  Many extensions of the Standard Model, including those with dark matter
particles, propose new mediator particles that decay into hadrons. This paper
presents a search for such low mass narrow resonances decaying into hadrons
using 140 fb$^{-1}$ of proton-proton collision data recorded with the ATLAS
detector at a centre-of-mass energy of 13 TeV. The resonances are searched for
in the invariant mass spectrum of large-radius jets with two-pronged
substructure that are recoiling against an energetic photon from initial state
radiation, which is used as a trigger to circumvent limitations on the maximum
data recording rate. This technique enables the search for boosted hadronically
decaying resonances in the mass range 20-100 GeV hitherto unprobed by the ATLAS
Collaboration. The observed data are found to agree with Standard Model
predictions and 95% confidence level upper limits are set on the coupling of a
hypothetical new spin-1 $Z'$ resonance with Standard Model quarks as a function
of the assumed $Z'$-boson mass in the range between 20 and 200 GeV.
