The properties of warm dark matter haloes (1308.1399v2)

Abstract: Well-motivated elementary particle candidates for the dark matter, such as the sterile neutrino, behave as warm dark matter (WDM).For particle masses of order a keV, free streaming produces a cutoff in the linear fluctuation power spectrum at a scale corresponding to dwarf galaxies. We investigate the abundance and structure of WDM haloes and subhaloes on these scales using high resolution cosmological N-body simulations of galactic haloes of mass similar to the Milky Way's. On scales larger than the free-streaming cutoff, the initial conditions have the same power spectrum and phases as one of the cold dark matter (CDM) haloes previously simulated by Springel et al as part of the Virgo consortium Aquarius project. We have simulated four haloes with WDM particle masses in the range 1.4-2.3keV and, for one case, we have carried out further simulations at varying resolution. N-body simulations in which the power spectrum cutoff is resolved are known to undergo artificial fragmentation in filaments producing spurious clumps which, for small masses (<10^7Msun in our case) outnumber genuine haloes. We have developed a robust algorithm to identify these spurious objects and remove them from our halo catalogues. We find that the WDM subhalo mass function is suppressed by well over an order magnitude relative to the CDM case for masses <10^9Msun. Requiring that there should be at least as many subhaloes as there are observed satellites in the Milky Way leads to a conservative lower limit to the (thermal equivalent) WDM particle mass of ~1.5\rmn{keV}. WDM haloes and subhaloes have cuspy density distributions that are well described by NFW or Einasto profiles. Their central densities are lower for lower WDM particle masses and none of the models we have considered suffer from the "too big to fail" problem recently highlighted by Boylan-Kolchin et al.

• The paper uses high-resolution N-body simulations to examine WDM halo properties and subhalo abundance.
• Simulations reveal a significant suppression of the subhalo mass function, addressing discrepancies in CDM models.
• The study constrains WDM particle masses by ruling out masses below 1.5 keV based on observed Milky Way satellite data.

Overview of "The Properties of Warm Dark Matter Haloes"

The paper by Lovell et al. examines the characteristics and implications of warm dark matter (WDM) haloes through high-resolution N-body simulations. Focusing on WDM candidates such as keV-mass sterile neutrinos, the paper explores the abundance, distribution, and internal structure of WDM subhaloes in galactic halos comparable to the size of the Milky Way. This work is contextualized in the broader effort to resolve discrepancies in current dark matter models, particularly cold dark matter (CDM), which struggles on small cosmological scales.

Key Highlights

• Simulation Setup and Methodology: The authors conduct simulations using a range of WDM particle masses from 1.5 to 2.3 keV, matching the initial conditions to the CDM simulations previously part of the Virgo Consortium’s Aquarius project. An essential improvement in WDM simulations involves correcting for artificial fragmentation in filaments by implementing a robust algorithm to identify and exclude spurious clumps from data analysis.
• Subhalo Mass Function: WDM subhalo mass function experiences significant suppression compared to CDM. The abundance of subhaloes is found to decrease with decreasing WDM particle mass, especially below $10^9 M_{\odot}$. This finding is crucial, as it may alleviate the "missing satellite problem" observed in CDM, where simulations predict more subhaloes than observed satellites.
• Halo Structure and Profiles: WDM haloes maintain cuspy density profiles, fitting well with Navarro-Frenk-White (NFW) or Einasto profiles. However, central densities decrease with decreasing particle mass, offering a potential solution to the "too big to fail" problem by populating dwarf galaxies with less dense haloes than those predicted by CDM.
• Constraints on WDM Particle Mass: Observational constraints on the abundance and structure of satellites in the Milky Way provide a strategy to constrain WDM particle mass. The paper confidently rules out WDM particle masses below 1.5 keV to maintain consistency with the number of observed Milky Way satellites, accounting for the observational biases of surveys such as SDSS.

Theoretical and Practical Implications

The research underscores the necessity of reevaluating dark matter paradigms on smaller scales. The disparities in halo substructure between WDM and CDM models illuminate significant alterations on sub-galactic scales, suggesting WDM as a viable candidate to address the overproduction of subhaloes in CDM. Additionally, the paper provides a framework for using satellite galaxy data to derive limits on dark matter particle characteristics.

Prospective Developments

Future work in this domain may focus on expanding the range of simulated WDM masses and employing even higher-resolution simulations to capture finer structures. Enhanced observational techniques, perhaps with upcoming telescopes, could provide further data on satellite galaxy populations, refining mass function constraints. Integrating baryonic physics into these simulations might also offer additional insights into the environmental effects on these small-scale structures.

Conclusion

Lovell et al.'s investigation contributes significantly to our understanding of WDM, challenging existing CDM assumptions and enlarging the spectrum of plausible dark matter models. It highlights not only the potential of WDM to resolve certain cosmological issues but also presents a robust methodological framework for future analysis in the field.