ALBERT

Description: We study of the role of ‘major’ mergers (mass ratios 〉1: 4) in driving size growth in high-redshift (1 〈 z 〈 2) spheroidal galaxies (SGs) with stellar masses between 10 9.5 and 10 10.7 M . This is a largely unexplored mass range at this epoch, containing the progenitors of more massive SGs on which the bulk of the size-evolution literature is based. We visually split our SGs into systems that are relaxed and those that exhibit tidal features indicative of a recent merger. Numerical simulations indicate that, given the depth of our images, only tidal features due to major mergers will be detectable at these epochs (features from minor mergers being too faint), making the disturbed SGs a useful route to estimating major-merger-driven size growth. The disturbed SGs are offset in size from their relaxed counterparts, lying close to the upper envelope of the local size–mass relation. The mean size ratio of the disturbed SGs to their relaxed counterparts is ~2. Combining this observed size growth with empirical major-merger histories from the literature suggests that the size evolution of a significant fraction (around two-thirds) of SGs in this mass range could be driven by major mergers. If, as is likely, our galaxies are progenitors of more massive ( M * 〉 10 10.7 M ) SGs at z 〈 1, then major mergers are also likely to play an important role in the size growth of at least some massive SGs in this mass range.

Description: Building galaxy merger trees from a state-of-the-art cosmological hydrodynamical simulation, Horizon-AGN, we perform a statistical study of how mergers and diffuse stellar mass acquisition processes drive galaxy morphologic properties above z  〉 1. By diffuse mass acquisition here, we mean both accretion of stars by unresolved mergers (relative stellar mass growth smaller than 4.5 per cent) as well as in situ star formation when no resolved mergers are detected along the main progenitor branch of a galaxy. We investigate how stellar densities, galaxy sizes and galaxy morphologies (defined via shape parameters derived from the inertia tensor of the stellar density) depend on mergers of different mass ratios. We investigate how stellar densities, effective radii and shape parameters derived from the inertia tensor depend on mergers of different mass ratios. We find strong evidence that diffuse stellar accretion and in situ formation tend to flatten small galaxies over cosmic time, leading to the formation of discs. On the other hand, mergers, and not only the major ones, exhibit a propensity to puff up and destroy stellar discs, confirming the origin of elliptical galaxies. We confirm that mergers grow galaxy sizes more efficiently than diffuse processes ( $r_{0.5}\propto M_{\rm s}^{0.85}$ and $r_{0.5}\propto M_{\rm s}^{0.1}$ on average, respectively) and we also find that elliptical galaxies are more susceptible to grow in size through mergers than disc galaxies with a size–mass evolution $r_{0.5}\propto M_{\rm s}^{1.2}$ instead of $r_{0.5}\propto M_{\rm s}^{-0.5}-M^{0.5}$ for discs depending on the merger mass ratio. The gas content drives the size–mass evolution due to merger with a faster size growth for gas-poor galaxies $r_{0.5}\propto M_{\rm s}^{2}$ than for gas-rich galaxies r 0.5 M s .

Description: Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies with masses above 2 x 10(10) times that of the Sun; the number of quiescent galaxies has increased by a factor of about 25 over the past ten billion years (refs 1-4). Once star formation has been shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole, an unknown mechanism must remove or heat the gas that is subsequently accreted from either stellar mass loss or mergers and that would otherwise cool to form stars. Energy output from a black hole accreting at a low rate has been proposed, but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centres of clusters, which are too rare to explain the vast majority of the quiescent population. Here we report bisymmetric emission features co-aligned with strong ionized-gas velocity gradients from which we infer the presence of centrally driven winds in typical quiescent galaxies that host low-luminosity active nuclei. These galaxies are surprisingly common, accounting for as much as ten per cent of the quiescent population with masses around 2 x 10(10) times that of the Sun. In a prototypical example, we calculate that the energy input from the galaxy's low-level active supermassive black hole is capable of driving the observed wind, which contains sufficient mechanical energy to heat ambient, cooler gas (also detected) and thereby suppress star formation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Cheung, Edmond -- Bundy, Kevin -- Cappellari, Michele -- Peirani, Sebastien -- Rujopakarn, Wiphu -- Westfall, Kyle -- Yan, Renbin -- Bershady, Matthew -- Greene, Jenny E -- Heckman, Timothy M -- Drory, Niv -- Law, David R -- Masters, Karen L -- Thomas, Daniel -- Wake, David A -- Weijmans, Anne-Marie -- Rubin, Kate -- Belfiore, Francesco -- Vulcani, Benedetta -- Chen, Yan-mei -- Zhang, Kai -- Gelfand, Joseph D -- Bizyaev, Dmitry -- Roman-Lopes, A -- Schneider, Donald P -- England -- Nature. 2016 May 25;533(7604):504-8. doi: 10.1038/nature18006.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Kavli Institute for the Physics and Mathematics of the Universe (World Premier International Research Center Initiative), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan. ; Sub-department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK. ; Institut d'Astrophysique de Paris (UMR 7095, CNRS and UPMC), 98 bis Boulevard Arago, F-75014 Paris, France. ; Department of Physics, Faculty of Science, Chulalongkorn University, 254 Phayathai Road, Pathumwan, Bangkok 10330, Thailand. ; Institute for Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK. ; Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, Kentucky 40506-0055, USA. ; Department of Astronomy, University of Wisconsin-Madison, 475 North Charter Street, Madison, Wisconsin 53706, USA. ; Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA. ; Center for Astrophysical Sciences, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA. ; McDonald Observatory, Department of Astronomy, University of Texas at Austin, 1 University Station, Austin, Texas 78712-0259, USA. ; Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA. ; Department of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK. ; School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS, UK. ; Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA. ; Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK. ; Kavli Institute for Cosmology, University of Cambridge, Cambridge CB3 0HE, UK. ; Department of Astronomy, Nanjing University, Nanjing 210093, China. ; New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates. ; Center for Cosmology and Particle Physics, New York University, Meyer Hall of Physics, 4 Washington Place, New York, New York 10003, USA. ; Apache Point Observatory and New Mexico State University, PO Box 59, Sunspot, New Mexico 88349-0059, USA. ; Sternberg Astronomical Institute, Moscow State University, Moscow, Russia. ; Departamento de Fisica y Astronomia, Facultad de Ciencias, Universidad de La Serena, Cisternas 1200, La Serena, Chile. ; Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ; Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/27225122" target="_blank"〉PubMed〈/a〉

Description: We use the Lyα Mass Association Scheme (LyMAS) to predict cross-correlations at z = 2.5 between dark matter haloes and transmitted flux in the Lyα forest, and compare to cross-correlations measured for quasars and damped Lyα systems (DLAs) from the Baryon Oscillation Spectroscopic Survey (BOSS) by Font-Ribera et al. We calibrate LyMAS using Horizon-AGN hydrodynamical cosmological simulations of a (100 h – 1 Mpc) 3 comoving volume. We apply this calibration to a (1 h – 1 Gpc) 3 simulation realized with 2048 3 dark matter particles. In the 100 h – 1 Mpc box, LyMAS reproduces the halo-flux correlations computed from the full hydrodynamic gas distribution very well. In the 1 h – 1 Gpc box, the amplitude of the large-scale cross-correlation tracks the halo bias b h as expected. We provide empirical fitting functions that describe our numerical results. In the transverse separation bins used for the BOSS analyses, LyMAS cross-correlation predictions follow linear theory accurately down to small scales. Fitting the BOSS measurements requires inclusion of random velocity errors; we find best-fitting rms velocity errors of 399 and $252\ \rm {km}\ \rm {s}^{-1}$ for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of $M_{\rm h}/10^{12}\ h^{-1}\,\mathrm{M}_{\odot }=2.19^{+0.16}_{-0.15}$ and $0.69^{+0.16}_{-0.14}$ for the host haloes of quasars and DLAs, with ~0.2 dex systematic uncertainty associated with redshift evolution, intergalactic medium parameters, and selection of data fitting range.

Description: We perform a detailed comparison of the phase-space density traced by the particle distribution in gadget simulations to the result obtained with a spherical Vlasov solver using the splitting algorithm. The systems considered are apodized Hénon spheres with two values of the virial ratio, R ~= 0.1 and 0.5. After checking that spherical symmetry is well preserved by the N -body simulations, visual and quantitative comparisons are performed. In particular, we introduce new statistics, correlators and entropic estimators, based on the likelihood of whether N -body simulations actually trace randomly the Vlasov phase-space density. When taking into account the limits of both the N -body and the Vlasov codes, namely collective effects due to the particle shot noise in the first case and diffusion and possible non-linear instabilities due to finite resolution of the phase-space grid in the second case, we find a spectacular agreement between both methods, even in regions of phase-space where non-trivial physical instabilities develop. However, in the colder case, R  = 0.1, it was not possible to prove actual numerical convergence of the N -body results after a number of dynamical times, even with N  = 10 8 particles.

Description: The velocity distribution function (VDF) of dark matter (DM) haloes in cold dark matter (CDM) dissipationless cosmological simulations, which must be non-separable in its radial and tangential components, is still poorly known. We present the first single-parameter, non-separable, anisotropic model for the VDF in CDM haloes, built from an isotropic q -Gaussian (Tsallis) VDF of the isotropic set of dimensionless spherical velocity components (after subtraction of streaming motions), normalized by the respective velocity dispersions. We test our VDF on 90 cluster-mass haloes of a dissipationless cosmological simulation. Beyond the virial radius, r vir , our model VDF adequately reproduces that measured in the simulated haloes, but no q -Gaussian model can adequately represent the VDF within r vir , as the speed distribution function is then flatter-topped than any q -Gaussian can allow. Nevertheless, our VDF fits significantly better the simulations than the commonly used Maxwellian (Gaussian) distribution, at virtually all radii within 5 r vir . Within 0.4 (1) r vir , the non-Gaussianity index q is (roughly) linearly related to the slope of the density profile and also to the velocity anisotropy profile. We provide a parametrization of the modulation of q with radius for both the median fits and the fit of the stacked halo. At radii of a few per cent of r vir , corresponding to the solar position in the Milky Way, our best-fitting VDF, although fitting better the simulations than the Gaussian one, overproduces significantly the fraction of high-velocity objects, indicating that one should not blindly use these q -Gaussian fits to make predictions on the direct detection rate of DM particles.

Description: The close relationship between mergers and the reorientation of the spin for galaxies and their host dark haloes is investigated using a cosmological hydrodynamical simulation (Horizon-AGN). Through a statistical analysis of merger trees, we show that spin swings are mainly driven by mergers along the filamentary structure of the cosmic web, and that these events account for the preferred perpendicular orientation of massive galaxies with respect to their nearest filament. By contrast, low-mass galaxies ( M s 〈 10 10 M at redshift 1.5) having undergone very few mergers, if at all, tend to possess a spin well aligned with their filament. Haloes follow the same trend as galaxies but display a greater sensitivity to smooth anisotropic accretion. The relative effect of mergers on magnitude is qualitatively different for minor and major mergers: mergers (and diffuse accretion) generally increase the magnitude of the specific angular momentum, but major mergers also give rise to a population of objects with less specific angular momentum left. Without mergers, secular accretion builds up the specific angular momentum of galaxies but not that of haloes. It also (re)aligns galaxies with their filament.

Description: The kinematic analysis of dark matter and hydrodynamical simulations suggests that the vorticity in large-scale structure is mostly confined to, and predominantly aligned with, their filaments, with an excess of probability of 20 per cent to have the angle between vorticity and filaments direction lower than 60° relative to random orientations. The cross-sections of these filaments are typically partitioned into four quadrants with opposite vorticity sign, arising from multiple flows, originating from neighbouring walls. The spins of haloes embedded within these filaments are consistently aligned with this vorticity for any halo mass, with a stronger alignment for the most massive structures up to an excess of probability of 165 per cent. The global geometry of the flow within the cosmic web is therefore qualitatively consistent with a spin acquisition for smaller haloes induced by this large-scale coherence, as argued in Codis et al. In effect, secondary anisotropic infall (originating from the vortex-rich filament within which these lower-mass haloes form) dominates the angular momentum budget of these haloes. The transition mass from alignment to orthogonality is related to the size of a given multi-flow region with a given polarity. This transition may be reconciled with the standard tidal torque theory if the latter is augmented so as to account for the larger scale anisotropic environment of walls and filaments.

Description: A large-scale hydrodynamical cosmological simulation, Horizon-AGN, is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 〈 z 〈 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 x 10 10 M . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

Description: Understanding how the intergalactic medium (IGM) was reionized at z 6 is one of the big challenges of current high-redshift astronomy. It requires modelling the collapse of the first astrophysical objects (Pop III stars, first galaxies) and their interaction with the IGM, while at the same time pushing current observational facilities to their limits. The observational and theoretical progress of the last few years have led to the emergence of a coherent picture in which the budget of hydrogen-ionizing photons is dominated by low-mass star-forming galaxies, with little contribution from Pop III stars and quasars. The reionization history of the Universe therefore critically depends on the number density of low-mass galaxies at high redshift. In this work, we explore how changes in the cosmological model, and in particular in the statistical properties of initial density fluctuations, affect the formation of early galaxies. Following Habouzit et al. ( 2014 ), we run five different N -body simulations with Gaussian and (scale-dependent) non-Gaussian initial conditions, all consistent with Planck constraints. By appealing to a phenomenological galaxy formation model and to a population synthesis code, we compute the far-UV galaxy luminosity function down to M FUV = -14 at redshift 7 ≤ z ≤ 15. We find that models with strong primordial non-Gaussianities on  Mpc scales show a far-UV luminosity function significantly enhanced (up to a factor of 3 at z  = 14) in low-mass galaxies. We adopt a reionization model calibrated from state-of-the-art hydrodynamical simulations and show that such scale-dependent non-Gaussianities leave a clear imprint on the Universe reionization history and electron Thomson scattering optical depth e . Although current uncertainties in the physics of reionization and on the determination of e still dominate the signatures of non-Gaussianities, our results suggest that e could ultimately be used to constrain the statistical properties of initial density fluctuations.