Dipole distortions in the intergalactic medium

Abstract

Baryonic feedback can significantly modify the spatial distribution of matter on small scales and create a bulk relative velocity between the dominant cold dark matter and the hot gas. We study the consequences of such bulk motions using two high resolution hydrodynamic simulations, one with no feedback and one with very strong feedback. We find that relative velocities of order 100 km/s are produced in the strong feedback simulation whereas it is much smaller when there is no feedback. Such relative motions induce dipole distortions to the gas, which we quantify by computing the dipole correlation function. We find halo coordinates and velocities are systematically changed in the direction of the relative velocity. Finally, we discuss potential to observe the relative velocity via large scale structure, Sunyaev-Zel'dovich and line emission measurements. Given the nonlinear nature of this effect, it should next be studied in simulations with different feedback implementations/strengths to determine the available model space. problem (Tornatore et al. 2010). We believe that feedback comes mainly in two flavors, arising from different physical processes and involving different energy scales. On one hand, supernova feedback involves small scales and is responsible for the suppression of the stellar mass function for small galaxies. On the other, Active Galactic Nuclei (AGN) feedback operates on large scales and is commonly believed to be the mechanism responsible for the stellar mass function suppression for large galaxies.Unfortunately, we do not have a complete physical understanding of these phenomena. In cosmological hydrodynamic simulations the large range of scales involved do not allow us to simulate these processes directly and instead subgrid models have been developed to capture them in a phenomenological, but physically motivated, manner (see the Horizon (Dubois et al. 2014), Illustris (Vogelsberger et al. 2014a), IllustrisTNG (Pillepich et al. 2018) or Eagle (Schaye et al. 2015) simulations as relevant examples). The "missing baryons" can then be found in simulations. As an example, Haider et al. (2016) found that around one quarter of baryons end up in halos, just under a half in filaments, and nearly a third in voids. These fractions are not replicated without feedback where more baryons end up in halos. Ad-

Measuring dark matter-neutrino relative velocity on cosmological scales

“…The baryon-CDM relative velocity and galaxy velocity bias can generate similar features in the cross-correlation of cosmological fields [39][40][41][42][43][44][45]. However, the velocity bias is severely constrained by the equivalence principle [45].…”

“…The large-scale baryon-CDM relative velocity can be predicted using perturbation theory and marginalized over [41][42][43]. On very small scales where baryon physics becomes important, we need to rely on hydrodynamical simulations, but the small-scale baryon-CDM relative velocity should not correlate with the neutrino-CDM velocity as it originates from nonlinear feedback processes [44]. However, measuring the CDMneutrino dipole signal only uses the direction of the relative velocity; therefore the measured dipole signal also includes the halo-CDM and baryon-CDM dipole which is degenerate with the small neutrino effect [19].…”