  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 $M_* = 10^{11}\ M_{\odot}/h$
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 $0.1\%$.
