arXiv subjects

Dylan Nelson

Publications and source records attributed to Dylan Nelson.

At least 19 recordsLinked to original sources

arXiv2026

ERGO-ML and how to directly infer the multi-temperature nature of intra-cluster gas: simulation-based inference with TNG-Cluster

Though galaxy clusters are virialized and mostly in thermodynamic equilibrium, their internal gas thermodynamics are complex. The hot gas is multiphase and spatially inhomogeneous. The exact state of the gas contains important information about the internal physics governing clusters, however, measuring the multiphase distribution of the X-ray emitting gas is hard if not impossible with current observational methods. In this work (part of the ERGO-ML series), we present a direct inference method of the multiphase temperature distribution of the gas in the intra-cluster medium (ICM), applicable over a large range of spatial and mass scales. It works using different spectral resolutions (XRISM-like and Chandra-like) and realistic photon noise. Our method is an application of simulation-based inference. We train a neural-network on X-ray spectra in the range 0.1-10 keV, generated for $\approx 10^5$ sightlines of massive clusters ($M_{\rm 200c} = 10^{14.2-15.2}~\mathrm{M_\odot}$) from the TNG-Cluster cosmological MHD galaxy simulation. We show that our method recovers the mass-weighted temperature of both individual sightlines and entire clusters with an average prediction error of $< 0.01 ~\mathrm{dex}$, and the total gas mass within $0.04 ~\mathrm{dex}$. On top of that it gives accurate measures of the fraction of sub- and super-virial gas. Our method is capable of extracting the cooler gas phase even if the flux contribution is 3 orders of magnitude below that of the virial gas phase. We then show that we can accurately recover the spatial distribution of multi-phase gas both in relaxed and merging systems. This opens the way to studies of the multiphase nature of the ICM and the violent physics of feedback, accretion and merging.

astro-ph.CO↗

arXiv2026

TNGC-SIDM I: Evolution of massive cluster mergers under the self-interacting dark matter paradigm

Merging galaxy clusters provide an ideal laboratory for testing the self-interacting nature of dark matter. In this paper, we present a pilot study of magnetohydrodynamical cosmological zoom-in simulations of two cluster mergers selected from TNG-Cluster, with $M_{\rm 200c} \sim 9\times10^{14}~M_\odot$ and $M_{\rm 200c}\sim 2\times10^{14}~M_\odot$. The simulations are performed with both standard collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM), assuming an isotropic, velocity-independent cross-section of $σ_{\rm T}/m_χ= 1~\rm cm^2~g^{-1}$, with an additional $0.5~\rm cm^2~g^{-1}$ run for the low-mass system, and the mass resolution and galaxy formation model are unchanged from TNG-Cluster. We show how dark matter self-interactions produce an isothermal core in the dark matter halo, generate unique substructures in the intracluster light, and introduce variation in the multi-wavelength observables of the intracluster plasma. Using these simulations, we test the validity of observational probes for dark matter self-interactions in cluster mergers. We find that the shock-to-shock separation shows similar evolution in the self-interaction models tested here, providing a reference for distinguishing the $\sim100-200~\rm kpc$-scale difference in halo separation induced by self-interactions. Moreover, the dark matter halo centroid of the main cluster in the massive cluster merger is offset from the brightest cluster galaxy by as much as $\sim20~\rm kpc$, larger than the offset in the sub-cluster. This offset persists beyond 4 Gyr after the first pericenter passage. Our results indicate that sample selection plays a key role in testing the possible self-interacting nature of dark matter using cluster mergers, and that double radio relic cluster mergers and massive clusters with evidence of an ancient major merger offer a promising testbed for this purpose.

astro-ph.CO↗

arXiv2026

Mass Proxy Quality of Halo Properties in IllustrisTNG and FLAMINGO Simulations: II. Galaxies, Stars, and Black Holes

We study the stellar, black hole, and satellite galaxy populations of group- and cluster-scale halos in the IllustrisTNG, TNG-Cluster, and FLAMINGO cosmological hydrodynamical simulations. We characterize the mass-conditioned scaling relations, covariance, conditional likelihoods, and mass proxy quality (MPQ) of integral properties within $R_{\rm 500c}$: total stellar mass ($M_{\star,\,\rm tot}$), brightest central galaxy stellar mass ($M_{\star,\,\rm BCG}$), intracluster light mass ($M_{\star,\,\rm ICL}$), total black hole mass ($M_{\rm BH}$), satellite occupation ($N_{\rm sat}$), and satellite velocity dispersion ($σ_{\rm sat,\,1D}$), at $z\in \{0,0.5,1,2\}$. While normalization differences of up to a factor of two emerge among simulations, many properties exhibit remarkably similar mass-localized slopes, intrinsic scatters, and MPQs across halo samples and redshifts. Correlations among stellar, satellite, and gas properties are generally consistent in sign and their dependence on halo mass and redshift, whereas those involving $M_{\rm BH}$ and $N_{\rm sat}$ show greater variation, particularly in TNG-Cluster. Conditional likelihoods are nearly lognormal for many properties, including $M_{\star,\,\rm tot}$, $N_{\rm sat}$, and $σ_{\rm sat,\,1D}$, while $M_{\star,\,\rm BCG}$ and $M_{\star,\,\rm ICL}$ are negatively skewed. Among the properties considered here, total stellar mass is the best mass proxy, with a halo mass scatter of $7$-$14\%$, followed by $N_{\rm sat}$ with a mass scatter of $9$-$18\%$. The consistent correlation behavior of $M_{\star,\,\rm ICL}$ highlights its potential for next-generation observational studies. Finally, $σ_{\rm sat,\,1D}$ exhibits highly consistent scaling, with mass-independent slope and scatter and a largely neutral correlation structure, suggesting that it is relatively insensitive to astrophysical complexity.

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arXiv2026

The star-forming past and quenching of brightest cluster galaxies in TNG-Cluster

The Brightest Cluster Galaxies (BCGs) in the most massive galaxy clusters of our Universe are, at present, mostly quiescent, elliptical, and red. However, their formation histories are unclear and likely different from those of massive ellipticals in lower-mass haloes. Simulations face the challenge of simultaneously requiring large volumes to sample massive clusters and sufficient resolution to model their stellar populations. Therefore, we make use of TNG-Cluster, one of the highest-resolution cosmological simulations of galaxy clusters, which returns 352 BCGs at $z=0$ with stellar masses $M_{*, <30 \mathrm{pkpc}} = 10^{11.6-12.9} ~\mathrm{M}_\odot$ and total halo masses $M_{\mathrm{200c}} = 10^{14.3-15.4}~\mathrm{M}_\odot$. 93 per cent of the BCGs are quenched at $z=0$, whereas at redshift $z>3$ all their main progenitors were still star forming. The star-formation histories of individual BCGs are complex, including multiple (re)quenching episodes. They rejuvenate on average $2.1 \pm 1.8$ times since $z=3$. BCGs quench for the first time $9.9^{+0.9}_{-1.4} ~\mathrm{Gyr}$ ago (10th-90th percentiles; $z=1.7^{+0.5}_{-0.5}$), while the last and final quenching event occurred $7.0^{+2.8}_{-5.0} ~\mathrm{Gyr}$ ago ($z=0.8^{+0.8}_{-0.6}$). We find a large scatter in the halo, stellar, and SMBH masses at both first and last quenching, suggesting no characteristic mass at which quenching occurs. Instead, BCGs quench for the first time when the cumulative kinetic energy injected by their SMBH equals the BCG binding energy, corresponding to a delay of $2.3^{+1.2}_{-1.2} ~ \mathrm{Gyr}$ after the first SMBH kinetic energy injection. Namely, TNG-Cluster BCGs quench when they experience significant gas expulsion. Our work shows that BCGs have complex and varied star-formation histories, with multiple rejuvenation episodes, but they ultimately quench due to SMBH feedback.

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arXiv2026

Inferring magnetic field strengths in high-redshift lensing galaxies from Faraday rotation measure observations

Observations of Faraday rotation in the lensing galaxy of CLASS B1152+199 at z = 0.439 suggest that efficient dynamo mechanisms had already amplified galactic magnetic fields by this epoch. We assess the reliability of this method for deriving magnetic field strengths and the likelihood of observing similarly large Faraday rotation differences. We select 64 star-forming disk galaxies from the TNG50 cosmological magnetohydrodynamical simulation, matched to CLASS B1152+199 in mass and redshift, and forward model them using polarized radiative transfer with POLARIS and subsequent RM synthesis. TNG50 predicts characteristic large-scale field strengths of B$_0$ = 1.9-15.6 $μ$G for individual galaxies, while the combined sample yields B$_0$ = 10.3 $μ$G. Field strengths B$_{obs}$ inferred from the synthetic observations generally recover B$_0$ within a factor of three. B$_{obs}$ depends only weakly on the assumed magnetic field geometry, whereas galaxy inclination and beam depolarization have stronger effects. The differential rotation measure recovered from RM synthesis remains correlated with the directly integrated value, although beam depolarization systematically reduces its magnitude. Only 1.41% of directly integrated sightline pairs reproduce the observed $Δ$RM = 1040 $\pm$ 60 rad m$^{-2}$ of CLASS B1152+199, increasing to 3.16% for sightlines at galactocentric distances comparable to the observed 2.6 and 6.5 kpc. Differential Faraday rotation of gravitationally lensed sources therefore provides a robust probe of large-scale magnetic fields, while the large $Δ$RM of CLASS B1152+199 is relatively rare among the considered TNG50 analog galaxies.

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arXiv2026

Quantifying the Impact of Incompleteness on Identifying and Interpreting Galaxy Protocluster Populations with the TNG-Cluster Simulation

We use the TNG-Cluster simulation to investigate how stellar mass and star formation rate (SFR) incompleteness affect the identification of density peaks within galaxy protoclusters at different redshifts. Our analysis focuses on a sample of $352$ protoclusters, defined as the progenitor populations of galaxies that reside within the virialized region of $z=0$ clusters with $M_{\rm{200}}^{z=0}\sim10^{14.3-15.5}~{\rm M}_{\odot}$. For comparison, we define our "baseline" protocluster population as galaxies with ${M}_{\star}> 10^{8.5}~{\rm M}_{\odot}$ at any redshift. We find that ${M}_{\star}$-limited (${M}_{\star} > 10^{9.5}~{\rm M}_{\odot}$) and SFR-limited ($\rm{SFR} > 10~{\rm M}_{\odot} \mathrm{yr}^{-1}$) subpopulations recover the baseline highest galaxy density peak in roughly $\sim60\%$ of cases within an accuracy of $1.0$ pMpc (corresponding to an angular scale of $\sim 2-2.5$ arcmin) at $z > 2$. This recovery fraction drops to $\sim40-50\%$ when restricting to galaxies with ${M}_{\star} > 10^{10.0}~{\rm M}_{\odot}$. We find that the baseline highest galaxy density peaks typically coincide with the highest dark matter and stellar mass density peaks, with separations less than $0.5$ pMpc in $\sim60-75\%$ of cases at $z>2$. This agreement drops to $\sim45-50\%$ when restricting to galaxies with ${M}_{\star} > 10^{10.0}~{\rm M}_{\odot}$. These results indicate that identifying the densest regions of protoclusters -- i.e., the core -- is highly sensitive to stellar mass and SFR completeness limits. Nevertheless, at $z>2$ we find that the baseline highest galaxy density peaks are generally sites of enhanced star formation and accelerated mass growth relative to the remainder of the protocluster, consistent with some observational studies.

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arXiv2026

Finding Halos in the Lyman-$α$ forest I. Introducing a hierarchical absorption framework

It has been demonstrated that one can track down galaxies in absorption 'hidden' in the Lyman-$α$ forest through the use of 'strong, blended Lyman-$α$' (or SBLA) absorption. Specifically a series of publications studied SBLA absorption systems with Lyman-$α$ flux transmission, $F_{Ly α} < 0.25$ on scales of 138 km s$^{-1}$ in the Sloan Digital Sky Survey (SDSS). In order to better understand the connection between halos and these SBLAs, we make use of several million synthetic absorption spectra from the TNG50 cosmological simulation, at z=2 and z=3. We explore spectra with SDSS-like resolution in order to understand the nature of SBLAs as defined thus far, as well as with high resolution (or 'resolved') spectra to generalise and optimise SBLAs as halo finders. For the SDSS SBLAs, we find that up to 67% of these absorption systems reside in hlaos, where the stronger the absorption and the lower the redshift, the higher the probability. We also manage to recover a mean halo mass of $10^{11.78} M_{\odot}$, in line with what is measured in observations. For the resolved SBLAs, we expand on the previous definition and allow the SBLA spectra size to vary between 54 km s$^{-1}$ and 483 km s$^{-1}$. We find that the largest absorbers have the highest probability of finding halos. When applying a hierarchical framework, where we allow the largest SBLAs to consume the smaller ones, we find that the halo mass distributions for each SBLA spectral size becomes narrower with respect to the non-hierarchical case. We are also able to probe halo masses from $M_h \approx 10^{9.5} M_{\odot}$ (for 100 km s$^{-1}$ SBLAs) to $M_h \approx 10^{11.5} M_{\odot}$ (for 450 km s$^{-1}$ SBLAs). With these results, we show that we are able to transform the Lyman-$α$ forest into a halo finding machine for not only identifying CGM regions, but also estimating their host halo masses.

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arXiv2026

Inferring the mass of the circumgalactic medium using X-ray resonant scattering

The circumgalactic medium (CGM) regulates galaxy growth and retains the imprint of feedback from supernovae and supermassive black holes. However, the bulk of the hot CGM produces little X-ray emission and is challenging to study with X-ray telescopes. We propose a novel method for evaluating the CGM mass using resonant scattering of the helium-like oxygen (\ovii) resonant line at $E=574$ eV. In a spherically symmetric and static CGM halo with a sharp central X-ray peak, the number of \ovii\ ions within an outer radial shell can be calculated from the ratio of the two directly observable quantities: the \ovii\ flux from the bright inner region and the scattered \ovii\ flux from the shell (where the scattered flux can be much higher than the intrinsic emission). To evaluate the accuracy of this geometric estimate for realistic galaxies -- with satellites, asymmetries, and gas velocities -- we use a sample of galaxies from the TNG50 cosmological simulation. We find that, when the most irregular systems are excluded based on their X-ray observables, we accurately predict the \ovii\ mass in the outer halo (e.g., in an $r=R_{\rm 500c}-R_{\rm 200c}$ shell) from the ratio of the fluxes in the corresponding annulus and the central peak region ($r<0.2R_{\rm 500c}$), with only a 10\% bias and an rms scatter of $\sim 0.2$ dex. As \ovii\ mass strongly correlates with the total oxygen and gas mass, this direct \ovii-counting method enables indirect estimates of those quantities by future X-ray microcalorimeter missions, such as {\em NewAthena}\/ and {\em HUBS}.

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arXiv2026

Turbulence in the multi-phase circumgalactic medium: bridging TNG50 and idealized kpc-scale simulations

The circumgalactic medium (CGM) is the extended multi-phase gaseous halo surrounding a galaxy. To study how its dynamics are shaped by the interplay of radiative cooling and turbulence, we first turn to the TNG50 cosmological magnetohydrodynamical simulation. We analyze the turbulent properties of the CGM of Milky Way-like galaxies at z=0, using a multi-scale filtering technique. We find that the CGM hosts predominantly subsonic flows with Mach number ~ 0.1 - 0.7 that are roughly balanced between compressive and solenoidal components with b = M_comp / M_tot ~ 0.3 - 0.6. We then use these physically motivated properties of turbulent CGM media to set the initial conditions and driving parameters for idealized, turbulent box simulations. These represent ~ 1 kpc^3 volumes of the CGM, but reach resolutions higher than available in cosmological volumes such as TNG50. To do so, we implement a turbulent driving algorithm into the AREPO code based on random but smoothly time-fluctuating acceleration fields, and then run a suite of turbulent boxes across the parameter space, including a model for metal-line radiative cooling. Using these simulations, we study how turbulence seeds density fluctuations that trigger thermal instability and promotes mixing between hot and cold phases, depending on the relative timescales of cooling and mixing. The abundance and evolution of cold ~10^4 K gas depends strongly on the strength of turbulence and the efficiency of cooling.

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arXiv2026

The host halo masses of AGNs and quasars at $z \sim 3-7$ with TNG-Cluster, FLAMINGO and other cosmological galaxy simulations

Most observations and clustering analyses suggest that quasars inhabit a narrow range of dark-matter halo masses ($10^{12-13}$ M$_{\odot}$) across cosmic time ($z\lesssim7$). Recent hydrodynamical simulations in gigaparsec-scale volumes now enable direct comparison of this picture with self-consistent galaxy-formation models. We quantify the relation between AGN bolometric luminosity and host halo mass before Cosmic Noon in TNG300, TNG-Cluster, FLAMINGO L1_m8 and L2p8_m9, and in smaller-volume simulations (Illustris, EAGLE, TNG100, and Simba). For AGNs with $L^{\mathrm{AGN}}_{\mathrm{bol}} \ge 10^{42}$ erg s$^{-1}$, more massive haloes host more luminous AGNs on average, but only up to a certain mass. The median luminosity-halo mass relation is highly non-linear, with large scatter, and flattens (FLAMINGO) or turns over (TNG300+TNG-Cluster) at halo mass, $M_{\mathrm{200,crit}} \gtrsim10^{12}$ M$_{\odot}$, at least at $z<5$-6. This high mass AGN quenching also manifests as a characteristic quasar host halo mass: in TNG300+TNG-Cluster, quasars ($L^{\mathrm{AGN}}_{\mathrm{bol}} \sim10^{45-47}$ erg s$^{-1}$) typically reside in haloes of mass $10^{12-12.5}$ M$_{\odot}$ at $z=3$-6, while FLAMINGO quasars extend to median masses of $\sim10^{12.8}$ M$_{\odot}$ at $z\sim3$-4. All simulations predict substantially larger scatter in AGN luminosity at fixed halo mass than in halo mass at fixed luminosity (up to 3 dex versus $\lesssim1$ dex between the 5th and 95th percentiles), implying weak coupling between halo growth and instantaneous SMBH accretion. Consequently, simulated quasar host masses broadly agree with observational estimates. The most luminous AGNs occupy increasingly rare haloes at earlier epochs but typically do not reside in the most massive haloes at any redshift up to $z\approx7$.

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arXiv2026

The Synthetic Absorption Line Spectral Almanac (SALSA)

We create the first large-scale mock spectroscopic survey of gas absorption sightlines traversing the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM) surrounding galaxies of virtual Universes. That is, we create mock, or synthetic, absorption spectra by drawing lines-of-sight through cosmological hydrodynamical simulations, using a new mesh-free Voronoi ray-tracing algorithm. The result is the Synthetic Absorption Line Spectral Almanac (SALSA), which is publicly released on a feature-rich online science platform (www.tng-project.org/spectra). It spans a range of ions, transitions, instruments, observational characteristics, assumptions, redshifts, and simulations. These include, but are not limited to: (ions) HI, OI, CI, MgI, MgII, FeII, SiII, CaII, ZnII, SiIII, SiIV, NV, CII, CIV, OVI; (instruments) SDSS-BOSS, KECK-HIRES, UVES, COS, DESI, 4MOST, WEAVE, XSHOOTER; (model choices) with/without dust depletion, noise, quasar continua, foregrounds; (redshift) from z=0 to z~6; (ancillary data) integrated equivalent widths, column densities, distances and properties of nearby galaxies; (simulations) IllustrisTNG including TNG50, TNG-Cluster, EAGLE, and SIMBA. This scope is not fixed, and will grow and evolve with community interest and requests over time -- suggestions are welcome. The resulting dataset is generic and broadly applicable, enabling diverse science goals such as: (i) studies of the underlying physical gas structures giving rise to particular absorption signatures, (ii) galaxy-absorber and halo-absorber correlations, (iii) virtual surveys and survey strategy optimization, (iv) stacking experiments and the identification of faint absorption features, (v) assessment of data reduction methods and completeness calculations, (vi) inference of physical properties from observables, and (vii) apples-to-apples comparisons between simulations and data.

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arXiv2026

Bulk vs. turbulent motions at the centres of galaxy clusters: AGN-driven turbulence according to TNG-Cluster

The highly dynamic intracluster medium (ICM) influences cluster thermodynamic evolution and probes key physical processes. Quantifying the non-thermal motions is therefore essential for understanding cluster physics and interpreting high spectral-resolution X-ray observations from telescopes like {\it XRISM}. We quantify bulk and turbulent gas motions in 352 galaxy clusters at $z=0$ (${\rm M_{200c}=10^{14.3-15.4}\, M_\odot}$) from the TNG-Cluster suite of magneto-hydrodynamical galaxy simulations. We use a multi-scale filtering Reynolds decomposition to separate total gas velocities into bulk (coherent) and turbulent (small-scale fluctuations) components. We primarily focus on the hot X-ray emitting gas in the central core regions. According to TNG-Cluster, majority of the ICM has subsonic turbulence but with broad velocity distributions reaching $\mathcal{M}_{\rm Turb}\sim 10$ and large cluster-to-cluster variations. In cluster centres, turbulence contributes less than half of the total velocity dispersion $(σ_{v\rm,Turb } \sim 0.5 ~σ_{v,\rm Total})$ for most clusters, with typical turbulent velocity dispersions of $50-75$ km s$^{-1}$ across the mass range, and with sub per cent levels of turbulent pressure support. Clusters that are strong cool cores, or have X-ray cavities, or experienced recent SMBH feedback energy injections exhibit systematically larger turbulent velocity dispersions and more prominent turbulent velocity tails. On average, the turbulent velocity dispersion peaks in cluster centres, decreases slightly to a minimum at $0.1-0.2 \, R_{\rm500c}$, then rises again. Our analysis shows that SMBH feedback is a key driver of turbulence in cluster cores, generating strong but short-lived motion alongside high-velocity outflows. It also calls for caution for interpreting {\it XRISM} observations.

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arXiv2026

Astronomical Advantages of a Boost Mission to Facilitate HST Science into the 2030s: Imaging the Circumgalactic Medium of Galaxies

We present the case for imaging ultraviolet line emission from highly ionized metals and HI Lya in the circumgalactic medium of galaxies, should the Hubble Space Telescope receive an orbital boost. Hubble can uniquely probe emission lines with ionization potentials between 13 and 200 electron-volts (Lya, CIV, OVI, NeVIII, etc). Spatial mapping of the diffuse material traced by these transitions is critical to constraining the physics of feedback and the energetic exchange between galaxies and their circumgalactic environments, as well as basic morphologies of the dominant mass component. Deep high-resolution mapping of these features will not be possible with any other observatory, existing or planned, until HWO is launched, which leaves HST as a critical observatory to test key science drivers for HWO. If HST receives an orbital boost, it can (a) provide the first statistical constraints on the spatial distribution of warm-hot CGM and (b) provide important avenues for science case development, as well as target/pointing selection, for HWO's upcoming spectroscopic facilities.

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arXiv2026

Analysis of synthetic OVI absorption associated with galaxy groups in SIMBA and TNG50 simulations

We compare OVI absorption in synthetic spectra from galaxy groups in the SIMBA and TNG50 cosmological hydrodynamic simulations against those observed from the COS-IGrM survey. We select 14 galaxy groups from each simulation with $12.89 \le \log(M_{\rm halo}/M_\odot) \le 13.61$, closely matching COS-IGrM, and create 90,000 synthetic spectra per group. We demonstrate the utility of synthetic absorption spectroscopy when comparing simulations with QSO absorption-based observations. We investigate absorber properties such as radial distributions and kinematics with respect to the group and nearest galaxy. The OVI covering fraction ($f_{\rm OVI}$) in TNG50 ($20.62 \pm 2.56\%$) and SIMBA ($5.98 \pm 0.82\%$) are both systematically lower than COS-IGrM ($44 \pm 5\%$). Kinematic analysis reveals that vast majority ($\sim 95\%$) of absorbers in both the simulations are gravitationally bound. In TNG50, strong absorbers ($\log N_{\rm OVI} > 15$) are located near star-forming galaxies ($\log {\rm sSFR} > -11$) within $\sim 200$kpc, suggesting physical connection to stellar feedback, whereas SIMBA shows no comparable trend. Furthermore, in TNG50 occurrence of OVI absorbers at small impact parameters increases with stellar mass of nearest galaxy, but shows no dependence on total stellar mass of group. In contrast, SIMBA shows no clear correlation with nearest galaxy's stellar mass, though groups with higher total stellar mass exhibit higher detection rate at larger impact parameters. Differences observed in simulations may arise from feedback models and resolution effects. Finally, we show absorber analysis methodology is important factor when comparing simulations with absorption spectroscopy observations.

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arXiv2026

Numerical effects on the stripping of dark matter and stars in IllustrisTNG galaxy groups and clusters

The stellar haloes and intra-cluster light around galaxies are crucial test beds for dark matter (DM) physics and galaxy formation models. We consider the role that the numerical resolution plays in the modelling of these systems by studying the stripping of satellites in the IllustrisTNG cosmological simulations. We focus on host haloes of total halo mass $M_{\mathrm 200c}=10^{12-15}M_{\odot}$ and satellites of stellar mass $>10^{7}$$M_{\odot}$, and compare stellar halo / satellite properties across 9 IllustrisTNG runs with baryonic particle mass resolution between $8.5\times10^4M_{\odot}$ and $7\times10^8$$M_{\odot}$, using a Lagrangian-region technique to identify counterpart satellites across different resolution simulations of the same volume. We publish the corresponding catalogues alongside this paper. We demonstrate that the stripping of DM from satellites that orbit in group- and cluster-mass hosts is largely independent of resolution at least until 90 per cent of their initial mass at infall has been stripped. We do not find evidence for spurious disruption of galaxies due to insufficient resolution for the satellite masses we consider. By contrast, the stripping of stellar mass is strongly resolution-dependent: each factor of 8 improvement in particle stellar mass typically adds 2Gyr to the stripping time. Improved numerical resolution within the IllustrisTNG model generally results in more compact satellites with larger stellar masses, which in turn generate more centrally concentrated stellar haloes and intra-cluster mass profiles. However, the concomitant increase in stellar mass with increased resolution of both satellites and hosts may still be the cause for the overprediction of the stellar halo mass at large host radii relative to observations seen in some previous studies.

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arXiv2026

Zooming in on the circumgalactic medium with GIBLE: Cloud-scale simulations with cosmological initial conditions

We conduct simulations of $\sim$kpc-scale cool clouds in the circumgalactic medium (CGM), using initial conditions sampled from a highly resolved cosmological magneto-hydrodynamical zoom-in of a Milky Way-like galaxy. We select ten distinct cold clouds with masses of $m_{\rm{cloud}}$ $\sim$ $10^{4.5-5}$ M$_\odot$, originally resolved at a mass resolution of $m_{\rm{gas}}$ $\sim$ $200$ M$_\odot$. To further resolve small-scale features and physics, we implement a targeted refinement scheme within spherical regions co-moving with each cloud, thereby boosting the local mass resolution by a factor of 1000, reaching $m_{\rm{gas}}$ $\sim$ $0.2$ M$_\odot$ (spatial resolution, $r_{\rm{gas,cloud}}$ $\sim$ $O(\rm{pc})$). The selected clouds have diverse properties, across a broad parameter space, resulting in heterogeneous evolution. For the clouds we study, radiative cooling is the dominant physical process enabling cloud survival, while magnetic fields play a comparatively smaller role. The motion of these clouds is governed not only by drag forces that decelerate them, but also by acceleration from momentum exchange with the complex background velocity field, which can cause them to move faster than ballistic projectiles set by their initial velocities. Our results suggest that the non-trivial details of realistic cosmological initial conditions -- specifically the complex density, temperature, and velocity fields -- may play an important role in subsequent cloud evolution, and that sampling the output of an existing large-scale simulation provides a self-consistent approach to capture these effects without ad hoc assumptions.

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arXiv2026

The temperature and metallicity distributions of the ICM: insights with TNG-Cluster for XRISM-like observations

The new era of high-resolution X-ray spectroscopy will significantly improve our understanding of the intra-cluster medium (ICM) by providing precise constraints on its underlying physical properties. However, spectral fitting requires reasonable assumptions on the thermal and chemical distributions of the gas. We use the output of TNG-Cluster, the newest addition to the IllustrisTNG suite of cosmological magnetohydrodynamical simulations, to provide theoretical expectations for the multi-phase nature of the ICM across hundreds of z=$ clusters (M$_{500c} = 10^{14.0-15.3}~M_\odot$) based upon a realistic model for galaxy formation and evolution. We create and analyse, in an observer-like manner, end-to-end XRISM/Resolve mock observations towards cluster centres. We then systematically compare the intrinsic temperature and Fe abundance of the simulated gas with the inferred ones from spectral fitting via a variety of commonly used spectral-emission models. Our analysis suggests that models with a distribution of temperatures, better describe the broad thermal distributions of the ICM, as predicted by TNG-Cluster, but still incur biases in the inferred temperature of 0.5-2 keV (16th-84th percentiles). However, all spectral-emission models systematically underestimate the Fe abundance of the central ICM by 0.12 Solar (22 per cent), almost an order of magnitude higher than the abundance errors reported in the literature, primarily due to projection effects. Selecting only strong cool core clusters leads to minor improvements on inference quality, removing the majority of outliers but maintaining similar overall biases and cluster-to-cluster scatter.

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arXiv2026

ERGO-ML: The assembly histories of HSC galaxy images via invertible neural networks, contrastive learning, and cosmological simulations

In this paper of ERGO-ML (Extracting Reality from Galaxy Observables with Machine Learning), we develop a model that infers the merger/assembly histories of galaxies directly from optical images. We apply the self-supervised contrastive learning framework NNCLR (Nearest-Neighbor Contrastive Learning of visual Representations) on realistic HSC mock images (g,r,i - bands) produced from galaxies simulated within the TNG50 and TNG100 flagship runs of the IllustrisTNG project. The resulting representation is then used as conditional input for a cINN (conditional Invertible Neural Network) to gain posteriors for merger/assembly statistics, particularly the lookback time and stellar mass of the last major merger and the fraction of ex-situ stars. Through validation against the ground truth available for simulated galaxies, we assess the performance of our model, achieving good accuracy in inferring the stellar ex-situ fraction ($\le \pm 10$ per cent for 80 per cent of the test sample) and the mass of the last major merger (within $\pm 0.5 \log \MSUN$ for stellar masses $>10^{9.5} \MSUN$ ). We successfully apply the TNG-trained model to simulated mocks from the EAGLE simulation, demonstrating that our model is applicable outside of the TNG domain. We use our simulation-based model to infer aspects of the history of observed galaxies, in particular for HSC images that are close to the domain of TNG ones. We recover the trend of increasing ex-situ stellar fraction with stellar mass and more spherical morphology, but we also identify a discrepancy between TNG and HSC: on average, observed galaxies generally exhibit lower ex-situ fractions. Despite challenges such as information loss (e.g. projection effects and surface brightness limits) and domain shifts (from simulations to observations), our results demonstrate the feasibility of extracting the merger past of galaxies from their optical images.

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