Elmo Tempel

We present measurements of galaxy spin-filament alignments at 0.20 < z < 0.42 using the MAGPI Survey, exploiting resolved stellar and ionised gas kinematics to reconstruct 3D spin axes. Cosmic filaments are identified using a stellar mass-matched constant-number-density population from the GAMA, MAGPI Redshift Survey and DESI datasets. We detect a flipping of the statistical alignment signal at z ∼ 0.3 as a function of M⋆: low-mass galaxies preferentially exhibit spins parallel to the direction of their host filaments, while high-mass galaxies tend towards perpendicular orientations. The transition mass is measured to be log (M⋆, t/M⊙) ≃ 10.8, approximately 0.6 dex higher than the value for the SAMI Galaxy Survey at z ∼ 0.05. Bootstrap analysis gives 99.5% confidence that the transition mass is higher for MAGPI than SAMI. At fixed stellar mass MAGPI galaxies exhibit stronger parallel alignments than their SAMI counterparts, suggesting evolution of spin–filament alignments over the past ∼ 3 Gyr. The gaseous component in MAGPI galaxies is more closely aligned with filaments than the stellar component, relative to a M⋆-matched SAMI sample. This indicates a stronger coupling between ionised gas and the large-scale structure at z ∼ 0.3, consistent with coherent gas accretion onto filaments remaining efficient at higher M⋆. At z ∼ 0 we detect a transition toward perpendicular alignments at lower M⋆, while the gas and stellar components exhibit similar alignments relative to filaments, likely due to the cumulative effects of mergers and secular evolution.

Non-gravitational processes that expel hot gas beyond the virial regions of groups and clusters of galaxies, known collectively as baryonic feedback, play a key role in reshaping the matter distribution of the Universe on megaparsec scales. Tracing the baryonic content of galaxy groups outside R ₅₀₀ provides a unique opportunity to constrain the modification of the matter distribution on these scales. Our aim is to quantify the baryon budget at the virial radii of galaxy groups and constrain the effect of active galactic nuclei feedback in redistributing matter on the largest scales ever probed with X-ray observations. We used eROSITA observations of a complete sample of 25 galaxy groups selected from the first public release of the eROSITA-DE data (eRASS1) and identified with the Two Micron Redshift Survey optical group catalogue (2MRS). We extracted and fitted surface brightness (SBx) profiles and present hot gas mass and hot gas fraction profiles out to R ₂₀₀ . We explored the physical properties of our sample by performing a Bayesian analysis of M _gas − M _tot , L _X − M _tot , and L _X − M _gas relations, taking into account the aperture covariance effects. At R ₅₀₀ , we report uniformly flat SBx profiles with a mean β parameter of 0.38 ± 0.04, steepening to β = 0.76 ± 0.19 beyond R ₅₀₀ . We measure a sub-cosmic hot gas fraction at the median mass of our sample, M ₅₀₀ = 2.54 × 10 ¹³ M _⊙ , of f _gas, 500 = 4.32 ± 0.42%. Similarly, at R ₂₀₀ and the median mass, M ₂₀₀ = 3.69 × 10 ¹³ M _⊙ , we obtain f _gas, 200 = 5.78 ± 0.69%. Our f _gas − M _tot and L _X − M _tot relations deviate significantly from predictions of the strong feedback variants of the FLAMINGO simulation (2.5 σ to 8.0 σ tension), whereas the fiducial FLAMINGO and BAHAMAS models provide the closest match to our measurements. Our results are consistent with prior X-ray observations but achieve twice the statistical precision of recent stacking studies. Using our measured baryon fractions and the hydrodynamical simulation-based model SP(k) , we infer a 10%−15% reduction in the matter power spectrum at k = 5 h Mpc ⁻¹ relative to a dark matter-only universe (0.85 ≤ P ( k )/ P _DMO ( k )≤0.90), in agreement with the predictions of fiducial FLAMINGO and BAHAMAS, while revealing a growing tension on smaller scales with the strong feedback variants.

Context . Galaxy groups and clusters are widely used to probe the evolution of the cosmic web and cosmology, while assuming that they are relaxed. Aims . We identify the properties of the brightest halo galaxies (BHGs) that can be used to predict the most likely sample of dynamically relaxed host halos. Our work combines thoroughly studied galaxy clusters with less frequently analysed groups. Methods . Our analysis was based on data from the IllustrisTNG simulations. We considered several observationally motivated parameters, including the offset of the BHG from the potential well of the host system ( d _off ) and from the r-band luminosity centre ( d _lum ), the distance between the brightest and second-brightest galaxies ( d ₁₂ ), and the r -band magnitude gap between them (∆ m ₁₂ ). The primary analysis was performed at redshift z = 0, with an additional investigation of the redshift evolution of halo relaxation up to z = 1. The observable proxies were applied to construct a halo mass function (HMF), which was then compared to the HMF of the relaxed sample defined from 3D information commonly used in theoretical approaches. Results . We find that d _off and ∆ m ₁₂ are effective indicators of group and cluster relaxation, particularly when used in combination. The selection criteria of d _off < 0.05 R ₂₀₀ and ∆ m ₁₂ > 1.6mag allowed us to reproduce an HMF that closely matches that of the theoretically relaxed halo population. These criteria can be applied to observations up to z ∼ 0.2 within a mass range M ₂₀₀ ≥ 10 ^12.5 M _⊙ (M _∗,BHG ≳ 10 ^10.9 M _⊙ ), including groups and clusters in the selection. In this mass range, 15–23% of the systems are considered fully relaxed at z = 0. The fraction of relaxed haloes decreases with redshift up to z ∼ 0.4, after which the decrease is far slower.

Context. Understanding baryonic physics at the scale of galaxy groups is essential for high-precision cosmological studies of the large-scale structure. Most models predict that the intergalactic medium in groups is hot and extends well beyond the virial radius, emitting X-rays on scales of up to several degrees in nearby systems. Early X-ray searches using ROSAT showed significant promise in recovering and identifying this emission. Aims. We aim to exploit the large area coverage, good sensitivity, and low instrumental background of eROSITA to detect the faint surface brightness emission of galaxy groups from the Two Micron All Sky Survey Redshift Survey (2MRS). Methods. Using the data from eROSITA-DE Data Release 1, including images, exposure maps, and local background maps, we performed a wavelet decomposition of image mosaics in the 0.6−2.3 keV band at angular scales of 1/8′−16′. We adopted 8′−16′ scales for source detection and 2′−4′ scales to improve catalog purity. A novel identification method based on the ranked partial Hausdorff distance fully exploits the X-ray image and group membership information. Random catalogs were used to control match purity, and the identification threshold was chosen to maximize the catalog size at a fixed purity. Results. We present a catalog of 619 X-ray galaxy groups with 80% purity, and define subsamples with 90% and 97% purity. Bright sources closely match the AXES-2MRS catalog (which is based on ROSAT All Sky Survey data analysis on spatial scales of 12′−24′). The X-ray luminosity function of our groups agrees with previous studies down to 5 × 10 ⁴¹ erg s ⁻¹ . Using dynamical mass estimates, we find that the X-ray counterpart completeness for groups with ≥4 members exceeds 60% for masses > 2 × 10 ¹³ M _⊙ . We modeled the 2MRS group catalog and justify the inclusion of two-member groups in the identification. Conclusions. This study demonstrates that large X-ray sources on spatial scales relevant for cosmological studies of baryonic distributions can be reliably detected and identified using nearby galaxy group catalogs.

Context . The chemical dichotomy in the [ α /Fe]-[Fe/H] plane, such as the one observed in the stellar Milky Way disc, is a consequence of the complex processes underlying the formation and evolution of disc galaxies. Aims . We determine the potential drivers behind the α -bimodality of the disc in a zoom-in hydrodynamical simulated galaxy with no prior major mergers and negligible radial migration. Methods . Using a Milky Way-mass galaxy from the FIRE-2 suite of simulations, we analysed gas flows in the disc, together with its star formation and merger history, as well as the chemical evolution of the hot corona. These data allowed us to investigate their links to transitions in the chemo-dynamical structure of the stellar disc and its radial distribution. Results . The simulated galaxy exhibits high and low- α sequences, without having experienced major mergers, nor significant radial migration in the past. A high- α thick disc forms during the early chaotic clustering phase. Afterwards, as the star formation rate declines, a dip in the stellar number density appears, coinciding with the dilution of the galactic corona by a minor merger, which subsequently halts the rise of [Fe/H] in the disc. Next, accreted gas onto the disc from minor mergers mildly enhances the star formation rate and generates the low- α sequence in the outer disc, with radial inward flows of this material feeding the low- α inner disc. Furthermore, we find that even at fixed radii, newly formed stars retain a sizable spread in their chemical abundances, reflecting chemical differences between the in situ and the infalling gas from which they formed. This serves as a further indication that the assumption of instantaneous gas mixing is invalid. Conclusions . Understanding the chemical evolution of stellar discs calls for their accretion merger history and interaction with the surrounding hot corona to be accounted for, as well as the vertical and radial gas flows that redistribute metals within the disc.

Context. Giant radio galaxies (GRGs) represent the largest individual astrophysical structures, rivalling galaxy clusters in physical extent. Understanding how they attain such scales demands an examination of their large-scale cosmic surroundings, particularly the under-explored filament environment. Aims. We quantify the three-dimensional (3D) distance of GRGs from the nearest filament spine; test how this distance correlates with their growth and formation of different morphological classes; assess whether their radio jets exhibit preferred orientations relative to filament axes; and examine how filament anisotropy from spine-to-periphery modulates radio morphology. Methods. We employed a filament catalogue from the Sloan Digital Sky Survey (SDSS) together with the largest GRG catalogue currently available. For each source, we measured the comoving distance to the nearest filament spine, the projected jet-spine orientation angle, and quantified lobe asymmetry via the arm-length ratio (ALR). These metrics trace proximity, directionality, and the impact of filamentary environment on morphology. We then compared GRGs with a control sample of small radio galaxies (SRGs) to constrain the environmental factors that regulate the attainment of giant sizes. We validated the robustness of our results via bootstrap resampling and non-parametric statistical tests. Results. Our results show that GRGs and SRGs have similar filament occupancy. By contrast, GRGs preferentially display larger alignment angles relative to filament spines, while SRG orientations are consistent with a random distribution. The GRGs further show enhanced morphological asymmetry, reflected in lower ALR values than SRGs. Conclusions. Attainment of giant sizes is not governed by proximity to filaments; rather, it correlates with jet-filament alignment. The GRGs are preferentially oriented at large angles to filament spines, consistent with the propagation of jets through lower-density void-facing channels that minimise environmental resistance. Consistently, lobe asymmetry mirrors this alignment, indicating that GRGs experience steeper transverse spine-to-void differential ram-pressure gradients along their paths.

We present the first statistical observational study detecting filaments in the immediate surroundings of galaxies, i.e. the local web of galaxies. Simulations predict that cold gas, the fuel for star formation, is channeled through filamentary structures into galaxies. Yet, direct observational evidence for this process has been limited by the challenge of mapping the cosmic web at small scales. Using miniJPAS spectro-photometric data combined with spectroscopic DESI redshifts when available, we constructed a high-density observational galaxy sample spanning 0.2 < z < 0.8. Local filaments are detected within a 3 Mpc physical radius of each galaxy with stellar mass M _★ > 10 ¹⁰ M _⊙ using all nearby galaxies as tracers, combined with a probabilistic adaptation of the DisPerSE algorithm designed to overcome limitations due to photometric redshift uncertainties. Our methodology was tested and validated using mock catalogues built with random forest models applied to a simulated lightcone. Besides recovering the expected increase in galaxy connectivity (defined as the number of filaments attached to a galaxy) with stellar mass, we show that our connectivity measurements agree with 3D reference estimates from the mock galaxies. Thanks to these filament reconstructions, we explore the relation between small-scale connectivity and galaxy star formation rate, finding a mild positive trend which needs to be confirmed by follow-up studies with larger sample sizes. We propose galaxy connectivity to local filaments as a powerful and physically motivated metric of environment, offering new insights into the role of cosmic structure in galaxy evolution.

Context. Hydrodynamical simulation predictions are often compared with observational data without fully accounting for systematics and biases specific to observational techniques. In this study, we used the magnetohydrodynamical simulation Magneticum to create a mock dataset that replicates the observational data available for analyzing hot gas properties in extensive galaxy group samples. Aims. Specifically, we simulated eROSITA eRASS:4 data along with a GAMA-like galaxy spectroscopic survey from the same lightcone and generated mock, optically selected galaxy catalogs using widely employed group-finding algorithms. We then applied an observational stacking technique to the mock eRASS:4 observations, and used the mock group catalogs as priors to determine the average properties of the underlying group population. This approach serves two primary purposes: (i) to produce predictions that incorporate observational systematics, and (ii) to assess these systematics and evaluate the reliability of the stacking technique in deriving the average X-ray properties of galaxy groups from eROSITA data. Methods. We provide the predicted X-ray emission of the Magneticum divided into all contributions of AGN, X-ray binaries (XRBs), and Intra-Group Medium (IGrM) per bin of halo mass. The predicted AGN and XRB contamination dominates the X-ray surface brightness profile emission in all halos with masses below 10 ¹³ M _⊙ , which contains the majority of low X-ray luminosity AGN. We tested the reliability of the stacking technique in reproducing the input X-ray surface brightness and electron density profile for all tested optical group selection algorithms. We considered completeness and contamination of the prior samples, miscentering of the optical group center, uncertainties in determining the X-ray emissivity due to the assumptions of mean gas temperature and metallicity, and systematics in the available halo mass proxy. Results. The primary source of systematics in our analysis arises from the precision of the halo mass proxy, which might impact the estimation of X-ray surface brightness profiles when utilized as a prior, and derived scaling relations. Our analysis displays the L _X –mass relationships produced by stacking various optically selected group priors and reveals that, in each instance, the slope of these relations appears somewhat flatter than the input relation, though still in agreement with observational data. We retrieve the f _gas –mass relation within R ₅₀₀ effectively and find agreement between the predictions and observational data. Conclusions. These systematic errors must be considered when comparing the results of any stacking technique with other works in the literature based on different prior catalogs, detections, or predictions.

Context. Superclusters of galaxies represent dynamically active environments in which galaxies and their systems form and evolve. Aims. We study the substructure, connectivity, and galaxy content of galaxy clusters A1656 and A1367 in the Coma supercluster and of A1185 in the Leo supercluster with the aim of understanding the evolution of clusters from turnaround to virialisation, and the evolution of whole superclusters. Methods. We used data from the Sloan Digital Sky Survey DR10 MAIN galaxy sample and from DESI cluster catalogues. The projected phase space diagram and the distribution of mass were used to identify regions of various infall stages (early and late infall, and regions of ongoing infall, i.e. regions of influence), their characteristic radii, embedded mass, and density contrasts in order to study the evolution of clusters with the spherical collapse model. We determined the substructure of clusters using normal mixture modelling and their connectivity by counting filaments in the cluster’s regions of influence. We analysed galaxy content in clusters and in their environment and derived scaling relations between cluster masses. Results. All three clusters have a substructure with two to five components and up to six filaments connected to them. The radii of regions of influence are R _inf ≈ 4 h ⁻¹ Mpc, and the density contrast at their borders is Δ ρ _inf ≈ 50 − 60. The scaling relations between the masses of clusters have a very small scatter. The galaxy content of the clusters and of their regions of influence vary from cluster to cluster. In high-density regions (superclusters), the percentage of quiescent galaxies is higher than in low-density regions between superclusters, where approximately one-fourth of the galaxies are still quiescent. Conclusions. The collapse of the regions of influence of clusters started at redshifts z ≈ 0.4 − 0.5. Clusters will be virialised approximately in ≈3.3 Gyrs. Clusters in superclusters will not merge, and their present-day turnaround regions will be virialised in ≈10 Gyrs. The large variety of properties of clusters suggests that they have followed different paths during evolution.