Strong lensing statistics in large, z~<0.2 surveys: bias in the lens galaxy population

We calculate the expected lensing statistics of the galaxy population in large, low-redshift surveys. Galaxies are modeled using realistic, multiple components: a dark matter halo, a bulge component and disc. We use semi-analytic models of galaxies coupled with dark matter haloes in the Millennium Run to model the lens galaxy population. We predict that a fraction of 1.4+/-0.18*10^-3 of radio sources will be lensed by galaxies within a survey like the 2dF below z<0.2. With a simulated sample of lensed radio sources, the predicted lensing galaxy population consists mainly of ellipticals (~80%) with an average lens velocity dispersion of 164+/-3 km/s, producing typical image separations of ~3 arcsec. The lens galaxy population lies on the fundamental plane but its velocity dispersion distribution is shifted to higher values compared to all early-type galaxies. Taking magnification bias into account, we predict that the ratio of 4:2 image systems is 30+/-5%, consistent with the observed ratio found in the Cosmic Lens All-Sky Survey. We also find that the population of 4-image lens galaxies is distinguishable from the population of lens galaxies in 2-image systems. Our key result is the explicit demonstration that the population of lens galaxies differs markedly from the galaxy population as a whole: lens galaxies have a higher average luminosity and reside in more massive haloes than the overall sample of ellipticals. This bias restricts our ability to infer galaxy evolution parameters from a sample of lensing galaxies. (abridged)Comment: 13 pages, 13 figures, submitted to MNRAS. Removed double figure and added correct figure

Detection of Non-Random Galaxy Orientations in X-ray Subclusters of the Coma Cluster

This study on the Coma cluster suggests that there are deviations from a completely random galaxy orientation on small scales. Since we found a significant coincidence of hot-gas features identified in the latest X-ray observations of Coma with these local anisotropies, they may indicate regions of recent mutual interaction of member galaxies within subclusters which are currently falling in on the main cluster.Comment: 4 pages, 4 figures, 3 tables v2: Rewritten introduction, amendments to the 'Interpretation' sectio

Galaxy formation and evolution in the Millennium Simulation

This Thesis addresses the topic of galaxy formation and evolution in the universe. In collaboration with D. Croton, G. de Lucia, V. Springel, and S.D.M. White, I made use of the Millennium simulation, a very large N-body simulation of dark-matter evolution in a cosmological volume carried out at the MPA in 2005 by Springel 2005, to explore the predictions made by the most recent generation of semi-analytic models for galaxy formation. These models are incorporating a new mode of feedback from active galactic nuclei (AGN), which have their origins in super-massive black holes accreting mass and turning it into energy. Because of its observational signature in the radio regime this feedback is called "radio mode" and it counteracts the cooling flows of cold gas in undisturbed dark-matter haloes hosting galaxy clusters, which would otherwise show much higher star-formation of their central object than is observed. Previous work by Croton 2006 and De Lucia 2006 has shown that with the new semi-analytic model the population of local galaxies can be reproduced quite accurately. In order to study the evolution of the population out to higher redshifts, the semi-analytic predictions have been compared to a number of observations in various filter bands, in particular to two recent efforts to get a comprehensive multi-wavelength dataset of high redshift galaxies carried out by the DEEP2 (Davis 2001) and COSMOS (Scoville 2006) collaborations. The approach taken was to perform as broad a comparison as possible to gain firm constraints on the assumed physics in our model. Therefore a multitude of observational properties was contrasted with the model predictions such as clustering, luminosity functions, stellar mass functions, number counts per area and redshift to a certain magnitude limit. In order to facilitate the comparison between simulations and recent intermediate and high-redshift surveys, it is very useful to have a number of independent mock observations of the simulated galaxies, which provide good enough statistics to get a handle on cosmic variance. To this end I have devised a computer program that calculates the simulated galaxies lying on the backward light cone of a hypothetical observer out to arbitrarily high redshifts, taking advantage of the periodicity of the simulation box but avoiding replications. The output provides accurately interpolated redshifts, positions, observer frame and rest-frame magnitudes, dust extinction, as well as all the intrinsic galaxy properties like stellar mass and star formation rate. Utilising this tool it is also possible to make predictions for future galaxy surveys, deeper in magnitude and redshift than current ones. Presently the mock catalogues are used by the DEEP2 and COSMOS teams as a comparison sample in general and as a means to assess their selection effects and improve their data reduction in particular. First comparisons of counts in apparent magnitude and redshift gave promising results, showing good agreement in the low and intermediate range. The same holds for the angular clustering analysis except for the faintest magnitudes. Thus we conclude that our current understanding of the processes governing galaxy formation and evolution from the very first objects to the present day population is realistic but still incomplete. In particular the treatment of the interplay between star formation and negative feedback and the various processes influencing satellite galaxies in big galaxy clusters have potential for improvement. In the following I will give a brief outline of the thesis. After setting the stage for any kind of model in Chapter 1 by defining the geometry of the universe and the cosmological parameters that determine it, I will describe our semi-analytical model of galaxy formation in Chapter 2, where it will be also explained how to construct realistic mock observations of the simulated galaxies. First in Chapter 3 it will be verified that a simple model which assumes that galaxies are conserved but evolve in luminosity due to their star formation histories cannot account for the observed evolution of the galaxy population in the universe. This fact can be understood in the context of hierarchical models where massive and luminous galaxies assembled from smaller objects. Chapter 4 proceeds with exploring the predictions from the considerably more sophisticated semi-analytic model based on an N-body simulation of the hierarchical growth of dark matter structures. For this analysis a set of mock light-cones was constructed for direct comparison with the data which shows reasonably good agreement between model and observations at low redshift and for bright apparent magnitudes. These light-cones represent one of the largest samples of realistic mock observations currently available. They can be used for testing data analysis techniques usually applied to real observations on a well defined sample of artificial galaxies to verify how well the derivation of galaxy properties from the data works. In Chapter 5 we will demonstrate how one can measure the evolution of the galaxy merger rate from observing close projected galaxy pairs. Interestingly we find that the calibration needed for the conversion is significantly different from what has typically been assumed in previous studies. Additionally we will demonstrate that galaxy merger rates and dark-matter merger rates show considerably different evolution with redshift. Consequently we conclude that merger rate studies are less suitable as a probe of cosmic structure formation than initially assumed, but nonetheless they can be of great help to understand the formation and evolution of galaxies in a hierarchical universe. Finally these results will be summarised and discussed in Chapter 6 where I will also give a brief outlook on the future of this work, a short glimpse of which is already presented in the Appendix

Optical Spectroscopy of IRAS 02091+6333

We present a detailed spectroscopic investigation, spanning four winters, of the asymptotic giant branch (AGB) star IRAS 02091+6333. Zijlstra & Weinberger (2002) found a giant wall of dust around this star and modelled this unique phenomenon. However their work suffered from the quality of the optical investigations of the central object. Our spectroscopic investigation allowed us to define the spectral type and the interstellar foreground extinction more precisely. Accurate multi band photometry was carried out. This provides us with the possibility to derive the physical parameters of the system. The measurements presented here suggest a weak irregular photometric variability of the target, while there is no evidence of a spectroscopic variability over the last four years.Comment: 5 pages, Latex, 3 tables, 4 figures, Astron. & Astrophys. - in pres

Emergence of rich-club topology and coordinated dynamics in development of hippocampal functional networks in vitro.

Recent studies demonstrated that the anatomical network of the human brain shows a "rich-club" organization. This complex topological feature implies that highly connected regions, hubs of the large-scale brain network, are more densely interconnected with each other than expected by chance. Rich-club nodes were traversed by a majority of short paths between peripheral regions, underlining their potential importance for efficient global exchange of information between functionally specialized areas of the brain. Network hubs have also been described at the microscale of brain connectivity (so-called "hub neurons"). Their role in shaping synchronous dynamics and forming microcircuit wiring during development, however, is not yet fully understood. The present study aimed to investigate the role of hubs during network development, using multi-electrode arrays and functional connectivity analysis during spontaneous multi-unit activity (MUA) of dissociated primary mouse hippocampal neurons. Over the first 4 weeks in vitro, functional connectivity significantly increased in strength, density, and size, with mature networks demonstrating a robust modular and small-world topology. As expected by a "rich-get-richer" growth rule of network evolution, MUA graphs were found to form rich-clubs at an early stage in development (14 DIV). Later on, rich-club nodes were a consistent topological feature of MUA graphs, demonstrating high nodal strength, efficiency, and centrality. Rich-club nodes were also found to be crucial for MUA dynamics. They often served as broker of spontaneous activity flow, confirming that hub nodes and rich-clubs may play an important role in coordinating functional dynamics at the microcircuit level.M.S.S. is supported by a PhD studentship funded by a Core Award from the Medical Research Council and the Wellcome Trust to the Behavioural and Clinical Neuroscience Institute (MRC Ref G1000183; WT Ref 093875/Z/10/Z) and by the Studienstiftung des deutschen Volkes. Additional support for this study from the Biotechnology and Biological Sciences Research Council (BBSRC Ref BB/H008608/1) is gratefully acknowledged.This is the final published version. It first appeared at http://www.jneurosci.org/content/35/14/5459.full

Masses for the Local Group and the Milky Way

We use the very large Millennium Simulation of the concordance $\Lambda$CDM cosmogony to calibrate the bias and error distribution of Timing Argument estimators of the masses of the Local Group and of the Milky Way. From a large number of isolated spiral-spiral pairs similar to the Milky Way/Andromeda system, we find the interquartile range of the ratio of timing mass to true mass to be a factor of 1.8, while the 5% and 95% points of the distribution of this ratio are separated by a factor of 5.7. Here we define true mass as the sum of the ``virial'' masses $M_{200}$ of the two dominant galaxies. For current best values of the distance and approach velocity of Andromeda this leads to a median likelihood estimate of the true mass of the Local Group of 5.27\times 10^{12}\msun, or $\log M_{LG}/M_\odot = 12.72$, with an interquartile range of $[12.58, 12.83]$ and a 5% to 95% range of $[12.26, 13.01]$. Thus a 95% lower confidence limit on the true mass of the Local Group is 1.81\times 10^{12}\msun. A timing estimate of the Milky Way's mass based on the large recession velocity observed for the distant satellite Leo I works equally well, although with larger systematic uncertainties. It gives an estimated virial mass for the Milky Way of 2.43 \times 10^{12}\msun with a 95% lower confidence limit of 0.80 \times 10^{12}\msun.Comment: 11 pages, 6 figures, MNRAS accepted. Added a new discussion paragraph and a new figure regarding the relative transverse velocity but conclusions unchange

Conservative and disruptive modes of adolescent change in human brain functional connectivity

Adolescent changes in human brain function are not entirely understood. Here, we used multiecho functional MRI (fMRI) to measure developmental change in functional connectivity (FC) of resting-state oscillations between pairs of 330 cortical regions and 16 subcortical regions in 298 healthy adolescents scanned 520 times. Participants were aged 14 to 26 y and were scanned on 1 to 3 occasions at least 6 mo apart. We found 2 distinct modes of age-related change in FC: “conservative” and “disruptive.” Conservative development was characteristic of primary cortex, which was strongly connected at 14 y and became even more connected in the period from 14 to 26 y. Disruptive development was characteristic of association cortex and subcortical regions, where connectivity was remodeled: connections that were weak at 14 y became stronger during adolescence, and connections that were strong at 14 y became weaker. These modes of development were quantified using the maturational index (MI), estimated as Spearman’s correlation between edgewise baseline FC (at 14 y, FC14) and adolescent change in FC (ΔFC14−26), at each region. Disruptive systems (with negative MI) were activated by social cognition and autobiographical memory tasks in prior fMRI data and significantly colocated with prior maps of aerobic glycolysis (AG), AG-related gene expression, postnatal cortical surface expansion, and adolescent shrinkage of cortical thickness. The presence of these 2 modes of development was robust to numerous sensitivity analyses. We conclude that human brain organization is disrupted during adolescence by remodeling of FC between association cortical and subcortical areas

Group-finding with photometric redshifts: The Photo-z Probability Peaks algorithm

We present a galaxy group-finding algorithm, the Photo-z Probability Peaks (P3) algorithm, optimized for locating small galaxy groups using photometric redshift data by searching for peaks in the signal-to-noise of the local overdensity of galaxies in a three-dimensional grid. This method is an improvement over similar two-dimensional matched-filter methods in reducing background contamination through the use of redshift information, allowing it to accurately detect groups at lower richness. We present the results of tests of our algorithm on galaxy catalogues from the Millennium Simulation. Using a minimum S/N of 3 for detected groups, a group aperture size of 0.25 Mpc/h, and assuming photometric redshift accuracy of sigma_z = 0.05 it attains a purity of 84% and detects ~295 groups/deg.^2 with an average group richness of 8.6 members. Assuming photometric redshift accuracy of sigma_z = 0.02, it attains a purity of 97% and detects ~143 groups/deg.^2 with an average group richness of 12.5 members. We also test our algorithm on data available for the COSMOS field and the presently-available fields from the CFHTLS-Wide survey, presenting preliminary results of this analysis.Comment: Accepted for publication by MNRAS, 16 pages, 11 color figure

The Cosmic Evolution Survey (COSMOS): a large-scale structure at z=0.73 and the relation of galaxy morphologies to local environment

We have identified a large-scale structure at z~0.73 in the COSMOS field, coherently described by the distribution of galaxy photometric redshifts, an ACS weak-lensing convergence map and the distribution of extended X-ray sources in a mosaic of XMM observations. The main peak seen in these maps corresponds to a rich cluster with Tx= 3.51+0.60/-0.46 keV and Lx=(1.56+/-0.04) x 10^{44} erg/s ([0.1-2.4] keV band). We estimate an X-ray mass within $r500$ corresponding to M500~1.6 x 10^{14} Msun and a total lensing mass (extrapolated by fitting a NFW profile) M(NFW)=(6+/-3) x 10^15 Msun. We use an automated morphological classification of all galaxies brighter than I_AB=24 over the structure area to measure the fraction of early-type objects as a function of local projected density Sigma_10, based on photometric redshifts derived from ground-based deep multi-band photometry. We recover a robust morphology-density relation at this redshift, indicating, for comparable local densities, a smaller fraction of early-type galaxies than today. Interestingly, this difference is less strong at the highest densities and becomes more severe in intermediate environments. We also find, however, local "inversions'' of the observed global relation, possibly driven by the large-scale environment. In particular, we find direct correspondence of a large concentration of disk galaxies to (the colder side of) a possible shock region detected in the X-ray temperature map and surface brightness distribution of the dominant cluster. We interpret this as potential evidence of shock-induced star formation in existing galaxy disks, during the ongoing merger between two sub-clusters.Comment: 15 pages (emulateapj style), 16 figs (low res.); to appear in the ApJ Supplement COSMOS Special Issue. Low-resolution figures; full resolution version available at: http://www.astro.caltech.edu/~cosmos/publications/files/guzzo_0701482.pd

Photo-z Performance for Precision Cosmology II : Empirical Verification

The success of future large scale weak lensing surveys will critically depend on the accurate estimation of photometric redshifts of very large samples of galaxies. This in turn depends on both the quality of the photometric data and the photo-z estimators. In a previous study, (Bordoloi et al. 2010) we focussed primarily on the impact of photometric quality on photo-z estimates and on the development of novel techniques to construct the N(z) of tomographic bins at the high level of precision required for precision cosmology, as well as the correction of issues such as imprecise corrections for Galactic reddening. We used the same set of templates to generate the simulated photometry as were then used in the photo-z code, thereby removing any effects of "template error". In this work we now include the effects of "template error" by generating simulated photometric data set from actual COSMOS photometry. We use the trick of simulating redder photometry of galaxies at higher redshifts by using a bluer set of passbands on low z galaxies with known redshifts. We find that "template error" is a rather small factor in photo-z performance, at the photometric precision and filter complement expected for all-sky surveys. With only a small sub-set of training galaxies with spectroscopic redshifts, it is in principle possible to construct tomographic redshift bins whose mean redshift is known, from photo-z alone, to the required accuracy of 0.002(1+z).Comment: 7 pages, 5 figures, accepted for publication in MNRA