## Zooming in on supermassive black holes: how resolving their gas cloud host renders their accretion episodic

Monthly Notices of the Royal Astronomical Society Oxford University Press 483 (2018) 3488–3509-

J Devriendt, RS Beckmann, A Slyz

Born in rapidly evolving mini-halos during the first billion years of the Universe, supermassive black holes (SMBH) feed from gas flows spanning many orders of magnitude, from the cosmic web in which they are embedded to their event horizon. As such, accretion onto SMBHs constitutes a formidable challenge to tackle numerically, and currently requires the use of sub-grid models to handle the flow on small, unresolved scales. In this paper, we study the impact of resolution on the accretion pattern of SMBHs initially inserted at the heart of dense galactic gas clouds, using a custom super-Lagrangian refinement scheme to resolve the black hole (BH) gravitational zone of influence. We find that once the self-gravitating gas cloud host is sufficiently well resolved, accretion onto the BH is driven by the cloud internal structure, independently of the BH seed mass, provided dynamical friction is present during the early stages of cloud collapse. For a pristine gas mix of hydrogen and helium, a slim disc develops around the BH on sub-parsec scales, turning the otherwise chaotic BH accretion duty cycle into an episodic one, with potentially important consequences for BH feedback. In the presence of such a nuclear disc, BH mass growth predominantly occurs when infalling dense clumps trigger disc instabilities, fuelling intense albeit short-lived gas accretion episodes.

## Simulating MOS science on the ELT: Ly alpha forest tomography

ASTRONOMY & ASTROPHYSICS 632 (2019) ARTN A94

JEG Devriendt, P Petitjean, Y Dubois, E Gendron, F Hammer, S Morris, L Kaper, R Sanchez-Janssen, SD Vergani, N Pirzkal, A Slyz, Y Yang, J Japelj, C Laigle, H Rahmani, C Pichon, M Puech

Mapping of the large-scale structure through cosmic time has numerous applications in the studies of cosmology and galaxy evolution. At $z &gt; 2$, the structure can be traced by the neutral intergalactic medium (IGM) by way of observing the Ly$\alpha$, forest towards densely-sampled lines-of-sight of bright background sources, such as quasars and star forming galaxies. We investigate the scientific potential of MOSAIC, a planned multi-object spectrograph on the European Extremely Large Telescope (ELT), for the 3D mapping of the IGM at $z \gtrsim 3$. We simulate a survey of $3 \lesssim z \lesssim 4$ galaxies down to a limiting magnitude of $m_{r}\sim 25.5$ mag in an area of 1 degree$^2$ in the sky. Galaxies and their spectra (including the line-of-sight Ly$\alpha$ absorption) are taken from the lightcone extracted from the Horizon-AGN cosmological hydrodynamical simulation. The quality of the reconstruction of the original density field is studied for different spectral resolutions and signal-to-noise ratios of the spectra. We demonstrate that the minimum $S/N$ (per resolution element) of the faintest galaxies that such survey has to reach is $S/N = 4$. We show that a survey with such sensitivity enables a robust extraction of cosmic filaments and the detection of the theoretically-predicted galaxy stellar mass and star-formation rate gradients towards filaments. By simulating the realistic performance of MOSAIC we obtain $S/N(T_{\rm obs}, R, m_{r})$ scaling relations. We estimate that $\lesssim 35~(65)$ nights of observation time are required to carry out the survey with the instrument's high multiplex mode and with the spectral resolution of $R=1000~(2000)$. A survey with a MOSAIC-concept instrument on the ELT is found to enable the mapping of the IGM at $z &gt; 3$ on Mpc scales, and as such will be complementary to and competitive with other planned IGM tomography surveys. [abridged]

## The formation and evolution of low-surface-brightness galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 485 (2019) 796–818-

G Martin, S Kaviraj, C Laigle, J Devriendt, RA Jackson, S Peirani, Y Dubois, C Pichon, A Slyz

Our statistical understanding of galaxy evolution is fundamentally driven by objects that lie above the surface-brightness limits of current wide-area surveys (μ ∼ 23 mag arcsec−2). While both theory and small, deep surveys have hinted at a rich population of low-surface-brightness galaxies (LSBGs) fainter than these limits, their formation remains poorly understood. We use Horizon-AGN, a cosmological hydrodynamical simulation to study how LSBGs, and in particular the population of ultra-diffuse galaxies (UDGs; μ &gt; 24.5 mag arcsec−2), form and evolve over time. For M∗&gt;108M⊙⁠, LSBGs contribute 47, 7, and 6 per cent of the local number, mass, and luminosity densities, respectively (∼85/11/10 per cent for M∗&gt;107M⊙⁠). Today’s LSBGs have similar dark-matter fractions and angular momenta to high-surface-brightness galaxies (HSBGs; μ &lt; 23 mag arcsec−2), but larger effective radii (×2.5 for UDGs) and lower fractions of dense, star-forming gas (more than ×6 less in UDGs than HSBGs). LSBGs originate from the same progenitors as HSBGs at z &gt; 2. However, LSBG progenitors form stars more rapidly at early epochs. The higher resultant rate of supernova-energy injection flattens their gas-density profiles, which, in turn, creates shallower stellar profiles that are more susceptible to tidal processes. After z ∼ 1, tidal perturbations broaden LSBG stellar distributions and heat their cold gas, creating the diffuse, largely gas-poor LSBGs seen today. In clusters, ram-pressure stripping provides an additional mechanism that assists in gas removal in LSBG progenitors. Our results offer insights into the formation of a galaxy population that is central to a complete understanding of galaxy evolution, and that will be a key topic of research using new and forthcoming deep-wide surveys.

## Horizon-AGN virtual observatory – 1. SED-fitting performance and forecasts for future imaging surveys

Monthly Notices of the Royal Astronomical Society Oxford University Press 486 (2019) 5104–5123-

C Laigle, I Davidzon, O Ilbert, J Devriendt, D Kashino, S Arnouts, C Pichon, P Capak, SDL Torre, Y Dubois, G Gozaliasl, DL Borgne, S Lilly, HJ McCracken, M Salvato, A Slyz

Using the light-cone from the cosmological hydrodynamical simulation HORIZON-AGN, we produced a photometric catalogue over 0 &lt; z &lt; 4 with apparent magnitudes in COSMOS, Dark Energy Survey, Large Synoptic Survey Telescope (LSST)-like, and Euclid-like filters at depths comparable to these surveys. The virtual photometry accounts for the complex star formation history (SFH) and metal enrichment of HORIZON-AGN galaxies, and consistently includes magnitude errors, dust attenuation, and absorption by intergalactic medium. The COSMOS-like photometry is fitted in the same configuration as the COSMOS2015 catalogue. We then quantify random and systematic errors of photometric redshifts, stellar masses, and star formation rates (SFR). Photometric redshifts and redshift errors capture the same dependencies on magnitude and redshift as found in COSMOS2015, excluding the impact of source extraction. COSMOS-like stellar masses are well recovered with a dispersion typically lower than 0.1 dex. The simple SFHs and metallicities of the templates induce a systematic underestimation of stellar masses at z &lt; 1.5 by at most 0.12 dex. SFR estimates exhibit a dust-induced bimodality combined with a larger scatter (typically between 0.2 and 0.6 dex). We also use our mock catalogue to predict photometric redshifts and stellar masses in future imaging surveys. We stress that adding Euclid near-infrared photometry to the LSST-like baseline improves redshift accuracy especially at the faint end and decreases the outlier fraction by a factor ∼2. It also considerably improves stellar masses, reducing the scatter up to a factor 3. It would therefore be mutually beneficial for LSST and Euclid to work in synergy.

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

Monthly Notices of the Royal Astronomical Society Oxford University Press 474 (2017) 4279-4301

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

We analyse the properties of circumgalactic gas around simulated galaxies in the redshift range z ≥ 3, utilizing a new sample of cosmological zoom simulations. These simulations are intended to be representative of the observed samples of Lyman α (Ly α) emitters recently obtained with the multi unit spectroscopic explorer (MUSE) instrument (halo masses ~ 10 10 - 10 11 M⊙). We show that supernova feedback has a significant impact on both the inflowing and outflowing circumgalactic medium (CGM) by driving outflows, reducing diffuse inflow rates, and by increasing the neutral fraction of inflowing gas. By temporally stacking simulation outputs, we find that significant net mass exchange occurs between inflowing and outflowing phases: none of the phases are mass-conserving. In particular, we find that the mass in neutral outflowing hydrogen declines exponentially with radius as gas flows outwards from the halo centre. This is likely caused by a combination of both fountain-like cycling processes and gradual photoionization/collisional ionization of outflowing gas. Our simulations do not predict the presence of fast-moving neutral outflows in the CGM. Neutral outflows instead move with modest radial velocities (~ 50 km s -1 ), and the majority of the kinetic energy is associated with tangential rather than radial motion.

## A three-phase amplification of the cosmic magnetic field in galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 479 (2018) 3343–3365-

S Martin-Alvarez, A Slyz, JEG Devriendt, R Teyssier

Arguably the main challenge of galactic magnetism studies is to explain how the interstellar medium of galaxies reaches energetic equipartition despite the extremely weak cosmic primordial magnetic fields that are originally predicted to thread the inter-galactic medium. Previous numerical studies of isolated galaxies suggest that a fast dynamo amplification might suffice to bridge the gap spanning many orders of magnitude in strength between the weak early Universe magnetic fields and the ones observed in high redshift galaxies. To better understand their evolution in the cosmological context of hierarchical galaxy growth, we probe the amplification process undergone by the cosmic magnetic field within a spiral galaxy to unprecedented accuracy by means of a suite of constrained transport magnetohydrodynamical adaptive mesh refinement cosmological zoom simulations with different stellar feedback prescriptions. A galactic turbulent dynamo is found to be naturally excited in this cosmological environment, being responsible for most of the amplification of the magnetic energy. Indeed, we find that the magnetic energy spectra of simulated galaxies display telltale inverse cascades. Overall, the amplification process can be divided in three main phases, which are related to different physical mechanisms driving galaxy evolution: an initial collapse phase, an accretion-driven phase, and a feedback-driven phase. While different feedback models affect the magnetic field amplification differently, all tested models prove to be subdominant at early epochs, before the feedback-driven phase is reached. Thus the three-phase evolution paradigm is found to be quite robust vis-a-vis feedback prescriptions.

## Bondi or not Bondi: The impact of resolution on accretion and drag force modelling for Supermassive Black Holes

Monthly Notices of the Royal Astronomical Society Oxford University Press 478 (2018) 995–1016-

RS Beckmann, A Slyz, JEG Devriendt

Whilst in galaxy-size simulations, supermassive black holes (SMBH) are entirely handled by sub-grid algorithms, computational power now allows the accretion radius of such objects to be resolved in smaller scale simulations. In this paper, we investigate the impact of resolution on two commonly used SMBH sub-grid algorithms; the Bondi-Hoyle-Lyttleton (BHL) formula for accretion onto a point mass, and the related estimate of the drag force exerted onto a point mass by a gaseous medium. We find that when the accretion region around the black hole scales with resolution, and the BHL formula is evaluated using local mass-averaged quantities, the accretion algorithm smoothly transitions from the analytic BHL formula (at low resolution) to a supply limited accretion (SLA) scheme (at high resolution). However, when a similar procedure is employed to estimate the drag force it can lead to significant errors in its magnitude, and/or apply this force in the wrong direction in highly resolved simulations. At high Mach numbers and for small accretors, we also find evidence of the advective-acoustic instability operating in the adiabatic case, and of an instability developing around the wake's stagnation point in the quasi-isothermal case. Moreover, at very high resolution, and Mach numbers above $\mathcal{M}_\infty \geq 3$, the flow behind the accretion bow shock becomes entirely dominated by these instabilities. As a result, accretion rates onto the black hole drop by about an order of magnitude in the adiabatic case, compared to the analytic BHL formula.

## 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, MLA Richardson, S Peirani, 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.

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

arXiv (2016)

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

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 &lt; z &gt; 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 &lt; z &gt; 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.

## 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>

## 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, JEG Devriendt, S Peirani, RS Beckmann, S Codis, L Miller, Y Dubois, CMC Laigle, A Slyz, C Pichon

## 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

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

<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>