Baryon-CDM isocurvature galaxy bias with IllustrisTNG

We study the impact that baryon-CDM relative density perturbations delta(bc) 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 delta(bc) perturbations in our simulations is mimicked by modifying the ratios of the cosmic densities of baryons Omega(b) and CDM Omega(c), at fixed total matter density Omega(m). We measure the corresponding galaxy bias parameter b(delta)(bc) as the response of galaxy abundances to delta(bc). When selecting by total host halo mass, b(delta)(bc) 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 delta(bc) on the halo mass function and stellar-to-halo-mass relations. We show that simple modeling of the latter two effects describes b(delta)(bc) for stellar-mass-selected objects well. We find b(delta)(bc) = 0:6 for M-* = 1011 M-circle dot/h and z = 0:5, which is representative of BOSS DR12 galaxies. For delta(bc) 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%.