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The angular momentum of baryons and dark matter halos revisited

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Taysun Kimm, Julien Devriendt, Adrianne Slyz, Christophe Pichon, Susan A. Kassin, Yohan Dubois

Cosmology and Nongalactic Astrophysics · Submitted 3 Jun 2011 · 75 references

Pith's verdict: The accretion-weighted spin measurement and the adiabatic null test are real contributions; the 'all masses, all redshifts' claim is broader than the 1–2 kpc runs can carry.

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arXiv ID1106.0538 (version 1)
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CitationAdrianne Slyz, Christophe Pichon, Julien Devriendt, Susan A. Kassin, Taysun Kimm, Yohan Dubois. "The angular momentum of baryons and dark matter halos revisited." arXiv:1106.0538 (2011). https://pith.science/paper/T2RLX3MB
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The paper

The paper claims

Gas and dark matter arrive at a galaxy with similar spin, but only the gas delivers it to the centre — so the standard picture in which a disc inherits its host halo's angular momentum has to be rebuilt.

The reading

The paper asks where a spiral galaxy's rotation comes from. The textbook answer is that gas and dark matter are torqued identically by the surrounding tidal field, shock-heat as the halo collapses, and settle into a disc that conserves the halo's specific angular momentum. Using adaptive-mesh cosmological simulations across five runs and a wide range of halo masses, the authors argue that this chain breaks at the first link. Gas and dark matter do cross the virial radius carrying about the same specific angular momentum, but that value is two to six times larger than the spin of the virialised dark matter halo taken as a whole, because dark matter particles keep orbiting through the centre and mix old low-spin material back into the outer halo while radiatively cooled gas streams straight in along cold filaments and stays put. The payoff is a revised origin story: the spin of a disc is set by large-scale structure motions of the cosmic web, carried deep inside the halo by gas, and only redistributed near the disc itself.

What carries the argument

Three pieces of machinery carry the argument. First, the accretion-weighted specific angular momentum $langle j rangle_{vr}$, computed over infalling material in the shell $0.95 le r/R_{rm vir} le 1.05$ using a step function on radial velocity, which isolates what the halo is actually being fed rather than what it currently contains. Second, a two-zone decomposition: $j_{rm gas,out}$ measured between $0.1$ and $1$ virial radius, and $j_{rm gas,cen}$ measured inside $0.1$ virial radius, whose comparison separates misalignment of the outer and inner spin axes from the time-delay of accretion history. Third, the reduced spin parameter $lambda' = j / (sqrt{2} R_{rm vir} V_c)$, which converts the measurements into the language semi-analytic models use, so that the fitted ratio of gas to dark matter spin can be dropped into existing galaxy-formation codes. A phase decomposition by temperature and density — cold dense, cold diffuse, hot dense, hot diffuse — is what pins the excess spin specifically on the cold filamentary component, which alone accounts for more than half of the gas angular momentum in the outer halo.

Extended reading notes

Core claim

The central claim is a decoupling of two quantities that disc-formation theory had treated as one. At the moment of accretion, gas and dark matter carry statistically indistinguishable specific angular momentum, and both carry more than the halo as a whole; the authors measure this with an accretion-weighted average over a thin shell at the virial radius, flagging each dark matter particle the first time it crosses so it is not counted twice. What differs is what happens afterwards. Cold, dense filamentary gas cools and reaches the inner halo on a dynamical time without being redistributed, so the outer halo (0.1 to 1 virial radius) is continuously refreshed with high-spin material and the gas angular momentum profile stays flat with radius. Dark matter is collisionless: particles pass through the centre and a fraction resettle in the outer halo, so the halo average is dragged down both by mixing of older, lower-spin material and by vector cancellation among misaligned orbits. The ratio of gas to dark matter spin is reported as at least two at essentially all masses and redshifts, with a fitting formula given in the paper. Crucially, when gas and stars are summed, the total baryon specific angular momentum tracks the dark matter halo value closely, which is why the simulated stellar disc still lands near the observed spin–velocity relation at redshift zero despite the injection of high-spin gas.

Load-bearing premise

The claim that the gas-to-dark-matter spin ratio holds at all masses rests on large-volume simulations with one- to two-kiloparsec resolution, and the authors themselves note that this resolution inflates gas spin by roughly twenty percent; if the inflation is larger and mass-dependent in low-mass halos, the fitted mass trend weakens.

Editorial extensions

If this is right

  • Semi-analytic and semi-empirical galaxy formation models that assign gas the same specific angular momentum as its host dark matter halo start from the wrong initial condition; the paper supplies a redshift- and mass-dependent fitting formula for the correction.
  • Disc sizes predicted by such models should be revised upward relative to the standard assumption, since the gas feeding the disc carries more spin than the halo average.
  • The observed agreement between simulated and real stellar disc angular momenta at redshift zero is explained not by conservation of halo spin but by the near-equality of total baryon and dark matter specific angular momentum, which is a different and more fragile coincidence.
  • Angular momentum redistribution is confined to the immediate vicinity of the disc; the outer halo is a transport channel, not a mixing chamber, at least wherever cooling is fast compared with a halo sound-crossing time.
  • The gas-to-dark-matter spin ratio should shrink with increasing halo mass, because hotter and more diffuse gas cools more slowly and has more time to mix with older low-spin material.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the transport is as lossless as claimed, the outer-halo gas in real galaxies should show a flat specific angular momentum profile out to the virial radius, which is in principle testable with absorption-line kinematics against background quasars rather than with emission from the disc.
  • The picture implies that the scatter in disc spin at fixed halo mass should trace the coherence of the filamentary inflow, so halos fed by a single dominant filament should spin faster than halos fed isotropically at the same mass.
  • Because the argument leans on a comparison between a high-resolution single-halo resimulation and coarser statistical runs, the mass trend in the fitted ratio is the part most exposed to a resolution-dependent inflation of simulated gas discs, and it is the part a future convergence study would most directly test.
  • Strong feedback that heats or stirs circumgalactic gas should narrow the gas–dark matter spin gap, which would make the gap a diagnostic of feedback strength in massive halos rather than a fixed prediction.
Original abstract

Recent theoretical studies have shown that galaxies at high redshift are fed by cold, dense gas filaments, suggesting angular momentum transport by gas differs from that by dark matter. Revisiting this issue using high-resolution cosmological hydrodynamics simulations with adaptive mesh refinement, we find that at the time of accretion, gas and dark matter do carry a similar amount of specific angular momentum, but that it is systematically higher than that of the dark matter halo as a whole. At high redshift, freshly accreted gas rapidly streams into the central region of the halo, directly depositing this large amount of angular momentum within a sphere of radius r=0.1rvir. In contrast, dark matter particles pass through the central region unscathed, and a fraction of them ends up populating the outer regions of the halo (r/rvir>0.1), redistributing angular momentum in the process. As a result, large-scale motions of the cosmic web have to be considered as the origin of gas angular momentum rather than its virialised dark matter halo host. This generic result holds for halos of all masses at all redshifts, as radiative cooling ensures that a significant fraction of baryons remain trapped at the centre of the halos. Despite this injection of angular momentum enriched gas, we predict an amount for stellar discs which is in fair agreement with observations at z=0. This arises because the total specific angular momentum of the baryons remains close to that of dark matter halos. We propose a new scenario where gas efficiently carries the angular momentum generated by large-scale structure motions deep inside dark matter halos, redistributing it only in the vicinity of the disc.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 7,142 input tokens, $0.0003 reported.

What the abstract itself states, as probabilities: claims first 0.04 · states limitation 0.17 · human participants 0.01 · controlled comparison 0.15 · randomized assignment 0.01 · states quantitative result 0.54 · compares with prior results 0.82 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Recent theoretical studies have shown that galaxies at high redshift are fed by cold, dense gas filaments, suggesting angular momentum transport by gas differs from that by dark matter. Revisiting this issue using high-resolution cosmological hydrodynamics simulations with adaptive mesh refinement, we find that at the time of accretion, gas and dark matter do carry a similar amount of specific angular momentum, but that it is systematically higher than that of the dark matter halo as a whole. At high redshift, freshly accreted gas rapidly streams into the central region of the halo, directly depositing this large amount of angular momentum within a sphere of radius r=0.1rvir. In contrast, dark matter particles pass through the central region unscathed, and a fraction of them ends up populating the outer regions of the halo (r/rvir>0.1), redistributing angular momentum in the process. As a result, large-scale motions of the cosmic web have to be considered as the origin of gas angular momentum rather than its virialised dark matter halo host. This generic result holds for halos of all masses at all redshifts, as radiative cooling ensures that a significant fraction of baryons remain trapped at the centre of the halos. Despite this injection of angular momentum enriched gas, we predict an amount for stellar discs which is in fair agreement with observations at z=0. This arises because the total specific angular momentum of the baryons remains close to that of dark matter halos. We propose a new scenario where gas efficiently carries the angular momentum generated by large-scale structure motions deep inside dark matter halos, redistributing it only in the vicinity of the disc.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Recent theoretical studies have shown that galaxies at high redshift are fed by cold, dense gas filaments, suggesting angular momentum transport by gas differs from that by dark matter. background 1.00 0.04 0.05 hedged possibility 0.95
s1 Revisiting this issue using high-resolution cosmological hydrodynamics simulations with adaptive mesh refinement, we find that at the time of accretion, gas and dark matter do carry a similar amount of specific angular momentum, but that it is systematically higher than that of the dark matter halo as a whole. result 0.94 0.97 0.41 asserted 1.00
s2 At high redshift, freshly accreted gas rapidly streams into the central region of the halo, directly depositing this large amount of angular momentum within a sphere of radius r=0.1rvir. result 1.00 0.87 0.11 asserted 1.00
s3 In contrast, dark matter particles pass through the central region unscathed, and a fraction of them ends up populating the outer regions of the halo (r/rvir>0.1), redistributing angular momentum in the process. result 0.99 0.87 0.08 asserted 1.00
s4 As a result, large-scale motions of the cosmic web have to be considered as the origin of gas angular momentum rather than its virialised dark matter halo host. interpretation 0.99 0.63 0.59 asserted 0.99
s5 This generic result holds for halos of all masses at all redshifts, as radiative cooling ensures that a significant fraction of baryons remain trapped at the centre of the halos. result 0.79 0.69 0.19 asserted 1.00
s6 Despite this injection of angular momentum enriched gas, we predict an amount for stellar discs which is in fair agreement with observations at z=0. result 0.99 0.90 0.10 asserted 0.65
s7 This arises because the total specific angular momentum of the baryons remains close to that of dark matter halos. interpretation 0.64 0.78 0.14 asserted 0.99
s8 We propose a new scenario where gas efficiently carries the angular momentum generated by large-scale structure motions deep inside dark matter halos, redistributing it only in the vicinity of the disc. interpretation 0.93 0.53 0.70 speculative 0.58

What kind of paper the classifier reports this is: simulation study 1.00 · evidence from simulation or computation 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Projected densities of the dark matter (top) and gas (bottom) cen￾tred on a halo of ≃ 1011M⊙ at z = 3 from the NUT simulation with super￾nova feedback (NutFB). A 6Rvir region is projected and a figure with as￾pect ratio 1:2 is displayed, where the longer length corresponds to = 6Rvir (= 224 kpc). At z = 3 the scale-height of the disc as well as the large scale filamentary structure is well resolved in the simulation. It can be seen that dark matter filaments are broader than gas filaments. The gas filaments are not destroyed by supernova explosions. with a comparison to…
Figure 2. Evolution of the specific angular momentum (j) of a dark matter halo hosting a Milky Way-type galaxy in the adiabatic run (NutAD). Dif￾ferent colours denote different components: dark matter (jdm, thick grey), halo gas (jgas,out, 0.1 ≤ r/Rvir ≤ 1, thick blue). Also plotted as a blue dashed line is j of all the gas inside the virial radius, jgas. We also dis￾play the specific angular momentum corresponding to a reduced spin value λ ′ = 0.04 (red dashed line) for the dark matter halo. The specific angular momentum of the halo gas (jgas,out) closely follows that of the dark matter halo…
Figure 3. Left panel: evolution of the specific angular momentum modulus (j) from the NutFB run. Different colours denote different components: dark matter (jdm, thick grey), gas at 0.1 ≤ r/Rvir ≤ 1 (jgas,out, thick blue), central gas (r/Rvir < 0.1, jgas,cen, thin blue), stars (jstar, thin orange), all baryons in the halo (jbar, dot-dash red). Note that the specific angular momentum calculations for dark matter (jdm), gas (jgas,out) and baryons (jbar) includes the contribution from satellite galaxies, while it is excluded for the measurement of jstar and jgas,cen. Angular momentum…
Figure 4. Fraction of total angular momentum of gas at 0.1 ≤ r/Rvir ≤ 1 from various components. We divide the gas into four different phases: i) cold (T < 105K), dense filamentary (nH,fil ≤ nH < nH,th), ii) cold diffuse (nH < nH,fil), iii) hot (T > 105K) dense, and iv) hot diffuse component (see Equation (2) for the definition of nH,fil). Note that these quantities do not include the gas belonging to satellite galaxies. Grey solid line corresponds to the total fraction of angular momentum of gas belonging to the host halo. Cold, dense filamentary accretion accounts for most of the angular…
Figure 7. Stacked distributions of normalised specific angular momentum as a function of radius from the NutCO run. The specific angular momentum of gas and dark matter is shown as solid and dashed lines. Blue lines display j at high redshift (z > 3), while orange lines show j at low redshift (z ≤ 3). Each point corresponds to the median, and interquartile ranges are included as error bars. The normalised j of gas is flat at 0.1 < ∼ r/Rvir < ∼ 1, and gets diminished in the region where the galactic disc is located, suggesting that j of gas is reasonably conserved up to r = 0.1Rvir. 3.4…
Figure 8. Spin parameter distributions of the gas within 0.1 ≤ r/Rvir ≤ 1 (top), the dark matter halo (middle), and the ratio of the two (bottom). Filled circles correspond to the results from the HORIZON-MareNostrum simula￾tion, and asterisks and diamonds show the results from the Cosmo25 and Cosmo50 run, respectively. Different colours denote different redshifts as indicated on the figure. The results from the NutCO run are also included as stars with the same colour-coding. Solid lines show linear fits to the sim￾ulated data at different redshifts as given by Equation (3). Statistically,…
Figure 9. True colour composite image of the NutCO galaxy in the U, B and V at z = 0. The image measures 15 kpc on a side. Note that internal extinction by dust is not accounted for in this image. Notice the presence of a large bulge (I-band B/D ratio 0.78) typical of a galaxy with morphological type Sa/b (see text for detail). λ ′ gas,out λ ′ DM ≃ max[3.5+24.3(1+z) −0.8 −1.97(1+z) −0.7 log Mvir, 2].(3) We emphasize that all the extra specific angular momentum brought in by the gas will not be devoted to spinning up the central galactic disc, given that its typical misalignment with the…
Figure 11. Accretion-weighted specific angular momentum for dark matter (black solid line) and gas (blue solid line) at the virial radius in the NutFB (left panel) and the NutCO (right panel) runs. The specific angular momenta of the dark matter and the halo gas (jgas,out) shown in
Figure 12. Distribution of angular momentum of DM particles as a function of their position for the virialised DM halo in the NutCO run at z = 0. Col￾ors indicate the lookback time at which particles were accreted for the first time. Dark and light grey contours enclose 50 and 90 percent of the mass respectively. Note the large range of angular momentum values spanned by individual particles at any radius. more mixing between low and high specific angular momentum gas to occur. As a result, the gap between jgas,out and jdm is reduced for the most massive objects. The redshift dependence of…
Figure 13. Mass assembly histories of the various components in the NutFB run (top) and the NutCO run (bottom). Stars, dark matter, and gas in satellite galaxies are included in the measurement. Note that the amount of gas mass in the central region (Mgas −Mgas(r > 0.1Rvir)) is comparable (a factor 2-3 lower) to the gas mass in the outer region at all times. c 0000 RAS, MNRAS 000, 000–000

Showing 10 of the paper's 13 figures. See all figures in the PDF ↗ Figures reproduced from arXiv:1106.0538 by the authors.

References
How the literature uses it

47 arXiv papers cite this in 79 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers citing it · 65 sentences without a confident label

Uses its method or tool 1 paper
  • Instead of choosing matter with recent infall times, we instead compute a velocity-weighted average for $j$ for all particles in the inner halo ($0.5 < R/< 1.0$) that posses in-flowing radial velocities with respect to the halo center: [equation] similar to the definition of “accretion-weighted” angular momentum performed by [ref] .

    Angular Momentum Acquisition in Galaxy Halos · Angular Momentum Acquisition · 0.98

Uses its data or values 2 papers
Compares with it 2 papers
Cites it as support 2 papers
  • In particular—as it relates to this chapter—under this developing paradigm of filamentary versus isotropic gas accretion, halo gas (and particularly gas accreted in the cold mode) tends to show considerably higher specific angular momentum than the dark matter in the halo [ref] .

    Gas Accretion and Angular Momentum · Introduction · 0.97

  • Motivated by recent findings that the specific angular momentum of gaseous halos are considerably higher than that of the dark matter halo [ref] we focus on how reservoirs of cold-mode gas in galaxy halos grow and evolve over time, and how the angular momentum of this material differs from the canonical scenario for angular momentum growth in galaxies.

    Angular Momentum Acquisition in Galaxy Halos · Conclusion · 0.99

Cites it as background 4 papers

Discussion

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