412 research outputs found
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Shape coexistence from lifetime and branching-ratio measurements in 68,70Ni
© 2016 The Author(s) Shape coexistence near closed-shell nuclei, whereby states associated with deformed shapes appear at relatively low excitation energy alongside spherical ones, is indicative of the rapid change in structure that can occur with the addition or removal of a few protons or neutrons. Near 68Ni (Z=28, N=40), the identification of shape coexistence hinges on hitherto undetermined transition rates to and from low-energy 0+ states. In 68,70Ni, new lifetimes and branching ratios have been measured. These data enable quantitative descriptions of the 0+ states through the deduced transition rates and serve as sensitive probes for characterizing their nuclear wave functions. The results are compared to, and consistent with, large-scale shell-model calculations which predict shape coexistence. With the firm identification of this phenomenon near 68Ni, shape coexistence is now observed in all currently accessible regions of the nuclear chart with closed proton shells and mid-shell neutrons
Classical dynamics of a two-species Bose-Einstein condensate in the presence of nonlinear maser processes
The stability analysis of a generalized Dicke model, in the semi-classical
limit, describing the interaction of a two-species Bose-Einstein condensate
driven by a quantized field in the presence of Kerr and spontaneous parametric
processes is presented. The transitions from Rabi to Josephson dynamics are
identified depending on the relative value of the involved parameters.
Symmetry-breaking dynamics are shown for both types of coherent oscillations
due to the quantized field and nonlinear optical processes.Comment: 12 pages, 5 figures. Accepted for publication as chapter in
"Spontaneous Symmetry Breaking, Self-Trapping, and Josephson Oscillations in
Nonlinear Systems
An overview of the current status of CMB observations
In this paper we briefly review the current status of the Cosmic Microwave
Background (CMB) observations, summarising the latest results obtained from CMB
experiments, both in intensity and polarization, and the constraints imposed on
the cosmological parameters. We also present a summary of current and future
CMB experiments, with a special focus on the quest for the CMB B-mode
polarization.Comment: Latest CMB results have been included. References added. To appear in
"Highlights of Spanish Astrophysics V", Proceedings of the VIII Scientific
Meeting of the Spanish Astronomical Society (SEA) held in Santander, 7-11
July, 200
Pairing in two-dimensional boson-fermion mixtures
The possibilities of pairing in two-dimensional boson-fermion mixtures are
carefully analyzed. It is shown that the boson-induced attraction between two
identical fermions dominates the p-wave pairing at low density. For a given
fermion density, the pairing gap becomes maximal at a certain optimal boson
concentration. The conditions for observing pairing in current experiments are
discussedComment: 10 pages, 5 figs, revtex
The evolution of galaxy groups and of galaxies therein
Properties of groups of galaxies depend sensitively on the algorithm for
group selection, and even the most recent catalogs of groups built from
redshift-space selection should suffer from projections and infalling galaxies.
The cosmo-dynamical evolution of groups from initial Hubble expansion to
collapse and virialization leads to a fundamental track (FT) in
virial-theorem-M/L vs crossing time. The increased rates of mergers, both
direct and after dynamical friction, in groups relative to clusters, explain
the higher fraction of elliptical galaxies at given local number density in
X-ray selected groups, relative to clusters, even when the hierarchical
evolution of groups is considered. Galaxies falling into groups and clusters
should later travel outwards to typically 2 virial radii, which is somewhat
less than the outermost radius where observed galaxy star formation
efficiencies are enhanced relative to field galaxies of same morphological
type. An ongoing analysis of the internal kinematics of X-ray selected groups
suggests that the radial profiles of line of sight velocity dispersion are
consistent with isotropic NFW distributions for the total mass density, with
higher (lower) concentrations than LambdaCDM predictions in groups of high
(low) mass. The critical mass, at M200 ~ 10^13 M_sun is consistent with
possible breaks in the X-ray luminosity-temperature and Fundamental Plane
relations. The internal kinematics of groups indicate that the M-T relation of
groups should agree with that extrapolated from clusters with no break at the
group scale. The analyses of observed velocity dispersion profiles and of the
FT both suggest that low velocity dispersion groups (compact and loose, X-ray
emitting or undetected) are quite contaminated by chance projections.Comment: Invited review, ESO workshop "Groups of Galaxies in the Nearby
Universe", held in Santiago, Chile, 5-9 December 2005, ed. I. Saviane, V.
Ivanov & J. Borissova, 16 page
Interacting Ghost Dark Energy in Non-Flat Universe
A new dark energy model called "ghost dark energy" was recently suggested to
explain the observed accelerating expansion of the universe. This model
originates from the Veneziano ghost of QCD. The dark energy density is
proportional to Hubble parameter, , where is a
constant of order and is
QCD mass scale. In this paper, we extend the ghost dark energy model to the
universe with spatial curvature in the presence of interaction between dark
matter and dark energy. We study cosmological implications of this model in
detail. In the absence of interaction the equation of state parameter of ghost
dark energy is always and mimics a cosmological constant in the
late time, while it is possible to have provided the interaction is
taken into account. When , all previous results of ghost dark energy in
flat universe are recovered. To check the observational consistency, we use
Supernova type Ia (SNIa) Gold sample, shift parameter of Cosmic Microwave
Background radiation (CMB) and the Baryonic Acoustic Oscillation peak from
Sloan Digital Sky Survey (SDSS). The best fit values of free parameter at
confidence interval are: ,
and . Consequently
the total energy density of universe at present time in this model at 68% level
equates to .Comment: 19 pages, 9 figures. V2: Added comments, observational consequences,
references, figures and major corrections. Accepted for publication in
General Relativity and Gravitatio
SPIDER: Probing the Early Universe with a Suborbital Polarimeter
We evaluate the ability of SPIDER, a balloon-borne polarimeter, to detect a
divergence-free polarization pattern ("B-modes") in the Cosmic Microwave
Background (CMB). In the inflationary scenario, the amplitude of this signal is
proportional to that of the primordial scalar perturbations through the
tensor-to-scalar ratio r. We show that the expected level of systematic error
in the SPIDER instrument is significantly below the amplitude of an interesting
cosmological signal with r=0.03. We present a scanning strategy that enables us
to minimize uncertainty in the reconstruction of the Stokes parameters used to
characterize the CMB, while accessing a relatively wide range of angular
scales. Evaluating the amplitude of the polarized Galactic emission in the
SPIDER field, we conclude that the polarized emission from interstellar dust is
as bright or brighter than the cosmological signal at all SPIDER frequencies
(90 GHz, 150 GHz, and 280 GHz), a situation similar to that found in the
"Southern Hole." We show that two ~20-day flights of the SPIDER instrument can
constrain the amplitude of the B-mode signal to r<0.03 (99% CL) even when
foreground contamination is taken into account. In the absence of foregrounds,
the same limit can be reached after one 20-day flight.Comment: 29 pages, 8 figures, 4 tables; v2: matches published version, flight
schedule updated, two typos fixed in Table 2, references and minor
clarifications added, results unchange
Shear viscous effects on the primordial power spectrum from warm inflation
We compute the primordial curvature spectrum generated during warm inflation,
including shear viscous effects. The primordial spectrum is dominated by the
thermal fluctuations of the radiation bath, sourced by the dissipative term of
the inflaton field. The dissipative coefficient \Upsilon, computed from first
principles in the close-to-equilibrium approximation, depends in general on the
temperature T, and this dependence renders the system of the linear
fluctuations coupled. Whenever the dissipative coefficient is larger than the
Hubble expansion rate H, there is a growing mode in the fluctuations before
horizon crossing. However, dissipation intrinsically means departures from
equilibrium, and therefore the presence of a shear viscous pressure in the
radiation fluid. This in turn acts as an extra friction term for the radiation
fluctuations that tends to damp the growth of the perturbations. Independently
of the T functional dependence of the dissipation and the shear viscosity, we
find that when the shear viscous coefficient \zeta_s is larger than 3 \rho_r/H
at horizon crossing, \rho_r being the radiation energy density, the shear
damping effect wins and there is no growing mode in the spectrum.Comment: 18 pages, 6 figure
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