The stellar-to-halo mass relation of central galaxies across three orders of halo mass

arXiv:2602.10193 · astro-ph.GA, astro-ph.HE · Submitted 2026-02-10 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "The stellar-to-halo mass relation of central galaxies across three orders of halo mass".

Jocelyn: The gist: This work presents an observational stellar-to-halo mass relation (SHMR) spanning three orders of halo mass,

Vera: First, who's behind it and why it matters.

Title and authors: Vera: The title itself tells you a lot about the scope, "The stellar-to-halo mass relation of central galaxies across three orders of halo mass." It suggests they aren't just looking at one regime; they're trying to capture the whole picture from small groups up to massive clusters.

Jocelyn: It really sounds like they are connecting the properties of the central galaxy, which we can measure through its light, to the properties of its entire host halo, which we get from X-ray data.

Subrahmanyan: That connection is key for us because it tells us how efficiently baryons turn into stars across different cosmic environments.

Vera: They are using X-ray data to derive halo masses and optical photometry to find those central galaxy stellar masses. It’s a combined approach that tries to be very comprehensive about what's happening in these systems.

Jocelyn: And the authors, Toptun et al., they are bringing together observational constraints from both X-ray surveys and galaxy light profiles.

Subrahmanyan: This combination lets them test how feedback mechanisms affect star formation efficiency as you move to deeper gravitational potentials.

The paper's summary: Jocelyn: So, what the paper actually does is look at the SHMR across three orders of halo mass, and they find a specific peak in that relation.

Vera: That peak happens near Mhalo ∼ one thousand twelve M⊙, and then the stellar fraction starts to decline as you move into higher mass halos <ref:2602.10193#pg2>.

Subrahmanyan: That declining trend is what they attribute to several physical processes working together—AGN feedback, reduced cooling efficiency, and the increasing role of ex-situ assembly in more massive halos.

Vera: They find that around that peak mass, where stellar and AGN feedback are roughly balanced, those halos are at their most efficient point for converting baryons into stars.

Jocelyn: It also confirms the expected link between the stellar content of the central galaxy and how deep the halo's potential well is.

Subrahmanyan: That direct link between galaxy properties and halo properties is what makes this result so important for our models of structure formation.

The paper's improvements: Vera: The authors suggest a few ways to make the analysis more robust, which I think are pretty smart. They use two independent binning schemes, one based on the central galaxy stellar mass and another based on an optical halo mass proxy.

Jocelyn: That bidirectional approach is interesting because it checks if the trend they find holds up regardless of which way you choose to cut the data.

Subrahmanyan: It’s a good way to ensure that their conclusion about where that peak is, near Mhalo ∼ one thousand twelve M⊙, isn't just an artifact of how they chose to group the data <ref:2602.10193#pg2>.

Vera: They also tackle the issue of getting reliable halo masses from X-ray data by stacking X-ray spectra of optically selected groups.

Jocelyn: That stacking method helps recover robust average gas temperatures for each bin, and then they use that mean temperature to convert it into a halo mass using the M–TX relation.

Subrahmanyan: This addresses a real limitation where individual X-ray measurements can be too faint for reliable halo mass estimates in many samples.

Conclusion: Vera: So, to wrap up on "The stellar-to-halo mass relation of central galaxies across three orders of halo mass," the main implication is that there's a clear peak efficiency point around Mhalo ∼ one thousand twelve M⊙ <ref:2602.10193#pg2>.

Jocelyn: And beyond that, the stellar fraction drops off as halo mass increases, which points to feedback mechanisms becoming more dominant in massive halos.

Subrahmanyan: It confirms our understanding of how star formation is regulated by the balance between cooling and quenching processes across different scales.

Vera: The paper does a solid job showing this correlation is consistent with what we see from other methods, like weak lensing and X-ray analyses of massive clusters.

Jocelyn: They also align well with semiempirical approaches like abundance matching when looking at the results within one sigma.

Subrahmanyan: This strengthens the picture because it shows that these detailed measurements fit into the larger cosmological framework we're building.

Vera: So, this paper provides a very clear map of where star formation efficiency peaks in galaxy halos, and it uses a multi-pronged approach to get those numbers.

Jocelyn: It's a solid piece of observational evidence connecting the smallest groups to the largest clusters.

Subrahmanyan: This SHMR study really helps us refine our theoretical models about how feedback shapes galaxy growth over cosmic time.

Victoria Toptun, Paola Popesso, Ilaria Marini, Stephan Vladutescu-Zopp, Klaus Dolag, Peter Behroozi, Lorenzo Lovisari, Stefano Ettori, Veronica Biffi,  Xiaohu Yang 12©, 13©, Natanael de Is퇭O 1, Я. Mazengo

European Southern Observatory · Excellence Cluster ORIGINS · Universitäts-Sternwarte Ludwig-Maximilians-University Munich · Max-Planck-Institut för Astrophysik Karl Schwarzschildstr. 1 Garching bei München · Department of Astronomy and Steward Observatory University of Arizona · INAF Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano · Center for Astrophysics Harvard & Smithsonian INAF Osservatorio di Astrofisica e Scienza dello Spazio INAF Osservatorio Astronomico di Trieste IFPU Institute for Fundamental Physics of the Universe State Key Laboratory of Dark Matter Physics Tsung-Dao Lee Institute School of Physics and Astronomy Shanghai Jiao Tong University Shanghai Key Laboratory for Particle Physics and Cosmology and Key Laboratory for Particle Physics Ministry of Education Shanghai Jiao Tong University Department of Astronomy and Mathematical Sciences The University of Dodoma

astro-ph.GA, astro-ph.HE

Submitted: 2026-02-10

Updated: 2026-10-14

Comments: Published in A&A; 14 pages, 11 figures

Journal ref: A&A, 712, A212 (2026)

DOI: 10.1051/0004-6361/202659359

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 77/100

The gist: The gist: This work presents an observational stellar-to-halo mass relation (SHMR) spanning three orders of halo mass, using X-ray data to derive halo masses and optical photometry to derive central

Key concepts

Stellar-to-Halo Mass Relation (SHMR)
This is a relationship showing how the total amount of stars in the brightest central galaxy (BCG) scales with the total mass of its surrounding dark matter halo. It helps scientists understand how galaxies assemble their stellar content based on the environment they live in.
Halo Gas Temperature ($T_X$)
Researchers used stacking X-ray spectra of galaxy groups to find the average temperature of the hot gas inside these halos. This temperature is a key property used to estimate the total mass of the dark matter halo through established physical relationships.
Feedback Processes
These are physical mechanisms, such as Active Galactic Nuclei (AGN) feedback, that inject energy into galaxy halos. These processes regulate how efficiently gas cools and forms stars, explaining why stellar growth efficiency changes depending on the halo's mass.
$M$-$T_X$ Relation
This is a mathematical relationship linking the measured X-ray gas temperature ($T_X$) to the total mass of the dark matter halo ($M_{ ext{halo}}$). By using this relation, researchers were able to convert the observed gas temperatures into accurate estimates for each halo's total mass.

Terminology

Summary

The gist: This work presents an observational stellar-to-halo mass relation (SHMR) spanning three orders of halo mass, using X-ray data to derive halo masses and optical photometry to derive central galaxy stellar masses.

Methodology

  1. The study uses halo masses derived from eROSITA eRASS1 X-ray data and BCG stellar masses based on SDSS photometry

  2. To recover robust average halo gas temperatures for each bin, the researchers used stacking X-ray spectra of optically selected groups

  3. These recovered gas temperatures are then converted to halo masses via the M–TX relation

  4. The analysis involves two independent binning schemes: one based on BCG stellar mass and one based on the optical halo mass proxy

  5. The spectral modeling for each bin utilized multiple components: an intracluster medium (ICM) component, a power-law component representing unresolved AGN, and Galactic line-of-sight absorption

  6. The ICM emission was described using the gadem2 model, which represents a multi-temperature plasma with a Gaussian distribution of emission measure

  7. The mean temperature was allowed to vary during the fitting process, while the width of the temperature distribution was fixed to reduce the number of free parameters

  8. The final SHMR is derived by fitting a power law or a double power law to describe the relation across halo mass ranges

Key Findings

The SHMR peaks near Mhalo ∼ 1012 M⊙, with a declining stellar fraction at higher masses

This trend reflects a combination of processes that reduce the efficiency of stellar mass growth in massive halos, such as AGN feedback, reduced cooling efficiency, and the increasing dominance of ex-situ assembly

Around the peak mass, where stellar and AGN feedback are comparably effective, halos reach their maximum efficiency in converting baryons into stars

The observed correlation confirms the expected connection between the stellar content of BCGs and the depth of the halo potential, providing a direct link between galaxy properties and halo properties

Comparison with Previous Studies

  1. The derived SHMR measurements are broadly consistent with previous observational studies based on weak lensing, X-ray analyses of massive clusters, and satellite kinematics

  2. The results are also consistent with semiempirical approaches such as abundance matching and empirical modeling within 1σ

Improvements for AI systems

  1. Based on We find that the SHMR peaks near Mhalo ∼ 1012 M⊙, with a declining stellar fraction at higher masses, an improved AI system can accurately predict how galaxy stellar content scales with host halo mass across three orders of magnitude, specifically identifying the peak efficiency point near the characteristic mass of group-scale halos.

  2. The system can now robustly estimate X-ray derived halo masses by using stacking X-ray spectra of optically selected groups and converting them via the M–TX relation, overcoming limitations where X-ray emission is generally too faint for reliable measurements of individual halos for the most of the sample.

  3. The AI can establish a robust benchmark for galaxy formation models by incorporating the derived SHMR, which is described as a relation that reflects a combination of processes that reduce the efficiency of stellar mass growth in massive halos, such as AGN feedback, reduced cooling efficiency, and the increasing dominance of ex-situ assembly.

  4. The system can perform cross-validation between different halo mass estimation methods by using bidirectional binning approach comparing results from BCG stellar mass and the optical halo mass proxy, ensuring that trends are consistent independent of the choice of binning variable.

  5. The improved system can analyze the sensitivity of galaxy formation models to physical assumptions by testing how results change based on metal abundance, noting that the lowest-mass bins, corresponding to a regime closer to individual galaxies than to clusters, are more sensitive to the assumed metal abundance in the spectral modeling.

  6. The AI can compare its predictions against hydrodynamical simulations like Magneticum and EAGLE, identifying discrepancies such as those where Illustris and Magneticum tend to slightly overpredict the stellar-to-halo mass ratio at specific high-mass regimes.

Abstract

The stellar content of galaxies is tightly connected to the mass and growth of their host dark matter halos. Observational constraints on this relation remain limited, particularly for low-mass groups, leaving uncertainties in how galaxies assemble their stars across halo mass scales. Accurately measuring the brightest central galaxy (BCG) stellar-to-halo mass relation (SHMR) over a wide mass range is therefore crucial for understanding galaxy formation and the role of feedback processes. Here we present the SHMR spanning M halo about 10 12 - 10 15,M, using halo masses derived from eROSITA eRASS1 X-ray data and BCG stellar masses based on SDSS photometry. By stacking X-ray spectra of optically selected groups, we recover robust average halo gas temperatures for each bin, which are then converted to halo masses via the M - T X relation. We find that the SHMR peaks near M halo about 10 12,M, with a declining stellar fraction at higher masses. This trend reflects a combination of processes that reduce the efficiency of stellar mass growth in massive halos, such as AGN feedback, reduced cooling efficiency, and the increasing dominance of ex-situ assembly, while halos continue to grow through mergers and accretion. Our measurements are consistent over the full mass range with previous observational studies, including weak lensing, X-ray analyses of individual clusters, and kinematical and dynamical methods. Comparisons with hydrodynamical simulations show good agreement at low masses but reveal significant discrepancies in the normalization at cluster scales, highlighting the sensitivity of BCG stellar growth to feedback prescriptions and halo assembly history. These results provide the first X-ray-based observational SHMR covering three orders of magnitude in halo mass, establish a robust benchmark for testing galaxy formation models.

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