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Thermal evolution of the primordial clouds in warm dark matter models with keV sterile neutrinos

Jaroslaw Stasielak, Peter L. Biermann, Alexander Kusenko

TL;DR

This study investigates how keV-scale sterile-neutrino warm dark matter, via radiative decays, shapes the thermal evolution of primordial gas clouds and the consequent star formation. By coupling a top-hat overdensity framework with a chemical network that includes ionization/heating from decay-generated X-rays, the authors quantify how increased $x_e$ and $x_{H_2}$ alter cooling, especially in small halos. Across benchmark models, they find that sterile-neutrino decays generally promote cooling and can enable collapse in halos that would otherwise fail, though very strong free-streaming (as in WDM3) can suppress small-scale structure and delay star formation. The results imply sterile-neutrino warm dark matter can influence reionization by facilitating early star formation in low-mass halos, though further refinement is needed to capture realistic collapse timings and radiative feedback.

Abstract

We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a two-fold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that, in all the cases we have examined, the overall effect of sterile dark matter is to facilitate the cooling of gas. Hence, we conclude that dark matter in the form of sterile neutrinos can help the early collapse of gas clouds and the subsequent star formation.

Thermal evolution of the primordial clouds in warm dark matter models with keV sterile neutrinos

TL;DR

This study investigates how keV-scale sterile-neutrino warm dark matter, via radiative decays, shapes the thermal evolution of primordial gas clouds and the consequent star formation. By coupling a top-hat overdensity framework with a chemical network that includes ionization/heating from decay-generated X-rays, the authors quantify how increased and alter cooling, especially in small halos. Across benchmark models, they find that sterile-neutrino decays generally promote cooling and can enable collapse in halos that would otherwise fail, though very strong free-streaming (as in WDM3) can suppress small-scale structure and delay star formation. The results imply sterile-neutrino warm dark matter can influence reionization by facilitating early star formation in low-mass halos, though further refinement is needed to capture realistic collapse timings and radiative feedback.

Abstract

We analyze the processes relevant for star formation in a model with dark matter in the form of sterile neutrinos. Sterile neutrino decays produce an X-ray background radiation that has a two-fold effect on the collapsing clouds of hydrogen. First, the X-rays ionize the gas and cause an increase in the fraction of molecular hydrogen, which makes it easier for the gas to cool and to form stars. Second, the same X-rays deposit a certain amount of heat, which could, in principle, thwart the cooling of gas. We find that, in all the cases we have examined, the overall effect of sterile dark matter is to facilitate the cooling of gas. Hence, we conclude that dark matter in the form of sterile neutrinos can help the early collapse of gas clouds and the subsequent star formation.

Paper Structure

This paper contains 11 sections, 62 equations, 4 figures.

Table of Contents

  1. Introduction
  2. H$_{2}$ cooling and sterile neutrinos decays
  3. Density evolution
  4. Temperature evolution
  5. Chemical reactions and cooling/heating function
  6. Initial conditions and numerical procedure
  7. Results
  8. Conclusions
  9. Specific intensity of the photons from the sterile neutrino decays
  10. Heating and ionization due to the sterile neutrino decays
  11. Cooling/heating function

Figures (4)

  • Figure 1: The parameter range favored by the pulsar kicks, the current limits from x-ray observations (see text for discussion), and the benchmark points chosen for model calculations.
  • Figure 2: Evolution of temperature with redshift for different models: $m_{s} =25$ keV and $\sin^2 \theta=3 \times 10^{-12}$ (WDM1), $m_{s} =15$ keV and $\sin^2 \theta=3 \times 10^{-12}$ (WDM2), $m_{s} =3.3$ keV and $\sin^2 \theta=3 \times 10^{-9}$ (WDM3). $M$ is the cloud mass and $z_{vir}$ is the redshift of virialization.
  • Figure 3: The same as in Fig. \ref{['fig:T']}, but for the ionization fraction.
  • Figure 4: The same as in Fig. \ref{['fig:T']}, but for the molecular hydrogen fraction.