# Publications by Julien Devriendt

## Predicting the observability of population III stars with ELT-HARMONI via the helium 1640 Å emission line

Monthly Notices of the Royal Astronomical Society Oxford University Press 501 (2021) 5517-5537

K Grisdale, N Thatte, J Devriendt, M Pereira Santaella, A Slyz, T Kimm, Y Dubois, S Yi

Population III (Pop. III) stars, as of yet, have not been detected, however as we move into the era of extremely large telescopes this is likely to change. One likely tracer for Pop. III stars is the He IIλ1640 emission line, which will be detectable by the HARMONI spectrograph on the European Extremely Large Telescope (ELT) over a broad range of redshifts (2 ≤ z ≤ 14). By post-processing galaxies from the cosmological, AMR-hydrodynamical simulation NEWHORIZON with theoretical spectral energy distributions (SED) for Pop. III stars and radiative transfer (i.e. the Yggdrasil Models and CLOUDY look-up tables, respectively) we are able to compute the flux of He IIλ1640 for individual galaxies. From mock 10 h observations of these galaxies we show that HARMONI will be able to detect Pop. III stars in galaxies up to z ∼ 10 provided Pop. III stars have a top heavy initial mass function (IMF). Furthermore, we find that should Pop. III stars instead have an IMF similar to those of the Pop. I stars, the He IIλ1640 line would only be observable for galaxies with Pop. III stellar masses in excess of 107M⊙⁠, average stellar age <1Myr at z = 4. Finally, we are able to determine the minimal intrinsic flux required for HARMONI to detect Pop. III stars in a galaxy up to z = 10.

## The Horizon Run 5 cosmological hydrodynamical simulation: probing galaxy formation from kilo- to gigaparsec scales

Astrophysical Journal IOP Publishing 908 (2021) 11

J Lee, J Shin, ON Snaith, Y Kim, CG Few, J Devriendt, Y Dubois, LM Cox, SE Hong, O-K Kwon, C Park, C Pichon, J Kim, BK Gibson, C Park

Horizon Run 5 (HR5) is a cosmological hydrodynamical simulation that captures the properties of the universe on a Gpc scale while achieving a resolution of 1 kpc. Inside the simulation box, we zoom in on a high-resolution cuboid region with a volume of 1049 × 119 × 127 cMpc3. The subgrid physics chosen to model galaxy formation includes radiative heating/cooling, UV background, star formation, supernova feedback, chemical evolution tracking the enrichment of oxygen and iron, the growth of supermassive black holes, and feedback from active galactic nuclei in the form of a dual jet-heating mode. For this simulation, we implemented a hybrid MPI-OpenMP version of RAMSES, specifically targeted for modern many-core many-thread parallel architectures. In addition to the traditional simulation snapshots, lightcone data were generated on the fly. For the post-processing, we extended the friends-of-friend algorithm and developed a new galaxy finder PGalF to analyze the outputs of HR5. The simulation successfully reproduces observations, such as the cosmic star formation history and connectivity of galaxy distribution, We identify cosmological structures at a wide range of scales, from filaments with a length of several cMpc, to voids with a radius of ~ 100 cMpc. The simulation also indicates that hydrodynamical effects on small scales impact galaxy clustering up to very large scales near and beyond the baryonic acoustic oscillation scale. Hence, caution should be taken when using that scale as a cosmic standard ruler: one needs to carefully understand the corresponding biases. The simulation is expected to be an invaluable asset for the interpretation of upcoming deep surveys of the universe.

## Spatially offset black holes in the Horizon-AGN simulation and comparison to observations

Monthly Notices of the Royal Astronomical Society Oxford University Press 500 (2020) staa3516

DJ Bartlett, H Desmond, J Devriendt, PG Ferreira, A Slyz

We study the displacements between the centres of galaxies and their supermassive black holes (BHs) in the cosmological hydrodynamical simulation Horizon-AGN, and in a variety of observations from the literature. The BHs in Horizon-AGN feel a subgrid dynamical friction force, sourced by the surrounding gas, which prevents recoiling BHs being ejected from the galaxy. We find that (i) the fraction of spatially offset BHs increases with cosmic time, (ii) BHs live on prograde orbits in the plane of the galaxy with an orbital radius that decays with time but stalls near z = 0, and (iii) the magnitudes of offsets from the galaxy centres are substantially larger in the simulation than in observations. We attribute the stalling of the infall and excessive offset magnitudes to the fact that dynamical friction from stars and dark matter is not modelled in the simulation, and hence provide a way to improve the BH dynamics of future simulations.

## The role of mergers and interactions in driving the evolution of dwarf galaxies over cosmic time

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 500 (2021) 4937-4957

G Martin, R Jackson, S Kaviraj, H Choi, J Devriendt, Y Dubois, T Kimm, K Kraljic, S Peirani, C Pichon, M Volonteri, S Yi

&#xA9; 2020 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. Dwarf galaxies (M&#x2217; &lt; 109 M&#xB7;) are key drivers of mass assembly in high-mass galaxies, but relatively little is understood about the assembly of dwarf galaxies themselves. Using the NewHorizon cosmological simulation (&#x223C;40 pc spatial resolution), we investigate how mergers and fly-bys drive the mass assembly and structural evolution of around 1000 field and group dwarfs up to z = 0.5. We find that, while dwarf galaxies often exhibit disturbed morphologies (5 and 20 per cent are disturbed at z = 1 and z = 3 respectively), only a small proportion of the morphological disturbances seen in dwarf galaxies are driven by mergers at any redshift (for 109 M&#xB7;, mergers drive under 20 per cent morphological disturbances). They are instead primarily the result of interactions that do not end in a merger (e.g. fly-bys). Given the large fraction of apparently morphologically disturbed dwarf galaxies which are not, in fact, merging, this finding is particularly important to future studies identifying dwarf mergers and post-mergers morphologically at intermediate and high redshifts. Dwarfs typically undergo one major and one minor merger between z = 5 and z = 0.5, accounting for 10 per cent of their total stellar mass. Mergers can also drive moderate star formation enhancements at lower redshifts (3 or 4 times at z = 1), but this accounts for only a few per cent of stellar mass in the dwarf regime given their infrequency. Non-merger interactions drive significantly smaller star formation enhancements (around two times), but their preponderance relative to mergers means they account for around 10 per cent of stellar mass formed in the dwarf regime.

## Cosmological simulations of the same spiral galaxy: the impact of baryonic physics

Monthly Notices of the Royal Astronomical Society Oxford University Press 501 (2020) staa3233

A Nuñez-Castiñeyra, E Nezri, J Devriendt, R Teyssier

The interplay of star formation (SF) and supernova (SN) feedback in galaxy formation is a key element for understanding galaxy evolution. Since these processes occur at small scales, it is necessary to have sub-grid models that recover their evolution and environmental effects at the scales reached by cosmological simulations. In this work, we present the results of the Mochima simulation, where we simulate the same spiral galaxy inhabiting a Milky Way (MW) size halo in a cosmological environment changing the sub-grid models for SN feedback and SF. We test combinations of the Schmidt law and a multifreefall based SF with delayed cooling feedback or mechanical feedback. We reach a resolution of 35 pc in a zoom-in box of 36 Mpc. For this, we use the code RAMSES with the implementation of gas turbulence in time and trace the local hydrodynamical features of the star-forming gas. Finally, we compare the galaxies at redshift 0 with global and interstellar medium observations in the MW and local spiral galaxies. The simulations show successful comparisons with observations. Nevertheless, diverse galactic morphologies are obtained from different numerical implementations. We highlight the importance of detailed modelling of the SF and feedback processes, especially for simulations with a resolution that start to reach scales relevant for molecular cloud physics. Future improvements could alleviate the degeneracies exhibited in our simulated galaxies under different sub-grid models.

## Dual effects of ram pressure on star formation in multiphase disk galaxies with strong stellar feedback

Astrophysical Journal IOP Science 905 (2020) 31

J Lee, T Kimm, H Katz, J Rosdahl, J Devriendt, A Slyz

We investigate the impact of ram pressure stripping due to the intracluster medium (ICM) on star-forming disk galaxies with a multiphase interstellar medium maintained by strong stellar feedback. We carry out radiation-hydrodynamic simulations of an isolated disk galaxy embedded in a 1011 M ⊙ dark matter halo with various ICM winds mimicking the cluster outskirts (moderate) and the central environment (strong). We find that both star formation quenching and triggering occur in ram pressure–stripped galaxies, depending on the strength of the winds. H i and H2 in the outer galactic disk are significantly stripped in the presence of moderate winds, whereas turbulent pressure provides support against ram pressure in the central region, where star formation is active. Moderate ICM winds facilitate gas collapse, increasing the total star formation rates by ~40% when the wind is oriented face-on or by ~80% when it is edge-on. In contrast, strong winds rapidly blow away neutral and molecular hydrogen gas from the galaxy, suppressing star formation by a factor of 2 within ~200 Myr. Dense gas clumps with n H gsim 10 M ⊙ pc−2 are easily identified in extraplanar regions, but no significant young stellar populations are found in such clumps. In our attempts to enhance radiative cooling by adopting a colder ICM of T = 106 K, only a few additional stars are formed in the tail region, even if the amount of newly cooled gas increases by an order of magnitude.

## Formation of compact galaxies in the Extreme-Horizon simulation

ASTRONOMY & ASTROPHYSICS 643 (2020) ARTN L8

S Chabanier, F Bournaud, Y Dubois, S Codis, D Chapon, D Elbaz, C Pichon, O Bressand, J Devriendt, R Gavazzi, K Kraljic, T Kimm, C Laigle, J-B Lekien, G Martin, N Palanque-Delabrouille, S Peirani, P-F Piserchia, A Slyz, M Trebitsch, C Yeche

&#xA9; S. Chabanier et al. 2020. We present the Extreme-Horizon (EH) cosmological simulation, which models galaxy formation with stellar and active galactic nuclei (AGN) feedback and uses a very high resolution in the intergalactic and circumgalactic medium. Its high resolution in low-density regions results in smaller-size massive galaxies at a redshift of z = 2, which is in better agreement with observations compared to other simulations. We achieve this result thanks to the improved modeling of cold gas flows accreting onto galaxies. In addition, the EH simulation forms a population of particularly compact galaxies with stellar masses of 1010-11 M that are reminiscent of observed ultracompact galaxies at z 2. These objects form primarily through repeated major mergers of low-mass progenitors and independently of baryonic feedback mechanisms. This formation process can be missed in simulations with insufficient resolution in low-density intergalactic regions.

## Black hole mergers from dwarf to massive galaxies with the NewHorizon and Horizon-AGN simulations

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 498 (2020) 2219-2238

M Volonteri, H Pfister, RS Beckmann, Y Dubois, M Colpi, CJ Conselice, M Dotti, G Martin, R Jackson, K Kraljic, C Pichon, M Trebitsch, SK Yi, J Devriendt, S Peirani

&#xA9; 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society Massive black hole (MBH) coalescences are powerful sources of low-frequency gravitational waves. To study these events in the cosmological context, we need to trace the large-scale structure and cosmic evolution of a statistical population of galaxies, from dim dwarfs to bright galaxies. To cover such a large range of galaxy masses, we analyse two complementary simulations: HORIZON-AGN with a large volume and low resolution that tracks the high-mass (&gt; 107 M&#x2609;) MBH population, and NEWHORIZON with a smaller volume but higher resolution that traces the low-mass (&lt; 107 M&#x2609;) MBH population. While HORIZON-AGN can be used to estimate the rate of inspirals for pulsar timing arrays, NEWHORIZON can investigate MBH mergers in a statistical sample of dwarf galaxies for LISA, which is sensitive to low-mass MBHs. We use the same method to analyse the two simulations, post-processing MBH dynamics to account for time delays mostly determined by dynamical friction and stellar hardening. In both simulations, MBHs typically merge long after galaxies do, so that the galaxy morphology at the time of the MBH merger is no longer determined by the structural disturbances engendered by the galaxy merger from which the MBH coalescence has originated. These time delays cause a loss of high-z MBH coalescences, shifting the peak of the MBH merger rate to z &#x223C; 1-2. This study shows how tracking MBH mergers in low-mass galaxies is crucial to probing the MBH merger rate for LISA and investigate the properties of the host galaxies.

## New methods for identifying Lyman continuum leakers and reionization-epoch analogues

Monthly Notices of the Royal Astronomical Society Oxford University Press 498 (2020) 164-180

H Katz, D Durovcikova, T Kimm, J Rosdahl, J Blaizot, MG Haehnelt, J Devriendt, A Slyz, R Ellis, N Laporte

Identifying low-redshift galaxies that emit Lyman continuum radiation (LyC leakers) is one of the primary, indirect methods of studying galaxy formation in the epoch of reionization. However, not only has it proved challenging to identify such systems, it also remains uncertain whether the low-redshift LyC leakers are truly ‘analogues’ of the sources that reionized the Universe. Here, we use high-resolution cosmological radiation hydrodynamics simulations to examine whether simulated galaxies in the epoch of reionization share similar emission line properties to observed LyC leakers at z ∼ 3 and z ∼ 0. We find that the simulated galaxies with high LyC escape fractions (fesc) often exhibit high O32 and populate the same regions of the R23–O32 plane as z ∼ 3 LyC leakers. However, we show that viewing angle, metallicity, and ionization parameter can all impact where a galaxy resides on the O32–fesc plane. Based on emission line diagnostics and how they correlate with fesc, lower metallicity LyC leakers at z ∼ 3 appear to be good analogues of reionization-era galaxies. In contrast, local [S II]-deficient galaxies do not overlap with the simulated high-redshift LyC leakers on the S II Baldwin–Phillips–Terlevich (BPT) diagram; however, this diagnostic may still be useful for identifying leakers. We use our simulated galaxies to develop multiple new diagnostics to identify LyC leakers using infrared and nebular emission lines. We show that our model using only [C II]158 μm and [O III]88 μm can identify potential leakers from non-leakers from the local Dwarf Galaxy Survey. Finally, we apply this diagnostic to known high-redshift galaxies and find that MACS 1149_JD1 at z = 9.1 is the most likely galaxy to be actively contributing to the reionization of the Universe.

## GalICS 2.1: a new semianalytic model for cold accretion, cooling, feedback, and their roles in galaxy formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 497 (2020) 279-301

A Cattaneo, I Koutsouridou, E Tollet, J Devriendt, Y Dubois

&#xA9; 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. Dekel &amp; Birnboim proposed that the mass-scale that separates late-type and early-type galaxies is linked to the critical halo mass $M_{\rm vir}^{\rm crit}$ for the propagation of a stable shock and showed that they could reproduce the observed bimodality scale for plausible values of the metallicity of the accreted gas Zaccr and the shock radius rs. Here, we take their analysis one step further and present a new semianalytic model that computes rs from first principles. This advancement allows us to compute $M_{\rm vir}^{\rm crit}$ individually for each halo. Separating cold-mode and hot-mode accretion has little effect on the final galaxy masses if feedback does not preferentially couple to the hot gas. We also present an improved model for stellar feedback where ${\sim }70{{\ \rm per\ cent}}$ of the wind mass is in a cold galactic fountain with a shorter reaccretion time-scale at high masses. The latter is the key mechanism that allows us to reproduce the low-mass end of the mass function of galaxies over the entire redshift range 0 &lt; z &lt; 2.5. Cooling must be mitigated to avoid overpredicting the number density of galaxies with stellar mass $M_{\rm stars}\gt 10^{11}\, {\rm M}_\odot$ but is important to form intermediate-mass galaxies. At $M_{\rm vir}\gt 3\times 10^{11}\, {\rm M}_\odot$, cold accretion is more important at high z, where gas is accreted from smaller solid angles, but this is not true at lower masses because high-z filaments have lower metallicities. Our predictions are consistent with the observed metallicity evolution of the intergalactic medium at 0 &lt; z &lt; 5.

## How primordial magnetic fields shrink galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 495 (2020) 4475-4495

S Martin-Alvarez, A Slyz, J Devriendt, C Gomez-Guijarro

As one of the prime contributors to the interstellar medium energy budget, magnetic fields naturally play a part in shaping the evolution of galaxies. Galactic magnetic fields can originate from strong primordial magnetic fields provided these latter remain below current observational upper limits. To understand how such magnetic fields would affect the global morphological and dynamical properties of galaxies, we use a suite of high-resolution constrained transport magnetohydrodynamic cosmological zoom simulations where we vary the initial magnetic field strength and configuration along with the prescription for stellar feedback. We find that strong primordial magnetic fields delay the onset of star formation and drain the rotational support of the galaxy, diminishing the radial size of the galactic disc and driving a higher amount of gas towards the centre. This is also reflected in mock UVJ observations by an increase in the light profile concentration of the galaxy. We explore the possible mechanisms behind such a reduction in angular momentum, focusing on magnetic braking. Finally, noticing that the effects of primordial magnetic fields are amplified in the presence of stellar feedback, we briefly discuss whether the changes we measure would also be expected for galactic magnetic fields of non-primordial origin.

## Why do extremely massive disc galaxies exist today?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 494 (2020) 5568-5575

R Jackson, G Martin, S Kaviraj, C Laigle, J Devriendt, Y Dubois, C Pichon

&#xA9; 2020 The Author(s). Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios. Since mergers drive disc-to-spheroid morphological transformation, and the most massive galaxies typically have the richest merger histories, it is surprising that discs exist at all at the highest stellar masses (e.g. beyond the knee of the mass function). Using Horizon-AGN, a cosmological hydroynamical simulation, we show that extremely massive (M&#x2217; &gt; 1011.4 M) discs are created via two channels. In the primary channel (accounting for 70 per cent of these systems and 8 per cent of massive galaxies), the most recent, significant (mass ratio &gt; 1:10) merger between a massive spheroid and a gas-rich satellite 'spins up' the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for 30 per cent of these systems and 3 per cent of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. Not unexpectedly, the fraction of massive discs increases towards higher redshift, due to the Universe being more gas-rich. The morphological mix of galaxies at the highest stellar masses is, therefore, a strong function of the gas fraction of the Universe. Finally, these massive discs have similar black hole masses and accretion rates to massive spheroids, providing a natural explanation for why some powerful AGN are surprisingly found in disc galaxies.

## Detecting the cosmic web: Ly alpha emission from simulated filaments at z=3

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 494 (2020) 5439-5448

LM Elias, S Genel, A Sternberg, J Devriendt, A Slyz, E Visbal, N Bouche

&#xA9; 2020 The Author(s). The standard cosmological model (&#x39B; cold dark matter, &#x39B;CDM) predicts the existence of the cosmic web: A distribution of matter into sheets and filaments connecting massive haloes. However, observational evidence has been elusive due to the low surface brightness levels of the filaments. Recent deep Multi Unit Spectroscopic Explorer (MUSE)/Very Large Telescope (VLT) data and upcoming observations offer a promising avenue for Ly&#x3B1; detection, motivating the development of modern theoretical predictions. We use hydrodynamical cosmological simulations run with the arepo code to investigate the potential detectability of large-scale filaments, excluding contributions from the haloes embedded in them. We focus on filaments connecting massive (M200c (1-3)&#xD7; 1012, M&#x2299;) haloes at z = 3, and compare different simulation resolutions, feedback levels, and mock image pixel sizes. We find increasing simulation resolution does not substantially improve detectability notwithstanding the intrinsic enhancement of internal filament structure. By contrast, for a MUSE integration of 31 h, including feedback increases the detectable area by a factor of &#x2265;5.5 on average compared with simulations without feedback, implying that even the non-bound components of the filaments have substantial sensitivity to feedback. Degrading the image resolution from the native MUSE scale of 0.2 arcsec2 pixel-1 to 5.3 arcsec2 apertures has the strongest effect, increasing the detectable area by a median factor of &#x2265;200 and is most effective when the size of the pixel roughly matches the width of the filament. Finally, we find the majority of Ly&#x3B1; emission is due to electron impact collisional excitations, as opposed to radiative recombination.

## Reionization history constraints from neural network based predictions of high-redshift quasar continua

Monthly Notices of the Royal Astronomical Society Oxford University Press 493 (2020) 4256-4275

D Ďurovčíková, H Katz, SEI Bosman, FB Davies, J Devriendt, A Slyz

Observations of the early Universe suggest that reionization was complete by z ∼ 6, however, the exact history of this process is still unknown. One method for measuring the evolution of the neutral fraction throughout this epoch is via observing the Lyα damping wings of high-redshift quasars. In order to constrain the neutral fraction from quasar observations, one needs an accurate model of the quasar spectrum around Lyα, after the spectrum has been processed by its host galaxy but before it is altered by absorption and damping in the intervening IGM. In this paper, we present a novel machine learning approach, using artificial neural networks, to reconstruct quasar continua around Lyα. Our QSANNDRA algorithm improves the error in this reconstruction compared to the state-of-the-art PCA-based model in the literature by 14.2% on average, and provides an improvement of 6.1% on average when compared to an extension thereof. In comparison with the extended PCA model, QSANNDRA further achieves an improvement of 22.1% and 16.8% when evaluated on low-redshift quasars most similar to the two high-redshift quasars under consideration, ULAS J1120+0641 at z = 7.0851 and ULAS J1342+0928 at z = 7.5413, respectively. Using our more accurate reconstructions of these two z > 7 quasars, we estimate the neutral fraction of the IGM using a homogeneous reionization model and find x¯H1=0.25+0.05−0.05 at z = 7.0851 and x¯H1=0.60+0.11−0.11 at z = 7.5413. Our results are consistent with the literature and favour a rapid end to reionization.

## Kinematic unrest of low mass galaxy groups

Astronomy and Astrophysics EDP Sciences 635 (2020) A36

J Devriendt, G Gozaliasl, A Finoguenov, HG Khosroshahi, C Laigle, CC Kirkpatrick, K Kiiveri, Y Dubois, J Ahoranta

In an effort to better understand the formation of galaxy groups, we examine the kinematics of a large sample of spectroscopically confirmed X-ray galaxy groups in the Cosmic Evolution Survey (COSMOS) with a high sampling of galaxy group members up to $z=1$. We compare our results with predictions from the cosmological hydrodynamical simulation of {\sc Horizon-AGN}. Using a phase-space analysis of dynamics of groups with halo masses of $M_{\mathrm{200c}}\sim 10^{12.6}-10^{14.50}M_\odot$, we show that the brightest group galaxies (BGG) in low mass galaxy groups ($M_{\mathrm{200c}}&lt;2 \times 10^{13} M_\odot$) have larger proper motions relative to the group velocity dispersion than high mass groups. The dispersion in the ratio of the BGG proper velocity to the velocity dispersion of the group, $\sigma_{\mathrm{BGG}}/\sigma_{group}$, is on average $1.48 \pm 0.13$ for low mass groups and $1.01 \pm 0.09$ for high mass groups. A comparative analysis of the {\sc Horizon-AGN} simulation reveals a similar increase in the spread of peculiar velocities of BGGs with decreasing group mass, though consistency in the amplitude, shape, and mode of the BGG peculiar velocity distribution is only achieved for high mass groups. The groups hosting a BGG with a large peculiar velocity are more likely to be offset from the $L_x-\sigma_{v}$ relation; this is probably because the peculiar motion of the BGG is influenced by the accretion of new members.

## The SAMI Galaxy Survey: first detection of a transition in spin orientation with respect to cosmic filaments in the stellar kinematics of galaxies

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 2864-2884

C Welker, J Bland-Hawthorn, J Van de Sande, C Lagos, P Elahi, D Obreschkow, J Bryant, C Pichon, L Cortese, Richards, SM Croom, M Goodwin, JS Lawrence, S Sweet, A Lopez-Sanchez, A Medling, Owers, Y Dubois, J Devriendt

We present the first detection of mass dependent galactic spin alignments with local cosmic filaments with &gt;2σ confidence using IFS kinematics. The 3D network of cosmic filaments is reconstructed on Mpc scales across GAMA fields using the cosmic web extractor DisPerSe. We assign field galaxies from the SAMI survey to their nearest filament segment in 3D and estimate the degree of alignment between SAMI galaxies’ kinematic spin axis and their nearest filament in projection. Low-mass galaxies align their spin with their nearest filament while higher mass counterparts are more likely to display an orthogonal orientation. The stellar transition mass from the first trend to the second is bracketed between 1010.4 M⊙ and 1010.9 M⊙, with hints of an increase with filament scale. Consistent signals are found in the Horizon-AGN cosmological hydrodynamic simulation. This supports a scenario of early angular momentum build-up in vorticity rich quadrants around filaments at low stellar mass followed by progressive flip of spins orthogonal to the cosmic filaments through mergers at high stellar mass. Conversely, we show that dark-matter only simulations post-processed with a semi-analytic model treatment of galaxy formation struggles to reproduce this alignment signal. This suggests that gas physics is key in enhancing the galaxy-filament alignment.

## When galaxies align: intrinsic alignments of the progenitors of elliptical galaxies in the Horizon-AGN simulation

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 4057-4068

J Bate, NE Chisari, S Codis, G Martin, Y Dubois, J Devriendt, C Pichon, A Slyz

Elliptical galaxies today appear aligned with the large-scale structure of the Universe, but it is still an open question when they acquire this alignment. Observational data is currently insufficient to provide constraints on the time evolution of intrinsic alignments, and hence existing models range from assuming that galaxies gain some primordial alignment at formation, to suggesting that they react instantaneously to tidal interactions with the large-scale structure. Using the cosmological hydrodynamical simulation Horizon-AGN, we measure the relative alignments between the major axes of galaxies and eigenvectors of the tidal field as a function of redshift. We focus on constraining the time evolution of the alignment of the main progenitors of massive $z=0$ elliptical galaxies, the main weak lensing contaminant at low redshift. We show that this population, which at $z=0$ has a stellar mass above $10^{10.4}$ M$_\odot$, transitions from having no alignment with the tidal field at $z=3$, to a significant alignment by $z=1$. From $z=0.5$ they preserve their alignment at an approximately constant level until $z=0$. We find a mass-dependence of the alignment signal of elliptical progenitors, whereby ellipticals that are less massive today ($10^{10.4}&lt;M/{\rm M}_\odot&lt;10^{10.7}$) do not become aligned till later redshifts ($z&lt;2$), compared to more massive counterparts. We also present an extended study of progenitor alignments in the parameter space of stellar mass and galaxy dynamics, the impact of shape definition and tidal field smoothing.

## The impact of the connectivity of the cosmic web on the physical properties of galaxies at its nodes

Monthly Notices of the Royal Astronomical Society Oxford University Press 491 (2019) 4294-4309

K Kraljic, C Pichon, S Codis, C Laigle, R Davé, Y Dubois, HS Hwang, D Pogosyan, S Arnouts, J Devriendt, M Musso, S Peirani, A Slyz, M Treyer

We investigate the impact of the number of filaments connected to the nodes of the cosmic web on the physical properties of their galaxies using the Sloan Digital Sky Survey. We compare these measurements to the cosmological hydrodynamical simulations Horizon-(no)AGN and Simba. We find that more massive galaxies are more connected, in qualitative agreement with theoretical predictions and measurements in dark matter only simulation. The star formation activity and morphology of observed galaxies both display some dependence on the connectivity of the cosmic web at fixed stellar mass: less star forming and less rotation supported galaxies also tend to have higher connectivity. These results qualitatively hold both for observed and virtual galaxies, and can be understood given that the cosmic web is the main source of fuel for galaxy growth. The simulations show the same trends at fixed halo mass, suggesting that the geometry of filamentary infall impacts galaxy properties beyond the depth of the local potential well. Based on simulations, it is also found that AGN feedback is key in reversing the relationship between stellar mass and connectivity at fixed halo mass. Technically, connectivity is a practical observational proxy for past and present accretion (minor mergers or diffuse infall).

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

Astronomy and Astrophysics EDP Sciences 632 (2019) A94

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

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]

## Massive spheroids can form in single minor mergers

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 4679-4689

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

<p>Understanding how rotationally supported discs transform into dispersion-dominated spheroids is central to our comprehension of galaxy evolution. Morphological transformation is largely merger-driven. While major mergers can efficiently create spheroids, recent work has highlighted the significant role of other processes, like minor mergers, in driving morphological change. Given their rich merger histories, spheroids typically exhibit large fractions of ‘<em>ex situ</em>’ stellar mass, i.e. mass that is accreted, via mergers, from external objects. This is particularly true for the most massive galaxies, whose stellar masses typically cannot be attained without a large number of mergers. Here, we explore an unusual population of extremely massive (<em>M</em><sub>*</sub> &gt; 10<sup>11</sup>M<sub>⊙</sub>) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low <em>ex situ</em> mass fractions, indicating that they form without recourse to significant merging. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11–0.33), with a specific orbital configuration, where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myr, coupled with strong <em>in situ</em> star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the <em>most massive</em> spheroids need not involve major mergers – or any significant merging at all – contrary to what is classically believed.</p>