Evolution And Assembly Of GaLaxies And Their Environments Research Articles

How do baryonic effects on the cosmic matter distribution vary with scale and local density environment?

ABSTRACT In this study, we investigate how the baryonic effects vary with scale and local density environment mainly by utilizing a novel statistic, the environment-dependent wavelet power spectrum (env-WPS). With four state-of-the-art cosmological simulation suites, EAGLE (Evolution and Assembly of GaLaxies and their Environments), SIMBA, Illustris, and IllustrisTNG, we compare the env-WPS of the total matter density field between the hydrodynamic and dark matter-only runs at z = 0. We find that the clustering is most strongly suppressed in the emptiest environment of $\rho _\mathrm{m}/\bar{\rho }_\mathrm{m} \ \lt \ 0.1$ with maximum amplitudes ∼67–89 per cent on scales ∼1.86–10.96 hMpc−1, and less suppressed in higher density environments on small scales (except Illustris). In the environments of $\rho _\mathrm{m}/\bar{\rho }_\mathrm{m}\geqslant 0.316$ (≥10 in EAGLE), the feedbacks also lead to enhancement features at intermediate and large scales, which is most pronounced in the densest environment of $\rho _\mathrm{m}/\bar{\rho }_\mathrm{m}\geqslant 100$ and reaches a maximum ∼7–15 per cent on scales ∼0.87–2.62 hMpc−1 (except Illustris). The baryon fraction of the local environment decreases with increasing density, denoting the feedback strength, and potentially explaining some differences between simulations. We also measure the volume and mass fractions of local environments, which are affected by ≳1 per cent due to baryon physics. In conclusion, our results show that the baryonic processes can strongly modify the overall cosmic structure on the scales of k > 0.1 hMpc−1, which encourages further research in this direction.

The dust attenuation scaling relation of star-forming galaxies in the eagle simulations

ABSTRACT Dust attenuation in star-forming galaxies (SFGs), as parametrized by the infrared excess (IRX ≡ LIR/LUV), is found to be tightly correlated with star formation rate, metallicity, and galaxy size, following a universal IRX relation up to z = 3. This scaling relation can provide a fundamental constraint for theoretical models to reconcile galaxy star formation, chemical enrichment, and structural evolution across cosmic time. We attempt to reproduce the universal IRX relation over 0.1 ≤ z ≤ 2.5 using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) hydrodynamical simulations and examine sensitive parameters in determining galaxy dust attenuation. Our findings show that while the predicted universal IRX relation from EAGLE approximately aligns with observations at z ≤ 0.5, noticeable disparities arise at different stellar masses and higher redshifts. Specifically, we investigate how modifying various galaxy parameters can affect the predicted universal IRX relation in comparison to the observed data. We demonstrate that the simulated gas-phase metallicity is the critical quantity for the shape of the predicted universal IRX relation. We find that the influence of the infrared luminosity and infrared excess is less important while galaxy size has virtually no significant effect. Overall, the EAGLE simulations are not able to replicate some of the observed characteristics between IRX and galaxy parameters of SFGs, emphasizing the need for further investigation and testing for our current state-of-the-art theoretical models.

A Comprehensive Study on the Relation between the Metal Enrichment of Ionized and Atomic Gas in Star-forming Galaxies

We study the relation between the metallicities of ionized and atomic gas in star-forming galaxies at z = 0–3 using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological, hydrodynamical simulations. This is done by constructing a dense grid of sight lines through the simulated galaxies and obtaining the star formation rate- and H i column density-weighted metallicities, Z SFR and Z H I, for each sightline as proxies for the metallicities of ionized and atomic gas, respectively. We find Z SFR ≳ Z H I for almost all sight lines, with their difference generally increasing with decreasing metallicity. The stellar masses of galaxies do not have a significant effect on this trend, but the positions of the sight lines with respect to the galaxy centers play an important role: the difference between the two metallicities decreases when moving toward the galaxy centers, and saturates to a minimum value in the central regions of galaxies, irrespective of redshift and stellar mass. This implies that the mixing of the two gas phases is most efficient in the central regions of galaxies where sight lines generally have high column densities of H i. However, a high H i column density alone does not guarantee a small difference between the two metallicities. In galaxy outskirts, the inefficiency of the mixing of star-forming gas with H i seems to dominate over the dilution of heavy elements in H i through mixing with the pristine gas. We find good agreement between the available observational data and the Z SFR–Z H I relation predicted by the EAGLE simulations. Though, observed regions with a nuclear starburst mode of star formation appear not to follow the same relation.

Shape, alignment, and mass distribution of baryonic and dark-matter halos in one EAGLE simulation

Context. Accurate knowledge of the morphology of halos and its evolution are key constraints on the galaxy formation model as well as a determinant parameter of the strong-lensing phenomenon. Large-scale cosmic simulations are a tailored tool used to obtain statistics on the shape and mass distributions of these halos according to redshift. Aims. Using the cosmological hydrodynamic simulation, the Evolution and Assembly of GaLaxies and their Environments (EAGLE), we aim to provide a comprehensive analysis of the evolution of the morphology of galaxy halos and of their mass distributions with a focus on the snapshot at redshift z = 0.5. Methods. We developed an iterative strategy involving a principal component analysis (PCA) to investigate the properties of the EAGLE halos and the differences in alignment between the various components. The semi-axes and orientation of the halos are estimated taking into account sub-halos in the simulation. The mass distributions of the dark-matter (DM), gas, and star halos are characterised by a half-mass radius, a concentration parameter and (projected) axis ratios. Results. We present statistics of the shape parameters of 336 540 halos from the EAGLE RefL0025N0376 simulation and describe their evolution from redshift z = 15 to z = 0. We measured the three-dimensional shape parameters for the DM, the gas, and the star components as well as for all particles. We also measured these parameters for two-dimensional projected distributions. At z = 0.5, the minor axis of gas aligns with the minor axis of DM for massive halos (M > 1012 M⊙), but this alignment is poorer for less massive halos. The DM halos axis ratios b/a and c/a have median values of 0.82 ± 0.11 and 0.64 ± 0.12, respectively. The gas in halos that also contain stars has a more flattened shape, with b/a = 0.70 ± 0.19 and c/a = 0.38 ± 0.20. The sphericity of gas in halos w/ and w/o stars appears to be negatively correlated to the total mass, while the sphericity of DM is insensitive to it. The measured projected axis ratios, bp/ap, of star halos at z = 0.5 have a median value of 0.80 ± 0.07, which is in good agreement with ground-based and space-based measurements within 1σ. For DM halos, we measure a value of 0.85 ± 0.06. The evolution of the concentration as a function of redshift is fairly homogeneous for the various components, except for the starless gas halos, which appear much more concentrated for z > 0.7.

The volume density of giant low surface brightness galaxies

ABSTRACT Rare giant low surface brightness galaxies (gLSBGs) act as a stress test for the current galaxy formation paradigm. To answer the question ‘How rare are they?’, we estimate their volume density in the local Universe. A visual inspection of 120 deg2 covered by deep Subaru Hyper Suprime-Cam data was performed independently by four team members. We detected 42 giant disky systems (30 of them isolated) at z ≤ 0.1 with either g-band 27.7 mag arcsec−2 isophotal radius or four disc scale lengths 4h ≥ 50 kpc, 37 of which (including 25 isolated) had low central surface brightness (μ0,g ≥ 22.7 mag arcsec−2). This corresponds to volume densities of 4.70 × 10−5 Mpc−3 for all galaxies with giant extended discs and 4.04 × 10−5 Mpc−3 for gLSBGs, which converts to ∼12 700 such galaxies in the entire sky out to z < 0.1. These estimates agree well with the result of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) cosmological hydrodynamical simulation. Giant disky galaxies represent the large-sized end of the volume density distribution of normal-sized spirals, suggesting the non-exceptional nature of giant discs. We observe a high active galactic nucleus fraction among the newly found gLSBGs.

The impact of galactic feedback on the shapes of dark matter haloes

ABSTRACT We quantify the impact of galaxy formation on dark matter halo shapes using cosmological simulations at redshift z = 0. Using magnetohydrodynamic simulations from the IllustrisTNG project, we focus on haloes of mass $10^{10\!-\!14} \, \rm M_{\odot }$ from the 50 Mpc (TNG50) and 100 Mpc (TNG100) boxes and compare them to dark matter-only (DMO) analogues and other simulations, e.g. Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) and Evolution and Assembly of GaLaxies and their Environments (EAGLE). We further quantify the prediction uncertainty by varying the feedback models using smaller 25 ${\rm Mpc}\, h^{-1}$ boxes. We find that (i) galaxy formation results in rounder haloes compared to DMO simulations, in qualitative agreement with past results. Haloes of mass ${\approx }2\times 10^{12} \, \rm M_{\odot }$ are most spherical, with an average minor-to-major axial ratio of $\langle s \rangle$ ≈ 0.75 in the inner halo, an increase of 40 per cent compared to their DMO counterparts. No significant difference is present for low-mass $10^{10} \, \rm M_{\odot }$ haloes; (ii) stronger feedback, e.g. increasing galactic wind speed, reduces the impact of baryons; (iii) the inner halo shape correlates with the stellar mass fraction, explaining the dependence of halo shapes on feedback models; and (iv) the fiducial and weaker feedback models are most consistent with observational estimates of the Milky Way halo shape. At fixed halo mass, very diverse and possibly unrealistic feedback models all predict inner shapes closer to one another than to the DMO results. Because of the large halo-to-halo variation in halo shape, a larger observational sample is required to statistically distinguish different baryonic prescriptions.

A deep learning approach to halo merger tree construction

ABSTRACT A key ingredient for semi-analytic models of galaxy formation is the mass assembly history of haloes, encoded in a tree structure. The most commonly used method to construct halo merger histories is based on the outcomes of high-resolution, computationally intensive N-body simulations. We show that machine learning (ML) techniques, in particular Generative Adversarial Networks (GANs), are a promising new tool to tackle this problem with a modest computational cost and retaining the best features of merger trees from simulations. We train our GAN model with a limited sample of merger trees from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation suite, constructed using two halo finders–tree builder algorithms: SUBFIND – D-TREES and ROCKSTAR – ConsistentTrees. Our GAN model successfully learns to generate well-constructed merger tree structures with high temporal resolution, and to reproduce the statistical features of the sample of merger trees used for training, when considering up to three variables in the training process. These inputs, whose representations are also learned by our GAN model, are mass of the halo progenitors and the final descendant, progenitor type (main halo or satellite), and distance of a progenitor to that in the main branch. The inclusion of the latter two inputs greatly improves the final learned representation of the halo mass growth history, especially for SUBFIND-like ML trees. When comparing equally sized samples of ML merger trees with those of the EAGLE simulation, we find better agreement for SUBFIND-like ML trees. Finally, our GAN-based framework can be utilized to construct merger histories of low- and intermediate-mass haloes, the most abundant in cosmological simulations.

The age gradients of galaxies in EAGLE: outside-in quenching as the origin of young bulges in cluster galaxies

ABSTRACT Many disc galaxies in clusters have been found with bulges of similar age or younger than their surrounding discs, at odds with field galaxies of similar morphology and their expected inside-out formation. We use the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to test potential origins for this difference in field and cluster galaxies. We find, in agreement with observations, that on average disc-dominated field galaxies in the simulations have older inner regions, while similar galaxies in groups and clusters have similarly aged or younger inner regions. This environmental difference is a result of outside-in quenching of the cluster galaxies. Prior to group/cluster infall, galaxies of a given present-day mass and morphology exhibit a similar evolution in their specific star formation rate (sSFR) profiles. Post-infall, the outer sSFRs of group and cluster galaxies significantly decrease due to interstellar medium stripping, while the central sSFR remains similar to field galaxies. Field disc galaxies instead generally retain radially increasing sSFR profiles. Thus, field galaxies continue to develop negative age gradients (younger discs), while cluster galaxies instead develop positive age gradients (younger bulges).

A Fast and Accurate Analytic Method of Calculating Galaxy Two-point Correlation Functions

Abstract We have developed a new analytic method to calculate the galaxy two-point correlation functions accurately and efficiently, applicable to surveys with finite, regular, and mask-free geometries. We have derived simple, accurate formulas of the normalized random–random pair counts RR as functions of the survey area dimensions. We have also suggested algorithms to compute the normalized data-random pair counts DR analytically. With all edge corrections fully accounted for analytically, our method computes RR and DR with perfect accuracy and zero variance in O(1) and O(N g) time, respectively. We test our method on a galaxy catalog from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation. Our method calculates RR + DR at a speed 3–6 orders of magnitude faster than the brute-force Monte Carlo method and 2.5 orders of magnitude faster than tree-based algorithms. For a galaxy catalog with 10 million data points in a cube, this reduces the computation time to under 1 minute on a laptop. Our analytic method is favored over the traditional Monte Carlo method whenever applicable. Some applications in the study of correlation functions and power spectra in cosmological simulations and galaxy surveys are discussed. However, we recognize that its applicability is very limited for realistic surveys with masks, irregular shapes, and/or weighted patterns.

The SAMI Galaxy Survey: Detection of Environmental Dependence of Galaxy Spin in Observations and Simulations Using Marked Correlation Functions

Abstract The existence of a kinematic morphology–density relation remains uncertain, and instead stellar mass appears the more dominant driver of galaxy kinematics. We investigate the dependence of the stellar spin parameter proxy λ R e on environment using a marked cross-correlation method with data from the Sydney Australian Astronomical Observatory Multi-object Integral-field Spectrograph (SAMI) Galaxy Survey. Our sample contains 710 galaxies with spatially resolved stellar velocity and velocity dispersion measurements. By utilizing the highly complete spectroscopic data from the Galaxy and Mass Assembly Survey, we calculate marked cross-correlation functions for SAMI galaxies using a pair count estimator and marks based on stellar mass and λ R e . We detect an anticorrelation of stellar kinematics with environment at the 3.2σ level, such that galaxies with low λ R e values are preferably located in denser galaxy environments. However, a significant correlation between stellar mass and environment is also found (correlation at 2.4σ), as found in previous works. We compare these results to mock observations from the cosmological Evolution and Assembly of Galaxies and their Environments (EAGLE) simulations, where we find a similar significant λ R e anticorrelation with environment, and a mass and environment correlation. We demonstrate that the environmental correlation of λ R e is not caused by the mass–environment relation. The significant relationship between λ R e and environment remains when we exclude slow rotators. The signals in SAMI and EAGLE are strongest on small scales (10–100 kpc) as expected from galaxy interactions and mergers. Our work demonstrates that the technique of marked correlation functions is an effective tool for detecting the relationship between λ R e and environment.

The survival of globular clusters in a cuspy Fornax

ABSTRACT It has long been argued that the globular clusters (GCs) in the Fornax dwarf galaxy indicate that its dark matter halo is likely to have a shallow density profile with a core of size ∼1 kpc. We revisit this argument by investigating analogues of Fornax formed in MOdelling Star cluster population Assembly In Cosmological Simulations within eagle (E-MOSAICS), a cosmological hydrodynamical simulation that follows the formation and evolution of GCs in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) galaxy formation model. In eagle, Fornax-mass haloes are cuspy and well described by the Navarro–Frenk–White profile. We post-process the E-MOSAICS to account for GC orbital decay by dynamical friction, which is not included in the original model. Dynamical friction causes 33 per cent of GCs with masses $M_{\rm GC}\ge 4\times 10^4{~\rm M_\odot }$ to sink to the centre of their host with the majority being tidally disrupted before forming a nuclear star cluster. Fornax has a total of five GCs, an exceptionally large number compared to other galaxies of similar stellar mass. In the simulations, we find that only 3 per cent of the Fornax analogues have five or more GCs, while 30 per cent have only one and 35 per cent have none. We find that GC systems in satellites are more centrally concentrated than in field dwarfs, and that those formed in situ (45 per cent) are more concentrated than those that were accreted. The present-day radial distribution of GCs in E-MOSAICS Fornax analogues is indistinguishable from that in Fornax, demonstrating that the presence of five GCs in the central kiloparsec of Fornax is consistent with a cuspy dark matter halo.

The cosmic dispersion measure in the EAGLE simulations

ABSTRACT The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique way to probe ionized baryons in the intergalactic medium (IGM). Cosmological models with different parameters lead to different DM–redshift (DM–z) relations. Additionally, the over/underdense regions in the IGM and the circumgalactic medium of intervening galaxies lead to scatter around the mean DM–z relations. We have used the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to measure the mean DM–z relation and the scatter around it using over 1 billion lines of sight at redshifts 0 < z < 3. We investigated two techniques to estimate line-of-sight DM: pixel scrambling and box transformations. We find that using box transformations (a technique from the literature) causes strong correlations due to repeated replication of structure. Comparing a linear and a non-linear model, we find that the non-linear model with a dependence on cosmological parameters provides a better fit to the DM–z relation. The differences between these models are the most significant at low redshifts (z < 0.5). The scatter around the DM–z relation is highly asymmetric, especially at low redshift (z < 0.5), and becomes more Gaussiana as redshift approaches z = 3, the limit of this study. The increase in Gaussianity with redshift is indicative of the large-scale structure that is better sampled with longer lines of sight. The DM–z relation measured in EAGLE is available with an easy-to-use python interface in the open-source FRB redshift estimation package fruitbat.

No Dependence of Radio Properties of Brightest Group Galaxies on the Luminosity Gap

Abstract We study the radio and optical properties of the brightest group galaxies (BGGs) in a sample of galaxy groups from the Sloan Digital Sky Survey (SDSS) DR7. The luminosity difference between the BGG and the second-ranked galaxy in the group (known as the luminosity, or magnitude, gap) has been used as a probe for the level of galaxy interaction for the BGG within the group. We study the properties of BGGs with magnitude gaps in the range of 0–2.7 mag, in order to investigate any relation between the luminosity gap and the radio properties of the BGG. In order to eliminate selection biases, we ensure that all variations in stellar mass are accounted for. We then confirm that, at fixed stellar mass, there are no significant variations in the optical properties of the BGGs over the full range of luminosity gaps studied. We compare these optical results with the Evolution and Assembly of GaLaxies and their Environments (EAGLE) hydrodynamical simulations and find broad consistency with the observational data. Using EAGLE we also confirm that no trends begin to arise in the simulated data at luminosity gaps beyond our observational limits. Finally, we find that, at a fixed stellar mass, the fraction of BGGs that are radio-loud also shows no trend as a function of luminosity gap. We examine how the BGG offset from the center of the group may affect the radio results and find no significant trend for the fraction of radio-loud BGGs with a magnitude gap in either the BGG samples with greater or less than 100 kpc offset from the center of the group.

SEAGLE – II. Constraints on feedback models in galaxy formation from massive early-type strong-lens galaxies

ABSTRACT We use nine different galaxy formation scenarios in ten cosmological simulation boxes from the EAGLE (Evolution and Assembly of GaLaxies and their Environments) suite of Lambda cold dark matter hydrodynamical simulations to assess the impact of feedback mechanisms in galaxy formation and compare these to observed strong gravitational lenses. To compare observations with simulations, we create strong lenses with M* > 1011 M⊙ with the appropriate resolution and noise level, and model them with an elliptical power-law mass model to constrain their total mass density slope. We also obtain the mass–size relation of the simulated lens-galaxy sample. We find significant variation in the total mass density slope at the Einstein radius and in the projected stellar mass–size relation, mainly due to different implementations of stellar and active galactic nucleus (AGN) feedback. We find that for lens-selected galaxies, models with either too weak or too strong stellar and/or AGN feedback fail to explain the distribution of observed mass density slopes, with the counter-intuitive trend that increasing the feedback steepens the mass density slope around the Einstein radius (≈3–10 kpc). Models in which stellar feedback becomes inefficient at high gas densities, or weaker AGN feedback with a higher duty cycle, produce strong lenses with total mass density slopes close to isothermal [i.e. −dlog (ρ)/dlog (r) ≈ 2.0] and slope distributions statistically agreeing with observed strong-lens galaxies in Sloan Lens ACS Survey and BOSS (Baryon Oscillation Spectroscopic Survey) Emission-Line Lens Survey. Agreement is only slightly worse with the more heterogeneous Strong Lensing Legacy Survey lens-galaxy sample. Observations of strong-lens-selected galaxies thus appear to favour models with relatively weak feedback in massive galaxies.

Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance

ABSTRACT The observed stellar initial mass function (IMF) appears to vary, becoming bottom-heavy in the centres of the most massive, metal-rich early-type galaxies. It is still unclear what physical processes might cause this IMF variation. In this paper, we demonstrate that the abundance of deuterium in the birth clouds of forming stars may be important in setting the IMF. We use models of disc accretion on to low-mass protostars to show that those forming from deuterium-poor gas are expected to have zero-age main-sequence masses significantly lower than those forming from primordial (high deuterium fraction) material. This deuterium abundance effect depends on stellar mass in our simple models, such that the resulting IMF would become bottom-heavy – as seen in observations. Stellar mass loss is entirely deuterium free and is important in fuelling star formation across cosmic time. Using the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation we show that stellar mass-loss-induced deuterium variations are strongest in the same regions where IMF variations are observed: at the centres of the most massive, metal-rich, passive galaxies. While our analysis cannot prove that the deuterium abundance is the root cause of the observed IMF variation, it sets the stage for future theoretical and observational attempts to study this possibility.

The star formation properties of the observed and simulated AGN Universe: BAT versus EAGLE

ABSTRACT In this paper, we present data from 72 low-redshift, hard X-ray selected active galactic nucleus (AGN) taken from the Swift–BAT 58 month catalogue. We utilize spectral energy distribution fitting to the optical to infrared photometry in order to estimate host galaxy properties. We compare this observational sample to a volume- and flux-matched sample of AGN from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) hydrodynamical simulations in order to verify how accurately the simulations can reproduce observed AGN host galaxy properties. After correcting for the known +0.2 dex offset in the SFRs between EAGLE and previous observations, we find agreement in the star formation rate (SFR) and X-ray luminosity distributions; however, we find that the stellar masses in EAGLE are 0.2–0.4 dex greater than the observational sample, which consequently leads to lower specific star formation rates (sSFRs). We compare these results to our previous study at high redshift, finding agreement in both the observations and simulations, whereby the widths of sSFR distributions are similar (∼0.4–0.6 dex) and the median of the SFR distributions lie below the star-forming main sequence by ∼0.3–0.5 dex across all samples. We also use EAGLE to select a sample of AGN host galaxies at high and low redshift and follow their characteristic evolution from z = 8 to z = 0. We find similar behaviour between these two samples, whereby star formation is quenched when the black hole goes through its phase of most rapid growth. Utilizing EAGLE we find that 23 per cent of AGN selected at z ∼ 0 are also AGN at high redshift, and that their host galaxies are among the most massive objects in the simulation. Overall, we find EAGLE reproduces the observations well, with some minor inconsistencies (∼0.2 dex in stellar masses and ∼0.4 dex in sSFRs).

Galaxy properties in the cosmic web of EAGLE simulation

ABSTRACT We investigate the dependence of the galaxy properties on cosmic web environments using the most up-to-date hydrodynamic simulation: Evolution and Assembly of Galaxies and their Environments (EAGLE). The baryon fractions in haloes and the amplitudes of the galaxy luminosity function decrease going from knots to filaments to sheets to voids. Interestingly, the value of L* varies dramatically in different cosmic web environments. At z = 0, we find a characteristic halo mass of $10^{12}\, {\rm h}^{-1}\rm M_{\odot }$, below which the stellar-to-halo mass ratio is higher in knots, while above which it reverses. This particular halo mass corresponds to a characteristic stellar mass of $1.8\times 10^{10} \,{\rm h}^{-1}\rm M_{\odot }$. Below the characteristic stellar mass, central galaxies have redder colours, lower sSFRs, and higher metallicities in knots than those in filaments, sheets and voids, while above this characteristic stellar mass, the cosmic web environmental dependences either reverse or vanish. Such dependences can be attributed to the fact that the active galaxy fraction decreases along voids, sheets, filaments, and knots. The cosmic web dependences get weaker towards higher redshifts for most of the explored galaxy properties and scaling relations, except for the gas metallicity versus stellar mass relation.

The warm-hot circumgalactic medium around EAGLE-simulation galaxies and its detection prospects with X-ray and UV line absorption

ABSTRACT We use the EAGLE (Evolution and Assembly of GaLaxies and their Environments) cosmological simulation to study the distribution of baryons, and far-ultraviolet (O vi), extreme-ultraviolet (Ne viii), and X-ray (O vii, O viii, Ne ix, and Fe xvii) line absorbers, around galaxies and haloes of mass $\,{M}_{\rm {200c}}= 10^{11}$–$10^{14.5} \, \rm {M}_{\odot}$ at redshift 0.1. EAGLE predicts that the circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards temperatures corresponding to the collisional ionization peak for each ion, and towards high metallicities. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O vi, photoionization becomes significant. When presenting observables, we work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Virial-temperature collisional ionization equilibrium ion fractions are good predictors of column density trends with halo mass, but underestimate the diversity of ions in haloes. Halo gas dominates the highest column density absorption for X-ray lines, but lower density gas contributes to strong UV absorption lines from O vi and Ne viii. Of the O vii (O viii) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $\,{M}_{\rm {200c}}= 10^{12.0}{-}10^{13.5} \, \rm {M}_{\odot}$. We predict that the X-IFU will detect O vii (O viii) in 77 (46) per cent of the sightlines passing $\,{M}_{\star }= 10^{10.5}{-}10^{11.0} \, \rm {M}_{\odot}$ galaxies within $100 \, \rm {pkpc}$ (59 (82) per cent for $\,{M}_{\star }\gt 10^{11.0} \, \rm {M}_{\odot}$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O vi.

An EAGLE’s view of ex situ galaxy growth

ABSTRACT Modern observational and analytical techniques now enable the direct measurement of star formation histories and the inference of galaxy assembly histories. However, current theoretical predictions of assembly are not ideally suited for direct comparison with such observational data. We therefore extend the work of prior examinations of the contribution of ex situ stars to the stellar mass budget of simulated galaxies. Our predictions are specifically tailored for direct testing with a new generation of observational techniques by calculating ex situ fractions as functions of galaxy mass and morphological type, for a range of surface brightnesses. These enable comparison with results from large field of view (FoV) Integral Field Unit (IFU) spectrographs, and increasingly accurate spectral fitting, providing a look-up method for the estimated accreted fraction. We furthermore provide predictions of ex situ mass fractions as functions of galaxy mass, galactocentric radius, and environment. Using z = 0 snapshots from the 100 and 25 cMpc3 EAGLE (Evolution and Assembly of GaLaxies and their Environments) simulations, we corroborate the findings of prior studies, finding that ex situ fraction increases with stellar mass for central and satellite galaxies in a stellar mass range of 2 × 107 to 1.9 × 1012 M⊙. For those galaxies of mass M* > 5 × 108 M⊙, we find that the total ex situ mass fraction is greater for more extended galaxies at fixed mass. When categorizing satellite galaxies by their parent group/cluster halo mass, we find that the ex situ fraction decreases with increasing parent halo mass at fixed galaxy mass. This apparently counterintuitive result may be due to high passing velocities within large cluster haloes inhibiting efficient accretion on to individual galaxies.

Environment from cross-correlations: connecting hot gas and the quenching of galaxies

ABSTRACT The observable properties of galaxies depend on both internal processes and the external environment. In terms of the environmental role, we still do not have a clear picture of the processes driving the transformation of galaxies. The use of proxies for environment (e.g. host halo mass, distance to the Nth nearest neighbour, etc.), as opposed to the real physical conditions (e.g. hot-gas density) may bear some responsibility for this. Here, we propose a new method that directly links galaxies to their local environments, by using spatial cross-correlations of galaxy catalogues with maps from large-scale structure surveys [e.g. thermal Sunyaev–Zel’dovich (tSZ) effect, diffuse X-ray emission, weak lensing of galaxies, or the cosmic microwave background (CMB)]. We focus here on the quenching of galaxies and its link to local hot gas properties. Maps of galaxy overdensity and quenched fraction excess are constructed from volume-limited Sloan Digital Sky Survey (SDSS) catalogues, which are cross-correlated with tSZ effect and X-ray maps from Planck and ROSAT, respectively. Strong signals out to Mpc scales are detected for most cross-correlations and are compared to predictions from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) and BAryons and Haloes of MAssive Systems (BAHAMAS) cosmological hydrodynamical simulations. The simulations successfully reproduce many, but not all, of the observed power spectra, with an indication that environmental quenching may be too efficient in the simulations. We demonstrate that the cross-correlations are sensitive to both the internal [e.g. active galactic nucleus (AGN) and stellar feedback] and external processes (e.g. ram pressure stripping, harassment, strangulation, etc.) responsible for quenching. The methods outlined in this paper can be adapted to other observables and, with upcoming surveys, will provide a stringent test of physical models for environmental transformation.