# Publications by Julien Devriendt

## Identifying the progenitors of present-day early-type galaxies in observational surveys: correcting 'progenitor bias' using the Horizon-AGN simulation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 474 (2018) 3140-3151

G Martin, S Kaviraj, JEG Devriendt, Y Dubois, C Pichon, C Laigle

## Gas flows in the circumgalactic medium around simulated high-redshift galaxies

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 473 (2018) 4279-4301

PD Mitchell, J Blaizot, J Devriendt, T Kimm, L Michel-Dansac, J Rosdahl, A Slyz

## Galaxy evolution in the metric of the cosmic web

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 474 (2018) 547-571

K Kraljic, S Arnouts, C Pichon, C Laigle, S de la Torre, D Vibert, C Cadiou, Y Dubois, M Treyer, C Schimd, S Codis, V de Lapparent, J Devriendt, HS Hwang, D Le Borgne, N Malavasi, B Milliard, M Musso, D Pogosyan, M Alpaslan, J Bland-Hawthorn, AH Wright

## Caught in the rhythm I. How satellites settle into a plane around their central galaxy

ASTRONOMY & ASTROPHYSICS 613 (2018) ARTN A4

C Welker, Y Dubois, C Pichon, J Devriendt, NE Chisari

## COSMOS2015 photometric redshifts probe the impact of filaments on galaxy properties

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 473 (2018) 5437-5458

C Laigle, C Pichon, S Arnouts, HJ McCracken, Y Dubois, J Devriendt, A Slyz, D Le Borgne, A Benoit-Levy, HS Hwang, O Ilbert, K Kraljic, N Malavasi, C Park, D Vibert

## The f(ℛ) halo mass function in the cosmic web

Journal of Cosmology and Astroparticle Physics Institute of Physics 2017 (2017) 012-

FV Braun-Bates, HA Winther, D Alonso, J Devriendt

An important indicator of modified gravity is the effect of the local environment on halo properties. This paper examines the influence of the local tidal structure on the halo mass function, the halo orientation, spin and the concentration-mass relation. We use the excursion set formalism to produce a halo mass function conditional on large-scale structure. Our simple model agrees well with simulations on large scales at which the density field is linear or weakly non-linear. Beyond this, our principal result is that f() does affect halo abundances, the halo spin parameter and the concentration-mass relationship in an environment-independent way, whereas we find no appreciable deviation from \text{ΛCDM} for the mass function with fixed environment density, nor the alignment of the orientation and spin vectors of the halo to the eigenvectors of the local cosmic web. There is a general trend for greater deviation from \text{ΛCDM} in underdense environments and for high-mass haloes, as expected from chameleon screening.

## Fluctuating feedback-regulated escape fraction of ionizing radiation in low-mass, high-redshift galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 470 (2017) 224-239

M Trebitsch, J Blaizot, J Rosdahl, J Devriendt, A Slyz

Low-mass galaxies are thought to provide the bulk of the ionizing radiation necessary to reionize the Universe. The amount of photons escaping the galaxies is poorly constrained theoretically, and difficult to measure observationally. Yet it is an essential parameter of reionization models.We study in detail how ionizing radiation can leak from high-redshift galaxies. For this purpose, we use a series of high-resolution radiation hydrodynamics simulations, zooming on three dwarf galaxies in a cosmological context. We find that the energy and momentum input from the supernova explosions has a pivotal role in regulating the escape fraction by disrupting dense star-forming clumps, and clearing sightlines in the halo. In the absence of supernovae, photons are absorbed very locally, within the birth clouds of massive stars. We follow the time evolution of the escape fraction and find that it can vary by more than six orders of magnitude. This explains the large scatter in the value of the escape fraction found by previous studies. This fast variability also impacts the observability of the sources of reionization: a survey even as deep as M 1500 = -14 would miss about half of the underlying population of Lyman-continuum emitters.

## Cosmic evolution of stellar quenching by AGN feedback: clues from the Horizon-AGN simulation

Monthly Notices of the Royal Astronomical Society Oxford University Press 472 (2017) 949-965

RS Beckmann, J Devriendt, AD Slyz, S Peirani, MLA Richardson, Y Dubois, C Pichon, NE Chisari, S Kaviraj, CMC Laigle, M Volonteri

The observed massive end of the local galaxy stellar mass function is steeper than its predicted dark matter (DM) halo counterpart in the standard $\Lambda$CDM paradigm. We investigate how active galactic nuclei (AGN) feedback can account for such a reduction in the stellar content of massive galaxies, through an influence on the gas content of their interstellar (ISM) and circum-galactic medium (CGM). We isolate the impact of AGNs by comparing two simulations from the HORIZON suite, which are identical except that one includes super massive black holes (SMBH) and related feedback. This allows us to cross-identify individual galaxies between these simulations and quantify the effect of AGN feedback on their properties, such as stellar mass and gas outflows. We find that the most massive galaxies ($\rm M_{*} \geq 3 \times 10^{11} M_\odot$) are quenched to the extent that their stellar masses decrease by about 80% at $z=0$. More generally, SMBHs affect their host halo through a combination of outflows that reduce their baryonic mass, particularly for galaxies in the mass range $\rm 10^9 M_\odot \leq M_{*} \leq 10^{11} M_\odot$, and a disruption of central gas inflows, which limits in-situ star formation, particularly massive galaxies with $\rm M_{*} \approx10^{11} M_\odot$. As a result of these processes, net gas inflows onto massive galaxies drop by up to 70%. Finally, we measure a redshift evolution in the stellar mass ratio of twin galaxies with and without AGN feedback, with galaxies of a given stellar mass showing stronger signs of quenching earlier on. This evolution is driven by a progressive flattening of the $\rm M_{SMBH}-M_*$ relation for galaxies with $\rm M_{*} \leq 10^{10} M_\odot$ as redshift decreases, which translates into smaller SBMHs being harboured by galaxies of any fixed stellar mass, and indicates stronger AGN feedback at higher redshift.

## Density profile of dark matter haloes and galaxies in the Horizon-AGN simulation: the impact of AGN feedback

Monthly Notices of the Royal Astronomical Society Oxford University Press 472 (2017) 2153–2169-

S Peirani, Y Dubois, M Volonteri, J Devriendt, K Bundy, J Silk, C Pichon, S Kaviraj, R Gavazzi, M Habouzit

Using a suite of three large cosmological hydrodynamical simulations, HORIZON-AGN, HORIZON-NOAGN (no AGN feedback) and HORIZON-DM (no baryons), we investigate how a typical sub-grid model for AGN feedback affects the evolution of the inner density profiles of massive dark matter haloes and galaxies. Based on direct object-to-object comparisons, we find that the integrated inner mass and density slope differences between objects formed in these three simulations (hereafter, HAGN, HnoAGN and HDM) significantly evolve with time. More specifically, at high redshift (z ~ 5), the mean central density profiles of HAGN and HnoAGN dark matter haloes tend to be much steeper than their HDM counterparts owing to the rapidly growing baryonic component and ensuing adiabatic contraction. By z ~ 1.5, these mean halo density profiles in HAGN have flattened, pummelled by powerful AGN activity (“quasarmode”): the integrated innermass difference gapswith HnoAGN haloes have widened, and those with HDM haloes have narrowed. Fast forward 9.5 billion years, down to z = 0, and the trend reverses: HAGN halo mean density profiles drift back to a more cusped shape as AGN feedback efficiency dwindles (“radio mode”), and the gaps in integrated central mass difference with HnoAGN and HDM close and broaden respectively.On the galaxy side, the story differs noticeably.Averaged stellar profile central densities and inner slopes are monotonically reduced by AGN activity as a function of cosmic time, resulting in better agreement with local observations. As both dark matter and stellar inner density profiles respond quite sensitively to the presence of a central AGN, there is hope that future observational determinations of these quantities can be used constrain AGN feedback models.

## Implications of strong intergalactic magnetic fields for ultrahigh-energy cosmic-ray astronomy

Physical Review D American Physical Society 96 (2017) 023010

R Batista, M-S Shin, J Devriendt, D Semikoz, G Sigl

We study the propagation of ultra-high-energy cosmic rays in the magnetised cosmic web. We focus on the particular case of highly magnetised voids ($B \sim \text{nG}$), using the upper bounds from the Planck satellite. The cosmic web was obtained from purely magnetohydrodynamical cosmological simulations of structure formation considering different power spectra for the seed magnetic field in order to account for theoretical uncertainties. We investigate the impact of these uncertainties on the propagation of cosmic rays, showing that they can affect the measured spectrum and composition by up to $\simeq 80\%$ and $\simeq 5\%$, respectivelly. In our scenarios, even if magnetic fields in voids are strong, deflections of 50 EeV protons from sources closer than $\sim\;$50 Mpc are less than $15^\circ$ in approximately 10-50% of the sky, depending on the distribution of sources and magnetic power spectrum. Therefore, UHECR astronomy might be possible in a significant portion of the sky depending on the primordial magnetic power spectrum, provided that protons constitute a sizeable fraction of the observed UHECR flux.

## The limited role of galaxy mergers in driving stellar mass growth over cosmic time

Monthly Notices of the Royal Astronomical Society Letters Oxford University Press 472 (2017) L50–L54-

G Martin, S Kaviraj, JEG Devriendt, Y Dubois, CMC Laigle, C Pichon

A key unresolved question is the role that galaxy mergers play in driving stellar mass growth over cosmic time. Recent observational work hints at the possibility that the overall contribution of `major' mergers (mass ratios $\gtrsim$1:4) to cosmic stellar mass growth may be small, because they enhance star formation rates by relatively small amounts at high redshift, when much of today's stellar mass was assembled. However, the heterogeneity and relatively small size of today's datasets, coupled with the difficulty in identifying genuine mergers, makes it challenging to $\textit{empirically}$ quantify the merger contribution to stellar mass growth. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation, to comprehensively quantify the contribution of mergers to the star formation budget over the lifetime of the Universe. We show that: (1) both major and minor mergers enhance star formation to similar amounts, (2) the fraction of star formation directly attributable to merging is small at all redshifts (e.g. $\sim$35 and $\sim$20 per cent at z$\sim$3 and z$\sim$1 respectively) and (3) only $\sim$25 per cent of today's stellar mass is directly attributable to galaxy mergers over cosmic time. Our results suggest that smooth accretion, not merging, is the dominant driver of stellar mass growth over the lifetime of the Universe.

## The new semi-analytic code GalICS 2.0 – reproducing the galaxy stellar mass function and the Tully–Fisher relation simultaneously

Monthly Notices of the Royal Astronomical Society Oxford University Press (2017)

A Cattaneo, J Blaizot, J Devriendt, GA Mamon, E Tollet, A Dekel, B Guiderdoni, M Kucukbas, ACR Thob

<p>GalICS 2.0 is a new semianalytic code to model the formation and evolution of galaxies in a cosmological context. <i>N</i>-body simulations based on a <i>Planck</i> cosmology are used to construct halo merger trees, track subhaloes, compute spins and measure concentrations. The accretion of gas onto galaxies and the morphological evolution of galaxies are modelled with prescriptions derived from hydrodynamic simulations. Star formation and stellar feedback are described with phenomenological models (as in other semianalytic codes). GalICS 2.0 computes rotation speeds from the gravitational potential of the dark matter, the disc and the central bulge. As the rotation speed depends not only on the virial velocity but also on the ratio of baryons to dark matter within a galaxy, our calculation predicts a different Tully-Fisher relation from models in which <i>v</i><sub>rot</sub> ∝ <i>v</i><sub>vir</sub>. This is why GalICS 2.0 is able to reproduce the galaxy stellar mass function and the Tully-Fisher relation simultaneously. Our results are also in agreement with halo masses from weak lensing and satellite kinematics, gas fractions, the relation between star formation rate (SFR) and stellar mass, the evolution of the cosmic SFR density, bulge-to-disc ratios, disc sizes and the Faber-Jackson relation.</p>

## The XXL survey: first results and future

Astronomische Nachrichten Wiley 338 (2017) 334–341-

M Pierre, C Adami, M Birkinshaw, J Devriendt, MJ Jarvis

<p>The XXL survey currently covers two 25 deg2 patches with XMM observations of ~ 10ks. We summarise the scientific results associated with the first release of the XXL data set, that occurred mid 2016. We review several arguments for increasing the survey depth to 40 ks during the next decade of XMM operations. X-ray (z &lt; 2) cluster, (z &lt; 4) AGN and cosmic background survey science will then benefit from an extraordinary data reservoir. This, combined with deep multi-λ observations, will lead to solid standalone cosmological constraints and provide a wealth of information on the formation and evolution of AGN, clusters and the X-ray background. In particular, it will offer a unique opportunity to pinpoint the z &gt; 1 cluster density. It will eventually constitute a reference study and an ideal calibration field for the upcoming eROSITA and Euclid missions.</p>

## nIFTy Cosmology: the clustering consistency of galaxy formation models

Monthly Notices of the Royal Astronomical Society Oxford University Press 469 (2017) 749-762

A Pujol, RA Skibba, E Gaztañaga, W Cui, D Cunnama, GD Lucia, J Devriendt, PJ Elahi, A Font, F Fontanot, J Garcia-Bellido, ID Gargiulo, V Gonzalez-Perez, J Helly, BMB Henriques, M Hirschmann, A Knebe, J Lee, GA Mamon, P Monaco, J Onions, PA Thomas, E Tollet, CA Vega-Martínez, SK Yi

We present a clustering comparison of 12 galaxy formation models (including Semi-Analytic Models (SAMs) and Halo Occupation Distribution (HOD) models) all run on halo catalogues and merger trees extracted from a single {\Lambda}CDM N-body simulation. We compare the results of the measurements of the mean halo occupation numbers, the radial distribution of galaxies in haloes and the 2-Point Correlation Functions (2PCF). We also study the implications of the different treatments of orphan (galaxies not assigned to any dark matter subhalo) and non-orphan galaxies in these measurements. Our main result is that the galaxy formation models generally agree in their clustering predictions but they disagree significantly between HOD and SAMs for the orphan satellites. Although there is a very good agreement between the models on the 2PCF of central galaxies, the scatter between the models when orphan satellites are included can be larger than a factor of 2 for scales smaller than 1 Mpc/h. We also show that galaxy formation models that do not include orphan satellite galaxies have a significantly lower 2PCF on small scales, consistent with previous studies. Finally, we show that the 2PCF of orphan satellites is remarkably different between SAMs and HOD models. Orphan satellites in SAMs present a higher clustering than in HOD models because they tend to occupy more massive haloes. We conclude that orphan satellites have an important role on galaxy clustering and they are the main cause of the differences in the clustering between HOD models and SAMs.

## Feedback-regulated star formation and escape of LyC photons from mini-haloes during reionisation

Monthly Notices of the Royal Astronomical Society Oxford University Press 466 (2017) 4826-4846

T Kimm, H Katz, M Haehnelt, J Rosdahl, J Devriendt, A Slyz

<p>Reionization in the early Universe is likely driven by dwarf galaxies. Using cosmological radiation-hydrodynamic simulations, we study star formation and the escape of Lyman continuum (LyC) photons from mini-haloes with Mhalo ≲ 10^8 M⊙. Our simulations include a new thermo-turbulent star formation model, non-equilibrium chemistry and relevant stellar feedback processes (photoionization by young massive stars, radiation pressure and mechanical supernova explosions). We find that feedback reduces star formation very efficiently in minihaloes, resulting in the stellar mass consistent with the slope and normalization reported in Kimm&amp;Cen; and the empirical stellarmass-to-halomass relation derived in the local Universe. Because star formation is stochastic and dominated by a few gas clumps, the escape fraction in mini-haloes is generally determined by radiation feedback (heating due to photoionization), rather than supernova explosions.We also find that the photon number-weighted mean escape fraction in mini-haloes is higher (∼20–40 per cent) than that in atomic-cooling haloes, although the instantaneous fraction in individual haloes varies significantly. The escape fraction from Pop III stars is found to be significant (≳10 per cent) only when the mass is greater than ∼100 M⊙. Based on simple analytic calculations, we showthat LyC photons from mini-haloes are, despite their high escape fractions, of minor importance for reionization due to inefficient star formation. We confirm previous claims that stars in atomic-cooling haloes with masses 10^8 M⊙ ≲ Mhalo ≲ 10^11 M⊙ are likely to be the most important source of reionization.</p>

## The rise and fall of stellar across the peak of cosmic star formation history: effects of mergers versus diffuse stellar mass acquisition

Monthly Notices of the Royal Astronomical Society Oxford University Press 465 (2016) 1241-1258

C Welker, Y Dubois, J Devriendt, C Pichon, S Kaviraj, S Peirani

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 &gt; 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 (r0.5 ∝ Ms0.85 and r0.5 ∝ Ms0.1 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 r0.5 ∝ Ms1.2 instead of r0.5 ∝ Ms-0.5 -M0.5 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 r0.5 ∝ Ms2 than for gas-rich galaxies r0.5 ∝ Ms.

## Galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation

Monthly Notices of the Royal Astronomical Society Oxford University Press (2017)

NE Chisari, N Koukoufilippas, A Jindal, S Peirani, RS Beckmann, S Codis, JEG Devriendt, L Miller, Y Dubois, CMC Laigle, A Slyz, C Pichon

Intrinsic alignments of galaxies are a significant astrophysical systematic affecting cosmological constraints from weak gravitational lensing. Obtaining numerical predictions from hydrodynamical simulations of expected survey volumes is expensive, and a cheaper alternative relies on populating large dark matter-only simulations with accurate models of alignments calibrated on smaller hydrodynamical runs. This requires connecting the shapes and orientations of galaxies to those of dark matter halos and to the large-scale structure. In this paper, we characterise galaxy-halo alignments in the Horizon-AGN cosmological hydrodynamical simulation. We compare the shapes and orientations of galaxies in the redshift range \$0

## How the cosmic web induces intrinsic alignments of galaxies

Proceedings of the International Astronomical Union Cambridge University Press 11 (2016) 437-442

S Codis, Y Dubois, C Pichon, J Devriendt, A Slyz

Intrinsic alignments are believed to be a major source of systematics for future generation of weak gravitational lensing surveys like Euclid or LSST. Direct measurements of the alignment of the projected light distribution of galaxies in wide field imaging data seem to agree on a contamination at a level of a few per cent of the shear correlation functions, although the amplitude of the effect depends on the population of galaxies considered. Given this dependency, it is difficult to use dark matter-only simulations as the sole resource to predict and control intrinsic alignments. We report here estimates on the level of intrinsic alignment in the cosmological hydrodynamical simulation Horizon-AGN that could be a major source of systematic errors in weak gravitational lensing measurements. In particular, assuming that the spin of galaxies is a good proxy for their ellipticity, we show how those spins are spatially correlated and how they couple to the tidal field in which they are embedded. We also present theoretical calculations that illustrate and qualitatively explain the observed signals.

## Why do galactic spins flip in the cosmic web? A Theory of Tidal Torques near saddles

Proceedings of the International Astronomical Union Cambridge University Press 11 (2016) 421-432

C Pichon, S Codis, D Pogosyan, Y Dubois, V Desjacques, J Devriendt

Filaments of the cosmic web drive spin acquisition of disc galaxies. The point process of filament-type saddle represent best this environment and can be used to revisit the Tidal Torque Theory in the context of an anisotropic peak (saddle) background split. The constrained misalignment between the tidal tensor and the Hessian of the density field generated in the vicinity of filament saddle points simply explains the corresponding transverse and longitudinal point-reflection symmetric geometry of spin distribution. It predicts in particular an <i>azimuthal</i> orientation of the spins of more massive galaxies and spin <i>alignment</i> with the filament for less massive galaxies. Its scale dependence also allows us to relate the transition mass corresponding to the alignment of dark matter halos spin relative to the direction of their neighboring filament to this geometry, and to predict accordingly it s scaling with the mass of non linearity, as was measured in simulations.

## How do galaxies build up their spin in the cosmic web?

Proceedings of the International Astronomical Union Cambridge University Press 11 (2016) 433-436

C Welker, Y Dubois, C Pichon, J Devriendt, S Peirani

Using the Horizon-AGN simulation we find a mass dependent spin orientation trend for galaxies: the spin of low-mass, rotation-dominated, blue, star-forming galaxies are preferentially aligned with their closest filament, whereas high-mass, velocity dispersion- supported, red quiescent galaxies tend to possess a spin perpendicular to these filaments. We explore the physical mechanisms driving galactic spin swings and quantify how much mergers and smooth accretion re-orient them relative to their host filaments.