Publications by Julien Devriendt


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 >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 ($10^{10.4}<M/{\rm M}_\odot<10^{10.7}$) do not become aligned till later redshifts ($z<2$), 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).


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 ($M_{\mathrm{200c}}<2 \times 10^{13} M_\odot$) 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.


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.


Population estimates for electromagnetically distinguishable supermassive binary black holes

Astrophysical Journal American Astronomical Society 879 (2019) 110

JH Krolik, M Volonteri, Y Dubois, J Devriendt

Distinguishing the photon output of an accreting supermassive black hole binary system from that of a single supermassive black hole accreting at the same rate is intrinsically difficult because the majority of the light emerges from near the innermost stable orbits of the black holes. However, there are two possible signals that can distinctively mark binaries, both arising from the gap formed in circumbinary accretion flows inside approximately twice the binary separation. One of these is a "notch" cut into the thermal spectra of these systems in the IR/optical/UV, the other a periodically varying excess hard X-ray luminosity whose period is of order the binary orbital period. Using data from detailed galaxy evolution simulations, we estimate the distribution function in mass, mass ratio, and accretion rate for accreting supermassive binary black holes (SMBBHs) as a function of redshift and then transform this distribution function into predicted source counts for these two potential signals. At flux levels >~10−13 erg cm−2 s−1, there may be ~O(102) such systems in the sky, mostly in the redshift range 0.5 <~ z <~ 1. Roughly 10% should have periods short enough (<~5 yr) to detect the X-ray modulation; this is also the period range accessible to Pulsar Timing Array observations.


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 (OUP) (2018)

RS Beckmann, J Devriendt, A Slyz

Born in rapidly evolving mini-halos during the first billion years of the Universe, super- massive 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 re- solved, 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 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 $z > 2$, 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 $z > 3$ on Mpc scales, and as such will be complementary to and competitive with other planned IGM tomography surveys. [abridged]


HORIZON-AGN virtual observatory – 2. Template-free estimates of galaxy properties from colours

Monthly Notices of the Royal Astronomical Society Oxford University Press 489 (2019) 4817–4835-

I Davidzon, C Laigle, PL Capak, O Ilbert, DC Masters, S Hemmati, N Apostolakos, J Coupon, SDL Torre, J Devriendt, Y Dubois, D Kashino, S Paltani, C Pichon

<p>Using the HORIZON-AGN hydrodynamical simulation and self-organizing maps (SOMs), we show how to compress the complex, high-dimensional data structure of a simulation into a 2D grid, which greatly facilitates the analysis of how galaxy observables are connected to intrinsic properties. We first verify the tight correlation between the observed 0.3–5 <em>μ</em>m broad-band colours of HORIZON-AGN galaxies and their high-resolution spectra. The correlation is found to extend to physical properties such as redshift, stellar mass, and star formation rate (SFR). This direct mapping from colour to physical parameter space still works after including photometric uncertainties that mimic the COSMOS survey. We then label the SOM grid with a simulated calibration sample to estimate redshift and SFR for COSMOS-like galaxies up to <em>z</em> ∼ 3. In comparison to state-of-the-art techniques based on synthetic templates, our method is comparable in performance but less biased at estimating redshifts, and significantly better at predicting SFRs. In particular, our ‘data-driven’ approach, in contrast to model libraries, intrinsically allows for the complexity of galaxy formation and can handle sample biases. We advocate that observations to calibrate this method should be one of the goals of next-generation galaxy surveys.</p>


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>


Galaxies flowing in the oriented saddle frame of the cosmic web

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) 483 (2019) 3227-3254

K Kraljic, C Pichon, Y Dubois, S Codis, C Cadiou, J Devriendt, M Musso, C Welker, S Arnouts, HS Hwang, C Laigle, S Peirani, A Slyz, M Treyer, D Vibert


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, C Pichon, P Capak, S Arnouts, 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.


Galaxy orientation with the cosmic web across cosmic time

Monthly Notices of the Royal Astronomical Society Oxford University Press 481 (2018) 4753–4774-

S Codis, A Jindal, NE Chisari, D Vibert, Y Dubois, C Pichon, J Devriendt

This work investigates the alignment of galactic spins with the cosmic web across cosmic time using the cosmological hydrodynamical simulation Horizon-AGN. The cosmic web structure is extracted via the persistent skeleton as implemented in the DISPERSE algorithm. It is found that the spin of low-mass galaxies is more likely to be aligned with the filaments of the cosmic web and to lie within the plane of the walls while more massive galaxies tend to have a spin perpendicular to the axis of the filaments and to the walls. The mass transition is detected with a significance of 9σ. This galactic alignment is consistent with the alignment of the spin of dark haloes found in pure dark matter simulations and with predictions from (anisotropic) tidal torque theory. However, unlike haloes, the alignment of low-mass galaxies is weak and disappears at low redshifts while the orthogonal spin orientation of massive galaxies is strong and increases with time, probably as a result of mergers. At fixed mass, alignments are correlated with galaxy morphology: the high-redshift alignment is dominated by spiral galaxies while elliptical centrals are mainly responsible for the perpendicular signal. The two regimes probed in this work induce competing galactic alignment signals for weak lensing, with opposite redshift and luminosity evolution. Understanding the details of these intrinsic alignments will be key to exploit future major cosmic shear surveys like Euclid or LSST.


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, JEG Devriendt, A Slyz, 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.


Cosmic CARNage II: the evolution of the galaxy stellar mass function in observations and galaxy formation models

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY 480 (2018) 1197-1210

R Asquith, FR Pearce, O Almaini, A Knebe, V Gonzalez-Perez, A Benson, J Blaizot, J Carretero, FJ Castander, A Cattaneo, SA Cora, DJ Croton, JE Devriendt, F Fontanot, ID Gargiulo, W Hartley, B Henriques, J Lee, GA Mamon, J Onions, ND Padilla, C Power, C Srisawat, ARH Stevens, PA Thomas, CA Vega-Martinez, SK Yi


Using Real and Simulated Measurements of the Thermal Sunyaev-Zel'dovich Effect to Constrain Models of AGN Feedback

ASTROPHYSICAL JOURNAL 865 (2018) ARTN 109

A Spacek, MLA Richardson, E Scannapieco, J Devriendt, Y Dubois, S Peirani, C Pichon


Universality of the halo mass function in screened gravity theories

JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS (2018) ARTN 028

F von Braun-Bates, J Devriendt


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 478 (2018) 995-1016

RS Beckmann, A Slyz, J Devriendt


The role of mergers in driving morphological transformation over cosmic time

Monthly Notices of the Royal Astronomical Society Oxford University Press (OUP) (2018)

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

Understanding the processes that trigger morphological transformation is central to understanding how and why the Universe transitions from being disc-dominated at early epochs to having the morphological mix that is observed today. We use Horizon-AGN, a cosmological hydrodynamical simulation, to perform a comprehensive study of the processes that drive morphological change in massive (M > 10^10 MSun) galaxies over cosmic time. We show that (1) essentially all the morphological evolution in galaxies that are spheroids at z=0 is driven by mergers with mass ratios greater than 1:10, (2) major mergers alone cannot produce today's spheroid population -- minor mergers are responsible for a third of all morphological transformation over cosmic time and are its dominant driver after z~1, (3) prograde mergers trigger milder morphological transformation than retrograde mergers -- while both types of events produce similar morphological changes at z>2, the average change due to retrograde mergers is around twice that due to their prograde counterparts at z~0, (4) remnant morphology depends strongly on the gas fraction of a merger, with gas-rich mergers routinely re-growing discs, and (5) at a given stellar mass, discs do not exhibit drastically different merger histories from spheroids -- disc survival in mergers is driven by acquisition of cold gas (via cosmological accretion and gas-rich interactions) and a preponderance of prograde mergers in their merger histories.

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