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The Effects of Dark Matter Annihilation and Dark Matter-Baryon Velocity Offsets at Cosmic Dawn

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Pith's one-line read Dark matter annihilation at cosmic dawn can shrink the gas content of the smallest star-forming halos and shift the 21cm absorption trough, with the sign of the effect on first-star formation depending on streaming velocity.

desk verdict A genuinely new combination of DM annihilation, molecular cooling, and streaming that yields solid analytic Mcool curves, but the undocumented z<25 rescaling in the 21cmvFAST runs is the one step a referee must force out into the open. read the letter →

arxiv 2411.10626 v2 pith:B65A7WWR submitted 2024-11-15 astro-ph.CO astro-ph.GAhep-ph

Liqiang Hou , Katherine J. Mack This is my paper · ORCID
classification astro-ph.COastro-ph.GAhep-ph
keywords darkmatterannihilationcosmicdawn21cmglobalsignalminihalosmolecularhydrogencoolingmatter-baryonstreamingvelocityPopulationIIIstarssemi-analyticmodel
topics Dark Matter
open problems Dark Matter
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper argues that dark matter annihilation into electron-positron pairs during cosmic dawn ($z\approx20$–$40$) does more than heat the intergalactic medium: it changes the supply of gas available to the first star-forming mini-halos and rewires the molecular-hydrogen cooling that lets Population III stars form. Using a semi-analytic model that feeds updated energy-deposition fractions into the CosmoRec recombination code and the 21cmvFAST simulation, the authors find that annihilation suppresses the gas fraction in halos below about $10^5\,M_\odot$ by 10–40% at $z=20$ and alters the minimum cooling mass $M_{\rm cool}$ in a redshift- and mass-dependent way. The sign of the effect on star formation is not fixed: annihilation lowers $M_{\rm cool}$ at $z>40$ but raises it by up to a factor of about 2 at $z=20$ when no streaming is present, while a dark matter–baryon velocity offset (streaming) reverses the trend and makes annihilation mostly lower $M_{\rm cool}$. These changes propagate into a global 21cm signal whose absorption trough is shifted by $\Delta z\sim 2$–5 and whose power spectrum is modified, giving concrete targets for 21cm experiments. The paper thus establishes that interpreting cosmic-dawn 21cm data requires treating DM annihilation, molecular cooling, Lyman-Werner feedback, and streaming together.

What carries the argument

The load-bearing object is the minimum cooling mass $M_{\rm cool}$, the lowest halo mass whose gas can cool via molecular hydrogen fast enough to form stars, here set by the criterion $t_{\rm cool}<0.2\,t_H$ with $t_{\rm cool}$ from the cooling-time formula. Around this object the paper assembles a chain of semi-analytic pieces: the filtering mass $M_F$, the time-averaged Jeans mass that sets the gas fraction a halo retains; the H2 chemistry with DM ionization added through $\Lambda_{\rm ion|DM}$; the deposition fractions $f_c(z)$ for heating, ionization, and Ly-$\alpha$ from transfer-function tables; and a boost factor $B(z)$ that accounts for annihilation power from collapsed halos. Streaming enters by replacing the effective sound speed with $c_s^{\prime2}=c_s^2+v_{\rm bc}^2$ and by raising the IGM temperature seen by halo gas, which lowers core gas densities. $M_{\rm cool}$ then controls the collapsed baryon fraction passed to 21cmvFAST, so every downstream prediction—gas fractions, star-formation timing, and the 21cm signal—flows through this one threshold.

What would settle it

Run the same pipeline with in-halo energy deposition switched on (local deposition efficiency $h\gtrsim -64$, the Appendix A regime) instead of $f_{\rm esc}\approx1$, and recompute the $z=20$ gas fractions and $M_{\rm cool}$ curves of Figures 6 and 9; if a $10^6\,M_\odot$ halo then shows gas temperatures and ionizations high enough to change the sign of $\Delta M_{\rm cool}$, the paper's central predictions are falsified.

Watch

Extended reading notes

Core claim

The paper's central claim is that dark matter annihilation in the 10 MeV–GeV mass range, with $\langle\sigma v\rangle/m_{\rm DM}=10^{-27}\,\mathrm{cm^3\,s^{-1}\,GeV^{-1}}$ and the $\chi\chi\to e^+e^-$ channel, leaves a compound imprint on the first structures. Energy deposited into the IGM raises the gas temperature and ionization fraction, which increases the time-averaged Jeans (filtering) mass $M_F$ and, through the gas-fraction prescription, reduces $f_{\rm gas}$ in low-mass halos by 10–40% at $z=20$ for $M_h<10^5\,M_\odot$. In the analytic molecular-cooling model, DM-induced ionization raises the electron abundance and accelerates H2 formation at early times, while DM heating lengthens the cooling time; the competition makes $M_{\rm cool}$ decrease slightly for $z>40$ and increase by up to a factor of about 2 at $z=20$ when streaming is absent. With a streaming velocity $v_{\rm bc}=v_{\rm rms}$, the gas density in mini-halos is lower, DM ionization dominates over heating, and annihilation mostly lowers $M_{\rm cool}$ across redshifts. When these effects are propagated through 21cmvFAST, the global 21cm absorption trough is shifted by $\Delta z\sim 2$ from streaming and by $\Delta z\approx 5$ relative to earlier DM models, and the 21cm power spectrum shows altered timing and amplitude. The paper also reports that local in-halo energy deposition, if efficient, could substantially raise gas temperature and ionization around a $10^6\,M_\odot$ halo, a channel it leaves for future work.

Load-bearing premise

The calculations assume that each halo's gas receives only the cosmic-average dark-matter annihilation background, with essentially all annihilation energy produced inside a halo escaping rather than heating that halo's own gas ($f_{\rm esc}\approx1$); if local deposition is efficient, the predicted gas-fraction suppression and the direction of the cooling-mass shift could change.

Editorial extensions

If this is right

  • Mini-halos below about $10^5\,M_\odot$ at $z=20$ retain 10–40% less gas, so Population III star formation in the smallest systems is suppressed while atomic-cooling halos above $10^8\,M_\odot$ are essentially unaffected.
  • The molecular-cooling threshold is non-monotonic in redshift: DM annihilation lowers $M_{\rm cool}$ above $z\approx40$ and raises it by up to a factor of about 2 at $z=20$ when streaming is absent.
  • Turning on a streaming velocity $v_{\rm bc}=v_{\rm rms}$ flips the trend: annihilation mostly lowers $M_{\rm cool}$ at most redshifts, so the same DM model that delays first stars without streaming can accelerate them with streaming.
  • Lyman-Werner feedback magnifies the DM effect on $M_{\rm cool}$ at low redshift, so 21cm fits that ignore this feedback will misattribute the DM contribution.
  • The global 21cm absorption trough is shifted by $\Delta z\sim 2$ with streaming and by $\Delta z\approx 5$ relative to earlier no-molecular-cooling models, giving an observable discriminator.

Reading between the lines

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

  • We infer that if the streaming-induced sign flip is real, single-band 21cm absorption measurements cannot separate DM annihilation from streaming-induced suppression; only the joint shape of the power spectrum can break the degeneracy.
  • We infer that at cross-sections below the benchmark, the qualitative structure-suppression effect does not disappear but shifts to lower redshift, so a null 21cm detection would constrain the model against this redshift-dependent sensitivity.
  • We infer that allowing the escape fraction $f_{\rm esc}$ to be a free parameter in the same pipeline is the natural next calculation; the Appendix A regime $h\gtrsim -64$ suggests the predicted trough position is sensitive to local energy transport.
  • We infer that the fixed-temperature assumption during cooling likely overestimates the cooling rate, and a time-dependent treatment would shift $M_{\rm cool}$ in a way testable against the simulation comparisons in the paper.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. This paper builds a semi-analytic model of dark matter annihilation during Cosmic Dawn (z≈20–40) and couples it to a simplified treatment of gas collapse, molecular hydrogen cooling, and star formation in mini-halos, using the public 21cmvFAST code to compute the global 21cm brightness temperature and power spectrum. The model considers s-wave annihilation of 9 MeV, 130 MeV, and 1.1 GeV dark matter into e+e− pairs, uses deposition fractions from Slatyer's tables via CosmoRec, and incorporates dark matter–baryon streaming velocities. The central results are: a 10–40% suppression of the gas fraction in halos below about 10^5 M☉ at z=20 (Fig. 6); a minimum cooling mass that decreases for z>40 but increases by up to a factor of about 2 at z=20 in the absence of streaming, with the effect reversing when streaming velocities are included (Figs. 3, 9, 10); and a shift of the global 21cm absorption trough by Δz ≈ 2–5 relative to no-annihilation models (Fig. 11).

Significance. If the predictions hold, the paper would provide a useful first unified estimate of DM annihilation and streaming effects on early structure formation and the 21cm signal, with a transparent analytic cooling model that reproduces the broad behavior of more expensive simulations. The use of independent energy-deposition tables, the CosmoRec thermal histories, and the public 21cmvFAST code are strengths, as is the explicit comparison with previous simulation fits for Mcool. However, the two main results—the z=20 gas fraction and the 21cm trough shift—depend on a rescaling of the deposition fraction for z<25 that is neither derived nor validated, and on the neglect of local halo energy deposition; these issues make the quantitative conclusions provisional.

major comments (3)
  1. [Section V, deposition-fraction rescaling] The rescaling fc(z<25) = f(z')/f(z) fc(z') is introduced without derivation, reference, or sensitivity analysis. This is load-bearing because it modifies the energy deposition in exactly the epoch of the headline results (z≈20 in Figs. 6, 10, 11). The Slatyer tables already provide redshift-dependent deposition fractions; replacing them below z=25 with a ratio evaluated at a matched electron fraction can change the injected heating and ionization by an order-unity, uncontrolled factor. Please either derive this step, replace it with the unmodified tables, or show that the z<25 predictions are insensitive to it.
  2. [Section IV A and Appendix A] The main pipeline assumes fesc≈1, so that each halo's gas experiences only the global DM annihilation background. Appendix A itself shows that for local deposition efficiency h ≳ −64, local heating and ionization around a 10^6 M☉ halo become significant. Since the z=20 gas fraction (Fig. 6) and the sign of the DM effect on Mcool (Figs. 9–10) depend on the gas density and electron fraction in mini-halos, the neglected local channel can alter the central conclusions. Please either include local deposition in the fiducial model or state, with a quantitative estimate, the local efficiency at which the headline results change.
  3. [Section V and Figs. 3, 6] The manuscript does not state whether the z=20 semi-analytic results (gas fraction in Fig. 6 and Mcool in Figs. 3, 9, 10) use the CosmoRec thermal history or the rescaled 21cmvFAST thermal history. The text says the gas thermal history for Fig. 6 is calculated with CosmoRec, while the simulation modifies the thermal evolution with Eqs. (16)–(17) and the z<25 rescaling. This ambiguity leaves the quantitative content of the central claims untracked; please clarify which thermal history enters each figure and, if different, show the impact.
minor comments (4)
  1. [Section IV B] The sentence 'The bottom panel of Fig. 1 shows the relative changes of minimum cooling mass' should refer to the bottom panel of Fig. 3.
  2. [Section II] Table I is introduced in the text but not explicitly cited; consider referencing it where the comparison with previous studies is discussed.
  3. [Section VII] The text reads 'Galacticforeground' and should read 'Galactic foreground'.
  4. [Fig. 4 caption] The caption states 'the H2 fractions was calculated'; the verb should agree with the plural subject.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the main derivation chain is forward (deposition tables into CosmoRec, then filtering mass, fgas, Mcool, and finally 21cmvFAST), with no parameter fitted to the target 21cm signal or to the fgas/Mcool predictions.

full rationale

Walking the claimed derivation chain: the annihilation power of Eq. 11 is combined with the independent Slatyer deposition tables [18, 65] and injected into CosmoRec to obtain Tg(z) and xe(z); those thermal quantities enter the Jeans/filtering-mass calculation (Eqs. 1-5), which yields fgas; the molecular-cooling model of Sec. IV B then produces Mcool; and fcoll from Eq. 8 is passed to the public 21cmvFAST code to produce the 21cm signal. No free parameter is tuned to reproduce the headline results: the annihilation cross-section benchmark, X-ray efficiency, star-formation efficiency, and cooling criterion are all adopted from prior literature or chosen as explicit benchmarks. The z<25 deposition-fraction rescaling in Sec. V (fc(z<25)=f(z')/f(z)fc(z'), with xe(z)=xe(z')) is the least documented step and deserves sensitivity testing, but it is not circular in the prohibited sense: z' is selected from the already-computed CosmoRec electron fraction rather than fitted to the 21cm output, and the headline z=20 fgas and Mcool figures use the CosmoRec thermal history rather than this rescaling. The Appendix's local-deposition estimate uses a self-cited simulation [41] to set the baseline efficiency h=-65, but the appendix is explicitly preliminary, h is varied by an order of magnitude in the figures, and the main text assumes fesc~1, so this self-citation is not load-bearing. The paper is therefore self-contained forward modeling with external benchmarks; the flagged items are modeling gaps and correctness risks, not circular reductions.

Assumptions & free parameters 7 free parameters · 11 assumptions · 0 invented entities

The central results depend on a chain of prior-model inputs and simplifying assumptions. The most consequential are the benchmark annihilation cross-section, the assumed e+e- channel, the star formation and X-ray efficiencies taken from LH16, the simplified H2 chemistry, and the assumption that halo gas receives only the background (not local) annihilation energy. None of these are fitted to the 21cm signal, so the model is not circular, but the quantitative predictions inherit the uncertainties of each input.

free parameters (7)
  • DM annihilation cross-section per mass (langle sigma v rangle / mDM) = 1e-27 cm^3 s^-1 GeV^-1
    Chosen benchmark near the Planck bound, allowed for feff < 0.1; all DM heating and ionization rates scale with this value.
  • DM particle mass (scan) = 9 MeV, 130 MeV, 1.1 GeV
    Scanned to show mass dependence; determines the energy deposition fractions fc(z) and the resulting thermal history.
  • Star formation efficiency f* for atomic cooling halos = 0.1
    Adopted from [17] (LH16); directly sets the star-forming baryon collapsed fraction and the timing of the 21cm signal.
  • X-ray efficiency zeta_X = 10^56
    Set following [17] to match the X-ray photon budget in 21cmvFAST; strongly affects the 21cm absorption and heating signatures.
  • Local energy deposition efficiency h = log10(epsilon_local / E_h) = -65
    Baseline value for the appendix estimate of local DM heating; the main results set fesc ~ 1, so this is not used in the central calculation.
  • Minimum halo mass for boost factor M_h^min = 10^-9 Msun
    Chosen lower cutoff for the halo mass function in the boost factor calculation; the boost factor B(z) is sensitive to this value.
  • Cooling core radius Rcore = 0.1 Rvir
    Assumed core radius for gas cooling in halos; changes the core gas density and hence the cooling time.
assumptions (11)
  • domain assumption Dark matter annihilates 100% into e+e- pairs in the 10 MeV to GeV mass range.
    Sec IV A: all deposition fractions and heating/ionization rates are computed for this channel; other channels (e.g., muons, photons) would change fc(z).
  • domain assumption Planck 2018 cosmological parameters (h=0.6766, Omega_m=0.3111, Omega_b=0.049) are adopted.
    Sec V: used in all thermal history and 21cm calculations.
  • domain assumption Sheth-Tormen halo mass function with a'=0.75, p'=0.3 and NFW profiles with Diemer-Joyce concentration are accurate.
    Sec IV A: determines boost factor B(z) and halo abundances; the boost factor is sensitive to these choices.
  • domain assumption Energy deposition fractions fc(z) from Slatyer tables are accurate for z=12-50.
    Sec IV A and V: the paper uses the tables of [18, 65] without recalculation; the z<25 rescaling modifies them ad hoc.
  • domain assumption Gas within halos receives the same per-baryon energy deposition as the IGM (global background only, fesc ~ 1).
    Sec IV A: central to the mini-halo gas fraction and cooling results; explicit statement: 'we assume the energy deposit rate per baryon in halos is the same as that in the IGM.'
  • domain assumption Molecular hydrogen chemistry is captured by the H- formation channel with equilibrium H-, case-B recombination, and rate coefficients k1-k4.
    Sec IV B 2: simplified network; omits e.g. H2+ channel and non-equilibrium H-; the cooling functions assume low density and ortho-para ratio 3:1.
  • ad hoc to paper The cooling criterion tcool < 0.2 tH defines the minimum cooling mass.
    Sec IV B 3, adopted from [48, 50]; the paper acknowledges that a criterion like tcool < 6 tff gives larger Mcool, so this choice directly shapes the results.
  • ad hoc to paper Core gas density in halos is min{rho_HM_gas,core, rho_LM_gas,core} with R_core = 0.1 Rvir.
    Sec IV B 1: an analytic modeling choice; the paper states that this assumption breaks down on small scales and uses the hydrostatic low-mass expression.
  • ad hoc to paper The ad hoc rescaling fc(z<25)=f(z')/f(z) fc(z') with xe(z)=xe(z') is a valid approximation.
    Sec V: stated without derivation or numerical validation; it couples fc to the ionization state and is used for all 21cmvFAST runs below z=25.
  • domain assumption DM annihilation does not produce a significant Lyman-Werner flux; only stellar LW feedback is included.
    Sec II and IV B 4: stated because LW photon deposition data for DM annihilation are unavailable.
  • domain assumption Streaming velocity vbc affects HMF, filtering mass, and IGM temperature via Eqs. 36-41 with Vcool fit from [56].
    Sec VI A: the prescription for vbc effects is taken from prior fits and applied to the analytic model.

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Cite this review

Pith. "Pith review of The Effects of Dark Matter Annihilation and Dark Matter-Baryon Velocity Offsets at Cosmic Dawn." pith.science (2026). https://pith.science/paper/B65A7WWR

@misc{pith2026241110626,
  author       = {Pith},
  title        = {Pith review of: The Effects of Dark Matter Annihilation and Dark Matter-Baryon Velocity Offsets at Cosmic Dawn},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/B65A7WWR}},
  note         = {Machine review of arXiv:2411.10626}
}
abstract

Dark matter annihilation has the potential to leave an imprint on the properties of the first luminous structures at Cosmic Dawn as well as the overall evolution of the intergalactic medium (IGM). In this work, we employ a semi-analytic method to model dark matter annihilation during Cosmic Dawn (approximately redshift $z=20$ to $40$), examining potential modifications to IGM evolution as well as gas collapse, cooling, and star formation in mini-halos. Our analysis takes into account the effects of dark matter-baryon velocity offsets, utilizing the public 21cmvFAST code, and producing predictions for the 21cm global signal. The results from our simplified model suggest that dark matter annihilation can suppress the gas fraction in small halos and alter the molecular cooling process, while the impact on star formation might be positive or negative depending on parameters of the dark matter model as well as the redshift and assumptions about velocity offsets. This underscores the need for more comprehensive simulations of the effects of exotic energy injection at Cosmic Dawn as observational probes are providing us new insights into the process of reionization and the formation of first stars and galaxies.

Figures

Figures reproduced from arXiv: 2411.10626 by the authors.

Figure 1
Figure 1. FIG. 1. Fraction of electron and molecular hydrogen H [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. The molecular hydrogen fraction as a function of [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. The minimum cooling mass, [PITH_FULL_IMAGE:figures/full_fig_p010_3.png] view at source ↗
Figures from the paper (11 more)
Figure 5
Figure 5. Figure 5: plots the minimum cooling mass with stellar LW feedback and dark matter annihilation. In the top panel, the solid black line represents the case with LW background alone. Compared to the case without LW radiation (as shown in [PITH_FULL_IMAGE:figures/full_fig_p011_5.png]
Figure 4
Figure 4. Figure 4: FIG. 4. Molecular hydrogen fraction in a 10 [PITH_FULL_IMAGE:figures/full_fig_p011_4.png]
Figure 6
Figure 6. Figure 6: FIG. 6. Gas fraction as a function of halo masses at redshift [PITH_FULL_IMAGE:figures/full_fig_p012_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. Filtering mass as a function of redshift with effects of [PITH_FULL_IMAGE:figures/full_fig_p014_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. Fractions of electrons and molecular hydrogen as a function of time during gas cooling. The halo has a mass 10 [PITH_FULL_IMAGE:figures/full_fig_p015_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Minimum cooling mass as a function of redshift, con [PITH_FULL_IMAGE:figures/full_fig_p015_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Minimum cooling mass in full scenario, include dark [PITH_FULL_IMAGE:figures/full_fig_p016_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. The simulated 21cm signal as a function of redshift [PITH_FULL_IMAGE:figures/full_fig_p016_11.png]
Figure 13
Figure 13. Figure 13: FIG. 13. Comparison of the 21cm brightness temperature [PITH_FULL_IMAGE:figures/full_fig_p017_13.png]
Figure 14
Figure 14. Figure 14: FIG. 14. The local boost factor in dark matter (DM) annihi [PITH_FULL_IMAGE:figures/full_fig_p023_14.png]
Figure 15
Figure 15. Figure 15: FIG. 15. Kinetic temperature and ionization fraction of gas surrounding a 10 [PITH_FULL_IMAGE:figures/full_fig_p024_15.png]

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Reference graph

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