature spectroscopic data of red giant stars and determined radial metallicity profiles for 30 Local Group dwarf galaxies. This is the largest compilation of this type to date. Results. The dwarf galaxies in our sample show a variety of metallicity profiles, most of them decreasing with radius and some with rather steep profiles. The derived metallicity gradients as a function of the half-light radius, [Fe/H](R/Re), show no statistical differences when compared with the morphological type of the galaxies, nor with their distance from the Milky Way or M31. No correlations are found with either stellar mass or star formation timescales. In particular, we do not find the linear relation between [Fe/H](R/Re) and the galaxy median age t50, which has been reported in the literature for a set of simulated systems. On the other hand, the high angular momentum in some of our galaxies does not seem to affect the gradient strengths. The strongest gradients in our sample are observed in systems that are likely to have experienced a past merger event. When these merger candidates are excluded, the analysed dwarf galaxies show mild gradients (ã-0.1 dex Re-1) with little scatter between them, regardless of their stellar mass, dynamical state, and their star formation history. These results agree well with different sets of simulations presented in the literature that were analysed using the same method as for the observed dwarf galaxies. Conclusions. The interplay between the multitude of factors that could drive the formation of metallicity gradients likely combine in complex ways to produce in general comparable mild [Fe/H](R/Re) values, regardless of stellar mass and star formation history. The strongest driver of steep gradients seems to be previous dwarf-dwarf merger events in a system

Microcantilever Studies of Angular Field Dependence of Vortex Dynamics in BSCCO

Using a nanogram-sized single crystal of BSCCO attached to a microcantilever we demonstrate in a direct way that in magnetic fields nearly parallel to the {\it ab} plane the magnetic field penetrates the sample in the form of Josephson vortices rather than in the form of a tilted vortex lattice. We further investigate the relation between the Josephson vortices and the pancake vortices generated by the perpendicular field component.Comment: 5 pages, 8 figure

Distinctive rings in the 21 cm signal of the epoch of reionization

It is predicted that sources emitting UV radiation in the Lyman band during the epoch of reionization (EoR) showed a series of discontinuities in their Ly-alpha flux radial profile as a consequence of the thickness of the Lyman line series in the primeval intergalactic medium. Through unsaturated Wouthuysen-Field coupling, these spherical discontinuities are also present in the 21 cm emission of the neutral IGM. In this article, we study the effects these discontinuities have on the differential brightness temperature of the 21 cm signal of neutral hydrogen in a realistic setting including all other sources of fluctuations. We focus on the early phases of the EoR, and we address the question of the detectability by the planned Square Kilometre Array. Such a detection would be of great interest, because these structures could provide an unambiguous diagnostic for the cosmological origin of the signal remaining after the foreground cleaning procedure. Also, they could be used as a new type of standard rulers. We determine the differential brightness temperature of the 21 cm signal in the presence of inhomogeneous Wouthuysen-Field effect using simulations which include (hydro)dynamics and both ionizing and Lyman lines 3D radiative transfer with the code LICORICE. We find that the Lyman horizons are clearly visible on the maps and radial profiles around the first sources of our simulations, but for a limited time interval, typically \Delta z \approx 2 at z \sim 13. Stacking the profiles of the different sources of the simulation at a given redshift results in extending this interval to \Delta z \approx 4. When we take into account the implementation and design planned for the SKA (collecting area, sensitivity, resolution), we find that detection will be challenging. It may be possible with a 10 km diameter for the core, but will be difficult with the currently favored design of a 5 km core.Comment: 10 pages, 10 figures; v2: Section 5.5 rewritten; some new references added; accepted for publication in Astronomy and Astrophysic

Kramers degeneracy in a magnetic field and Zeeman spin-orbit coupling in antiferromagnets

In this article, I analyze the symmetries and degeneracies of electron eigenstates in a commensurate collinear antiferromagnet. In a magnetic field transverse to the staggered magnetization, a hidden anti-unitary symmetry protects double degeneracy of the Bloch eigenstates at a special set of momenta. In addition to this `Kramers degeneracy' subset, the manifold of momenta, labeling the doubly degenerate Bloch states in the Brillouin zone, may also contain an `accidental degeneracy' subset, that is not protected by symmetry and that may change its shape under perturbation. These degeneracies give rise to a substantial momentum dependence of the transverse g-factor in the Zeeman coupling, turning the latter into a spin-orbit interaction. I discuss a number of materials, where Zeeman spin-orbit coupling is likely to be present, and outline the simplest properties and experimental consequences of this interaction, that may be relevant to systems from chromium to borocarbides, cuprates, hexaborides, iron pnictides, as well as organic and heavy fermion conductors.Comment: 16+ pages, extended version of arXiv:0805.0378; revised versio

Experimental evidence for fast cluster formation of chain oxygen vacancies in YBa2Cu3O7-d being at the origin of the fishtail anomaly

We report on three different and complementary measurements, namely magnetisation measurements, positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin.Comment: 10 pages, 4 figures, submitted to PR