arXiv subjects

Maan H. Hani

Publications and source records attributed to Maan H. Hani.

At least 19 recordsLinked to original sources

arXiv2024

Interacting galaxies in the IllustrisTNG simulations -- VI: Reconstructed orbits, close encounters and mergers

Cosmological simulations have been used to study interacting galaxies as a function of galaxy pair separation, enabling comparisons with observational studies of galaxy pairs. The study of interacting galaxies as a function of time (i.e. merger stage) has mostly been limited to high resolution merger simulations, due to the poor time sampling available in cosmological simulations. Building on an earlier study of galaxy pairs in the IllustrisTNG cosmological simulations, we reconstruct the orbits of galaxy pairs involving massive galaxies ($M_* > 10^{10}M_{\odot}$) at redshifts of $0 \leq z < 1$, using a novel kinematic interpolation scheme to model the orbits in between the IllustrisTNG snapshots (which are separated by 162 Myr on average). We assess the accuracy of these interpolations using a pre-existing suite of merger simulations, and find that kinematic interpolations provide a remarkable improvement in accuracy compared with interpolations that use only radial separations or 3D positions. We find that nearly 90 per cent of the closest pairs ($r < 25$ kpc) have had a pericentre encounter within the past Gyr. Many of these close pairs are found on rapidly shrinking orbits, and roughly 85 per cent of these pairs will merge within 1 Gyr. However, approximately 3 per cent of these close pairs appear to be flyby systems that will never merge. These reconstructed orbits will be used in future studies to investigate how and when galaxy properties change during close encounters and mergers between galaxies in IllustrisTNG.

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arXiv2022

The interconnection between galaxy mergers, AGN activity and rapid quenching of star formation in simulated post-merger galaxies

We investigate the role of galaxy mergers on supermassive black hole (SMBH) accretion and star formation quenching in three state-of-the-art cosmological simulations with contrasting physics models: EAGLE, Illustris and IllustrisTNG. We find that recently coalesced 'post-mergers' in all three simulations have elevated SMBH accretion rates by factors of ~2-5. However, rapid (within 500 Myr of coalescence) quenching of star formation is rare, with incidence rates of 0.4% in Illustris, 4.5% in EAGLE and 10% in IllustrisTNG. The rarity of quenching in post-mergers results from substantial gas reservoirs that remain intact after the merger. The post-mergers that do successfully quench tend to be those that had both low pre-merger gas fractions as well as those that experience the largest gas losses. Although rare, the recently quenched fraction of post-mergers is still elevated compared to a control sample of non-mergers by factors of two in IllustrisTNG and 11 in EAGLE. Conversely, quenching is rarer in Illustris post-mergers than in their control. Recent observational results by Ellison et al. have found rapid quenching to be at least 30 times more common in post-mergers, a significantly higher excess than found in any of the simulations. Our results, therefore, indicate that whilst merger-induced SMBH accretion is a widespread prediction of the simulations, its link to quenching depends sensitively on the physics models, and that none of the subgrid models of the simulations studied here can fully capture the connection between mergers and rapid quenching seen in observations.

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arXiv2022

Kinematics of the Local Group gas and galaxies in the Hestia simulations

We investigate the kinematic properties of gas and galaxies in the Local Group (LG) using high-resolution simulations performed by the {\sc Hestia} (High-resolution Environmental Simulations of The Immediate Area) collaboration. Our simulations include the correct cosmography surrounding LG-like regions consisting of two main spiral galaxies of $\sim 10^{12}$~M$_\odot$, their satellites and minor isolated galaxies, all sharing the same large-scale motion within a volume of a few Mpc. We characterise the gas and galaxy kinematics within the simulated LGs, from the perspective of the Sun, to compare with observed trends from recent HST/COS absorption-line observations and LG galaxy data. To analyse the velocity pattern of LG gas and galaxies seen in the observational data, we build sky maps from the local standard of rest, and the galactic and local group barycentre frames. Our findings show that the establishment of a radial velocity dipole at low/high latitudes, near the preferred barycentre direction, is a natural outcome of simulation kinematics for material {\it outside} the Milky Way virial radius after removing galaxy rotation when the two main LG galaxies are approaching. Our results favour a scenario where gas and galaxies stream towards the LG barycentre producing a velocity dipole resembling observations. While our study shows in a qualitative way the global matter kinematics in the LG as part of its on-going assembly, quantitative estimates of gas-flow rates and physical conditions of the LG gas have to await a more detailed modeling of the ionization conditions, which will be presented in a follow-up paper.

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arXiv2022

Realistic synthetic integral field spectroscopy with RealSim-IFS

The most direct way to confront observed galaxies with those formed in numerical simulations is to forward-model simulated galaxies into synthetic observations. Provided that synthetic galaxy observations include similar constraints and limitations as real observations, they can be used to (1) carry out even-handed comparisons of observation and theory and (2) map the observable characteristics of simulated galaxies to their a priori known origins. In particular, integral field spectroscopy (IFS) expands the scope of such comparisons and mappings to an exceptionally broad set of physical properties. We therefore present RealSim-IFS: a tool for forward-modelling galaxies from hydrodynamical simulations into synthetic IFS observations. The core components of RealSim-IFS model the detailed spatial sampling mechanics of any fibre-bundle, image slicer, or lenslet array IFU and corresponding observing strategy, real or imagined, and support the corresponding propagation of noise adopted by the user. The code is highly generalized and can produce cubes in any light- or mass-weighted quantity (e.g. specific intensity, gas/stellar line-of-sight velocity, stellar age/metallicity, etc.). We show that RealSim-IFS exactly reproduces the spatial reconstruction of specific intensity and variance cubes produced by the MaNGA survey Data Reduction Pipeline using the calibrated fibre spectra as input. We then apply RealSim-IFS by producing a public synthetic MaNGA stellar kinematic survey of 893 galaxies with $\log M_{\star}/M_{\odot}>10$ from the TNG50 cosmological hydrodynamical simulation.

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arXiv2022

The observability of galaxy merger signatures in nearby gas-rich spirals

Galaxy mergers are crucial to understanding galaxy evolution, therefore we must determine their observational signatures to select them from large IFU galaxy samples such as MUSE and SAMI. We employ 24 high-resolution idealised hydrodynamical galaxy merger simulations based on the "Feedback In Realistic Environment" (FIRE-2) model to determine the observability of mergers to various configurations and stages using synthetic images and velocity maps. Our mergers cover a range of orbital configurations at fixed 1:2.5 stellar mass ratio for two gas rich spirals at low redshift. Morphological and kinematic asymmetries are computed for synthetic images and velocity maps spanning each interaction. We divide the interaction sequence into three: (1) the pair phase; (2) the merging phase; and (3) the post-coalescence phase. We correctly identify mergers between first pericentre passage and 500 Myr after coalescence using kinematic asymmetry with 66% completeness, depending upon merger phase and the field-of-view of the observation. We detect fewer mergers in the pair phase (40%) and many more in the merging and post-coalescence phases (97%). We find that merger detectability decreases with field-of-view, except in retrograde mergers, where centrally concentrated asymmetric kinematic features enhances their detectability. Using a cut-off derived from a combination of photometric and kinematic asymmetry, we increase these detections to 89% overall, 79% in pairs, and close to 100% in the merging and post-coalescent phases. By using this combined asymmetry cut-off we mitigate some of the effects caused by smaller fields-of-view subtended by massively multiplexed integral field spectroscopy programmes.

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arXiv2022

Cold and Hot gas distribution around the Milky-Way-M31 system in the HESTIA simulations

Recent observations have revealed remarkable insights into the gas reservoir in the circumgalactic medium (CGM) of galaxy haloes. In this paper, we characterise the gas in the vicinity of Milky Way and Andromeda analogues in the HESTIA (High resolution Environmental Simulations of The Immediate Area) suite of constrained Local Group (LG) simulations. The HESTIA suite comprise of a set of three high-resolution {\sc arepo}-based simulations of the LG, run using the Auriga galaxy formation model. For this paper, we focus only on the $z = 0$ simulation datasets and generate mock skymaps along with a power spectrum analysis to show that the distributions of ions tracing low-temperature gas (HI and SiIII) are more clumpy in comparison to warmer gas tracers (OVI, OVII and OVIII). We compare to the spectroscopic CGM observations of M31 and low-redshift galaxies. HESTIA under-produces the column densities of the M31 observations, but the simulations are consistent with the observations of low-redshift galaxies. A possible explanation for these findings is that the spectroscopic observations of M31 are contaminated by gas residing in the CGM of the Milky Way.

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arXiv2021

VERTICO: The Virgo Environment Traced In CO Survey

We present the Virgo Environment Traced in CO (VERTICO) survey, a new effort to map $^{12}$CO($2-1$), $^{13}$CO($2-1$), and C$^{18}$O($2-1$) in 51 Virgo Cluster galaxies with the Atacama Compact Array, part of the Atacama Large Millimeter/submillimeter Array (ALMA). The primary motivation of VERTICO is to understand the physical mechanisms that perturb molecular gas disks, and therefore star formation and galaxy evolution, in dense environments. This first paper contains an overview of VERTICO's design and sample selection, $^{12}$CO($2-1$) observations, and data reduction procedures. We characterize global $^{12}$CO($2-1$) fluxes and molecular gas masses for the 49 detected VERTICO galaxies, provide upper limits for the two non-detections, and produce resolved $^{12}$CO($2-1$) data products (median resolution $= 8^{\prime\prime} \approx 640~{\rm pc}$). Azimuthally averaged $^{12}$CO($2-1$) radial intensity profiles are presented along with derived molecular gas radii. We demonstrate the scientific power of VERTICO by comparing the molecular gas size--mass scaling relation for our galaxies with a control sample of field galaxies, highlighting the strong effect that radius definition has on this correlation. We discuss the drivers of the form and scatter in the size--mass relation and highlight areas for future work. VERTICO is an ideal resource for studying the fate of molecular gas in cluster galaxies and the physics of environment-driven processes that perturb the star formation cycle. Upon public release, the survey will provide a homogeneous legacy dataset for studying galaxy evolution in our closest cluster.

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arXiv2021

Gas flows in galaxy mergers: supersonic turbulence in bridges, accretion from the circumgalactic medium, and metallicity dilution

In major galaxy mergers, the orbits of stars are violently perturbed, and gas is torqued to the centre, diluting the gas metallicity and igniting a starburst. In this paper, we study the gas dynamics in and around merging galaxies using a series of cosmological magneto-hydrodynamical (MHD) zoom-in simulations. We find that the gas bridge connecting the merging galaxies pre-coalescence is dominated by turbulent pressure, with turbulent Mach numbers peaking at values of 1.6-3.3. This implies that bridges are dominated by supersonic turbulence, and are thus ideal candidates for studying the impact of extreme environments on star formation. We also find that gas accreted from the circumgalactic medium (CGM) during the merger significantly contributes (27-51 per cent) to the star formation rate (SFR) at the time of coalescence and drives the subsequent reignition of star formation in the merger remnant. Indeed, 19-53 per cent of the SFR at z=0 originates from gas belonging to the CGM prior the merger. Finally, we investigate the origin of the metallicity-diluted gas at the centre of merging galaxies. We show that this gas is rapidly accreted onto the galactic centre with a time-scale much shorter than that of normal star-forming galaxies. This explains why coalescing galaxies are not well-captured by the fundamental metallicity relation.

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arXiv2021

Towards robust determination of non-parametric morphologies in marginal astronomical data: resolving uncertainties with cosmological hydrodynamical simulations

Quantitative morphologies, such as asymmetry and concentration, have long been used as an effective way to assess the distribution of galaxy starlight in large samples. Application of such quantitative indicators to other data products could provide a tool capable of capturing the 2-dimensional distribution of a range of galactic properties, such as stellar mass or star-formation rate maps. In this work, we utilize galaxies from the Illustris and IllustrisTNG simulations to assess the applicability of concentration and asymmetry indicators to the stellar mass distribution in galaxies. Specifically, we test whether the intrinsic values of concentration and asymmetry (measured directly from the simulation stellar mass particle maps) are recovered after the application of measurement uncertainty and a point spread function (PSF). We find that random noise has a non-negligible systematic effect on asymmetry that scales inversely with signal-to-noise, particularly at signal-to-noise less than 100. We evaluate different methods to correct for the noise contribution to asymmetry at very low signal-to-noise, where previous studies have been unable to explore due to systematics. We present algebraic corrections for noise and resolution to recover the intrinsic morphology parameters. Using Illustris as a comparison dataset, we evaluate the robustness of these fits in the presence of a different physics model, and confirm these correction methods can be applied to other datasets. Lastly, we provide estimations for the uncertainty on different correction methods at varying signal-to-noise and resolution regimes.

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arXiv2021

Interacting galaxies in the IllustrisTNG simulations -- III: (the rarity of) quenching in post-merger galaxies

Galaxy mergers are traditionally one of the favoured mechanisms for quenching star formation. To test this paradigm in the context of modern cosmological simulations, we use the IllustrisTNG simulation to investigate the impact of individual merger events on quenching (i.e. star formation rate at least 3sig below the star-forming main sequence) within 500Myr after the coalescence phase.The rate of quenching amongst recently merged galaxies is compared with a control sample that is matched in redshift, stellar mass, star formation rate (SFR), black hole mass and environment.We find quenching to be uncommon among the descendants of post-merger galaxies, with only 5% of galaxies quenching within 500 Myr after the merger.Despite this low absolute rate, we find that quenching occurs in post-mergers at twice the rate of the control galaxies.The fraction of quenched post-merger descendants 1.5 Gyr after the merger becomes statistically indistinguishable from that of non-post-mergers, suggesting that mergers could speed up the quenching process in those post-mergers whose progenitors had physical conditions able to sustain effective active galactic nuclei (AGN) kinetic feedback, thus capable of removing gas from galaxies.Our results indicate that although quenching does not commonly occur promptly after coalescence, mergers nonetheless do promote the cessation of star formation in some post-mergers. We find that, in IllustrisTNG, it is the implementation of the AGN kinetic feedback that is responsible for quenching post-mergers, as well as non-post-merger controls.As a result of the released kinetic energy, galaxies experience gas loss and eventually, they will quench.Galaxies with an initially low gas fraction show a preferable pre-disposition towards quenching.The primary distinguishing factor between quenched and star-forming galaxies is gas fraction, with a sharp boundary at fgas=0.1 in TNG.

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arXiv2021

The Structural Evolution of Isolated Galaxies at Low Redshift in the IllustrisTNG Simulation

We study the structural evolution of isolated star-forming galaxies in the Illustris TNG100-1 hydrodynamical simulation, with a focus on investigating the growth of the central core density within 2 kpc ($Σ_{*,2kpc}$) in relation to total stellar mass ($M_*$) at z < 0.5. First, we show that several observational trends in the $Σ_{*,2kpc}$-$M_*$ plane are qualitatively reproduced in IllustrisTNG, including the distributions of AGN, star forming galaxies, quiescent galaxies, and radial profiles of stellar age, sSFR, and metallicity. We find that galaxies with dense cores evolve parallel to the $Σ_{*,2kpc}$-$M_*$ relation, while galaxies with diffuse cores evolve along shallower trajectories. We investigate possible drivers of rapid growth in $Σ_{*,2kpc}$ compared to $M_*$. Both the current sSFR gradient and the BH accretion rate are indicators of past core growth, but are not predictors of future core growth. Major mergers (although rare in our sample; $\sim$10%) cause steeper core growth, except for high mass ($M_*$ >$\sim$ $10^{10} M_{\odot}$) mergers, which are mostly dry. Disc instabilities, as measured by the fraction of mass with Toomre Q < 2, are not predictive of rapid core growth. Instead, rapid core growth results in more stable discs. The cumulative black hole feedback history sets the maximum rate of core growth, preventing rapid growth in high-mass galaxies ($M_*$ >$\sim$ $10^{9.5} M_{\odot}$). For massive galaxies the total specific angular momentum of accreting gas is the most important predictor of future core growth. Our results suggest that the angular momentum of accreting gas controls the slope, width and zero-point evolution of the $Σ_{*,2kpc}$-$M_*$ relation.

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arXiv2021

Convolutional neural network identification of galaxy post-mergers in UNIONS using IllustrisTNG

The Canada-France Imaging Survey (CFIS) will consist of deep, high-resolution r-band imaging over ~5000 square degrees of the sky, representing a first-rate opportunity to identify recently-merged galaxies. Due to the large number of galaxies in CFIS, we investigate the use of a convolutional neural network (CNN) for automated merger classification. Training samples of post-merger and isolated galaxy images are generated from the IllustrisTNG simulation processed with the observational realism code RealSim. The CNN's overall classification accuracy is 88 percent, remaining stable over a wide range of intrinsic and environmental parameters. We generate a mock galaxy survey from IllustrisTNG in order to explore the expected purity of post-merger samples identified by the CNN. Despite the CNN's good performance in training, the intrinsic rarity of post-mergers leads to a sample that is only ~6 percent pure when the default decision threshold is used. We investigate trade-offs in purity and completeness with a variable decision threshold and find that we recover the statistical distribution of merger-induced star formation rate enhancements. Finally, the performance of the CNN is compared with both traditional automated methods and human classifiers. The CNN is shown to outperform Gini-M20 and asymmetry methods by an order of magnitude in post-merger sample purity on the mock survey data. Although the CNN outperforms the human classifiers on sample completeness, the purity of the post-merger sample identified by humans is frequently higher, indicating that a hybrid approach to classifications may be an effective solution to merger classifications in large surveys.

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arXiv2020

Spatially resolved star formation and fuelling in galaxy interactions

We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations (stellar mass ratio = 2.5:1), which employs the "Feedback In Realistic Environments-" model (fire-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally-concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR-enhancement in the central kiloparsec is fuel-driven (55% for the secondary, 71% for the primary) -- whilst central SFR-suppression is efficiency-driven (91% for the secondary, 97% for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially-resolved galaxy surveys -- capable of examining vast and diverse samples of interacting systems -- coupled with multi-wavelength campaigns aimed to capture their internal ISM structure.

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arXiv2020

Interacting galaxies in the IllustrisTNG simulations -- I: Triggered star formation in a cosmological context

We use the IllustrisTNG cosmological hydrodynamical simulations to investigate how the specific star formation rates (sSFRs) of massive galaxies $(M_* > 10^{10} M_\odot)$ depend on the distance to their closest companions. We estimate sSFR enhancements by comparing with control samples that are matched in redshift, stellar mass, local density and isolation, and we restrict our analysis to pairs with stellar mass ratios of 0.1 to 10. At small separations (~15 kpc), the mean sSFR is enhanced by a factor of 2.0 $\pm$ 0.1 in the flagship (110.7 Mpc)$^3$ simulation (TNG100-1). Statistically significant enhancements extend out to 3D separations of 280 kpc in the (302.6 Mpc)$^3$ simulation (TNG300-1). We find similar trends in the EAGLE and Illustris simulations, although their sSFR enhancements are lower than those in TNG100-1 by about a factor of two. Enhancements in IllustrisTNG galaxies are seen throughout the redshift range explored ($0 \leq z < 1$), with the strength of the enhancements decreasing with increasing redshift for galaxies with close companions. In order to more closely compare with observational results, we separately consider 2D projected distances between galaxies in IllustrisTNG. We detect significant sSFR enhancements out to projected separations of 260 kpc in TNG300-1, with projection effects diluting the size of the enhancements by about 20 per cent below 50 kpc. We find similar sSFR enhancements in TNG100-1 and Sloan Digital Sky Survey galaxies, with enhancements extending out to projected separations of about 150 kpc for star-forming galaxies at $z < 0.2$. Finally, by summing over all separations, we estimate that the presence of closest companions boosts the average sSFR of massive galaxies in TNG100-1 by 14.5 per cent.

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arXiv2020

Interacting galaxies in the IllustrisTNG simulations -- II: Star formation in the post-merger stage

Galaxy mergers are a major evolutionary transformation whose effects are borne out by a plethora of observations and numerical simulations. However, most previous simulations have used idealised, isolated, binary mergers and there has not been significant progress on studying statistical samples of galaxy mergers in large cosmological simulations. We present a sample of 27,691 post-merger (PM) galaxies ($0\le z \le 1$) identified from IllustrisTNG: a cosmological, large box, magneto-hydrodynamical simulation suite. The PM sample spans a wide range of merger and galaxy properties ($M_\star$, $μ$, $f_\mathrm{gas}$). We demonstrate that star forming (SF) PMs exhibit enhanced star formation rates (SFRs) on average by a factor of $\sim 2$, while the passive PMs show no statistical enhancement. We find that the SFR enhancements: (1) show no dependence on redshift, (2) anti-correlate with the PM's stellar mass, and (3) correlate with the gas fraction of the PM's progenitors. However, SF PMs show stronger enhancements which may indicate other processes being at play (e.g., gas phase, feedback efficiency). Although the SFR enhancement correlates mildly with the merger mass ratio, the more abundant minor mergers ($0.1 \le μ< 0.3 $) still contribute $\sim 50\%$ of the total SFR enhancement. By tracing the PM sample forward in time, we find that galaxy mergers can drive significant SFR enhancements which decay over $\sim 0.5$ Gyr independent of the merger mass ratio, although the decay timescale is dependent on the simulation resolution. The strongest merger-driven starburst galaxies evolve to be passive/quenched on faster timescales than their controls.

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arXiv2020

Rapid early coeval star formation and assembly of the most massive galaxies in the universe

The current consensus on the formation and evolution of the brightest cluster galaxies is that their stellar mass forms early ($z \gtrsim 4$) in separate galaxies that then eventually assemble the main structure at late times ($z \lesssim 1$). However, advances in observational techniques have led to the discovery of protoclusters out to $z \sim 7$, suggesting that the late-assembly picture may not be fully complete. If these protoclusters assemble rapidly in the early universe, they should form the brightest cluster galaxies much earlier than suspected by the late-assembly picture. Using a combination of observationally constrained hydrodynamical and dark-matter-only simulations, we show that the stellar assembly time of a sub-set of the brightest cluster galaxies occurs at high redshifts ($z > 3$) rather than at low redshifts ($z < 1$), as is commonly thought. We find, using isolated non-cosmological hydrodynamical simulations, that highly overdense protoclusters assemble their stellar mass into brightest cluster galaxies within $\sim 1$ $\mathrm{Gyr}$ of evolution -- producing massive blue elliptical galaxies at high redshifts ($z \gtrsim 1.5$). We argue that there is a downsizing effect on the cluster scale wherein some of the brightest cluster galaxies in the cores of the most-massive clusters assemble earlier than those in lower-mass clusters. In those clusters with $z = 0$ virial mass $\geqslant 5\times10^{14}$ M$_\mathrm{\odot}$, we find that $9.8$% have their cores assembly early, and a higher fraction of $16.4$% in those clusters above $10^{15}$ M$_\mathrm{\odot}$. The James Webb Space Telescope will be able to detect and confirm our prediction in the near future, and we discuss the implications to constraining the value of $σ_\mathrm{8}$.

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arXiv2020

Variations in the slope of the resolved star-forming main sequence: a tool for constraining the mass of star-forming regions

The correlation between galaxies' integrated stellar masses and star formation rates (the `star formation main sequence'; SFMS) is a well-established scaling relation. Recently, surveys have found a relationship between the star formation rate and stellar mass surface densities on kpc and sub-kpc scales (the `resolved SFMS'; rSFMS). In this work, we demonstrate that the rSFMS emerges naturally in FIRE-2 zoom-in simulations of Milky Way-mass galaxies. We make SFR and stellar mass maps of the simulated galaxies at a variety of spatial resolutions and star formation averaging time-scales and fit the rSFMS using multiple methods from the literature. While the absolute value of the SFMS slope depends on the fitting method, the slope is steeper for longer star formation time-scales and lower spatial resolutions regardless of the fitting method employed. We present a toy model that quantitatively captures the dependence of the simulated galaxies' rSFMS slope on spatial resolution and use it to illustrate how this dependence can be used to constrain the characteristic mass of star-forming clumps.

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arXiv2019

Deep learning predictions of galaxy merger stage and the importance of observational realism

Machine learning is becoming a popular tool to quantify galaxy morphologies and identify mergers. However, this technique relies on using an appropriate set of training data to be successful. By combining hydrodynamical simulations, synthetic observations and convolutional neural networks (CNNs), we quantitatively assess how realistic simulated galaxy images must be in order to reliably classify mergers. Specifically, we compare the performance of CNNs trained with two types of galaxy images, stellar maps and dust-inclusive radiatively transferred images, each with three levels of observational realism: (1) no observational effects (idealized images), (2) realistic sky and point spread function (semi-realistic images), (3) insertion into a real sky image (fully realistic images). We find that networks trained on either idealized or semi-real images have poor performance when applied to survey-realistic images. In contrast, networks trained on fully realistic images achieve 87.1% classification performance. Importantly, the level of realism in the training images is much more important than whether the images included radiative transfer, or simply used the stellar maps (87.1% compared to 79.6% accuracy, respectively). Therefore, one can avoid the large computational and storage cost of running radiative transfer with a relatively modest compromise in classification performance. Making photometry-based networks insensitive to colour incurs a very mild penalty to performance with survey-realistic data (86.0% with r-only compared to 87.1% with gri). This result demonstrates that while colour can be exploited by colour-sensitive networks, it is not necessary to achieve high accuracy and so can be avoided if desired. We provide the public release of our statistical observational realism suite, RealSim, as a companion to this paper.

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