arXiv subjects

Benedikt Diemer

Publications and source records attributed to Benedikt Diemer.

At least 19 recordsLinked to original sources

arXiv2026

The Connection between Halo Assembly History and the Stellar Distribution I: The Central Stellar Mass - Richness Plane

Recent observations suggest intrinsic links between the stellar components of massive halos and their assembly histories. Using massive halos with $\log[M_{\rm halo}/M_\odot] \geq 13.0$ from the IllustrisTNG-300 simulation, we explore differences in secondary halo properties between samples selected by central galaxy stellar mass and satellite richness. For group-like masses, we find that halos selected by the outskirts stellar mass of their central galaxies between 50 and 100 kpc ($M_{\star,[50,100]}$) have similar halo mass distributions to those selected by intrinsic satellite richness ($λ_{10, R_{200}}$), but differ significantly in assembly history: $M_{\star,[50,100]}$-selected halos are more concentrated and formed earlier. At higher masses ($\sim10^{14}~M_\odot$), intrinsic richness becomes the better halo-mass proxy, but only when projection effects are ignored. We find that the difference-set test selects comparable halo-mass samples most effectively when the two proxies have similar scatter. Projection effects and baryonic physics do not alter the main trends but remain important caveats for observational applications. Our findings show that the central stellar mass-richness plane is a powerful tool for selecting halos with particular accretion histories, offering new insights into the galaxy-halo connection in the era of deep imaging and precision lensing surveys such as Euclid and LSST.

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arXiv2026

Shock-heated Away: The Impact of Radiative Cooling on Gas-Phase Transitions in Supernova Remnants

Supernova (SN) feedback plays a central role in regulating the structure of the interstellar medium (ISM) through the injection of energy and momentum. The amount of hot gas produced by a supernova is a key quantity that determines how efficiently SN feedback heats the ISM and drives mass exchange between its different gas phases, here defined as cold ($T < 10^3\, \mathrm{K}$), warm ($10^3\, \mathrm{K} < T < 2\times10^4\, \mathrm{K}$), and hot ($T > 2\times10^4\, \mathrm{K}$) gas. However, previous studies have reported discrepant amounts of hot gas formed under otherwise similar ambient conditions. To resolve these disagreements, we quantify the amount of hot gas produced by individual SN explosions using a suite of controlled simulations spanning a broad range of ISM environments that include both uniform and turbulent, multiphase backgrounds. We show that radiative cooling is a key factor regulating hot-gas production, and that differences in cooling efficiency can account for some of the discrepancies reported in the literature. We derive a simple predictive relation for the peak hot-gas mass attained during the evolution of a supernova remnant in terms of the mean ambient density, the initial phase distribution, and the efficiency of gas cooling, which we parameterize as the cooling time over a key temperature range of $10^{4.5}\,\mathrm{K} \lesssim T \lesssim 10^{5.1}\,\mathrm{K}$. Finally, using tracer particles, we distinguish the evaporation of cold and warm gas into the hot phase and derive physically motivated expressions for the evaporation efficiency. Our results provide simple, predictive relations for hot-gas production and phase transitions that can be incorporated into subgrid models of SN feedback in galaxy formation simulations.

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arXiv2026

Cluster finding with outskirt stellar masses and percolation

The abundance of galaxy clusters is a powerful cosmological probe, but optical cluster cosmology is limited by selection systematics, in particular the projection effects that affect cluster finders based on galaxy populations such as the red sequence. The outer stellar mass ($M_\mathrm{out}$) of cluster central galaxies -- e.g., the stellar mass in a 50-100 kpc annulus -- offers an alternative selection that relies only on the central galaxy and is therefore largely free from projection effects. Its primary systematic is instead satellite contamination, since massive clusters can host more than one galaxy with high outer stellar mass. Using the IllustrisTNG300 simulation at $z=0.4$, we quantify this contamination and investigate a simple, proximity-based percolation method to mitigate it, in which galaxies with lower outer stellar mass lying within a given radius of a more massive galaxy are removed from the sample. We find that the satellite fraction defined by the friends-of-friends (FoF) algorithm is modest even without percolation ($\leq 15\%$ for $M_\mathrm{out} > 10^{10}\,\mathrm{M}_\odot$ and $<10\%$ for $M_\mathrm{out} > 10^{11}\,\mathrm{M}_\odot$), and that percolation reduces it further, with the improvement increasing for percolation radii up to $3.0\,R_{200c}$. For a moderately high outer stellar mass cut ($\sim 4\times10^{10}\,\mathrm{M}_\odot$) and percolation radius ($\sim 2.0\,R_{200c}$), we recover a cluster sample that is both highly complete and pure for halo masses $\gtrsim 10^{14}\,\mathrm{M}_\odot$. These results indicate that outer stellar mass, combined with simple percolation, has the potential to provide a clean and readily calibratable selection of massive galaxy clusters.

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arXiv2026

A universal model for the accretion rates and formation times of dark matter halos

The formation histories of halos set the baseline rate at which galaxies accrete gas over cosmic time. While a number of models describe these histories and their derivative, the mass accretion rate (MAR), a simple and universal formula has remained elusive. Here we measure the median MARs and half-mass formation times of halos in dark matter-only and hydrodynamical simulations, in extremely different cosmologies ($Λ$CDM and Einstein-de Sitter), and across a wide range of redshifts ($z = 0$-$14$). We confirm that MARs increase with mass and redshift, and that they are virtually identical in hydrodynamical and dark matter-only simulations. We show that MARs are accurately described by a universal six-parameter function of three physical variables: the peak height $ν$, the slope of the linear power spectrum $n_{\rm eff}$, and the effective linear growth rate $α_{\rm eff}$. A complementary two-parameter fit for the formation redshift improves on the function of Lacey \& Cole by fixing one parameter to its physical value and adding a dependence on $n_{\rm eff}$. Our model is broadly consistent with some prescriptions from the literature but provides a larger range and higher accuracy at high redshifts and low masses. Our fitting functions are implemented in the publicly available \textsc{Colossus} toolkit.

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arXiv2026

One Halo, Two Boundaries: Relating Accretion Shocks and Splashback Radii in Galaxy Clusters

The boundaries of dark matter and gas in clusters are delineated by the splashback radius and the accretion shock, respectively. Theoretically, both of these boundaries are expected to coincide at the outskirts of halos. However, hydrodynamic cosmological simulations have highlighted significant displacement between them. In this study, we utilise the IllustrisTNG simulation suite to investigate the statistical relationship between the splashback and shock surfaces in a sample of 812 cluster-mass halos. We compute the full angular distribution of both boundaries and examine their relationship, also considering how different moments of this distribution correlate with halo properties. We employ a dispersion-based measure for the splashback boundary and the maximum entropy distance for the shock location. Despite examining various boundary definitions, we consistently observe an offset between the splashback and shock boundaries, with $R_{\rm sh}/R_{\rm sp} \sim 1.3-2$, depending on specific methodological choices. This offset predominantly occurs along void directions. We analyse the redshift evolution of these boundaries for a subset of halos and find that splashback and shock boundaries are not necessarily distinct at earlier times. During mergers, gas dissipates energy and resists contraction via pressure, unlike collisionless dark matter, leading to the observed boundary offset. We also find that the feature in pressure profiles arising from the outer accretion shock is sensitive to the exact method of stacking, which has important implications for observations.

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arXiv2026

Distinguishing Orbiting and Infalling Dark Matter Particles with Machine Learning

Dark matter halos are typically defined as spheres that enclose some overdensity, but these sharp, somewhat arbitrary boundaries introduce non-physical artifacts such as backsplash halos, pseudo-evolution, and an incomplete accounting of halo mass. A more physically motivated alternative is to define halos as the collection of particles that are physically orbiting within their potential well. However, existing methods to classify particles as orbiting or infalling suffer from trade-offs between accuracy, computational cost, and generalizability across cosmologies. We present an efficient, yet accurate, supervised machine learning approach using decision trees. The classification is based on only the particle radii and velocities at two epochs. Compared to detailed analysis of particle trajectories, we find that our model matches the classification of 97\% of particles. Consequently, we are able to quickly and accurately reproduce the density profiles of the orbiting and infalling components out to many virial radii. We demonstrate that our model generalizes to a significantly different cosmology that lies outside the training dataset. We make publicly available both our final model and the code to train similar models.

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arXiv2025

Reaching for the Edge II: Stellar Halos out to Large Radii as a Tracer of Dark Matter Halo Mass

The diffuse outskirts of brightest cluster galaxies (BCGs) encode valuable information about the assembly history and mass of their host dark matter halos. However, the low surface brightness of these stellar halos has historically made them difficult to observe. Recent deep imaging, particularly with Hyper Suprime-Cam (HSC), has shown that the stellar mass within relatively large projected annuli, such as within $50$ and $100$ kpc, is a promising proxy for halo mass. However, the optimal radial definition of this "outskirt mass" remains uncertain. We construct an HSC-like mock observing pipeline to measure the stellar mass density profiles of BCGs in the IllustrisTNG simulations. Our mock observations closely reproduce HSC profiles across six orders of magnitude in surface density. We then systematically measure stellar masses within different annuli and how tightly they are connected to halo mass. We find that stellar masses measured within simple apertures exhibit considerably more scatter in the stellar mass-halo mass relation than those measured within projected ellipsoidal annuli. We identify an optimal range of definitions, with inner radii between $\sim 70$-$200$ kpc and outer radii between $\sim 125$-$500$ kpc. We also introduce two halo-mass-dependent Sérsic models for the average stellar halo profiles. We present a Sérsic-based fitting function that describes the profiles as a function of the halo mass, $M_{\rm vir}$, with a median error of $54\%$. Adding the central stellar mass of the BCG as a second parameter slightly improves the accuracy to a median error of $39\%$. Together, these results provide fitting functions for BCG stellar halos that can be applied to future wide-field surveys to infer halo masses from deep imaging data.

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arXiv2025

Lost in the FoG: Pitfalls of Models for Large-Scale Hydrogen Distributions

Large-scale HI surveys and their cross-correlations with galaxy distributions have immense potential as cosmological probes. Interpreting these measurements requires theoretical models that must incorporate redshift-space distortions (RSDs), such as the Kaiser and fingers-of-God (FoG) effect, and differences in the tracer and matter distributions via the tracer bias. These effects are commonly approximated with assumptions that should be tested on simulated distributions. In this work, we use the hydrodynamical simulation suite IllustrisTNG to assess the performance of models of $z \leq 1$ HI auto and HI-galaxy cross-power spectra, finding that the models employed by recent observations introduce errors comparable to or exceeding their measurement uncertainties. In particular, neglecting FoG causes $\gtrsim 10\%$ deviations between the modeled and simulated power spectra at $k \gtrsim 0.1$ $h$ / Mpc, larger than assuming a constant bias which reaches the same error threshold at slightly smaller scales. However, even without these assumptions, models can still err by $\sim 10\%$ on relevant scales. These remaining errors arise from multiple RSD damping sources on HI clustering, which are not sufficiently described with a single FoG term. Overall, our results highlight the need for an improved understanding of RSDs to harness the capabilities of future measurements of HI distributions.

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arXiv2025

The Outskirt Stellar Mass of Low-Redshift Massive Galaxies is an Excellent Halo Mass Proxy in Illustris/IllustrisTNG Simulations

Recent observations suggest that the extended stellar halos of low-redshift massive galaxies are tightly connected to the assembly of their dark matter halos. In this paper, we use the Illustris, IllustrisTNG100, and IllustrisTNG300 simulations to compare how different stellar aperture masses trace halo mass. For massive central galaxies ($M_\star\geq 10^{11.2}M_\odot$), we find that a 2D outskirt stellar mass measured between 50 to 100 kpc ($M_{\star,[50,100]}$) consistently outperforms other aperture-based stellar masses. We further show that $M_{\star,[50,100]}$ correlates better with halo mass than the total amount of accreted stars (the ex situ mass), which suggests that not all accreted stars connect to halo assembly equally. While the galaxy formation recipes are different between Illustris and IllustrisTNG100, the two simulations yield consistent ex situ outskirt fractions for massive galaxies (about 70% in $M_{\star,[50,100]}$). These results demonstrate the potential of using the outskirt stellar mass to deepen our understanding of galaxy-halo connection in massive dark matter halos and trace dark matter halos better.

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arXiv2025

ULULA: An ultra-lightweight 2D hydrodynamics code for teaching and experimentation

Hydrodynamics is a difficult subject to teach in the classroom because most relevant problems must be solved numerically rather than analytically. While there are numerous public hydrodynamics codes, the complexity of production-level software obscures the underlying physics and can be overwhelming to first-time users. Here we present ULULA, an ultra-lightweight python code to solve hydrodynamics and gravity in 2D. The main goal is for the code to be easy to understand, extend, and experiment with. The simulation framework consists of fewer than 800 active lines of pure python code, but it includes a robust MUSCL-Hancock scheme with exchangeable components such as Riemann solvers, reconstruction schemes, boundary conditions, and equations of state. Numerous well-known hydrodynamics problems are provided and can be run in a few minutes on a laptop. The code is open-source, generously commented, and extensively documented.

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arXiv2025

The mass-dependent UVJ diagram at cosmic noon: A challenge for galaxy evolution models and dust radiative transfer

Context. The UVJ color-color diagram is a widely used diagnostic to separate star-forming and quiescent galaxies. Observational data from photometric surveys reveal a strong stellar mass trend, with higher-mass star-forming galaxies being systematically more dust-reddened. Aims. We analyze the UVJ diagram in the TNG100 cosmological simulation at cosmic noon ($z\approx2$). Specifically, we focus on the trend between UVJ colors and mass which has not been reproduced in any cosmological simulation thus far. Methods. We applied the SKIRT dust radiative transfer code to the TNG100 simulation to generate rest-frame UVJ fluxes. These UVJ colors were then compared to observational data from several well-studied extragalactic fields from the CANDELS/3D-HST programs, augmented by recent JWST/NIRCam photometry. Results. Quiescent and low-mass ($M_\star\lesssim10^{10.5}\,\mathrm{M}_\odot$) galaxies at cosmic noon do not require significant levels of dust reddening, as opposed to massive ($M_\star\gtrsim10^{11}\,\mathrm{M}_\odot$) star-forming galaxies. An extensive range of possible dust models fall short of the required dust reddening in V-J color for massive star-forming galaxies, with the simulated galaxies being too blue by $\approx0.9\,\mathrm{mag}$. Conclusions. We find that only variations in the star-to-dust geometries of the simulated galaxies can yield V-J colors that are red enough to match the observations. A toy model with isolated dust screens around younger stellar populations (with ages below $\sim1\,\mathrm{Gyr}$) can reproduce the observational data, while all conventional dust radiative transfer models (where the dust distribution follows the metals in the interstellar medium) fail to achieve the required V-J colors.

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arXiv2025

Relationship between 2D and 3D Galaxy Stellar Mass and Correlations with Halo Mass

Recent studies suggest that the stars in the outer regions of massive galaxies trace halo mass better than the inner regions and that an annular stellar mass provides a low scatter method of selecting galaxy clusters. However, we can only observe galaxies as projected two-dimensional objects on the sky. In this paper, we use a sample of simulated galaxies to study how well galaxy stellar mass profiles in three dimensions correlate with halo mass, and what effects arise when observationally projecting stellar profiles into two dimensions. We compare 2D and 3D outer stellar mass selections and find that they have similar performance as halo mass proxies and that, surprisingly, a 2D selection sometimes has marginally better performance. We also investigate whether the weak lensing profiles around galaxies selected by 2D outer stellar mass suffer from projection effects. We find that the lensing profiles of samples selected by 2D and 3D definitions are nearly identical, suggesting that the 2D selection does not create a bias. These findings underscore the promise of using outer stellar mass as a tool for identifying galaxy clusters.

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arXiv2024

A Multi-Wavelength Technique for Estimating Galaxy Cluster Mass Accretion Rates

The mass accretion rate of galaxy clusters is a key factor in determining their structure, but a reliable observational tracer has yet to be established. We present a state-of-the-art machine learning model for constraining the mass accretion rate of galaxy clusters from only X-ray and thermal Sunyaev-Zeldovich observations. Using idealized mock observations of galaxy clusters from the MillenniumTNG simulation, we train a machine learning model to estimate the mass accretion rate. The model constrains 68% of the mass accretion rates of the clusters in our dataset to within 33% of the true value without significant bias, a ~58% reduction in the scatter over existing constraints. We demonstrate that the model uses information from both radial surface brightness density profiles and asymmetries.

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arXiv2024

A dynamics-based density profile for dark haloes -- III. Parameter space

In the previous paper of this series, we proposed a new function to fit halo density profiles out to large radii. This truncated Einasto profile models the inner, orbiting matter as $ρ_{\rm orb} \propto \exp \left[-2/α (r / r_{\rm s})^α- 1/β (r / r_{\rm t})^β\right]$ and the outer, infalling term as a power-law overdensity. In this paper, we analyse the resulting parameter space of scale radius $r_{\rm s}$, truncation radius $r_{\rm t}$, steepening $α$, truncation sharpness $β$, infalling normalisation $δ_1$, and infalling slope $s$. We show that these parameters are non-degenerate in averaged profiles, and that fits to the total profiles generally recover the underlying properties of the orbiting and infalling terms. We study the connection between profile parameters and halo properties such as mass (or peak height) and accretion rate. We find that the commonly cited dependence of $α$ on peak height is an artefact of fitting Einasto profiles to the actual, truncated profiles. In our fits, $α$ is independent of mass but dependent on accretion rate. When fitting individual halo profiles, the parameters exhibit significant scatter but otherwise follow the same trends. We confirm that the entire profiles are sensitive to the accretion history of haloes, and that the two radial scales $r_{\rm s}$ and $r_{\rm t}$ particularly respond to the formation time and recent accretion rate. As a result, $r_{\rm t}$ is a more accurate measure of the accretion rate than the commonly used radius where the density slope is steepest.

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arXiv2024

Dynamics-based halo model for large scale structure

Accurate modelling of the one-to-two halo transition has long been difficult to achieve. We demonstrate that physically motivated halo definitions that respect the bimodal phase-space distribution of dark matter particles near halos resolves this difficulty. Specifically, the two phase-space components are overlapping and correspond to: 1) particles \it orbiting \rm the halo; and 2) particles \it infalling \rm into the halo for the first time. Motivated by this decomposition, García [R. García et. al., MNRAS 521, 2464 (2023)] advocated for defining haloes as the collection of particles orbiting their self-generated potential. This definition identifies the traditional one-halo term of the halo--mass correlation function with the distribution of orbiting particles around a halo, while the two-halo term governs the distribution of infalling particles. We use dark matter simulations to demonstrate that the distribution of orbiting particles is finite and can be characterised by a single physical scale $r_{\rm h}$, which we refer to as the \it halo radius. \rm The two-halo term is described using a simple yet accurate empirical model based on the Zel'dovich correlation function. We further demonstrate that the halo radius imprints itself on the distribution of infalling particles at small scales. Our final model for the halo--mass correlation function is accurate at the $\approx 2\%$ level for $r \in [0.1,50]\ h^{-1}\ Mpc$. The Fourier transform of our best fit model describes the halo--mass power spectrum with comparable accuracy for $k\in [0.06, 6.0]\ h\ Mpc^{-1}$.

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arXiv2024

Atomic Hydrogen Shows its True Colours: Correlations between HI and Galaxy Colour in Simulations

Intensity mapping experiments are beginning to measure the spatial distribution of neutral atomic hydrogen (HI) to constrain cosmological parameters and the large-scale distribution of matter. However, models of the behaviour of HI as a tracer of matter are complicated by galaxy evolution. In this work, we examine the clustering of HI in relation to galaxy colour, stellar mass, and HI mass in IllustrisTNG at $z$ = 0, 0.5, and 1. We compare the HI-red and HI-blue galaxy cross-power spectra, finding that HI-red has an amplitude 1.5 times higher than HI-blue at large scales. The cross-power spectra intersect at $\approx 3$ Mpc in real space and $\approx 10$ Mpc in redshift space, consistent with $z \approx 0$ observations. We show that HI clustering increases with galaxy HI mass and depends weakly on detection limits in the range $M_{\mathrm{HI}} \leq 10^8 M_\odot$. In terms of $M_\star$, we find blue galaxies in the greatest stellar mass bin cluster more than blue galaxies in other stellar mass bins. Red galaxies in the greatest stellar mass bin, however, cluster the weakest amongst red galaxies. These trends arise due to central-satellite compositions. Centrals correlate less with HI for increasing stellar mass, whereas satellites correlate more, irrespective of colour. Despite the clustering relationships with stellar mass, we find that the cross-power spectra are largely insensitive to detection limits in HI and galaxy surveys. Counter-intuitively, all auto and cross-power spectra for red and blue galaxies and HI decrease with time at all scales in IllustrisTNG. We demonstrate that processes associated with quenching contribute to this trend. The complex interplay between HI and galaxies underscores the importance of understanding baryonic effects when interpreting the large-scale clustering of HI, blue, and red galaxies at $z \leq 1$.

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arXiv2023

VERTICO and IllustrisTNG: The spatially resolved effects of environment on galactic gas

It has been shown in previous publications that the TNG100 simulation quantitatively reproduces the observed reduction in each of the total atomic and total molecular hydrogen gas for galaxies within massive halos, i.e.~dense environments. In this Letter, we study how well TNG50 reproduces the resolved effects of a Virgo-like cluster environment on the gas surface densities of satellite galaxies with $m_* > \! 10^9\,{\rm M}_\odot$ and ${\rm SFR} \! > 0.05\,{\rm M}_\odot\,{\rm yr}^{-1}$. We select galaxies in the simulation that are analogous to those in the HERACLES and VERTICO surveys, and mock-observe them to the common specifications of the data. Although TNG50 does not quantitatively match the observed gas surface densities in the centers of galaxies, the simulation does qualitatively reproduce the trends of gas truncation and central density suppression seen in VERTICO in both HI and H$_2$. This result promises that modern cosmological hydrodynamic simulations can be used to reliably model the post-infall histories of cluster satellite galaxies.

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arXiv2023

Haunted haloes: tracking the ghosts of subhaloes lost by halo finders

Dark matter subhaloes are key for the predictions of simulations of structure formation, but their existence frequently ends prematurely due to two technical issues, namely numerical disruption in N-body simulations and halo finders failing to identify them. Here we focus on the second issue, using the phase-space friends-of-friends halo finder ROCKSTAR as a benchmark (though we expect our results to translate to comparable codes). We confirm that the most prominent cause for losing track of subhaloes is tidal distortion rather than a low number of particles. As a solution, we present a flexible post-processing algorithm that tracks all subhalo particles over time, computes subhalo positions and masses based on those particles, and progressively removes stripped matter. If a subhalo is lost by the halo finder, this algorithm keeps tracking its so-called ghost until it has almost no particles left or has truly merged with its host. We apply this technique to a large suite of N-body simulations and restore lost subhaloes to the halo catalogues, which has a dramatic effect on key summary statistics of large-scale structure. Specifically, the subhalo mass function increases by about 50% and the halo correlation function increases by a factor of two at small scales. While these quantitative results are somewhat specific to our algorithm, they demonstrate that particle tracking is a promising way to reliably follow haloes and reduce the need for orphan models. Our algorithm and augmented halo catalogues are publicly available.

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