Semi-Analytical Jeans Filtering Functions for Baryonic Density and Velocity Fields in Second-Order Cosmological Perturbation Theory
Cosmological perturbation theory provides the framework for describing the evolution of matter density fluctuations and the formation of large-scale structure in the $Λ$CDM paradigm. We present a semi-analytical approach for a mixed fluid composed of cold dark matter (CDM) and baryons. Assuming General Relativity, we derive the equations of motion from the Vlasov equation, incorporating baryonic effects through the stress tensor and retaining only baryonic pressure. We introduce Jeans Filtering Functions (JFF) as a biasing tool to describe baryonic fluctuations using CDM as a tracer. First- and second-order solutions are obtained with a semi-analytical method based on a single iteration of the equations of motion. These solutions can be evaluated at low computational cost and allow us to study the shift of the filtering scale beyond linear order and its impact on the matter power spectrum without computing the spectrum explicitly. In contrast to approaches based directly on the power spectrum, we quantify baryonic effects through the fluctuation fields themselves. We derive semi-analytical expressions for baryonic density and velocity fluctuations, with the velocity divergence field providing the most significant result. The method offers a readily evaluable description of baryonic effects in large-scale structure and shows how pressure shifts the filtering scale, with implications for quantities such as the filtering mass and the temperature of pressure-supported components.