  A search is performed for localized excesses in the low-mass dijet invariant
mass distribution, targeting a hypothetical new particle decaying into two jets
and produced in association with either a high transverse momentum photon or a
jet. The search uses the full Run 2 data sample from LHC proton-proton
collisions collected by the ATLAS experiment at a center-of-mass energy of 13
TeV during 2015-2018. Two variants of the search are presented for each type of
initial-state radiation: one that makes no jet flavor requirements and one that
requires both of the jets to have been identified as containing $b$-hadrons. No
excess is observed relative to the Standard Model prediction, and the data are
used to set upper limits on the production cross-section for a benchmark $Z'$
model and, separately, for generic, beyond the Standard Model scenarios which
might produce a Gaussian-shaped contribution to dijet invariant mass
distributions. The results extend the current constraints on dijet resonances
to the mass range between 200 and 650 GeV.
