Baryon-CDM isocurvature galaxy bias with IllustrisTNG
Alexandre Barreira, Giovanni Cabass, Dylan Nelson, Fabian Schmidt
Abstract
We study the impact that baryon-CDM relative density perturbations δ<sub>bc</sub> have on galaxy formation using cosmological simulations with the IllustrisTNG model. These isocurvature (non-adiabatic) perturbations can be induced primordially, if multiple fields are present during inflation, and are generated before baryon-photon decoupling when baryons did not comove with CDM. The presence of long-wavelength δ<sub>bc</sub> perturbations in our simulations is mimicked by modifying the ratios of the cosmic densities of baryons Ω<sub>b</sub> and CDM Ω<sub>c</sub>, at fixed total matter density Ω<sub>m</sub>. We measure the corresponding galaxy bias parameter b<sub>δ</sub><sup>bc</sup> as the response of galaxy abundances to δ<sub>bc</sub>. When selecting by total host halo mass, b<sub>δ</sub><sup>bc</sup> is negative and it decreases with mass and redshift. Stellar-mass selected simulated galaxies show a weaker or even the opposite trend because of the competing effects of δ<sub>bc</sub> on the halo mass function and stellar-to-halo-mass relations. We show that simple modeling of the latter two effects describes b<sub>δ</sub><sup>bc</sup> for stellar-mass-selected objects well. We find b<sub>δ</sub><sup>bc</sup> =0.6 for M<sub>*</sub> = 10<sup>11</sup> M<sub>⊙</sub>/h and z=0.5, which is representative of BOSS DR12 galaxies. For δ<sub>bc</sub> modes generated by baryon-photon interactions, we estimate the impact on the DR12 power spectrum to be below 1%, and shifts on inferred distance and growth rate parameters should not exceed 0.1%.