Milky Way type galaxies in a LCDM cosmology

We analyse a sample of 52,000 Milky Way (MW) type galaxies drawn from the publicly available galaxy catalogue of the Millennium Simulation with the aim of studying statistically the differences and similarities of their properties in comparison to our Galaxy. Model galaxies are chosen to lie in haloes with maximum circular velocities in the range 200-250 km/seg and to have bulge-to-disk ratios similar to that of the Milky Way. We find that model MW galaxies formed quietly through the accretion of cold gas and small satellite systems. Only 12 per cent of our model galaxies experienced a major merger during their lifetime. Most of the stars formed in situ, with only about 15 per cent of the final mass gathered through accretion. Supernovae and AGN feedback play an important role in the evolution of these systems. At high redshifts, when the potential wells of the MW progenitors are shallower, winds driven by supernovae explosions blow out a large fraction of the gas and metals. As the systems grow in mass, SN feedback effects decrease and AGN feedback takes over, playing a more important role in the regulation of the star formation activity at lower redshifts. Although model Milky Way galaxies have been selected to lie in a narrow range of maximum circular velocities, they nevertheless exhibit a significant dispersion in the final stellar masses and metallicities. Our analysis suggests that this dispersion results from the different accretion histories of the parent dark matter haloes. Statically, we also find evidences to support the Milky Way as a typical Sb/Sc galaxy in the same mass range, providing a suitable benchmark to constrain numerical models of galaxy formationComment: 10 pages, 7 figures, mne2.cls, MNRAS, replaced with accepted versio

The Color-Magnitude Relation in Coma: Clues to the Age and Metallicity of Cluster Populations

We have observed three fields of the Coma cluster of galaxies with a narrow band (modified Stromgren) filter system. Observed galaxies include 31 in the vicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839 complete to M_5500=-18 in all three subclusters. Spectrophotometric classification finds all three subclusters of Coma to be dominated by red, E type (ellipticals/S0's) galaxies with a mean blue fraction, f_B, of 0.10. The blue fraction increases to fainter luminosities, possible remnants of dwarf starburst population or the effects of dynamical friction removing bright, blue galaxies from the cluster population by mergers. We find the color-magnitude (CM) relation to be well defined and linear over the range of M_5500=-13 to -22. After calibration to multi-metallicity models, bright ellipticals are found to have luminosity weighted mean [Fe/H] values between -0.5 and +0.5, whereas low luminosity ellipticals have [Fe/H] values ranging from -2 to solar. The lack of CM relation in our continuum color suggests that a systematic age effect cancels the metallicity effects in this bandpass. This is confirmed with our age index which finds a weak correlation between luminosity and mean stellar age in ellipticals such that the stellar populations of bright ellipticals are 2 to 3 Gyrs younger than low luminosity ellipticals.Comment: 26 pages AAS LaTeX, 6 figures, accepted for publication in A

The Properties of Satellite Galaxies in External Systems. I. Morphology and Structural Parameters

We present the first results of an ongoing project to study the morphological, kinematical, dynamical, and chemical properties of satellite galaxies of external giant spiral galaxies. The sample of objects has been selected from the catalogue by Zaritsky et al. (1997). The paper analyzes the morphology and structural parameters of a subsample of 60 such objects. The satellites span a great variety of morphologies and surface brightness profiles. About two thirds of the sample are spirals and irregulars, the remaining third being early-types. Some cases showing interaction between pairs of satellites are presented and briefly discussed.Comment: Accepted for publication in Astrophys. Journal Supp. Se

Spectral and morphological properties of quasar hosts in SPH simulations of AGN feeding by mergers

We present a method for generating virtual observations from smoothed-particle-hydrodynamics (SPH) simulations. This method includes stellar population synthesis models and the reprocessing of starlight by dust to produce realistic galaxy images. We apply this method and simulate the merging of two identical giant Sa galaxies. The merger remnant is an elliptical galaxy. The merger concentrates the gas content of the two galaxies into the nuclear region. The gas that flows into the nuclear region refuels the central black holes of the merging galaxies. We follow the refuelling of the black holes during the merger semi-analytically. In the simulation presented in this article, the black holes grow from 3 x 10^7 to 1.8X 10^8 Solar masses, with a peak AGN luminosity of M_B ~ -23.7. We study how the morphological and spectral properties of the system evolve during the merger and work out the predictions of this scenario for the properties of host galaxies during the active phase. The peak of AGN activity coincides with the merging of the two galactic nuclei and occurs at a stage when the remnant looks like a lenticular galaxy. The simulation predicts the formation of a circumnuclear starburst ring/dusty torus with an opening angle of 30-40 degrees and made of clouds with n_H=10^24 cm^-2. The average optical depth of the torus is quite high, but the obscuring medium is patchy, so that there still exist lines of sight where the AGN is visible in a nearly edge-on view. For the same reason, there are lines of sight where the AGN is completely obscured in the face-on view.Comment: 14 pages, 11 figure

Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies

We show that the observed upper bound on the line-of-sight velocity dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a simple dynamical origin within the LCDM cosmological model, under two main hypotheses. The first is that most of the stars now in the luminous parts of a giant elliptical formed at redshift z>6. Subsequently, the stars behaved dynamically just as an additional component of the dark matter. The second hypothesis is that the mass distribution characteristic of a newly formed dark matter halo forgets such details of the initial conditions as the stellar "collisionless matter" that was added to the dense parts of earlier generations of halos. We also assume that the stellar velocity dispersion does not evolve much at z<6, because a massive host halo grows mainly by the addition of material at large radii well away from the stellar core of the galaxy. These assumptions lead to a predicted number density of ellipticals as a function of stellar velocity dispersion that is in promising agreement with the Sloan Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio

The Kinematics of the Ultra-Faint Milky Way Satellites: Solving the Missing Satellite Problem

We present Keck/DEIMOS spectroscopy of stars in 8 of the newly discovered ultra-faint dwarf galaxies around the Milky Way. We measure the velocity dispersions of Canes Venatici I and II, Ursa Major I and II, Coma Berenices, Hercules, Leo IV and Leo T from the velocities of 18 - 214 stars in each galaxy and find dispersions ranging from 3.3 to 7.6 km/s. The 6 galaxies with absolute magnitudes M_V < -4 are highly dark matter-dominated, with mass-to-light ratios approaching 1000. The measured velocity dispersions are inversely correlated with their luminosities, indicating that a minimum mass for luminous galactic systems may not yet have been reached. We also measure the metallicities of the observed stars and find that the 6 brightest of the ultra-faint dwarfs extend the luminosity-metallicity relationship followed by brighter dwarfs by 2 orders of magnitude in luminosity; several of these objects have mean metallicities as low as [Fe/H] = -2.3 and therefore represent some of the most metal-poor known stellar systems. We detect metallicity spreads of up to 0.5 dex in several objects, suggesting multiple star formation epochs. Having established the masses of the ultra-faint dwarfs, we re-examine the missing satellite problem. After correcting for the sky coverage of the SDSS, we find that the ultra-faint dwarfs substantially alleviate the discrepancy between the predicted and observed numbers of satellites around the Milky Way, but there are still a factor of ~4 too few dwarf galaxies over a significant range of masses. We show that if galaxy formation in low-mass dark matter halos is strongly suppressed after reionization, the simulated circular velocity function of CDM subhalos can be brought into approximate agreement with the observed circular velocity function of Milky Way satellite galaxies. [slightly abridged]Comment: 22 pages, 15 figures (12 in color), 6 tables, minor revisions in response to referee report. Accepted for publication in Ap

An Isocurvature CDM Cosmogony. II. Observational Tests

A companion paper presents a worked model for evolution through inflation to initial conditions for an isocurvature model for structure formation. It is shown here that the model is consistent with the available observational constraints that can be applied without the help of numerical simulations. The model gives an acceptable fit to the second moments of the angular fluctuations in the thermal background radiation and the second through fourth moments of the measured large-scale fluctuations in galaxy counts, within the possibly significant uncertainties in these measurements. The cluster mass function requires a rather low but observationally acceptable mass density, 0.1\lsim\Omega\lsim 0.2 in a cosmologically flat universe. Galaxies would be assembled earlier in this model than in the adiabatic version, an arguably good thing. Aspects of the predicted non-Gaussian character of the anisotropy of the thermal background radiation in this model are discussed.Comment: 14 pages, 3 postscript figures, uses aas2pp4.st

Communication intentionnelle dans le domaine du leadership

L’existence de toute organisation repose sur le soutien que lui accordent ses membres. Le fonctionnement optimal d’un système est tributaire de la qualité des relations entre ses composantes, c’est-à-dire, pour une organisation, entre ses différents paliers. La qualité de ces relations présente des avantages concurrentiels sur les plans économique, politique et social, car elles se traduisent par une communication claire, directe et régulière entre les niveaux hiérarchiques. Pour les dirigeants, il s’agit de leur capacité à exercer leur leadership. Cette communication a un impact sur le développement des compétences des employés, la productivité de l’entreprise et sa visibilité stratégique dans la sphère politique et, enfin, sur l’attraction et le maintien d’une main-d’œuvre qualifiée et engagée. Il va sans dire qu’au 21e siècle, l’avènement des médias sociaux rend plus délicate la tâche des gestionnaires et présente ainsi un défi bien plus important que la mise en place de bonnes relations de travail qu’on préconisait au siècle dernier. Les auteurs aborderont le leadership autoral comme alternative en vue d’un leadership de conscience plus adapté aux défis des organisations du siècle actuel

Magnetic nanostructures by adaptive twinning in strained epitaxial films

We exploit the intrinsic structural instability of the Fe70Pd30 magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic and ferroelectric materials.Comment: Final version. Supplementary information available on request or at the publisher's websit

The evolution of field early-type galaxies to z~0.7

We have measured the Fundamental Plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0) in the redshift range 0.1<z<0.66. We find that: i) the FP is defined and tight out to the highest redshift bin; ii) the intercept \gamma evolves as d\gamma/dz=0.58+0.09-0.13 (for \Omega=0.3, \Omega_{\Lambda}=0.7), or, in terms of average effective mass to light ratio, as d\log(M/L_B)/dz=-0.72+0.11-0.16, i.e. faster than is observed for cluster E/S0 -0.49+-0.05. In addition, we detect [OII] emission >5\AA in 22% of an enlarged sample of 42 massive E/S0 in the range 0.1<z<0.73, in contrast with the quiescent population observed in clusters at similar z. We interpret these findings as evidence that a significant fraction of massive field E/S0 experiences secondary episodes of star-formation at z<1.Comment: ApJ Letters, in pres