Publications by Julien Devriendt


The Horizon-AGN simulation: evolution of galaxy properties over cosmic time

arXiv (2016)

S Kaviraj, C Laigle, T Kimm, J Devriendt, Y Dubois, C Pichon, A Slyz, E Chisari, S Peirani

We compare the predictions of Horizon-AGN, a hydro-dynamical cosmological simulation that uses an adaptive mesh refinement code, to observational data in the redshift range 0 < z > 6. We study the reproduction, by the simulation, of quantities that trace the aggregate stellar-mass growth of galaxies over cosmic time: luminosity and stellar-mass functions, the star formation main sequence, rest-frame UV-optical-near infrared colours and the cosmic star-formation history. We show that Horizon-AGN, which is not tuned to reproduce the local Universe, produces good overall agreement with these quantities, from the present day to the epoch when the Universe was 5% of its current age. By comparison to Horizon-noAGN, a twin simulation without AGN feedback, we quantify how feedback from black holes is likely to help shape galaxy stellar-mass growth in the redshift range 0 < z > 6, particularly in the most massive galaxies. Our results demonstrate that Horizon-AGN successfully captures the evolutionary trends of observed galaxies over the lifetime of the Universe, making it an excellent tool for studying the processes that drive galaxy evolution and making predictions for the next generation of galaxy surveys.


Modelling Lyman α forest cross-correlations with LyMAS

Monthly Notices of the Royal Astronomical Society Oxford University Press 461 (2016) 4353-4373

C Lochhaas, DH Weinberg, S Peirani, Y Dubois, S Colombi, J Blaizot, A Font-Ribera, C Pichon, J Devriendt

We use the Lya Mass Association Scheme (LyMAS) to predict cross-correlations at z = 2.5 between dark matter haloes and transmitted flux in the Lya forest, and compare to crosscorrelations measured for quasars and damped Lya 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 20483 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 bh 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 km s-1 for quasars and DLAs, respectively. We infer bias-weighted mean halo masses of Mh/1012 h-1M⊙ = 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.


Simulated observations of high-redshift galaxies with the HARMONI spectrograph for the European Extremely Large Telescope

GROUND-BASED AND AIRBORNE INSTRUMENTATION FOR ASTRONOMY VI 9908 (2016) ARTN 99089Y

S Kendrew, S Zieleniewski, RCW Houghton, N Thatte, J Devriendt, M Tecza, F Clarke, K O'Brien, B Haussler


Radio continuum surveys and galaxy evolution: modelling and simulations

Proceedings of Science Sissa Medialab 267 (2016) 1-12

A Slyz, J Devriendt, M Jarvis, Y Dubois, C Pichon

We predict the evolution of the radio continuum sky at 1.4 GHz from the Horizon-AGN Adaptive Mesh Refinement (AMR) cosmological hydrodynamical simulation of a cubic volume of the Universe 100h−1 Mpc on a side. With empirically motivated models for the radio continuum emission due to both star formation and Active Galactic Nuclei (AGN), we estimate the contribution of each of these processes to the local radio continuum luminosity function (LF) and describe its evolution up to redshift 4. Despite the simplicity of these models, we find that our predictions for the local luminosity function are fairly consistent with Mauch & Sadler (2007) observations, with the faint end of the luminosity function dominated by star forming galaxies and the bright end by radio loud AGNs. At redshift one, a decent match to Smolcic et al. (2009) VLA data in the COSMOS field can only be achieved when we account for radio continuum emission from AGNs. We predict that the strongest evolution across the peak epoch of cosmic activity happens for low luminosity star forming galaxies L1.4GHz < 1022 W Hz−1 , whose contribution rises until z ∼ 2 and declines at higher redshifts. The contribution of low luminosity AGNs L1.4GHz < 1022 W Hz−1 steadily declines from z = 0 throughout the redshift range, whilst that of radio loud objects with luminosities in the range 1022 W Hz−1 < L1.4GHz < 1024 W Hz−1 rises dramatically until z = 4. Finally, high-luminosity radio loud AGNs, with L1.4GHz > 1024 W Hz−1 show surprisingly little evolution from z = 0 to z = 4.


The XXL Survey: I. Scientific motivations - XMM-Newton observing plan - Follow-up observations and simulation programme

Astronomy and Astrophysics EDP Sciences 592 (2016) A1-

M Pierre, F Pacaud, C Adami, S Alis, B Altieri, B Baran, C Benoist, M Birkinshaw, A Bongiorno, MN Bremer, M Brusa, A Butler, P Ciliegi, L Chiappetti, N Clerc, PS Corasaniti, J Coupon, CD Breuck, J Democles, S Desai, J Delhaize, J Devriendt, Y Dubois, D Eckert, A Elyiv

<p><b>Context.</b> The quest for the cosmological parameters that describe our universe continues to motivate the scientific community to undertake very large survey initiatives across the electromagnetic spectrum. Over the past two decades, the <i>Chandra</i> and <i>XMM-Newton</i> observatories have supported numerous studies of X-ray-selected clusters of galaxies, active galactic nuclei (AGNs), and the X-ray background. The present paper is the first in a series reporting results of the XXL-XMM survey; it comes at a time when the Planck mission results are being finalised. </p><p><b>Aims.</b> We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg<sup>2</sup> each at a point-source sensitivity of ~5 × 10<sup>-15</sup> erg s<sup>-1</sup> cm<sup>-2</sup> in the [0.5−2] keV band (completeness limit). The survey’s main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes. </p><p><b>Methods.</b> We describe the 542 XMM observations along with the associated multi-λ and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis. </p><p><b>Results.</b> The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-λ associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses. </p><p><b>Conclusions.</b> The XXL multi-λ data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps.</p>


The cosmic evolution of massive black holes in the Horizon-AGN simulation

Monthly Notices of the Royal Astronomical Society Oxford University Press 460 (2016) 2979-2996

M Volonteri, Y Dubois, C Pichon, J Devriendt

We analyse the demographics of black holes (BHs) in the large-volume cosmological hydrodynamical simulation Horizon-AGN. This simulation statistically models how much gas is accreted on to BHs, traces the energy deposited into their environment and, consequently, the back-reaction of the ambient medium on BH growth. The synthetic BHs reproduce a variety of observational constraints such as the redshift evolution of the BH mass density and the mass function. Strong self-regulation via AGN feedback, weak supernova feedback, and unresolved internal processes result in a tight BH-galaxy mass correlation. Starting at z ~ 2, tidal stripping creates a small population of BHs over-massive with respect to the halo. The fraction of galaxies hosting a central BH or an AGN increases with stellar mass. The AGN fraction agrees better with multi-wavelength studies, than single-wavelength ones, unless obscuration is taken into account. The most massive haloes present BH multiplicity, with additional BHs gained by ongoing or past mergers. In some cases, both a central and an off-centre AGN shine concurrently, producing a dual AGN. This dual AGN population dwindles with decreasing redshift, as found in observations. Specific accretion rate and Eddington ratio distributions are in good agreement with observational estimates. The BH population is dominated in turn by fast, slow, and very slow accretors, with transitions occurring at z = 3 and z = 2, respectively.


Simulated stellar kinematics studies of high-redshift galaxies with the HARMONI Integral Field Spectrograph

Monthly Notices of the Royal Astronomical Society Oxford University Press 458 (2016) 2405-2422

S Kendrew, S Zieleniewski, RCW Houghton, N Thatte, J Devriendt, M Tecza, F Clarke, K O'Brien, B Häussler

We present a study into the capabilities of integrated and spatially resolved integral field spectroscopy of galaxies at z = 2–4 with the future HARMONI spectrograph for the European Extremely Large Telescope (E-ELT) using the simulation pipeline, HSIM. We focus particularly on the instrument's capabilities in stellar absorption line integral field spectroscopy, which will allow us to study the stellar kinematics and stellar population characteristics. Such measurements for star-forming and passive galaxies around the peak star formation era will provide a critical insight into the star formation, quenching and mass assembly history of high-z, and thus present-day galaxies. First, we perform a signal-to-noise study for passive galaxies at a range of stellar masses for z = 2–4, assuming different light profiles; for this population, we estimate that integrated stellar absorption line spectroscopy with HARMONI will be limited to galaxies with M* ≳ 1010.7 M⊙. Secondly, we use HSIM to perform a mock observation of a typical star-forming 1010 M⊙ galaxy at z = 3 generated from the high-resolution cosmological simulation NUTFB. We demonstrate that the input stellar kinematics of the simulated galaxy can be accurately recovered from the integrated spectrum in a 15-h observation, using common analysis tools. Whilst spatially resolved spectroscopy is likely to remain out of reach for this particular galaxy, we estimate HARMONI's performance limits in this regime from our findings. This study demonstrates how instrument simulators such as HSIM can be used to quantify instrument performance and study observational biases on kinematics retrieval; and shows the potential of making observational predictions from cosmological simulation output data.


Redshift and luminosity evolution of the intrinsic alignments of galaxies in Horizon-AGN

Monthly Notices of the Royal Astronomical Society Oxford University Press 461 (2016) 2702–2721-

N Chisari, C Laigle, S Codis, Y Dubois, J Devriendt, L Miller, K Benabed, A Slyz, R Gavazzi, C Pichon

Intrinsic galaxy shape and angular momentum alignments can arise in cosmological large-scale structure due to tidal interactions or galaxy formation processes. Cosmological hydrodynamical simulations have recently come of age as a tool to study these alignments and their contamination to weak gravitational lensing. We probe the redshift and luminosity evolution of intrinsic alignments in Horizon-AGN between z=0 and z=3 for galaxies with an r-band absolute magnitude of &lt;-20. Alignments transition from being radial at low redshifts and high luminosities, dominated by the contribution of ellipticals, to being tangential at high redshift and low luminosities, where discs dominate the signal. This cannot be explained by the evolution of the fraction of ellipticals and discs alone: intrinsic evolution in the amplitude of alignments is necessary. The alignment amplitude of elliptical galaxies alone is smaller in amplitude by a factor of ~2, but has similar luminosity and redshift evolution as in current observations and in the nonlinear tidal alignment model at projected separations of &gt; 1 Mpc. Alignments of discs are null in projection and consistent with current low redshift observations. The combination of the two populations yields an overall amplitude a factor of ~4 lower than observed alignments of luminous red galaxies with a steeper luminosity dependence. The restriction on accurate galaxy shapes implies that the galaxy population in the simulation is complete only to an r-band absolute magnitude of &lt;-20. Higher resolution simulations will be necessary to avoid extrapolation of the intrinsic alignment predictions to the range of luminosities probed by future surveys.


The Horizon-AGN simulation: morphological diversity of galaxies promoted by AGN feedback

Monthly Notices of the Royal Astronomical Society Oxford University Press 463 (2016) 3948-3964

Y Dubois, S Peirani, C Pichon, J Devriendt, R Gavazzi, C Welker, M Volonteri

The interplay between cosmic gas accretion on to galaxies and galaxy mergers drives the observed morphological diversity of galaxies. By comparing the state-of-the-art hydrodynamical cosmological simulations Horizon-AGN and Horizon-noAGN, we unambiguously identify the critical role of active galactic nuclei (AGN) in setting up the correct galaxy morphology for the massive end of the population. With AGN feedback, typical kinematic and morpho-metric properties of galaxy populations as well as the galaxy-halo mass relation are in much better agreement with observations. Only AGN feedback allows massive galaxies at the centre of groups and clusters to become ellipticals, while without AGN feedback those galaxies reform discs. It is the merger-enhanced AGN activity that is able to freeze the morphological type of the post-merger remnant by durably quenching its quiescent star formation. Hence morphology is shown to be driven not only by mass but also by the nature of cosmic accretion: at constant galaxy mass, ellipticals are galaxies that are mainly assembled through mergers, while discs are preferentially built from the in situ star formation fed by smooth cosmic gas infall.


Towards simulating star formation in turbulent high-z galaxies with mechanical supernova feedback (vol 451, pg 2900, 2015)

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 459 (2016) 256-256

T Kimm, R Cen, J Devriendt, Y Dubois, A Slyz


COMPARING SIMULATIONS OF AGN FEEDBACK

ASTROPHYSICAL JOURNAL 825 (2016) ARTN 83

MLA Richardson, E Scannapieco, J Devriendt, A Slyz, RJ Thacker, Y Dubois, J Wurster, J Silk


Bursty star formation feedback and cooling outflows

Monthly Notices of the Royal Astronomical Society Oxford University Press 462 (2016) 994-1001

T Suarez, A Pontzen, HV Peiris, A Slyz, J Devriendt

<p>We study how outflows of gas launched from a central galaxy undergoing repeated starbursts propagate through the circumgalactic medium (CGM), using the simulation code RAMSES. We assume that the outflow from the disk can be modelled as a rapidly moving bubble of hot gas at ~ 1 kpc above disk, then ask what happens as it moves out further into the halo around the galaxy on ~ 100 kpc scales. To do this we run 60 two-dimensional simulations scanning over parameters of the outflow. Each of these is repeated with and without radiative cooling, assuming a primordial gas composition to give a lower bound on the importance of cooling. In a large fraction of radiative-cooling cases we are able to form rapidly outflowing cool gas from in situ cooling of the flow. We show that the amount of cool gas formed depends strongly on the ‘burstiness’ of energy injection; sharper, stronger bursts typically lead to a larger fraction of cool gas forming in the outflow. The abundance ratio of ions in the CGM may therefore change in response to the detailed historical pattern of star formation. For instance, outflows generated by star formation with short, intense bursts contain up to 60 per cent of their gas mass at temperatures &lt; 5 X 10^4 K; for near-continuous star formation the figure is ≲ 5 per cent. Further study of cosmological simulations, and of idealised simulations with e.g., metal-cooling, magnetic fields and/or thermal conduction, will help to understand the precise signature of bursty outflows on observed ion abundances.</p>


Intrinsic alignments of galaxies in the Horizon-AGN cosmological hydrodynamical simulation

Monthly Notices of the Royal Astronomical Society Oxford University Press 454 (2015) 2736-2753

N Chisari, S Codis, C Laigle, Y Dubois, C Pichon, J Devriendt, A Slyz, L Miller, R Gavazzi, K Benabed

The intrinsic alignments of galaxies are recognised as a contaminant to weak gravitational lensing measurements. In this work, we study the alignment of galaxy shapes and spins at low redshift ($z\sim 0.5$) in Horizon-AGN, an adaptive-mesh-refinement hydrodynamical cosmological simulation box of 100 Mpc/h a side with AGN feedback implementation. We find that spheroidal galaxies in the simulation show a tendency to be aligned radially towards over-densities in the dark matter density field and other spheroidals. This trend is in agreement with observations, but the amplitude of the signal depends strongly on how shapes are measured and how galaxies are selected in the simulation. Disc galaxies show a tendency to be oriented tangentially around spheroidals in three-dimensions. While this signal seems suppressed in projection, this does not guarantee that disc alignments can be safely ignored in future weak lensing surveys. The shape alignments of luminous galaxies in Horizon-AGN are in agreement with observations and other simulation works, but we find less alignment for lower luminosity populations. We also characterize the systematics of galaxy shapes in the simulation and show that they can be safely neglected when measuring the correlation of the density field and galaxy ellipticities.


Towards simulating star formation in turbulent high-z galaxies with mechanical supernova feedback

Monthly Notices of the Royal Astronomical Society Oxford University Press 451 (2015) 2900-2921

T Kimm, R Cen, J Devriendt, Y Dubois, A Slyz

<p>To better understand the impact of supernova (SN) explosions on the evolution of galaxies, we perform a suite of high-resolution (12 pc), zoom-in cosmological simulations of a Milky Way-like galaxy at <i>z</i> = 3 with adaptive mesh refinement. We find that SN explosions can efficiently regulate star formation, leading to the stellar mass and metallicity consistent with the observed mass–metallicity relation and stellar mass–halo mass relation at <i>z</i> ~ 3. This is achieved by making three important changes to the classical feedback scheme: (i) the different phases of SN blast waves are modelled directly by injecting radial momentum expected at each stage, (ii) the realistic time delay of SNe is required to disperse very dense gas before a runaway collapse sets in, and (iii) a non-uniform density distribution of the interstellar medium (ISM) is taken into account below the computational grid scale for the cell in which an SN explodes. The simulated galaxy with the SN feedback model shows strong outflows, which carry approximately 10 times larger mass than star formation rate, as well as smoothly rising circular velocity. Although the metallicity of the outflow depends sensitively on the feedback model used, we find that the accretion rate and metallicity of the cold flow around the virial radius is impervious to SN feedback. Our results suggest that understanding the structure of the turbulent ISM may be crucial to assess the role of SN and other feedback processes in galaxy formation theory.</p>


A detailed study of feedback from a massive star

Monthly Notices of the Royal Astronomical Society Oxford University Press 448 (2015) 3248-3264

S Geen, J Rosdahl, J Blaizot, J Devriendt, A Slyz

<p>We present numerical simulations of a 15 M<sub>⊙</sub> star in a suite of idealized environments in order to quantify the amount of energy transmitted to the interstellar medium (ISM). We include models of stellar winds, UV photoionization and the subsequent supernova based on theoretical models and observations of stellar evolution. The system is simulated in 3D using RAMSES-RT, an Adaptive Mesh Refinement Radiation Hydrodynamics code. We find that stellar winds have a negligible impact on the system owing to their relatively low luminosity compared to the other processes. The main impact of photoionization is to reduce the density of the medium into which the supernova explodes, reducing the rate of radiative cooling of the subsequent supernova. Finally, we present a grid of models quantifying the energy and momentum of the system that can be used to motivate simulations of feedback in the ISM unable to fully resolve the processes discussed in this work.</p>


Black hole evolution: I. Supernova-regulated black hole growth

Monthly Notices of the Royal Astronomical Society Oxford University Press 452 (2015) 1502-1518

Y Dubois, M Volonteri, J Silk, J Devriendt, A Slyz, R Teyssier

The growth of a supermassive black hole (BH) is determined by how much gas the host galaxy is able to feed it, which in turn is controlled by the cosmic environment, through galaxy mergers and accretion of cosmic flows that time how galaxies obtain their gas, but also by internal processes in the galaxy, such as star formation and feedback from stars and the BH itself. In this paper, we study the growth of a 10^12 Msun halo at z=2, which is the progenitor of al group of galaxies at z=0, and of its central BH by means of a high-resolution zoomed cosmological simulation, the Seth simulation. We study the evolution of the BH driven by the accretion of cold gas in the galaxy, and explore the efficiency of the feedback from supernovae (SNe). For a relatively inefficient energy input from SNe, the BH grows at the Eddington rate from early times, and reaches self-regulation once it is massive enough. We find that at early cosmic times z&gt;3.5, efficient feedback from SNe forbids the formation of a settled disc as well as the accumulation of dense cold gas in the vicinity of the BH and starves the central compact object. As the galaxy and its halo accumulate mass, they become able to confine the nuclear inflows provided by major mergers and the BH grows at a sustained near-to-Eddington accretion rate. We argue that this mechanism should be ubiquitous amongst low-mass galaxies, corresponding to galaxies with a stellar mass below &lt;10^9 Msun in our simulations.


nIFTy cosmology: comparison of galaxy formation models

Monthly Notices of the Royal Astronomical Society Oxford University Press 451 (2015) 4029-4059

A Knebe, FR Pearce, PA Thomas, A Benson, J Blaizot, R Bower, J Carretero, FJ Castander, A Cattaneo, Cora, DJ Croton, W Cui, D Cunnama, GD Lucia, J Devriendt, PJ Elahi, A Font, F Fontanot, J Garcia-Bellido, ID Gargiulo, V Gonzalez-Perez, J Helly, B Henriques, M Hirschmann, J Lee

We present a comparison of 14 galaxy formation models: 12 different semi-analytical models and 2 halo-occupation distribution models for galaxy formation based upon the same cosmological simulation and merger tree information derived from it. The participating codes have proven to be very successful in their own right but they have all been calibrated independently using various observational data sets, stellar models, and merger trees. In this paper we apply them without recalibration and this leads to a wide variety of predictions for the stellar mass function, specific star formation rates, stellar-to- halo mass ratios, and the abundance of orphan galaxies. The scatter is much larger than seen in previous comparison studies primarily because the codes have been used outside of their native environment within which they are well tested and calibrated. The purpose of the `nIFTy comparison of galaxy formation models' is to bring together as many different galaxy formation modellers as possible and to investigate a common approach to model calibration. This paper provides a unified description for all participating models and presents the initial, uncalibrated comparison as a baseline for our future studies where we will develop a common calibration framework and address the extent to which that reduces the scatter in the model predictions seen here.


Galaxy merger histories and the role of merging in driving star formation at z &gt; 1

Monthly Notices of the Royal Astronomical Society Oxford University Press 452 (2015) 2845-2850

S Kaviraj, J Devriendt, Y Dubois, A Slyz, C Welker, C Pichon, S Peirani, DL Borgne

<p>We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (<em>M</em><sub>*</sub> &gt; 10<sup>10</sup> M<sub>⊙</sub>) galaxies around the epoch of peak cosmic star formation (1 &lt; <em>z</em> &lt; 4). The fraction of massive galaxies in major mergers (mass ratio <em>R</em> &lt; 4: 1) is around 3 per cent, a factor of ∼2.5 lower than minor mergers (4: 1 &lt; <em>R</em> &lt; 10: 1) at these epochs, with no trend with redshift. At <em>z</em> ∼ 1, around a third of massive galaxies have undergone a major merger, while all remaining systems have undergone a minor merger. While almost all major mergers at <em>z</em> &gt; 3 are ‘blue’ (i.e. have significant associated star formation), the proportion of ‘red’ mergers increases rapidly at <em>z</em> &lt; 2, with most merging systems at <em>z</em> ∼ 1.5 producing remnants that are red in rest-frame UV–optical colours. The star formation enhancement during major mergers is mild (∼20–40 per cent) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (<em>R</em> &lt; 10: 1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard Λ cold dark matter paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.</p>


Intrinsic alignment of simulated galaxies in the cosmic web: implications for weak lensing surveys

Monthly Notices of the Royal Astronomical Society Oxford University Press 448 (2015) 3391-3404

S Codis, R Gavazzi, Y Dubois, C Pichon, K Benabed, V Desjacques, D Pogosyan, J Devriendt, A Slyz

<p>The intrinsic alignment of galaxy shapes (by means of their angular momentum) and their cross-correlation with the surrounding dark matter tidal field are investigated using the 160 000, <em>z</em> = 1.2 synthetic galaxies extracted from the high-resolution cosmological hydrodynamical simulation HORIZON-AGN. One- and two-point statistics of the spin of the stellar component are measured as a function of mass and colour. For the low-mass galaxies, this spin is <em>locally</em> aligned with the tidal field ‘filamentary’ direction while, for the high-mass galaxies, it is <em>perpendicular</em> to both filaments and walls. The bluest galaxies of our synthetic catalogue are more strongly correlated with the surrounding tidal field than the reddest galaxies, and this correlation extends up to ∼10 <em>h</em><sup>− 1</sup> Mpc comoving distance. We also report a correlation of the projected ellipticities of blue, intermediate-mass galaxies on a similar scale at a level of 10<sup>−4</sup> which could be a concern for cosmic shear measurements. We do not report any measurable intrinsic alignments of the reddest galaxies of our sample. This work is a first step towards the use of very realistic catalogue of synthetic galaxies to evaluate the contamination of weak lensing measurement by the intrinsic galactic alignments.</p>


Milking the spherical cow - on aspherical dynamics in spherical coordinates

Monthly Notices of the Royal Astronomical Society Oxford University Press 451 (2015) 1366-1379

A Pontzen, JI Read, R Teyssier, F Governato, A Gualandris, N Roth, J Devriendt

Galaxies and the dark matter haloes that host them are not spherically symmetric, yet spherical symmetry is a helpful simplifying approximation for idealized calculations and analysis of observational data. The assumption leads to an exact conservation of angular momentum for every particle, making the dynamics unrealistic. But how much does that inaccuracy matter in practice for analyses of stellar distribution functions, collisionless relaxation, or dark matter core-creation? We provide a general answer to this question for a wide class of aspherical systems; specifically, we consider distribution functions that are 'maximally stable', i.e. that do not evolve at first order when external potentials (which arise from baryons, large-scale tidal fields or infalling substructure) are applied. We show that a spherically symmetric analysis of such systems gives rise to the false conclusion that the density of particles in phase space is ergodic (a function of energy alone). Using this idea we are able to demonstrate that: (a) observational analyses that falsely assume spherical symmetry are made more accurate by imposing a strong prior preference for near-isotropic velocity dispersions in the centre of spheroids; (b) numerical simulations that use an idealized spherically symmetric setup can yield misleading results and should be avoided where possible; and (c) triaxial dark matter haloes (formed in collisionless cosmological simulations) nearly attain our maximally stable limit, but their evolution freezes out before reaching it.

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