We study stellar core growth in simulations of merging massive (M_⋆>10^11 M_⊙) elliptical galaxies by a supermassive black hole (SMBH) displaced by gravitational wave induced recoil velocity. With controlled, dense sampling of the SMBH recoil velocity, we find the core radius originally formed by SMBH binary scouring can grow by a factor of 2-3 when the recoil velocity exceeds ∼50 per cent of the central escape velocity, and the mass deficit grows by up to a factor of ∼4. Using Bayesian inference we predict the distribution of stellar core sizes formed through this process to peak at ∼1 kpc. An orbital decomposition of stellar particles within the core reveals that radial orbits dominate over tube orbits when the recoil velocity exceeds the velocity dispersion of the core, whereas tube orbits dominate for the lowest recoil kicks. A change in orbital structure is reflected in the anisotropy parameter, with a central tangential bias present only for recoil velocities less than the local stellar velocity dispersion. Emulating current integral field unit observations of the stellar line-of-sight velocity distribution, we uncover a distinct signature in the Gauss-Hermite symmetric deviation coefficient h_4 that uniquely constrains the core size due to binary scouring. This signature is insensitive to the later evolution of the stellar mass distribution due to SMBH recoil. Our results provide a novel method to estimate the SMBH recoil magnitude from observations of local elliptical galaxies, and implies these galaxies primarily experienced recoil velocities less than the stellar velocity dispersion of the core.

In recent years, cosmological hydrodynamical simulations have proven their utility as key interpretative tools in the study of galaxy formation and evolution. In this work, we present a like-for-like comparison between the baryon cycle in three publicly available, leading cosmological simulation suites: EAGLE, IllustrisTNG, and SIMBA. While these simulations broadly agree in terms of their predictions for the stellar mass content and star formation rates of galaxies at $z\approx0$, they achieve this result for markedly different reasons. In EAGLE and SIMBA, we demonstrate that at low halo masses ($M_{\rm 200c}\lesssim 10^{11.5}\, M_{\odot}$), stellar feedback (SF)-driven outflows can reach far beyond the scale of the halo, extending up to $2-3\times R_{\rm 200c}$. In contrast, in TNG, SF-driven outflows, while stronger at the scale of the ISM, recycle within the CGM (within $R_{\rm 200c}$). We find that AGN-driven outflows in SIMBA are notably potent, reaching several times $R_{\rm 200c}$ even at halo masses up to $M_{\rm 200c}\approx10^{13.5}\, M_{\odot}$. In both TNG and EAGLE, AGN feedback can eject gas beyond $R_{\rm 200c}$ at this mass scale, but seldom beyond $2-3\times R_{\rm 200c}$. We find that the scale of feedback-driven outflows can be directly linked with the prevention of cosmological inflow, as well as the total baryon fraction of haloes within $R_{\rm 200c}$. This work lays the foundation to develop targeted observational tests that can discriminate between feedback scenarios, and inform sub-grid feedback models in the next generation of simulations.

In this study of the `Resolving supermAssive Black hole Binaries In galacTic hydrodynamical Simulations' (RABBITS) series, we investigate the orbital evolution of supermassive black holes (SMBHs) during galaxy mergers. We simulate both disc and elliptical galaxy mergers using the KETJU code, which can simultaneously follow galaxy (hydro-)dynamics and small-scale SMBH dynamics with post-Newtonian corrections. With our SMBH binary subgrid model, we show how active galactic nuclei (AGNs) feedback affects galaxy properties and SMBH coalescence. We find that simulations without AGN feedback exhibit excessive star formation, resulting in merger remnants that deviate from observed properties. Kinetic AGN feedback proves more effective than thermal AGN feedback in expelling gas from the centre and quenching star formation. The different central galaxy properties, which are a result of distinct AGN feedback models, lead to varying rates of SMBH orbital decay. In the dynamical friction phase, galaxies with higher star formation and higher SMBH masses possess denser centres, become more resistant to tidal stripping, experience greater dynamical friction, and consequently form SMBH binaries earlier. As AGN feedback reduces gas densities in the centres, dynamical friction by stars dominates over gas. In the SMBH hardening phase, compared to elliptical mergers, disc mergers exhibit higher central densities of newly formed stars, resulting in accelerated SMBH hardening and shorter merger time-scales (i.e. $\lesssim 500$ Myr versus $\gtrsim 1$ Gyr). Our findings highlight the importance of AGN feedback and its numerical implementation in understanding the SMBH coalescing process, a key focus for low-frequency gravitational wave observatories.

Utilizing the apostle-auriga simulations, which start from the same zoom-in initial conditions of Local Group-like systems, but run with different galaxy formation subgrid models and hydrodynamic solvers, we study the impact of stellar feedback models on the evolution of angular momentum in disc galaxies. At z=0, auriga disc galaxies tend to exhibit higher specific angular momenta compared to their cross-matched apostle counterparts. By tracing the evolution history of the Lagrangian mass tracers of the in-situ star particles in the z=0 galaxies, we find that the specific angular momentum distributions of the gas tracers from the two simulations at the halo accretion time are relatively similar. The present-day angular momentum difference is mainly driven by the physical processes occurring inside dark matter haloes, especially galactic fountains. Due to the different subgrid implementations of stellar feedback processes, auriga galaxies contain a high fraction of gas that has gone through recycled fountain (similar to 65 per cent) which could acquire angular momentum through mixing with the high angular momentum circumgalactic medium (CGM). In apostle, however, the fraction of gas that has undergone the recycled fountain process is significantly lower (down to similar to 20 per cent for Milky Way-sized galaxies) and the angular momentum acquisition from the CGM is marginal. As a result, the present-day auriga galaxies overall have higher specific angular momenta.

ABSTRACT In this study of the ‘Resolving supermAssive Black hole Binaries In galacTic hydrodynamical Simulations’ (RABBITS) series, we focus on the hardening and coalescing process of supermassive black hole (SMBH) binaries in galaxy mergers. For simulations including different galaxy formation processes (i.e. gas cooling, star formation, SMBH accretion, stellar, and AGN feedback), we systematically control the effect of stochastic eccentricity by fixing it to similar values during the SMBH hardening phase. We find a strong correlation between the SMBH merger time-scales and the presence of nuclear star formation. Throughout the galaxy merging process, gas condenses at the centre due to cooling and tidal torques, leading to nuclear star formation. These recently formed stars, which inherit low angular momenta from the gas, contribute to the loss cone and assist in the SMBH hardening via three-body interactions. Compared to non-radiative hydrodynamical runs, the SMBH merger time-scales measured from the runs including cooling, stellar, and SMBH physical processes tend to be shortened by a factor of ∼1.7. After fixing the eccentricity to the range of e ∼ 0.6–0.8 during the hardening phase, the simulations with AGN feedback reveal merger time-scales of ∼100–500 Myr for disc mergers and ∼1–2 Gyr for elliptical mergers. With a semi-analytical approach, we find that the torque interaction between the binary and its circumbinary disc has minimal impact on the shrinking of the binary orbit in our retrograde galaxy merger. Our results are useful in improving the modelling of SMBH merger time-scales and gravitational-wave event rates.

We present a comprehensive study of the star formation histories of massive-quenched galaxies at z = 3 in three semi-analytical models (SHARK, GAEA, GALFORM) and three cosmological hydrodynamical simulations (EAGLE, ILLUSTRISTNG, SIMBA). We study the predicted number density and stellar mass function of massive-quenched galaxies, their formation and quenching time-scales and star formation properties of their progenitors. Predictions are disparate in all these diagnostics, for instance: (i) some simulations reproduce the observed number density of very massive-quenched galaxies (> 10(11) M-circle dot) but underpredict the high density of intermediate-mass ones, while others fit well the lower masses but underpredict the higher ones; (ii) in most simulations, except for GAEA and EAGLE, most massive-quenched galaxies had starburst periods, with the most intense ones happening at 4 < z < 5; however, only in SHARK and ILLUSTRISTNG we do find a large number of progenitors with star formation rates > 300 M-circle dot yr(-1); (iii) quenching time-scales are in the range approximate to 20-150 Myr depending on the simulation; among other differences. These disparate predictions can be tied to the adopted active galactic nucleus (AGN) feedback model. For instance, the explicit black hole (BH) mass dependence to trigger the 'radio mode' in ILLUSTRISTNG and SIMBA makes it difficult to produce quenched galaxies with intermediate stellar masses, also leading to higher baryon collapse efficiencies (approximate to 15-30 per cent); while the strong bolometric luminosity dependence of the AGN outflow rate in GAEA leads to BHs of modest mass quenching galaxies. Current observations are unable to distinguish between these different predictions due to the small sample sizes. However, these predictions are testable with current facilities and upcoming observations, allowing a 'true physics experiment' to be carried out.

The metal content of galaxies is a direct probe of the baryon cycle. A hallmark example is the relationship between a galaxy's stellar mass, star formation rate (SFR), and gas-phase metallicity: the fundamental metallicity relation (FMR). While low-redshift (z less than or similar to 4) observational studies suggest that the FMR is redshift-invariant, recent high-zJWST data indicate deviations from the FMR established at low-z. In this study, we utilize the FMR to predict the evolution of the normalization of the mass-metallicity relation (MZR) using the cosmological simulations Illustris, IllustrisTNG, EAGLE, and SIMBA. Our findings demonstrate that a z=0 calibrated FMR struggles to predict the evolution in the MZR of each simulation. To quantify the divergence of the predictions, we introduce the concepts of a 'static' FMR, where the role of the SFR in setting the normalization of the MZR does not change with redshift, and a 'dynamic' FMR, where the role of SFR evolves over time. We find static FMRs in SIMBA and dynamic FMRs in Illustris, IllustrisTNG, and EAGLE. We suggest that the differences between these models likely points to the subtle differences in the implementation of the baryon cycle. Moreover, we echo recent JWST results at z>4 by finding significant offsets from the FMR in IllustrisTNG and EAGLE, suggesting that the observed FMR may have a similar dynamic trend as these simulations. Overall, our findings imply that the current FMR framework neglects important time variations of these simulations' baryon cycles.

The scatter about the mass-metallicity relation (MZR) has a correlation with the star formation rate (SFR) of galaxies. The lack of evidence of evolution in correlated scatter at z less than or similar to 2.5 leads many to refer to the relationship between mass, metallicity, and SFR as the Fundamental Metallicity Relation (FMR). Yet, recent high-redshift (z > 3) JWST observations have challenged the fundamental (i.e. redshift-invariant) nature of the FMR. In this work, we show that the cosmological simulations Illustris, IllustrisTNG, and Evolution and Assembly of GaLaxies and their Environment (EAGLE) all predict MZRs that exhibit scatter with a secondary dependence on SFR up to z = 8. We introduce the concept of a 'strong' FMR, where the strength of correlated scatter does not evolve with time, and a 'weak' FMR, where there is some time evolution. We find that each simulation analysed has a statistically significant weak FMR - there is non-negligible evolution in the strength of the correlation with SFR. Furthermore, we show that the scatter is reduced an additional similar to 10-40 per cent at z greater than or similar to 3 when using a weak FMR, compared to assuming a strong FMR. These results highlight the importance of avoiding coarse redshift binning when assessing the FMR.

ABSTRACT We study environmental quenching in the eagle/c-eagle cosmological hydrodynamic simulations over the last 11 Gyr (i.e. z = 0–2). The simulations are compared with observations from the SAMI Galaxy Survey at z = 0. We focus on satellite galaxies in galaxy groups and clusters ($10^{12}\, {\rm M}_{\odot }$ ≲ M200 < $3 \times 10^{15}\, {\rm M}_{\odot }$). A star-formation concentration index [C-index = log10(r50, SFR/r50, rband)] is defined, which measures how concentrated star formation is relative to the stellar distribution. Both eagle/c-eagle and SAMI show a higher fraction of galaxies with low C-index in denser environments at z = 0–0.5. Low C-index galaxies are found below the SFR–M⋆ main sequence (MS), and display a declining specific star formation rate (sSFR) with increasing radii, consistent with ‘outside-in’ environmental quenching. Additionally, we show that C-index can be used as a proxy for how long galaxies have been satellites. These trends become weaker at increasing redshift and are absent by z = 1–2. We define a quenching time-scale tquench as how long it takes satellites to transition from the MS to the quenched population. We find that simulated galaxies experiencing ‘outside-in’ environmental quenching at low redshift (z = 0 ∼ 0.5) have a long quenching time-scale (median tquench > 2 Gyr). The simulated galaxies at higher redshift (z = 0.7 ∼ 2) experience faster quenching (median tquench < 2 Gyr). At z ≳ 1–2 galaxies undergoing environmental quenching have decreased sSFR across the entire galaxy with no ‘outside-in’ quenching signatures and a narrow range of C-index, showing that on average environmental quenching acts differently than at z ≲ 1.

We use a sample of $z=0$ galaxies visually classified as slow rotators (SRs) in the EAGLE hydrodynamical simulations to explore the effect of galaxy mergers on their formation, characterise their intrinsic galaxy properties, and study the connection between quenching and kinematic transformation. SRs that have had major or minor mergers (mass ratios $\ge 0.3$ and $0.1-0.3$, respectively) tend to have a higher triaxiality parameter and ex-situ stellar fractions than those that had exclusively very minor mergers or formed in the absence of mergers (no-merger SRs). No-merger SRs are more compact, have lower black hole-to-stellar mass ratios and quenched later than other SRs, leaving imprints on their $z=0$ chemical composition. For the vast majority of SRs we find that quenching, driven by active galactic nuclei feedback, precedes kinematic transformation, except for satellite SRs, in which these processes happen in tandem. However, in $\approx 50$\% of these satellites, satellite-satellite mergers are responsible for their SR fate, while environment (i.e. tidal field and interactions with the central) can account for the transformation in the rest. By splitting SRs into kinematic sub-classes, we find that flat SRs prefer major mergers; round SRs prefer minor or very minor mergers; prolate SRs prefer gas-poor mergers. Flat and prolate SRs are more common among satellites hosted by massive halos ($>10^{13.6}\,\rm M_{\odot}$) and centrals of high masses ($M_{\star} > 10^{10.5}\, \rm M_{\odot}$). Although EAGLE galaxies display kinematic properties that broadly agree with observations, there are areas of disagreement, such as inverted stellar age and velocity dispersion profiles. We discuss these and how upcoming simulations can solve them.

We study the shapes of spatially integrated H I emission line profiles of galaxies in the EAGLE simulation using three separate measures of the profile's asymmetry. We show that the subset of EAGLE galaxies whose gas fractions and stellar masses are consistent with those in the xGASS survey also have similar H I line asymmetries. Central galaxies with symmetric H I line profiles typically correspond to rotationally supported H I and stellar disks, but those with asymmetric line profiles may or may not correspond to dispersion-dominated systems. Galaxies with symmetric H I emission lines are, on average, more gas rich than those with asymmetric lines, and also exhibit systematic differences in their specific star formation rates, suggesting that turbulence generated by stellar or AGN feedback may be one factor contributing to H I line asymmetry. The line asymmetry also correlates strongly with the dynamical state of a galaxy's host dark matter halo: older, more relaxed haloes host more-symmetric galaxies than those hosted by unrelaxed ones. At fixed halo mass, asymmetric centrals tend to be surrounded by a larger number of massive subhaloes than their symmetric counterparts, and also experience higher rates of gas accretion and outflow. At fixed stellar mass, central galaxies have, on average, more symmetric H I emission lines than satellites; for the latter, ram pressure and tidal stripping are significant sources of asymmetry.

ABSTRACT Using the eagle (Evolution and Assembly of GaLaxies and their Environments) suite of simulations, we demonstrate that both cold gas stripping and starvation of gas inflow play an important role in quenching satellite galaxies across a range of stellar and halo masses, M⋆ and M200. Quantifying the balance between gas inflows, outflows, and star formation rates, we show that even at z = 2, only $\approx 30{{\ \rm per\ cent}}$ of satellite galaxies are able to maintain equilibrium or grow their reservoir of cool gas – compared to $\approx 50{{\ \rm per\ cent}}$ of central galaxies at this redshift. We find that the number of orbits completed by a satellite on first-infall to a group environment is a very good predictor of its quenching, even more so than the time since infall. On average, we show that intermediate-mass satellites with M⋆ between will be quenched at $10^{9}\, {\rm M}_{\odot }\, {\rm and}\, 10^{10}\, {\rm M}_{\odot }$ first pericenter in massive group environments, $M_{200}\gt 10^{13.5}\, {\rm M}_{\odot }$; and will be quenched at second pericenter in less massive group environments, $M_{200}\lt 10^{13.5}\, {\rm M}_{\odot }$. On average, more massive satellites ($M_{\star }\gt 10^{10}\, {\rm M}_{\odot }$) experience longer depletion time-scales, being quenched between first and second pericenters in massive groups, while in smaller group environments, just $\approx 30{{\ \rm per\ cent}}$ will be quenched even after two orbits. Our results suggest that while starvation alone may be enough to slowly quench satellite galaxies, direct gas stripping, particularly at pericenters, is required to produce the short quenching time-scales exhibited in the simulation.

ABSTRACT The inflow of cosmological gas on to haloes, while challenging to directly observe and quantify, plays a fundamental role in the baryon cycle of galaxies. Using the eagle suite of hydrodynamical simulations, we present a thorough exploration of the physical properties of gas accreting on to haloes – namely, its spatial characteristics, density, temperature, and metallicity. Classifying accretion as ‘hot’ or ‘ cold’ based on a temperature cut-off 105.5 K, we find that the covering fraction (fcov) of cold-mode accreting gas is significantly lower than the hot-mode, with z = 0 fcov values of ${\approx}50{{\ \rm per\ cent}}$ and ${\approx}80{{\ \rm per\ cent}}$, respectively. Active galactic nucleus (AGN) feedback in eagle reduces inflow fcov values by ${\approx}10{{\ \rm per\ cent}}$, with outflows decreasing the solid angle available for accretion flows. Classifying inflow by particle history, we find that gas on first-infall on to a halo is metal depleted by ≈2 dex compared to pre-processed gas, which we find to mimic the circum-galactic medium (CGM) in terms of metal content. We also show that high (low) halo-scale gas accretion rates are associated with metal-poor (rich) CGM in haloes below $10^{12}\, \mathrm{M}_{\odot }$, and that variation in halo-scale gas accretion rates may offer a physical explanation for the enhanced scatter in the star-forming main sequence at low (${\lesssim}10^{9}\, \mathrm{M}_{\odot }$) and high (${\gtrsim}10^{10}\, \mathrm{M}_{\odot }$) stellar masses. Our results highlight how gas inflow influences several halo- and galaxy-scale properties, and the need to combine kinematic and chemical data in order to confidently break the degeneracy between accreting and outgoing gas in CGM observations.

ABSTRACT We use the eagle suite of hydrodynamical simulations to analyse accretion rates (and the breakdown of their constituent channels) on to haloes over cosmic time, comparing the behaviour of baryons and dark matter (DM). We also investigate the influence of sub-grid baryon physics on halo-scale inflow, specifically the consequences of modelling radiative cooling, as well as feedback from stars and active galactic nuclei (AGNs). We find that variations in halo baryon fractions at fixed mass (particularly their circumgalactic medium gas content) are very well correlated with variations in the baryon fraction of accreting matter, which we show to be heavily suppressed by stellar feedback in low-mass haloes, Mhalo ≲ 1011.5 M⊙. Breaking down accretion rates into first infall, recycled, transfer, and merger components, we show that baryons are much more likely to be smoothly accreted than to have originated from mergers when compared to DM, finding (averaged across halo mass) a merger contribution of $\approx 6{{\ \rm per\ cent}}$ for baryons, and $\approx 15{{\ \rm per\ cent}}$ for DM at z ≈ 0. We also show that the breakdown of inflow into different channels is strongly dependent on sub-grid physics, particularly the contribution of recycled accretion (accreting matter that has been previously ejected from progenitor haloes). Our findings highlight the dual role that baryonic feedback plays in regulating the evolution of galaxies and haloes: by (i) directly removing gas from haloes, and (ii) suppressing gas inflow to haloes.

We use the EAGLE simulations to study the connection between the quenching time-scale, tau(Q), and the physical mechanisms that transform star-forming galaxies into passive galaxies. By quantifying tau(Q) in two complementary ways - as the time over which (i) galaxies traverse the green valley on the colour-mass diagram, or (ii) leave the main sequence of star formation and subsequently arrive on the passive cloud in specific star formation rate (SSFR)-mass space - we hod that the tau(Q) distribution of high-mass centrals, low-mass centrals, and satellites are divergent. In the low stellar mass regime where M-star < 10(9.6) M-circle dot, centrals exhibit systematically longer quenching time-scales than satellites (approximate to 4 Gyr compared to approximate to 2 Gyr). Satellites with low stellar mass relative to their halo mass cause this disparity, with ram pressure stripping quenching these galaxies rapidly. Low-mass centrals are quenched as a result of stellar feedback, associated with long tau(Q) greater than or similar to 3 Gyr. At intermediate stellar masses where 10(9.7) < M-star < 10(10.3) M-circle dot, tau(Q) are the longest for both centrals and satellites, particularly for galaxies with higher gas fractions. At M-star greater than or similar to 10(10.3) M-circle dot, galaxy merger counts and black hole activity increase steeply for all galaxies. Quenching time-scales for centrals and satellites decrease with stellar mass in this regime to tau(Q) less than or similar to 2 Gyr. In anticipation of new intermediate redshift observational galaxy surveys, we analyse the passive and star-forming fractions of galaxies across redshift, and find that the tau(Q) peak at intermediate stellar masses is responsible for a peak (inflection point) in the fraction of green valley central (satellite) galaxies at z approximate to 0.5-0.7.