  We describe the first demonstration of a sub-keV electron recoil energy
threshold in a dual-phase liquid argon time projection chamber. This is an
important step in an effort to develop a detector capable of identifying the
ionization signal resulting from nuclear recoils with energies of order a few
keV and below. We obtained this result by observing the peaks in the energy
spectrum at 2.82 keV and 0.27 keV, following the K- and L-shell electron
capture decay of Ar-37, respectively. The Ar-37 source preparation is described
in detail, since it enables calibration that may also prove useful in dark
matter direct detection experiments. An internally placed Fe-55 x-ray source
simultaneously provided another calibration point at 5.9 keV. We discuss the
ionization yield and electron recombination in liquid argon at those three
calibration energies.
