The effect of neutrinos on the matter distribution as probed by the Intergalactic Medium (1003.2422v2)

Abstract: We present a suite of full hydrodynamical cosmological simulations that quantitatively address the impact of neutrinos on the (mildly non-linear) spatial distribution of matter and in particular on the neutral hydrogen distribution in the Intergalactic Medium (IGM), which is responsible for the intervening Lyman-alpha absorption in quasar spectra. The free-streaming of neutrinos results in a (non-linear) scale-dependent suppression of power spectrum of the total matter distribution at scales probed by Lyman-alpha forest data which is larger than the linear theory prediction by about 25% and strongly redshift dependent. By extracting a set of realistic mock quasar spectra, we quantify the effect of neutrinos on the flux probability distribution function and flux power spectrum. The differences in the matter power spectra translate into a ~2.5% (5%) difference in the flux power spectrum for neutrino masses with Sigma m_{\nu} = 0.3 eV (0.6 eV). This rather small effect is difficult to detect from present Lyman-alpha forest data and nearly perfectly degenerate with the overall amplitude of the matter power spectrum as characterised by sigma_8. If the results of the numerical simulations are normalized to have the same sigma_8 in the initial conditions, then neutrinos produce a smaller suppression in the flux power of about 3% (5%) for Sigma m_{\nu} = 0.6$ eV (1.2 eV) when compared to a simulation without neutrinos. We present constraints on neutrino masses using the Sloan Digital Sky Survey flux power spectrum alone and find an upper limit of Sigma m_{\nu} < 0.9$ eV (2 sigma C.L.), comparable to constraints obtained from the cosmic microwave background data or other large scale structure probes.

The Effect of Neutrinos on the Matter Distribution as Probed by the Intergalactic Medium

This paper presents a detailed investigation of the impact of neutrinos on the spatial distribution of matter, particularly focusing on the Intergalactic Medium (IGM) and its implications for the Lyman-$\alpha$ forest observed in quasar spectra. The research utilizes a suite of full hydrodynamical cosmological simulations to quantify this impact. Crucially, neutrino free-streaming results in a scale-dependent suppression of the matter power spectrum that deviates from linear theory predictions. This suppression is found to be approximately 25% larger and strongly redshift dependent at scales probed by Lyman-$\alpha$ forest data.

Numerical Simulations and Predictions

Neutrinos, known for their non-zero mass and flavor oscillations, have significant implications beyond the Standard Model of particle physics. Their ability to stream freely through the cosmos affects both cosmic expansion and the growth of structure. Early neutrinos were relativistic, but those in the mass range 0.05 eV $\le \Sigma m_{\nu} \le 1.5$ eV become non-relativistic at redshifts 100 to 3000 and contribute as a form of hot dark matter. While linear theory can approximate neutrino effects on large-scale structure, small scales where neutrino effects are most pronounced require non-linear numerical simulation.

This research leverages the {\small GADGET-3} hydrodynamical code to model the neutrino effects in cosmological simulations. Two techniques are implemented: a particle-based approach and a grid-based method, each with distinct advantages and limitations. The grid-based approach, while free of Poisson noise, assumes linear evolution of neutrino density perturbations, which is insufficient for capturing all non-linear effects. Conversely, the particle-based approach captures these non-linearities but suffers from shot noise.

Key Findings

1. Matter Power Spectrum Suppression: At z=3, the simulations reveal that neutrinos suppress power in the matter distribution beyond predictions of linear theory by about $10.5\, f_{\nu}$ at scales $k \,(h/{\rm Mpc}) \in [0.3,3]$. This suppression increases with neutrino mass and varies with redshift, demonstrating that non-linear effects are significantly stronger.
2. Comparative Analysis: The paper contrasts simulations without neutrinos calibrated to match the initial $\sigma_8$ values of neutrino-inclusive models. It reports that compared to simulations with neutrinos, those with adjusted $\sigma_8$ underpredict power on small scales by up to 5-10%, indicating that simple $\sigma_8$ corrections fail to capture the full complexity of neutrino effects.
3. Flux Power Spectrum Insights: Extracting mock Lyman-$\alpha$ forest spectra, the analysis reveals that suppressions in matter power lead to a notable reduction in flux power, possibly observable with future data precision levels. Specifically, the flux power spectrum shows a $2.5\%$ (up to $5\%$) decrease for neutrino masses $\Sigma m_{\nu} = 0.3$ eV (0.6 eV) — a non-trivial effect for current observational capabilities.

Implications and Future Directions

The paper culminates in setting an upper limit on neutrino masses based solely on Lyman-$\alpha$ forest data, affirming an upper limit of $\Sigma m_{\nu} < 0.9$ eV at 2$\sigma$ confidence. This limit is competitive with constraints derived from cosmic microwave background measurements. Importantly, the findings highlight the necessity for consistency between large- and small-scale measurements for robust neutrino mass constraints.

Given these results, the paper emphasizes the pivotal role of hydrodynamical simulations in improving our understanding of neutrino cosmology. The work suggests that future advancements in computational techniques and observational data precision will enable a finely-tuned measurement of neutrino masses. These findings are critical for refining models of cosmic structure formation and represent an essential step towards integrating neutrino physics with the broader cosmological framework.