Distinguishing the nature of dark matter by mapping cosmic filaments from Lyman-alpha emission

Technical Summary: Distinguishing the Nature of Dark Matter by Mapping Cosmic Filaments from Lyman-alpha Emission

Problem Statement
The standard $\Lambda$CDM cosmological model successfully explains large-scale observations but faces persistent challenges on small scales, such as the missing satellites, cusp-core, and too-big-to-fail problems. While baryonic processes (e.g., stellar feedback) can alleviate these discrepancies, modifications to the dark matter paradigm, such as Warm Dark Matter (WDM), offer alternative explanations. WDM, characterized by non-negligible thermal velocities (e.g., sterile neutrinos), suppresses small-scale structure formation. However, a degeneracy exists between baryonic effects and intrinsic dark matter properties, making small-scale challenges alone insufficient to validate or rule out specific models. Existing tools like the Lyman-$\alpha$ (Ly$\alpha$) forest and gravitational lensing provide constraints but require additional, independent strategies. This work investigates whether the morphological and brightness differences in cosmic filaments—specifically the distribution of neutral hydrogen (HI) traced by Ly$\alpha$ emission—can serve as a direct probe to distinguish between Cold Dark Matter (CDM) and WDM.

Methodology
The authors utilize high-resolution "zoom-in" hydrodynamical simulations of a Milky Way-like halo (Halo A from the Aquarius project) performed with the GADGET-3 code. Two simulations are run with identical Gaussian initial conditions (random phases) to ensure a direct comparison:

1. CDM Scenario: Standard Cold Dark Matter.
2. WDM Scenario: Warm Dark Matter with a thermal relic mass of 1.5 keV (chosen to be lower than current Ly$\alpha$ forest constraints to clearly highlight model differences).

Both simulations include cooling, photoheating from a UV/X-ray background, and a simplified star formation criterion ($n_H > 0.1 \text{ cm}^{-3}$ and $\rho/\bar{\rho} > 2000$), but explicitly exclude feedback processes. The analysis focuses on redshifts $z=2.5$ and $z=4$.

To generate observable Ly$\alpha$ surface brightness maps, the authors:

• Recalculate neutral hydrogen ($n_{HI}$), ionized hydrogen ($n_{HII}$), and electron ($n_e$) densities in each gas cell, applying a self-shielding suppression factor (Rahmati et al. 2013) to correct for the lack of self-shielding in the raw simulation data.
• Compute Ly$\alpha$ emissivity via recombination and collisional excitation mechanisms using temperature and density data.
• Project the total Ly$\alpha$ luminosity onto a 2D plane using the Cloud-In-Cell (CIC) method.
• Apply a transmission factor to account for line-of-sight attenuation, specific to the adopted cosmological parameters.
• Generate mock observations for current (VLT/MUSE) and future (ELT) telescopes by adding Gaussian noise consistent with instrumental sensitivities.

Key Results

• Structural Differences: At $z=4$, the HI gas in WDM filaments is significantly smoother and denser than in CDM filaments. In contrast, CDM filaments exhibit a more clumpy, structured distribution. By $z=2.5$, small Ly$\alpha$ clumps begin to appear in WDM filaments, reducing the morphological contrast between the two models.
• Surface Brightness Signatures: The study finds that WDM filaments are not only smoother but also systematically brighter than their CDM counterparts at high redshifts.
• Observational Feasibility:
  • Sensitivity: Distinguishing the models requires sensitivity levels achievable by next-generation 30-meter class telescopes (ELT). The authors estimate that CDM filaments require sensitivities around $10^{-20} \text{ erg s}^{-1} \text{ cm}^{-2} \text{ arcsec}^{-2}$, while WDM filaments are marginally detectable with current 8-meter class telescopes (VLT/MUSE) at noise levels of $\sim 3.5 \times 10^{-20} \text{ erg s}^{-1} \text{ cm}^{-2} \text{ arcsec}^{-2}$.
  • Instrumentation: Integral Field Spectrographs (IFS) with large fields of view (FOV) are identified as critical. MUSE ($1' \times 1'$ FOV) is highlighted as sufficient to capture filaments extending 1.5–2 $h^{-1}$ cMpc at $z=2.5-4$.
• Model Robustness: Even in the presence of observational noise, the smoother and brighter nature of WDM filaments remains a distinct signature compared to the clumpier, fainter CDM structures.

Significance and Claims
The paper claims that Ly$\alpha$ emission from cosmic filaments provides a promising observational probe for constraining the nature of dark matter. The primary contribution is the identification of filament smoothness and surface brightness as distinctive signatures that can differentiate between CDM and WDM models, particularly at high redshifts ($z=4$).

The authors emphasize that their findings rely on a conservative estimate of intrinsic Ly$\alpha$ surface brightness; thus, the detectability of filaments is unlikely to be overestimated. While the simulations omit explicit feedback processes due to current uncertainties in star formation efficiency within filaments, the authors argue that the relative differences between WDM and CDM signals should remain robust, as feedback would likely impact the distinct small-scale structures of the two models differently.

The study concludes that the upcoming generation of 30-meter class telescopes will be critical for enabling these measurements. By mapping the structure of cosmic filaments, these instruments offer a compelling opportunity to break the degeneracy between baryonic effects and dark matter properties, potentially validating or ruling out specific dark matter paradigms. The authors also note that these results may extend to Fuzzy Dark Matter (FDM) models, which exhibit similar suppression of small-scale fluctuations.