13 research outputs found
Clusters of galaxies : observational properties of the diffuse radio emission
Clusters of galaxies, as the largest virialized systems in the Universe, are
ideal laboratories to study the formation and evolution of cosmic
structures...(abridged)... Most of the detailed knowledge of galaxy clusters
has been obtained in recent years from the study of ICM through X-ray
Astronomy. At the same time, radio observations have proved that the ICM is
mixed with non-thermal components, i.e. highly relativistic particles and
large-scale magnetic fields, detected through their synchrotron emission. The
knowledge of the properties of these non-thermal ICM components has increased
significantly, owing to sensitive radio images and to the development of
theoretical models. Diffuse synchrotron radio emission in the central and
peripheral cluster regions has been found in many clusters. Moreover
large-scale magnetic fields appear to be present in all galaxy clusters, as
derived from Rotation Measure (RM) studies. Non-thermal components are linked
to the cluster X-ray properties, and to the cluster evolutionary stage, and are
crucial for a comprehensive physical description of the intracluster medium.
They play an important role in the cluster formation and evolution. We review
here the observational properties of diffuse non-thermal sources detected in
galaxy clusters: halos, relics and mini-halos. We discuss their classification
and properties. We report published results up to date and obtain and discuss
statistical properties. We present the properties of large-scale magnetic
fields in clusters and in even larger structures: filaments connecting galaxy
clusters. We summarize the current models of the origin of these cluster
components, and outline the improvements that are expected in this area from
future developments thanks to the new generation of radio telescopes.Comment: Accepted for the publication in The Astronomy and Astrophysics
Review. 58 pages, 26 figure
Extragalactic Radio Continuum Surveys and the Transformation of Radio Astronomy
Next-generation radio surveys are about to transform radio astronomy by
discovering and studying tens of millions of previously unknown radio sources.
These surveys will provide new insights to understand the evolution of
galaxies, measuring the evolution of the cosmic star formation rate, and
rivalling traditional techniques in the measurement of fundamental cosmological
parameters. By observing a new volume of observational parameter space, they
are also likely to discover unexpected new phenomena. This review traces the
evolution of extragalactic radio continuum surveys from the earliest days of
radio astronomy to the present, and identifies the challenges that must be
overcome to achieve this transformational change.Comment: To be published in Nature Astronomy 18 Sept 201
Precision spectroscopy of helium in a magic wavelength optical dipole trap
Improvements in both theory and frequency metrology of few-electron systems
such as hydrogen and helium have enabled increasingly sensitive tests of
quantum electrodynamics (QED), as well as ever more accurate determinations of
fundamental constants and the size of the nucleus. At the same time advances in
cooling and trapping of neutral atoms have revolutionized the development of
increasingly accurate atomic clocks. Here, we combine these fields to reach the
highest precision on an optical tranistion in the helium atom to date by
employing a Bose-Einstein condensate confined in a magic wavelength optical
dipole trap. The measured transition accurately connects the ortho- and
parastates of helium and constitutes a stringent test of QED theory. In
addition we test polarizability calculations and ultracold scattering
properties of the helium atom. Finally, our measurement probes the size of the
nucleus at a level exceeding the projected accuracy of muonic helium
measurements currently being performed in the context of the proton radius
puzzle
The clustering of massive galaxies at z similar to 1
We use the angular two-point correlation function to estimate the spatial correlation length of radio sources taken from the large-area 1.4 GHz NVSS radio survey. At the median survey redshift of z similar to 1, r(0) is found to be increasing with flux density. This is consistent with a scenario in which powerful (i.e. FRII) radio galaxies probe significantly more massive spatial structures than less powerful radio galaxies. The large spatial correlation length that we derive for FRIIs is remarkably close to that of extremely red objects (EROs). This implies that powerful radio galaxies and EROs trace equally massive structures at z similar to 1. Moreover, because powerful radio galaxies and EROs are both associated with luminous early-type galaxies we propose that they could be the same objects seen at different evolutionary stages. The correlation length of massive, luminous galaxies at z similar to 1 is comparable to that of bright ellipticals locally, suggesting that r(0) (comoving) of these massive galaxies has changed little from z similar to 1 to z approximate to 0. This is in excellent agreement with current ACDM hierarchical model predictions
Magic wavelengths for the 2 S-3 -> 2 S-1 transition in helium
We have calculated ac polarizabilities of the 2 3 S and 2 1 S states of both He 4 and He 3 in the range 318 nm to 2.5 µm and determined the magic wavelengths at which these polarizabilities are equal for either isotope. The calculations, only based on available ab initio tables of level energies and Einstein A coefficients, do not require advanced theoretical techniques. The polarizability contribution of the continuum is calculated using a simple extrapolation beyond the ionization limit, yet the results agree to better than 1% with such advanced techniques. Several promising magic wavelengths are identified around 320 nm with sufficient accuracy to design an appropriate laser system. The extension of the calculations to He 3 is complicated due to the additional hyperfine structure, but we show that the magic wavelength candidates around 320 nm are predominantly shifted by the isotope shift
Ultracold metastable helium: Ramsey fringes and atom interferometry
We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a 1:5 μm optical dipole trap. Applying a single π/2 rf pulse we demonstrate that 50% of the atoms initially in the m = +1 state can be transferred to the magnetic field insensitive m = 0 state. Two π/2 pulses with varying time delay allow a Ramseytype measurement of the Zeeman shift for a high precision measurement of the 2 3S1-2 1S0 transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the m = 0 state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant