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The mass-size plane of EAGLE galaxies

T0 review · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read EAGLE simulated galaxies reproduce the observed mass-size plane, with stellar age varying along lines of constant velocity dispersion and age acting as a spin proxy in flattened galaxies.

arxiv 1908.00416 v1 pith:PPR43YDM submitted 2019-08-01 astro-ph.GA astro-ph.CO

María Sol Rosito , Patricia Tissera , Susana Pedrosa , Claudia Lagos This is my paper · ORCID
classification astro-ph.GAastro-ph.CO
keywords galaxiesplanedispersionmass-sizevelocityparameterseaglefound
verification ladder T0 review T1 audit T2 compute T3 formal

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The reading

Galaxies come in two broad flavors: flat rotating disks like the Milky Way, and round, slowly rotating ellipticals. Astronomers have recently found that both kinds obey a mass-size plane: a galaxy's mass and radius are linked, and the scatter around that link correlates with age, velocity dispersion, and metal content. This paper asks whether a leading computer model of the universe, the EAGLE simulation, can reproduce these patterns.

The authors took 1,700 well-resolved galaxies from a simulated box of space 100 megaparsecs on a side. They split them into disk-dominated and spheroid-dominated galaxies, then mapped each galaxy's age, velocity dispersion, chemical content, and spin across the mass-size plane.

The main results: simulated galaxies do fall on the observed mass-size plane, especially at intermediate masses. Older stars sit in more compact galaxies at a fixed dynamical mass. Stellar ages vary along lines of constant velocity dispersion, exactly as seen in the MaNGA survey, except for the fastest galaxies, where the trend flattens. Extended disks have negative age gradients (younger stars further out), matching inside-out growth. But early-type galaxies in the simulation show too many inverted age profiles, and the simulated metallicity gradients are much weaker than observed.

A new finding is that a galaxy's stellar age tracks its spin parameter in flattened systems: slow rotators are old, fast rotators are young. The spin parameter distribution is bimodal, with disks of young stars on one peak and old slow rotators on the other. The authors interpret this as a shared origin: the processes that shut off star formation and age a galaxy also redistribute its angular momentum and change its shape.

Extended reading notes

Core claim

The central claim is that the EAGLE simulation reproduces the observed mass-size plane correlations: galaxies with similar median stellar ages move approximately along lines of constant sigma_e, in agreement with observations (Section 3, Fig. 2), and stellar-weighted ages are good proxies for lambda in galaxies with low ellipticity (Abstract). If true, the current cosmological paradigm explains the scatter on the plane and links star formation history, angular momentum, and morphology.

Load-bearing premise

The EAGLE subgrid prescriptions for star formation, chemical enrichment, and feedback produce reliable z=0 stellar age, metallicity, and kinematic gradients, even though these outputs are predictions rather than calibration targets of the model (Section 2, after Schaye et al. 2015). The age-gradient and age-spin conclusions would be invalidated if the subgrid choices imprint artificial radial or kinematic structure. The authors flag this residual uncertainty themselves in Section 4: further work on the multi-phase nature of the ISM in simulations could naturally lead to more realistic internal kinematics and profiles of galaxies.

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Assumptions & free parameters 5 free parameters · 5 assumptions · 0 invented entities

No new physical entities are introduced. The analysis depends on five chosen thresholds or fitting choices (morphological split, slow-rotator boundary, radial fit interval, LOESS smoothing, resolution cut) and on the EAGLE model itself as a domain assumption. The virialization assumption in Section 2 is the least avoidable of the axioms and affects both axes of the mass-size plane.

free parameters (5)
  • B/T = 0.5 morphological split = 0.5
    Threshold separating spheroid-dominated (E-SDG) from disc-dominated (E-DDG) galaxies, adopted from Rosito et al. 2018. Results such as the slow-rotator fraction and the age-morphology link depend on this cut.
  • lambda = 0.2 slow/fast rotator boundary = 0.2
    Boundary separating slow from fast rotators, taken from Fig. 3 of Lagos et al. 2018a. The reported slow-rotator fractions (55 percent, 8 percent, 28 percent, 2 percent) depend on which threshold definition is used.
  • Radial fitting interval for gradients = 0.7 kpc to Rhm
    Gradients are fit from three times the gravitational softening to the half-mass radius, following Li2018. The measured age and metallicity gradient values depend on this interval.
  • LOESS smoothing parameters
    Two-dimensional locally weighted regression smoothing is applied to all plane distributions, but the span and degree are not specified and colorbars use first and third quartiles as limits. The apparent sharpness of trends on the plane depends on these unspecified choices.
  • Sample resolution cut = 10,000 star particles
    Galaxies with more than 10,000 star particles within the optical radius are selected for analysis. The plane trends and gradient statistics could change with this resolution threshold.
assumptions (5)
  • domain assumption Lambda-CDM cosmology with Planck parameters
    The EAGLE simulation assumes a Lambda-CDM universe with Planck cosmology (Section 2). All results inherit this cosmological framework.
  • domain assumption EAGLE subgrid physics produces realistic galaxy populations
    The analysis assumes the EAGLE implementations of radiative cooling, star formation, chemical evolution, and stellar and AGN feedback yield galaxies representative of the observed universe (Section 2).
  • domain assumption Virialization for dynamical mass estimates
    Dynamical masses are estimated assuming virialization for both simulations and observations, as in Rosito et al. 2018 (Section 2). Both axes of the dynamical mass-size plane depend on this assumption.
  • domain assumption Projected mock observations mimic real IFS measurements
    Lambda and epsilon are r-band luminosity-weighted line-of-sight stellar values measured at the projected half-light radius with random inclinations, taken from Lagos et al. 2018a to best mimic observational procedures (Section 2). Comparability with MaNGA rests on this.
  • domain assumption Stellar mass-weighted ages are the right age proxy
    The paper uses stellar mass-weighted ages throughout, while observed ages from surveys such as Li2018 are typically luminosity-weighted. This weighting mismatch is not discussed as a systematic in the comparison.

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Pith. "Pith review of The mass-size plane of EAGLE galaxies." pith.science (2026). https://pith.science/paper/PPR43YDM

@misc{pith2026190800416,
  author       = {Pith},
  title        = {Pith review of: The mass-size plane of EAGLE galaxies},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/PPR43YDM}},
  note         = {Machine review of arXiv:1908.00416}
}
read the original abstract

Current observational results show that both late-and-early-type galaxies follow tight mass-size planes, on which physical properties such as age, velocity dispersion and metallicities correlate with the scatter on the plane. We study the mass-size plane of galaxies in cosmological hydrodynamical simulations, as a function of velocity dispersion, age, chemical abundances, ellipticity and spin parameters with the aim at assessing to what extent the current cosmological paradigm can reproduce these observations and provide a physical interpretation of them. We select a sample of well-resolved galaxies from the (100 Mpc)^3 simulation of the EAGLE Project. This sample is composed by 508 spheroid-dominated galaxies and 1213 disc-dominated galaxies. The distributions of velocity dispersion, age, metallicity indicators and gradients and spin parameters across the mass-size plane are analysed. Furthermore, we study the relation between shape and kinematic parameters. The results are compared with observations. The mass-weighted ages of the EAGLE galaxies are found to vary along lines of constant velocity dispersion on the mass-size plane, except for galaxies with velocity dispersion larger than aprox 150 km s^(-1) . Negative age gradients tend to be found in extended disc galaxies in agreement with observations. However, the age distributions of early-type galaxies show a larger fraction with inverted radial profiles. The distribution of metallicity gradients does not show any clear dependence on this plane. Galaxies with similar spin parameters ({\lambda}) display larger sizes as their dynamical masses increase. Stellar-weighted ages are found to be good proxies for {\lambda} in galaxies with low ellipticity ({\epsilon}). Abridged

Figures

Figures reproduced from arXiv: 1908.00416 by the authors.

Figure 1
Figure 1. Velocity dispersion (σe) LOESS-smoothed distributions on the stellar mass-size plane for E-SDGs (left panel), E-DDGs (middle panel) and all galaxies (right panel) in the EAGLE simulation at z = 0. The median relations for the EAGLE galaxies (pink rhombus) are also shown. For comparison the median relations for passive (left panel), active (middle panel) and all galaxies together (right panel) reported by van der Wel… view at source ↗
Figure 2
Figure 2. Stellar mass-weighted average age (lower panels) LOESS-smoothed distributions on the dynamical mass-size plane for E-SDGs (left panel), E-DDGs (middle panel) and all galaxies (right panel) in the EAGLE simulation at z = 0. The dashed lines show the predicted distributions for systems with constant σe at 100, 125, 150, 200, 225, 250 and 300 km s−1 (from left to right). plane on a variety of galaxy properties. Our mai… view at source ↗
Figure 3
Figure 3. Age (upper panels) and [O/H] (lower panels) gradients LOESS-smoothed distributions on the mass-size plane for E-SDGs (left panels), E-DDGs (middle panels) and all galaxies (right panels) in EAGLE simulation at z = 0. Dashed lines are the same as given in [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Upper panel: Spin parameter (λ) distribution on the mass-size plane for E-SDGs (left panel), E-DDGs (middle panel) and all galaxies (right panel) in the EAGLE simulation. Lines are the same as given in [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]

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