IllustrisTNG Simulations Research Articles

The Evolving Interstellar Medium of Star-forming Galaxies, as Traced by Stardust*

Abstract We analyze the far-infrared (FIR) properties of ∼5000 star-forming galaxies at z < 4.5, drawn from the deepest, super-deblended catalogs in the GOODS-N and COSMOS fields. We develop a novel panchromatic spectral energy distribution fitting algorithm, Stardust, that models the emission from stars, active galactic nuclei (AGNs), and infrared dust emission, without relying on energy balance assumptions. Our code provides robust estimates of the UV−optical and FIR physical parameters, such as the stellar mass (M *), dust mass (M dust), infrared luminosities (L IR) arising from AGN and star formation activity, and the average intensity of the interstellar radiation field (〈U〉). Through a set of simulations we quantify the completeness of our data in terms of M dust, L IR, and 〈U〉 and subsequently characterize the distribution and evolution of these parameters with redshift. We focus on the dust-to-stellar mass ratio (f dust), which we parameterize as a function of cosmic age, stellar mass, and specific star formation rate. The f dust is found to increase by a factor of 10 from z = 0 to z = 2 and appears to remain flat at higher z, mirroring the evolution of the gas fraction. We also find a growing fraction of warm to cold dust with increasing distance from the main sequence, indicative of more intense interstellar radiation fields, higher star formation efficiencies, and more compact star-forming regions for starburst galaxies. Finally, we construct the dust mass functions (DMFs) of star-forming galaxies up to z = 1 by transforming the stellar mass function to DMF through the scaling relations derived here. The evolution of f dust and the recovered DMFs are in good agreement with the theoretical predictions of the Horizon-AGN and IllustrisTNG simulations.

Toward Precise Galaxy Evolution: A Comparison between Spectral Indices of z ∼1 Galaxies in the IllustrisTNG Simulation and the LEGA-C Survey

Abstract We present the first comparison of observed stellar continuum spectra of high-redshift galaxies and mock galaxy spectra generated from hydrodynamical simulations. The mock spectra are produced from the IllustrisTNG TNG100 simulation combined with stellar population models and take into account dust attenuation and realistic observational effects (aperture effects and noise). We compare the simulated Dn4000 and EW(Hδ) of galaxies with 10.5 ≤ log ( M * / M ⊙ ) ≤ 11.5 at 0.6 ≤ z ≤ 1.0 to the observed distributions from the LEGA-C survey. TNG100 globally reproduces the observed distributions of spectral indices, implying that the age distribution of galaxies in TNG100 is generally realistic. Yet there are small but significant differences. For old galaxies, TNG100 shows small Dn4000 when compared to LEGA-C, while LEGA-C galaxies have larger EW(Hδ) at fixed Dn4000 . There are several possible explanations: (1) LEGA-C galaxies have overall older ages combined with small contributions (a few percent in mass) from younger (<1 Gyr) stars, while TNG100 galaxies may not have such young subpopulations; (2) the spectral mismatch could be due to systematic uncertainties in the stellar population models used to convert stellar ages and metallicities to observables. In conclusion, the latest cosmological galaxy-formation simulations broadly reproduce the global age distribution of galaxies at z ∼ 1 and, at the same time, the high quality of the latest observed and simulated data sets help constrain stellar population synthesis models as well as the physical models underlying the simulations.

Ultrahigh energy cosmic ray deflection by the intergalactic magnetic field

The origin and composition of ultrahigh energy cosmic rays (UHECRs) remains a mystery. The common lore is that UHECRs are deflected from their primary directions by the Galactic and extragalactic magnetic fields. Here we describe an extragalactic contribution to the deflection of UHECRs that does not depend on the strength and orientation of the initial seed field. Using the IllustrisTNG simulations, we show that outflow-driven magnetic bubbles created by feedback processes during galaxy formation deflect approximately half of all ${10}^{20}\text{ }\text{ }\mathrm{eV}$ protons by 1\ifmmode^\circ\else\textdegree\fi{} or more, and up to 20\ifmmode^\circ\else\textdegree\fi{}--30\ifmmode^\circ\else\textdegree\fi{}. This implies that the deflection in the intergalactic medium must be taken into account in order to identify the sources of UHECRs.

Galaxy and mass assembly (GAMA): The environmental impact on SFR and metallicity in galaxy groups

ABSTRACT We present a study of the relationships and environmental dependencies between stellar mass, star formation rate, and gas metallicity for more than 700 galaxies in groups up to redshift 0.35 from the Galaxy And Mass Assembly (GAMA) survey. To identify the main drivers, our sample was analysed as a function of group-centric distance, projected galaxy number density, and stellar mass. By using control samples of more than 16 000 star-forming field galaxies and volume-limited samples, we find that the highest enhancement in SFR (0.3 dex) occurs in galaxies with the lowest local density. In contrast to previous work, our data show small enhancements of ∼0.1 dex in SFR for galaxies at the highest local densities or group-centric distances. Our data indicates quenching in SFR only for massive galaxies, suggesting that stellar mass might be the main driver of quenching processes for star forming galaxies. We can discard a morphological driven quenching, since the Sérsic index distribution for group and control galaxies are similar. The gas metallicity does not vary drastically. It increases ∼0.08 dex for galaxies at the highest local densities, and decreases for galaxies at the highest group-centric distances, in agreement with previous work. Altogether, the local density, rather than group-centric distance, shows the stronger impact in enhancing both, the SFR and gas metallicity. We applied the same methodology to galaxies from the IllustrisTNG simulations, and although we were able to reproduce the general observational trends, the differences between group and control samples only partially agree with the observations.

Predictions for anisotropic X-ray signatures in the circumgalactic medium: imprints of supermassive black hole driven outflows

ABSTRACT The circumgalactic medium (CGM) encodes signatures of the galaxy-formation process, including the interaction of galactic outflows driven by stellar and supermassive black hole (SMBH) feedback with the gaseous halo. Moving beyond spherically symmetric radial profiles, we study the angular dependence of CGM properties around z = 0 massive galaxies in the IllustrisTNG simulations. We characterize the angular signal of density, temperature, and metallicity of the CGM as a function of galaxy stellar mass, halo mass, distance, and SMBH mass, via stacking. TNG predicts that the CGM is anisotropic in its thermodynamical properties and chemical content over a large mass range, $M_*\sim 10^{10-11.5}\, \mathrm{M}_\odot$. Along the minor axis directions, gas density is diluted, whereas temperature and metallicity are enhanced. These feedback-induced anisotropies in the CGM have a magnitude of 0.1−0.3 dex, extend out to the halo virial radius, and peak at Milky Way-like masses, $M_*\sim 10^{10.8}\, \mathrm{M}_\odot$. In TNG, this mass scale corresponds to the onset of efficient SMBH feedback and the production of strong outflows. By comparing the anisotropic signals predicted by TNG versus other simulations – Illustris and EAGLE – we find that each simulation produces distinct signatures and mass dependencies, implying that this phenomenon is sensitive to the underlying physical models. Finally, we explore X-ray emission as an observable of this CGM anisotropy, finding that future X-ray observations, including the eROSITA all-sky survey, will be able to detect and characterize this signal, particularly in terms of an angular modulation of the X-ray hardness.

The Impacts of Modeling Choices on the Inference of Circumgalactic Medium Properties from Sunyaev–Zeldovich Observations

Abstract As the signal-to-noise of Sunyaev–Zeldovich (SZ) cross-correlation measurements of galaxies improves our ability to infer properties about the circumgalactic medium (CGM), we will transition from being limited by statistical uncertainties to systematic uncertainties. Using thermodynamic profiles of the CGM created from the IllustrisTNG (The Next Generation) simulations we investigate the importance of specific choices in modeling the galaxy sample. These choices include different sample selections in the simulation (stellar versus halo mass, color selections) and different fitting models (matching by the shape of the mass distribution, inclusion of a two-halo term). We forward model a mock galaxy sample into projected SZ observable profiles and fit these profiles to a generalized Navarro–Frenk–White profile using forecasted errors of the upcoming Simons Observatory experiment. We test the number of free parameters in the fits and show that this is another modeling choice that yields different results. Finally, we show how different fitting models can reproduce parameters of a fiducial profile, and show that the addition of a two-halo term and matching by the mass distribution of the sample are extremely important modeling choices to consider.

Formation of massive disc galaxies in the IllustrisTNG simulation

ABSTRACT We investigate the formation history of massive disc galaxies in hydrodynamical simulation – the IllustrisTNG, to study why massive disc galaxies survive through cosmic time. 83 galaxies in the simulation are selected with M*,z = 0 > 8 × 1010 M⊙ and kinematic bulge-to-total ratio less than 0.3. We find that 8.4 per cent of these massive disc galaxies have quiet merger histories and preserve disc morphology since formed. 54.2 per cent have a significant increase in bulge components in history, then become discs again till present time. The rest 37.3 per cent experience prominent mergers but survive to remain discy. While mergers and even major mergers do not always turn disc galaxies into ellipticals, we study the relations between various properties of mergers and the morphology of merger remnants. We find a strong dependence of remnant morphology on the orbit type of major mergers. Specifically, major mergers with a spiral-in falling orbit mostly lead to disc-dominant remnants, and major mergers of head-on galaxy–galaxy collision mostly form ellipticals. This dependence of remnant morphology on orbit type is much stronger than the dependence on cold gas fraction or orbital configuration of merger system as previously studied.

An Optimal Estimator of Intrinsic Alignments for Star-forming Galaxies in IllustrisTNG Simulation

Abstract Emission line galaxies (ELGs), more generally star-forming galaxies, are valuable tracers of large-scale structure and therefore main targets of upcoming wide-area spectroscopic galaxy surveys. We propose a fixed-aperture shape estimator of each ELG for extracting the intrinsic alignment (IA) signal, and assess the performance of the method using image simulations of ELGs generated from the IllustrisTNG simulation including observational effects such as the sky background noise. We show that our method enables a significant detection of the IA power spectrum with the linear-scale coefficient A IA ≃ (13–15) ± 3.0 up to z = 2, even from the small simulation volume ∼ 0.009 ( h − 1 Gpc ) 3 , in contrast to the null detection with the standard method. Thus the ELG IA signal, measured with our method, opens up opportunities to exploit cosmology and galaxy physics in high-redshift universe.

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

ABSTRACT 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 two-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 ratio (S/N), particularly at an S/N less than 100. We evaluate different methods to correct for the noise contribution to asymmetry at very low S/N, 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 data set, 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 data sets. Lastly, we provide estimations for the uncertainty on different correction methods at varying S/N and resolution regimes.

Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

ABSTRACT Local variations in the intergalactic medium (IGM) neutral hydrogen fraction will affect the Ly α absorption signature of protoclusters identified in tomographic surveys. Using the IllustrisTNG simulations, we investigate how the AGN proximity effect and hot, collisionally ionized gas arising from gravitational infall and black hole feedback changes the Ly α absorption associated with $M_{z=0}\simeq 10^{14}\, {\rm M}_\odot$ protoclusters at z ≃ 2.4. We find that protocluster galaxy overdensities exhibit a weak anticorrelation with Ly α transmission in IGM transmission maps, but local H$\, \rm \scriptstyle I$ ionization enhancements due to hot $T\gt 10^{6}\rm \, K$ gas or nearby AGN can disrupt this relationship within individual protoclusters. On average, however, we find that strong reductions in the IGM neutral fraction are limited to within $\lesssim 5h^{-1}\, \textrm {cMpc}$ of the dark matter haloes. Local ionization enhancements will therefore have a minimal impact on the completeness of protocluster identification in tomographic surveys if smoothing Ly α transmission maps over scales of $\sim 4 h^{-1}\, \textrm {cMpc}$, as is typically done in observations. However, if calibrating the relationship between the matter density and Ly α transmission in tomographic maps using simple analytical models for the Ly α forest opacity, the presence of hot gas around haloes can still result in systematically lower estimates of Mz = 0 for the most massive protoclusters.

Quenched fractions in the IllustrisTNG simulations: comparison with observations and other theoretical models

ABSTRACT We make an in-depth comparison of the IllustrisTNG cosmological simulations with observed quenched fractions of central and satellite galaxies, for Mstars = 109–12 M⊙ at 0 ≤ z ≤ 3. We show how measurement choices [aperture, quenched definition, and star formation rate (SFR) indicator time-scale], as well as sample selection issues (projection effects, satellite/central misclassification, and host mass distribution sampling), impact this comparison. The quenched definition produces differences of up to 70 (30) percentage points for centrals (satellites) above ∼1010.5 M⊙. At z ≳ 2, a larger aperture within which SFR is measured suppresses the quenched fractions by up to ∼50 percentage points. Proper consideration of the stellar and host mass distributions is crucial: Naive comparisons to volume-limited samples from simulations lead to misinterpretation of the quenched fractions as a function of redshift by up to 20 percentage points. Including observational uncertainties to theoretical values of Mstars and SFR changes the quenched fraction values and their trend and/or slope with mass. Taking projected rather than three-dimensional distances for satellites decreases the quenched fractions by up to 10 per cent. TNG produces quenched fractions for both centrals and satellites broadly consistent with observations and predicts up to ∼80 (90) per cent of quenched centrals at z = 0 (z = 2), in line with recent observations, and higher than other theoretical models. The quantitative agreement of TNG and Sloan Digital Sky Survey for satellite quenched fractions in groups and clusters depends strongly on the galaxy and host mass range. Our mock comparison highlights the importance of properly accounting for observational effects and biases.

Galaxy and mass assembly (GAMA): the clustering of galaxy groups

ABSTRACT We explore the clustering of galaxy groups in the Galaxy and Mass Assembly (GAMA) survey to investigate the dependence of group bias and profile on separation scale and group mass. Due to the inherent uncertainty in estimating the group selection function, and hence the group autocorrelation function, we instead measure the projected galaxy–group cross-correlation function. We find that the group profile has a strong dependence on scale and group mass on scales $r_\bot \lesssim 1 h^{-1}$. We also find evidence that the most massive groups live in extended, overdense, structures. In the first application of marked clustering statistics to groups, we find that group-mass marked clustering peaks on scales comparable to the typical group radius of r⊥ ≈ 0.5 h−1. While massive galaxies are associated with massive groups, the marked statistics show no indication of galaxy mass segregation within groups. We show similar results from the IllustrisTNG simulations and the L-Galaxies model, although L-Galaxies shows an enhanced bias and galaxy mass dependence on small scales.

Magnetization of the intergalactic medium in the IllustrisTNG simulations: the importance of extended, outflow-driven bubbles

ABSTRACT We study the effects of galaxy formation physics on the magnetization of the intergalactic medium (IGM) using the IllustrisTNG simulations. We demonstrate that large-scale regions affected by the outflows from galaxies and clusters contain magnetic fields that are several orders of magnitude stronger than in unaffected regions with the same electron density. Moreover, like magnetic fields amplified inside galaxies, these magnetic fields do not depend on the primordial seed, i.e. the adopted initial conditions for magnetic field strength. We study the volume filling fraction of these strong field regions and their occurrence in random lines of sight. As a first application, we use these results to put bounds on the photon–axion conversion from spectral distortion of the CMB. As photon–axion coupling grows with energy, stronger constraints could potentially be obtained using data on the propagation of gamma-ray photons through the IGM. Finally, we also briefly discuss potential applications of our results to the Faraday Rotation measurements.

A novel approach to investigate chemical inhomogeneities in GRB host galaxies: the Zabs–Zemiss relation

ABSTRACT Models of chemical enrichment and inhomogeneity in high-redshift galaxies are challenging to constrain observationally. In this work, we discuss a novel approach to probe chemical inhomogeneities within long gamma-ray burst (GRB) host galaxies, by comparing the absorption metallicity, Zabs, from the GRB afterglow (which probes the environment along the line of sight) with the emission-line metallicity, Zemiss, measured via slit spectroscopy. Using the IllustrisTNG simulation, the theoretical relationship between these metallicity metrics is explored for a range of GRB formation models, varying the GRB progenitor metallicity threshold. For galaxies with fixed Zemiss, the median value of Zabs depends strongly on the GRB progenitor threshold metallicity, with Zabs significantly lower than Zemiss for high-metallicity hosts. Conversely, at fixed Zabs, the median value of Zemiss depends primarily on the metallicity distribution of galaxies in IllustrisTNG and their chemical inhomogeneities, offering a GRB model-independent way to constrain these processes observationally. Currently, only one host galaxy has data for both absorption and emission metallicities (GRB121014A). We re-analyse the emission spectrum and compare the inferred metallicity Zemiss to a recent Bayesian determination of Zabs, finding $\log (Z_{\rm emiss}/Z_{\odot }) = \log (Z_{\rm abs}/Z_{\odot }) +0.35^{+ 0.14}_{- 0.25}$, within ∼2 standard deviations of predictions from the IllustrisTNG simulation. Future observations with the James Webb Space Telescope will be able to measure Zemiss for four other GRB hosts with known Zabs values, using ∼2-h observations. While small, the sample will provide preliminary constraints on the Zabs–Zemiss relation to test chemical enrichment schemes in cosmological simulations.

Properties of gas phases around cosmic filaments atz= 0 in the IllustrisTNG simulation

We present the study of gas phases around cosmic-web filaments detected in the TNG300-1 hydro-dynamical simulation at redshiftz = 0. We separate the gas into five different phases according to temperature and density. We show that filaments are essentially dominated by gas in the warm-hot intergalactic medium (WHIM), which accounts for more than 80% of the baryon budget atr ∼ 1 Mpc. Apart from WHIM gas, cores of filaments (r ≤ 1 Mpc) also host large contributions from other hotter and denser gas phases, whose fractions depend on the filament population. By building temperature and pressure profiles, we find that gas in filaments is isothermal up tor ∼ 1.5 Mpc, with average temperatures ofTcore = 4−13 × 105K, depending on the large-scale environment. Pressure at cores of filaments is on averagePcore = 4−12 × 10−7keV.cm−3, which is ∼1000 times lower than pressure measured in observed clusters. We also estimate that the observed Sunyaev-Zel’dovich signal from cores of filaments should range between 0.5 < y < 4.1 × 10−8, and these results are compared with recent observations. Our findings show that the state of the gas in filaments depends on the presence of haloes and on the large-scale environment.

Measuring the Mass and Concentration of Dark Matter Halos from the Velocity Dispersion Profile of their Stars

Abstract We use the IllustrisTNG (TNG) simulations of galaxy formation to measure the velocity dispersion profiles of dark matter and stars in Milky Way–mass, galaxy group, and cluster-scale dark matter halos. The mean profiles calculated from both tracers are similar in shape, exhibiting a large halo-to-halo scatter around the average profile. The so-called “splashback” radius demarcates the outer boundary of the halo, and manifests as a kink in the velocity dispersion profile, located on average between ∼1.0–1.5r 200m , where r 200m is the radius within which the density of the halo equals 200 times the background density of the universe. We find that this location may also be identified as the radius at which the (stacked) dispersion profile drops to 60% of its peak value (for line-of-sight motions in TNG halos). We further show that the scatter in the dispersion profiles may be attributed to the variations in the assembly history of the host halos. In particular, this segregates the profile into two regimes: one within ∼0.1r 200m , where the scatter is set by the early assembly history of the halo; and the other beyond this radius, where the scatter is influenced more strongly by its late-time assembly. Finally, we show that a two-parameter model can be used to fit the measured velocity dispersion profiles and the fit parameters can be related directly to two fundamental halo properties: mass and concentration. We describe a simple model that allows us to express the stellar velocity dispersion profile in terms of these halo properties only.

The splashback boundary of haloes in hydrodynamic simulations

ABSTRACT The splashback radius, Rsp, is a physically motivated halo boundary that separates infalling and collapsed matter of haloes. We study Rsp in the hydrodynamic and dark matter-only IllustrisTNG simulations. The most commonly adopted signature of Rsp is the radius at which the radial density profiles are steepest. Therefore, we explicitly optimize our density profile fit to the profile slope and find that this leads to a $\sim 5{{\ \rm per\ cent}}$ larger radius compared to other optimizations. We calculate Rsp for haloes with masses between 1013 and 15 M⊙ as a function of halo mass, accretion rate, and redshift. Rsp decreases with mass and with redshift for haloes of similar M200 m in agreement with previous work. We also find that Rsp/R200 m decreases with halo accretion rate. We apply our analysis to dark matter, gas, and satellite galaxies associated with haloes to investigate the observational potential of Rsp. The radius of steepest slope in gas profiles is consistently smaller than the value calculated from dark matter profiles. The steepest slope in galaxy profiles, which are often used in observations, tends to agree with dark matter profiles but is lower for less massive haloes. We compare Rsp in hydrodynamic and N-body dark matter-only simulations and do not find a significant difference caused by the addition of baryonic physics. Thus, results from dark matter-only simulations should be applicable to realistic haloes.

Interacting galaxies in the IllustrisTNG simulations – III. (The rarity of) quenching in post-merger galaxies

ABSTRACT Galaxy mergers are traditionally one of the favoured mechanisms for the transformation of spiral galaxies to spheroids and 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 star formation [i.e. star formation rate (SFR) at least 3σ below the star-forming main sequence] within 500 Myr 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, SFR, black hole mass, and environment. We find quenching to be uncommon among the descendants of post-merger galaxies, with only ${\sim} 5{{\ \rm per\ cent}}$ 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 become 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 none the less 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.

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

ABSTRACT The Canada–France Imaging Survey (CFIS) will consist of deep, high-resolution r-band imaging over ∼5000 deg2 of the sky, representing a first-rate opportunity to identify recently merged galaxies. Because of 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 per cent, 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 per cent 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.

Living with Neighbors. III. The Origin of the Spin–Orbit Alignment of Galaxy Pairs: A Neighbor versus the Large-scale Structure

Abstract Recent observations revealed a coherence between the spin vector of a galaxy and the orbital motion of its neighbors. We refer to the phenomenon as “the spin–orbit alignment (SOA)” and explore its physical origin via the IllustrisTNG simulation. This is the first study to utilize a cosmological hydrodynamic simulation to investigate the SOA of galaxy pairs. In particular, we identify paired galaxies at z = 0 having the nearest neighbor with mass ratios from 1/10 to 10 and calculate the spin–orbit angle for each pair. Our results are as follows. (a) There exists a clear preference for prograde orientations (i.e., SOA) for galaxy pairs, qualitatively consistent with observations. (b) The SOA is significant for both baryonic and dark matter spins, being the strongest for gas and the weakest for dark matter. (c) The SOA is stronger for less massive targets and for targets having closer neighbors. (d) The SOA strengthens for galaxies in low-density regions, and the signal is dominated by central–satellite pairs in low-mass halos. (e) There is an explicit dependence of the SOA on the duration of interaction with its current neighbor. Taken together, we propose that the SOA witnessed at z = 0 has been developed mainly by interactions with a neighbor for an extended period of time, rather than tidal torque from the ambient large-scale structure.