  Cosmological data can be used to search for---and characterize---light
particles in the standard model, if these populate our Universe. In addition to
the well-known effect of these light relics in the background cosmology,
usually parametrized through a change in the effective number $N_{\rm eff}$ of
neutrino species, these particles can become nonrelativistic at later times,
affecting the growth of matter fluctuations due to their thermal velocities. An
extensively studied example is that of massive neutrinos, which are known to
produce a suppression in the matter power spectrum due to their free streaming.
Galaxies, as biased traces of matter fluctuations, can therefore provide us
with a wealth of information about both known and unknown degrees of freedom in
the standard model. To harness this information, however, the galaxy bias has
to be determined in the presence of massive relics, which is expected to vary
with scale. Here we present the code RelicFast, which efficiently computes the
scale-dependent bias induced by relics of different masses, spins, and
temperatures, through spherical collapse and the peak-background split. Using
this code, we find that, in general, the bias induced by light relics partially
compensates the suppression of power, and should be accounted for in any search
for relics with galaxy data. In particular, for the case of neutrinos, we find
that both the normal and inverted hierarchies present a percent-level step in
the Lagrangian bias, with a size scaling linearly with the neutrino-mass sum,
in agreement with recent simulations. RelicFast can compute halo bias in under
a second, allowing for this effect to be properly included for different
cosmologies, and light relics, at little computational cost.
