Evolution of the fσ8f\sigma_8 tension with the Planck15/Λ\LambdaCDM determination and implications for modified gravity theories

We construct an updated extended compilation of distinct (but possibly correlated) $f\sigma_8(z)$ Redshift Space Distortion (RSD) data published between 2006 and 2018. It consists of 63 datapoints and is significantly larger than previously used similar datasets. After fiducial model correction we obtain the best fit $\Omega_{0m}-\sigma_8$ $\Lambda$CDM parameters and show that they are at a $5\sigma$ tension with the corresponding Planck15/$\Lambda$CDM values. Introducing a nontrivial covariance matrix correlating randomly $20\%$ of the RSD datapoints has no significant effect on the above tension level. We show that the tension disappears (becomes less than $1\sigma$) when a subsample of the 20 most recently published data is used. A partial cause for this reduced tension is the fact that more recent data tend to probe higher redshifts (with higher errorbars) where there is degeneracy among different models due to matter domination. Allowing for a nontrivial evolution of the effective Newton's constant as $G_{\textrm{eff}}(z)/G_{\textrm{N}} = 1 + g_a \left(\frac{z}{1+z}\right)^2 - g_a \left(\frac{z}{1+z}\right)^4$ ($g_a$ is a parameter) and fixing a \plcdm background we find $g_a=-0.91\pm 0.17$ from the full $f\sigma_8$ dataset while the 20 earliest and 20 latest datapoints imply $g_a=-1.28^{+0.28}_{-0.26}$ and $g_a=-0.43^{+0.46}_{-0.41}$ respectively. Thus, the more recent $f\sigma_8$ data appear to favor GR in contrast to earlier data. Finally, we show that the parametrization $f\sigma_8(z)=\lambda \sigma_8 \Omega(z)^\gamma /(1+z)^\beta$ provides an excellent fit to the solution of the growth equation for both GR ($g_a=0$) and modified gravity ($g_a\neq 0$).Comment: Version published in Phys. Rev. D 97, 103503 (3 May 2018). 17 pages, 11 figures. Added a Table indicating the consistency of RSD data with other growth of perturbation probes (weak lensing). The Mathematica files with the numerical analysis may be downloaded from http://leandros.physics.uoi.gr/growth-tomograph

Pseudo-intelectualismo y melancolía. La poética de la bilis negra en el Lexífanes de Luciano

En el mundo altamente competitivo y exhibicionista de Luciano, el hiper-aticismo, el (ab)uso de palabras recónditas y arcaicas a fin de impactar deviene en una especie de plaga. En este artículo discuto cómo el Lexífanes se centra precisamente en las asociaciones literales y metafóricas del hiperaticismo como una enfermedad, prestando especial atención al dictamen médico -articulado en el texto por el doble del autor, Licinio-, según el cual el personaje epónimo del diálogo sufre de melancolía. En lugar de constituir una referencia de pasada al vocabulario coloquial de la locura, la melancolía -sostengo- ayuda a Luciano a asimilar a la fuerza la acumulación de palabras pretenciosas para el desvarío de un loco, un parloteo no sensitivo carente de significado. Al mismo tiempo, Luciano también pretende exponer la presencia de la melancolía en el lenguaje cultural y médico de su época como una enfermedad que de manera especial afecta a los “grandes espíritus”, a las personas de inteligencia excepcional. Al llamar a Lexífanes ‘melancólico’, Luciano reprende la pretensión de Lexiphanes a la vez como hiper-aticista y, no menos importante, como pseudointelectual que está configurando su imagen pública, temperamental y fisiológicamente, como un genio cuya atrabiliaria conformación le permite actuar y hablar en modos extraños.In Lucian’s highly competitive and exhibitionist world, hyper-Atticism, the (ab)use of recondite, archaic words for the sake of impression, has become a sort of plague. In this article, I discuss how Lexiphanes focuses precisely on the literal and metaphorical associations of hyper-Atticism as a disease, by paying particular attention on the medical verdict - articulated in the text by Lucian’s authorial double, Lycinus - that the dialogue’s eponymous character suffers from melancholia. Rather than constitute a passing reference to the colloquial vocabulary of insanity, melancholia, I argue, helps Lucian forcefully assimilate the accumulation of pretentious words to the raving of the insane, a non-sensical blabbering that is void of meaning. At the same time, Lucian aims also to expose the presence of melancholy in the cultural and medical idiom of his time as a disease that typically affects ‘great spirits’, people of exceptional intelligence. By calling Lexiphanes ‘melancholic’, Lucian scolds Lexiphanes’ pretentiousness both as a hyper-Atticist and, no less importantly, as a pseudo-intellectual who is shaping his public image, temperamentally and physiognomically, as a genius whose atrabilious constitution entitles him to act and speak in strange ways

Dark satellites and the morphology of dwarf galaxies

One of the strongest predictions of the LambdaCDM cosmological model is the presence of dark satellites orbiting all types of galaxies. We focus here on the dynamical effects of such satellites on disky dwarf galaxies, and demonstrate that these encounters can be dramatic. Although mergers with M_sat > M_d are not very common, because of the lower baryonic content they occur much more frequently on the dwarf scale than for L_*-galaxies. As an example, we present a numerical simulation of a 20% (virial) mass ratio merger between a dark satellite and a disky dwarf (akin to the Fornax dwarf galaxy in luminosity) that shows that the merger remnant has a spheroidal morphology. We conclude that perturbations by dark satellites provide a plausible path for the formation of dSph systems and also could trigger starbursts in gas rich dwarf galaxies. Therefore the transition from disky to the often amorphous, irregular, or spheroidal morphologies of dwarfs could be a natural consequence of the dynamical heating of hitherto unobservable dark satellites.Comment: Submitted to ApJ Letters. 5 pages, 4 figures. A movie showing an encounter between a disky dwarf galaxy and a dark satellite can be found at http://www.astro.rug.nl/~ahelmi/dwarfs-morphologies.htm

N-body realizations of cuspy dark matter haloes

We describe an algorithm for generating equilibrium initial conditions for numerical experiments with dark matter haloes. Our haloes are modelled using a general form for the mass density p{r), making it possible to represent most of the popular density profiles in the literature. The finite mass 7-models and the cuspy density profiles found in recent high-resolution cosmological TV-body simulations having a density power-law fall-off at large distances proportional to are included as special cases. The algorithm calculates the phase-space distribution function of each model assuming spherical symmetry and either an isotropic velocity dispersion tensor or an anisotropic velocity dispersion tensor of the type proposed by Osipkov and Merritt. The particle velocities are assigned according to the exact velocity distribution, making this method ideal for experiments requiring a high degree of stability. Numerical tests confirm that the resulting models are highly stable. This approach is motivated by the instabilities that arise when a local Maxwellian velocity distribution is adopted. For example, after approximating the velocity distribution by a Gaussian we show that a Hernquist halo with an initial r(^-1) density cusp immediately develops a constant density core. Moreover, after a single crossing time the orbital anisotropy has evolved over the entire system. Previous studies that use this approximation to construct halo or galaxy models could be compromised by this behaviour. Using the derived distribution functions we show the exact 1-d velocity distributions and we compare them with the Gaussian velocity distributions with the same second moment for different distances from the halo centre. We show that instabilities arise because a Gaussian velocity distribution is a very poor approximation to the true velocity distribution of particles. We also perform a series of numerical simulations evolving several dark matter halo models in isolation, with the intention of checking the stability of the initialization procedure in both configuration and velocity space. A subset of the models are evolved under the assumption that the velocity distribution at any given point is a Gaussian and the time evolution of the density profiles and velocity structure is monitored. Finally, a number of applications are discussed, including issues of relaxation in dark matter haloes as well as mergers of haloes in scattering experiments

Birth of massive black hole binaries

If massive black holes (BHs) are ubiquitous in galaxies and galaxies experience multiple mergers during their cosmic assembly, then BH binaries should be common albeit temporary features of most galactic bulges. Observationally, the paucity of active BH pairs points toward binary lifetimes far shorter than the Hubble time, indicating rapid inspiral of the BHs down to the domain where gravitational waves lead to their coalescence. Here, we review a series of studies on the dynamics of massive BHs in gas-rich galaxy mergers that underscore the vital role played by a cool, gaseous component in promoting the rapid formation of the BH binary. The BH binary is found to reside at the center of a massive self-gravitating nuclear disc resulting from the collision of the two gaseous discs present in the mother galaxies. Hardening by gravitational torques against gas in this grand disc is found to continue down to sub-parsec scales. The eccentricity decreases with time to zero and when the binary is circular, accretion sets in around the two BHs. When this occurs, each BH is endowed with it own small-size (< 0.01 pc) accretion disc comprising a few percent of the BH mass. Double AGN activity is expected to occur on an estimated timescale of < 1 Myr. The double nuclear point-like sources that may appear have typical separation of < 10 pc, and are likely to be embedded in the still ongoing starburst. We note that a potential threat of binary stalling, in a gaseous environment, may come from radiation and/or mechanical energy injections by the BHs. Only short-lived or sub-Eddington accretion episodes can guarantee the persistence of a dense cool gas structure around the binary necessary for continuing BH inspiral.Comment: To appear in "2007 STScI Spring Symposium: Black Holes", eds. M. Livio & A. M. Koekemoer, Cambridge University Press, 25 pages, 12 figure

Thick-disk evolution induced by the growth of an embedded thin disk

We perform collisionless N-body simulations to investigate the evolution of the structural and kinematical properties of simulated thick disks induced by the growth of an embedded thin disk. The thick disks used in the present study originate from cosmologically-common 5:1 encounters between initially-thin primary disk galaxies and infalling satellites. The growing thin disks are modeled as static gravitational potentials and we explore a variety of growing-disk parameters that are likely to influence the response of thick disks. We find that the final thick-disk properties depend strongly on the total mass and radial scale-length of the growing thin disk, and much less sensitively on its growth timescale and vertical scale-height as well as the initial sense of thick-disk rotation. Overall, the growth of an embedded thin disk can cause a substantial contraction in both the radial and vertical direction, resulting in a significant decrease in the scale-lengths and scale-heights of thick disks. Kinematically, a growing thin disk can induce a notable increase in the mean rotation and velocity dispersions of thick-disk stars. We conclude that the reformation of a thin disk via gas accretion may play a significant role in setting the structure and kinematics of thick disks, and thus it is an important ingredient in models of thick-disk formation.Comment: 19 pages, 17 figures, 1 table, LaTeX (uses emulateapj.cls). Replaced to match the version accepted for publication in ApJ. Fig. 3 adde

The Sphericalization of Dark Matter Halos by Galaxy Disks

Cosmological simulations indicate that cold dark matter (CDM) halos should be triaxial. Verifying observationally this theoretical prediction is, however, less than straightforward because the assembly of galaxies is expected to modify the halo shapes and to render them more axisymmetric. We use a suite of N-body simulations to investigate quantitatively the effect of the growth of a central disk galaxy on the shape of triaxial dark matter halos. As expected, the halo responds to the presence of the disk by becoming more spherical. The net effect depends only weakly on the orientation of the disk relative to the halo principal axes or the timescale of disk assembly, but strongly on the overall gravitational importance of the disk. Our results show that exponential disks whose contribution peaks at less than ~50% of their circular velocity are unable to modify noticeably the shape of the gravitational potential of their surrounding halos. Many dwarf and low surface brightness galaxies are expected to be in this regime, and therefore their detailed kinematics could be used to probe halo triaxiality, one of the basic predictions of the CDM paradigm. We argue that the complex disk kinematics of the dwarf galaxy NGC 2976 might be the reflection of a triaxial halo. Such signatures of halo triaxiality should be common in galaxies where the luminous component is subdominant.Comment: 7 pages, 4 figures, LaTeX (uses emulateapj.cls). Replaced to match the version accepted for publication in ApJL. This version includes an additional figure in response to referee. Conclusions remain unchange

The stellar structure and kinematics of dwarf spheroidal galaxies formed by tidal stirring

Using high-resolution N-body simulations we study the stellar properties of dwarf spheroidal galaxies resulting from the tidally induced morphological transformation of disky dwarfs on a cosmologically motivated eccentric orbit around the Milky Way. Dwarf galaxy models initially consist of an exponential stellar disk embedded in an extended spherical dark matter halo. Depending on the initial orientation of the disk with respect to the orbital plane, different final configurations are obtained. The least evolved dwarf is triaxial and retains a significant amount of rotation. The more evolved dwarfs are prolate spheroids with little rotation. We show that the final density distribution of stars can be approximated by a simple modification of the Plummer law. The kinematics of the dwarfs is significantly different depending on the line of sight which has important implications for mapping the observed stellar velocity dispersions of dwarfs to subhalo circular velocities. When the dwarfs are observed along the long axis, the measured velocity dispersion is higher and decreases faster with radius. In the case where rotation is significant, when viewed perpendicular to the long axis, the effect of minor axis rotation is detected, as expected for triaxial systems. We model the velocity dispersion profiles and rotation curves of the dwarfs by solving the Jeans equations for spherical and axisymmetric systems and adjusting different sets of free parameters. We find that the mass is typically overestimated when the dwarf is seen along the long axis and underestimated when the observation is along the short or intermediate axis. The effect of non-sphericity cannot however bias the inferred mass by more than 60 percent in either direction, even for the most strongly stripped dwarf which is close to disruption.Comment: 17 pages, 15 figures, revised version accepted for publication in Ap