Accurate Predictions for the Scale-Dependent Galaxy Bias from Primordial Non-Gaussianity

The large-scale clustering of galaxies can serve as a probe of primordial non-Gaussianity in the Universe competitive with the anisotropies of the CMB. Here, we present improved theoretical predictions which include an important, previously overlooked correction to the bias. We demonstrate that the new predictions are able to reproduce the results of N-body simulations, explaining the significant departures seen from previous theoretical results. These refined predictions open the way to accurate constraints on primordial physics with large-scale structure surveys.Comment: 4 pages, 3 figures (v2): in press in PR

Halo Clustering with Non-Local Non-Gaussianity

We show how the peak-background split can be generalized to predict the effect of non-local primordial non-Gaussianity on the clustering of halos. Our approach is applicable to arbitrary primordial bispectra. We show that the scale-dependence of halo clustering predicted in the peak-background split (PBS) agrees with that of the local-biasing model on large scales. On smaller scales, k >~ 0.01 h/Mpc, the predictions diverge, a consequence of the assumption of separation of scales in the peak-background split. Even on large scales, PBS and local biasing do not generally agree on the amplitude of the effect outside of the high-peak limit. The scale dependence of the biasing - the effect that provides strong constraints to the local-model bispectrum - is far weaker for the equilateral and self-ordering-scalar-field models of non-Gaussianity. The bias scale dependence for the orthogonal and folded models is weaker than in the local model (~ 1/k), but likely still strong enough to be constraining. We show that departures from scale-invariance of the primordial power spectrum may lead to order-unity corrections, relative to predictions made assuming scale-invariance - to the non-Gaussian bias in some of these non-local models for non-Gaussianity. An Appendix shows that a non-local model can produce the local-model bispectrum, a mathematical curiosity we uncovered in the course of this investigation.Comment: 12 pages, 4 figures; submitted to Phys. Rev. D; v2: references added; v3: some more comments on kernel-bispectrum relation in appendi

Assembly bias and the dynamical structure of dark matter halos

Based on the Millennium Simulation we examine assembly bias for the halo properties: shape, triaxiality, concentration, spin, shape of the velocity ellipsoid and velocity anisotropy. For consistency we determine all these properties using the same set of particles, namely all gravitationally self-bound particles belonging to the most massive sub-structure of a given friends-of-friends halo. We confirm that near-spherical and high-spin halos show enhanced clustering. The opposite is true for strongly aspherical and low-spin halos. Further, below the typical collapse mass, M*, more concentrated halos show stronger clustering whereas less concentrated halos are less clustered which is reversed for masses above M*. Going beyond earlier work we show that: (1) oblate halos are more strongly clustered than prolate ones; (2) the dependence of clustering on the shape of the velocity ellipsoid coincides with that of the real-space shape, although the signal is stronger; (3) halos with weak velocity anisotropy are more clustered, whereas radially anisotropic halos are more weakly clustered; (4) for all highly clustered subsets we find systematically less radially biased velocity anisotropy profiles. These findings indicate that the velocity structure of halos is tightly correlated with environment.Comment: 5 pages, 2 figures, accepted for publication in Ap

Magnetic Resonance Imaging-Based Assessment of Breast Cancer-Related Lymphoedema Tissue Composition.

OBJECTIVES: The aim of this study was to propose a magnetic resonance imaging acquisition and analysis protocol that uses image segmentation to measure and depict fluid, fat, and muscle volumes in breast cancer-related lymphoedema (BCRL). This study also aims to compare affected and control (unaffected) arms of patients with diagnosed BCRL, providing an analysis of both the volume and the distribution of the different tissue components. MATERIALS AND METHODS: The entire arm was imaged with a fluid-sensitive STIR and a 2-point 3-dimensional T1W gradient-echo-based Dixon sequences, acquired in sagittal orientation and covering the same imaging volume. An automated image postprocessing procedure was developed to simultaneously (1) contour the external volume of the arm and the muscle fascia, allowing separation of the epifacial and subfascial volumes; and to (2) separate the voxels belonging to the muscle, fat, and fluid components. The total, subfascial, epifascial, muscle (subfascial), fluid (epifascial), and fat (epifascial) volumes were measured in 13 patients with unilateral BCRL. Affected versus unaffected volumes were compared using a 2-tailed paired t test; a value of P < 0.05 was considered to be significant. Pearson correlation was used to investigate the linear relationship between fat and fluid excess volumes. The distribution of fluid, fat, and epifascial excess volumes (affected minus unaffected) along the arm was also evaluated using dedicated tissue composition maps. RESULTS: Total arm, epifascial, epifascial fluid, and epifascial fat volumes were significantly different (P < 0.0005), with greater volume in the affected arms. The increase in epifascial volume (globally, 94% of the excess volume) constituted the bulk of the lymphoedematous swelling, with fat comprising the main component. The total fat excess volume summed over all patients was 2.1 times that of fluid. Furthermore, fat and fluid excess volumes were linearly correlated (Pearson r = 0.75), with the fat excess volume being greater than the fluid in 11 subjects. Differences in muscle compartment volume between affected and unaffected arms were not statistically significant, and contributed only 6% to the total excess volume. Considering the distribution of the different tissue excess volumes, fluid accumulated prevalently around the elbow, with substantial involvement of the upper arm in only 3 cases. Fat excess volume was generally greater in the upper arm; however, the relative increase in epifascial volume, which considers the total swelling relative to the original size of the arm, was in 9 cases maximal within the forearm. CONCLUSIONS: Our measurements indicate that excess of fat within the epifascial layer was the main contributor to the swelling, even when a substantial accumulation of fluid was present. The proposed approach could be used to monitor how the internal components of BCRL evolve after presentation, to stratify patients for treatment, and to objectively assess treatment response. This methodology provides quantitative metrics not currently available during the standard clinical assessment of BCRL and shows potential for implementation in clinical practice

Weak Lensing Effects on the Galaxy Three-Point Correlation Function

We study the corrections to the galaxy three-point correlation function (3PCF) induced by weak lensing magnification due to the matter distribution along the line of sight. We consistently derive all the correction terms arising up to second order in perturbation theory and provide analytic expressions as well as order of magnitude estimates for their relative importance. The magnification contributions depend on the geometry of the projected triangle on the sky plane, and scale with different powers of the number count slope and redshift of the galaxy sample considered. We evaluate all terms numerically and show that, depending on the triangle configuration as well as the galaxy sample considered, weak lensing can in general significantly contribute to and alter the three-point correlation function observed through galaxy and quasar catalogs.Comment: 24 pages, 11 figures; version accepted for publication in Phys. Rev. D; v2: typos corrected, figure caption clarifie

The Correlation Between Star Formation and 21cm Emission During the Reionization Epoch

Reionization is thought to be dominated by low mass galaxies, while direct observations of resolved galaxies probe only the most massive, rarest objects. The cross-correlation between fluctuations in the surface brightness of the cumulative Ly-alpha emission (which serves as a proxy for the star formation rate) and the redshifted 21cm signal from neutral hydrogen in the intergalactic medium (IGM), will directly probe the causal link between the production of ionizing photons in galaxies and the reionization of the IGM. We discuss the prospects for detecting this cross-correlation for unresolved galaxies. We find that on angular scales <10 arc-minutes, detection will be practical using widefield near-IR imaging from space in combination with the forthcoming Mileura Widefield Array - Low Frequency Demonstrator. When redshifted 21cm observations of the neutral IGM are combined with space-based near-IR imaging of Ly-alpha emission, the detection on angular scales <3 arc-minutes will be limited by the sensitivity of the 21cm signal, even when a small aperture optical telescope (~2m) and a moderate field of view (~10 square degrees) are used. On scales >3 arc-minutes, the measurement of cross-correlation will be limited by the accuracy of the foreground sky subtraction.Comment: 12 pages. 5 figures. Submitted to MNRA

Dynamically-Driven Star Formation In Models Of NGC 7252

We present new dynamical models of the merger remnant NGC 7252 which include star formation simulated according to various phenomenological rules. By using interactive software to match our model with the observed morphology and gas velocity field, we obtain a consistent dynamical model for NGC 7252. In our models, this proto-elliptical galaxy formed by the merger of two similar gas-rich disk galaxies which fell together with an initial pericentric separation of ~2 disk scale lengths approximately 620 Myr ago. Results from two different star formation rules--- density-dependent and shock-induced--- show significant differences in star formation during and after the first passage. Shock-induced star formation yields a prompt and wide-spread starburst at the time of first passage, while density-dependent star formation predicts a more slowly rising and centrally concentrated starburst. A comparison of the distributions and ages of observed clusters with results of our simulations favors shock-induced mechanism of star formation in NGC 7252. We also present simulated color images of our model of NGC 7252, constructed by incorporating population synthesis with radiative transfer and dust attenuation. Overall the predicted magnitudes and colors of the models are consistent with observations, although the simulated tails are fainter and redder than observed. We suggest that a lack of star formation in the tails, reflected by the redder colors, is due to an incomplete description of star formation in our models rather than insufficient gas in the tails.Comment: 11 pages, 9 figures, to be published in MNRA

The ROSAT-ESO Flux-Limited X-Ray (REFLEX) Galaxy Cluster Survey III: The Power Spectrum

We present a measure of the power spectrum on scales from 15 to 800 Mpc/h using the ROSAT-ESO Flux-Limited X-Ray(REFLEX) galaxy cluster catalogue. The REFLEX survey provides a sample of the 452 X-ray brightest southern clusters of galaxies with the nominal flux limit S=3.0 10^{-12}erg/s/cm2 for the ROSAT energy band (0.1-2.4)keV. Several tests are performed showing no significant incompletenesses of the REFLEX clusters with X-ray luminosities brighter than 10^{43}erg/s up to scales of about 800 Mpc/h. They also indicate that cosmic variance might be more important than previous studies suggest. We regard this as a warning not to draw general cosmological conclusions from cluster samples with a size smaller than REFLEX. Power spectra, P(k), of comoving cluster number densities are estimated for flux- and volume-limited subsamples. The most important result is the detection of a broad maximum within the comoving wavenumber range 0.022<k<0.030 h/Mpc. The data suggest an increase of the power spectral amplitude with X-ray luminosity. Compared to optically selected cluster samples the REFLEX P(k)is flatter for wavenumbers k<0.05 h/Mpc thus shifting the maximum of P(k) to larger scales. The smooth maximum is not consistent with the narrow peak detected at k=0.05 h/Mpc using the Abell/ACO richness $\ge 0$ data. In the range 0.02<k<0.4 h/Mpc general agreement is found between the slope of the REFLEX P(k) and those obtained with optically selected galaxies. A semi-analytic description of the biased nonlinear power spectrum in redshift space gives the best agreement for low-density Cold Dark Matter models with or without a cosmological constant.Comment: 22 pages, 20 figures, (A&A accepted), also available at http://www.xray.mpe.mpg.de/theorie/REFLEX

Targeted Activation of Hippocampal Place Cells Drives Memory-Guided Spatial Behavior

The hippocampus is crucial for spatial navigation and episodic memory formation. Hippocampal place cells exhibit spatially selective activity within an environment and have been proposed to form the neural basis of a cognitive map of space that supports these mnemonic functions. However, the direct influence of place cell activity on spatial navigation behavior has not yet been demonstrated. Using an ‘all-optical’ combination of simultaneous two-photon calcium imaging and two-photon optogenetics, we identified and selectively activated place cells that encoded behaviorally relevant locations in a virtual reality environment. Targeted stimulation of a small number of place cells was sufficient to bias the behavior of animals during a spatial memory task, providing causal evidence that hippocampal place cells actively support spatial navigation and memory