Keck Deep Fields. I. Observations, Reductions, and the Selection of Faint Star-Forming Galaxies at Redshifts z~4, 3, and 2

We introduce a very deep, R_lim~27, multicolor imaging survey of very faint star-forming galaxies at z~4, z~3, z~2.2, and z~1.7. This survey, carried out on the Keck I telescope, uses the very same UGRI filter system that is employed by the Steidel team to select galaxies at these redshifts, and thus allows us to construct identically-selected, but much fainter, samples. However, our survey reaches ~1.5 mag deeper than the work of Steidel and his group, letting us probe substantially below the characteristic luminosity L* and thus study the properties and redshift evolution of the faint component of the high-z galaxy population. The survey covers 169 square arcminutes in three spatially independent patches on the sky and -- to R<~27 -- contains 427 GRI-selected z~4 LBGs, 1481 UGR-selected z~3 LBGs, 2417 UGR-selected z~2.2 star-forming galaxies, and 2043 UGR-selected z~1.7 star-forming galaxies. In this paper, the first in a series, we introduce the survey, describe our observing and data reduction strategies, and outline the selection of our z~4, z~3, z~2.2, and z~1.7 samples.Comment: To appear in Ap

A Parameterized Post-Friedmann Framework for Modified Gravity

We develop a parameterized post-Friedmann (PPF) framework which describes three regimes of modified gravity models that accelerate the expansion without dark energy. On large scales, the evolution of scalar metric and density perturbations must be compatible with the expansion history defined by distance measures. On intermediate scales in the linear regime, they form a scalar-tensor theory with a modified Poisson equation. On small scales in dark matter halos such as our own galaxy, modifications must be suppressed in order to satisfy stringent local tests of general relativity. We describe these regimes with three free functions and two parameters: the relationship between the two metric fluctuations, the large and intermediate scale relationships to density fluctuations and the two scales of the transitions between the regimes. We also clarify the formal equivalence of modified gravity and generalized dark energy. The PPF description of linear fluctuation in f(R) modified action and the Dvali-Gabadadze-Porrati braneworld models show excellent agreement with explicit calculations. Lacking cosmological simulations of these models, our non-linear halo-model description remains an ansatz but one that enables well-motivated consistency tests of general relativity. The required suppression of modifications within dark matter halos suggests that the linear and weakly non-linear regimes are better suited for making complementary test of general relativity than the deeply non-linear regime.Comment: 12 pages, 9 figures, additional references reflect PRD published versio

On constraining the speed of gravitational waves following GW150914

We point out that the observed time delay between the detection of the signal at the Hanford and Livingston LIGO sites from the gravitational wave event GW150914 places an upper bound on the speed of propagation of gravitational waves, $c_{gw}\lesssim 1.7$ in the units of speed of light. Combined with the lower bound from the absence of gravitational Cherenkov losses by cosmic rays that rules out most of subluminal velocities, this gives a model-independent double-sided constraint $1\lesssim c_{gw}\lesssim 1.7$. We compare this result to model-specific constraints from pulsar timing and cosmology.Comment: 3 pages, 1 figure, references adde

Multipartite quantum correlations: symplectic and algebraic geometry approach

We review a geometric approach to classification and examination of quantum correlations in composite systems. Since quantum information tasks are usually achieved by manipulating spin and alike systems or, in general, systems with a finite number of energy levels, classification problems are usually treated in frames of linear algebra. We proposed to shift the attention to a geometric description. Treating consistently quantum states as points of a projective space rather than as vectors in a Hilbert space we were able to apply powerful methods of differential, symplectic and algebraic geometry to attack the problem of equivalence of states with respect to the strength of correlations, or, in other words, to classify them from this point of view. Such classifications are interpreted as identification of states with `the same correlations properties' i.e. ones that can be used for the same information purposes, or, from yet another point of view, states that can be mutually transformed one to another by specific, experimentally accessible operations. It is clear that the latter characterization answers the fundamental question `what can be transformed into what \textit{via} available means?'. Exactly such an interpretations, i.e, in terms of mutual transformability can be clearly formulated in terms of actions of specific groups on the space of states and is the starting point for the proposed methods.Comment: 29 pages, 9 figures, 2 tables, final form submitted to the journa

Interval identification of FMR parameters for spin reorientation transition in (Ga,Mn)As

In this work we report results of ferromagnetic resonance studies of a 6% 15 nm (Ga,Mn)As layer, deposited on (001)-oriented GaAs. The measurements were performed with in-plane oriented magnetic field, in the temperature range between 5K and 120K. We observe a temperature induced reorientation of the effective in-plane easy axis from [-110] to [110] direction close to the Curie temperature. The behavior of magnetization is described by anisotropy fields, H_{eff} (= 4\piM -H_{2\perp}), H_{2\parallel}, and H_{4\parallel}. In order to precisely investigate this reorientation, numerical values of anisotropy fields have been determined using powerful - but still largely unknown - interval calculations. In simulation mode this approach makes possible to find all the resonance fields for arbitrarily oriented sample, which is generally intractable analytically. In 'fitting' mode we effectively utilize full experimental information, not only those measurements performed in special, distinguished directions, to reliably estimate the values of important physical parameters as well as their uncertainties and correlations.Comment: 3 pages, 3 figures. Presented at The European Conference "Physics of Magnetism 2011" (PM'11), June 27 - July 1, 2011, Poznan, Polan

Models of f(R) Cosmic Acceleration that Evade Solar-System Tests

We study a class of metric-variation f(R) models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in f(R) models do not have substantially deeper potentials than expected in LCDM, then cosmological field amplitudes |f_R| > 10^{-6} will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of f(R) models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on f(R) are phrased in a nearly model-independent manner.Comment: 13 pages, 10 figures. Submitted to Phys. Rev.

How Future Space-Based Weak Lensing Surveys Might Obtain Photometric Redshifts Independently

We study how the addition of on-board optical photometric bands to future space-based weak lensing instruments could affect the photometric redshift estimation of galaxies, and hence improve estimations of the dark energy parameters through weak lensing. Basing our study on the current proposed Euclid configuration and using a mock catalog of galaxy observations, various on-board options are tested and compared with the use of ground-based observations from the Large Synoptic Survey Telescope (LSST) and Pan-STARRS. Comparisons are made through the use of the dark energy Figure of Merit, which provides a quantifiable measure of the change in the quality of the scientific results that can be obtained in each scenario. Effects of systematic offsets between LSST and Euclid photometric calibration are also studied. We find that adding two (U and G) or even one (U) on-board optical band-passes to the space-based infrared instrument greatly improves its photometric redshift performance, bringing it close to the level that would be achieved by combining observations from both space-based and ground-based surveys while freeing the space mission from reliance on external datasets.Comment: Accepted for publication in PASP. A high-quality version of Fig 1 can be found on http://www.ap.smu.ca/~sawicki/DEphoto

Thickness dependence of magnetic properties of (Ga,Mn)As

We report on a monotonic reduction of Curie temperature in dilute ferromagnetic semiconductor (Ga,Mn)As upon a well controlled chemical-etching/oxidizing thinning from 15 nm down to complete removal of the ferro- magnetic response. The effect already starts at the very beginning of the thinning process and is accompanied by the spin reorientation transition of the in-plane uniaxial anisotropy. We postulate that a negative gradient along the growth direction of self-compensating defects (Mn interstitial) and the presence of surface donor traps gives quantitative account on these effects within the p-d mean field Zener model with adequate mod- ifications to take a nonuniform distribution of holes and Mn cations into account. The described here effects are of practical importance for employing thin and ultrathin layers of (Ga,Mn)As or relative compounds in concept spintronics devices, like resonant tunneling devices in particular.Comment: 4 pages, 4 figures and supplementary information 2 pages, 1 figur