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Baryon-CDM isocurvature galaxy bias with IllustrisTNG

We study the impact that baryon-CDM relative density perturbations $δ_{bc}$ 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 $δ_{bc}$ perturbations in our simulations is mimicked by modifying the ratios of the cosmic densities of baryons $Ω_b$ and CDM $Ω_c$, at fixed total matter density $Ω_m$. We measure the corresponding galaxy bias parameter $b_δ^{bc}$ as the response of galaxy abundances to $δ_{bc}$. When selecting by total host halo mass, $b_δ^{bc}$ 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 $δ_{bc}$ on the halo mass function and stellar-to-halo-mass relations. We show that simple modeling of the latter two effects describes $b_δ^{bc}$ for stellar-mass-selected objects well. We find $b_δ^{bc} =0.6$ for $M_* = 10^{11}\ M_{\odot}/h$ and $z=0.5$, which is representative of BOSS DR12 galaxies. For $δ_{bc}$ 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 $0.1\%$.