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We present a comprehensive multi-wavelength study of 5 poststarburst galaxies
with $M_\ast > 10^{11} M_\odot$ at $z\sim 0.7$, examining their stars, gas, and
current and past star-formation activities. Using optical images from the
Subaru telescope and Hubble Space Telescope, we observe a high incidence of
companion galaxies and low surface brightness tidal features, indicating that
quenching is closely related to interactions between galaxies. From optical
spectra provided by the LEGA-C survey, we model the stellar continuum to derive
the star-formation histories and show that the stellar masses of progenitors
ranging from $2\times10^9 M_\odot$ to $10^{11} M_\odot$, undergoing a burst of
star formation several hundred million years prior to observation, with a decay
time scale of $\sim100$ million years. Our ALMA observations detect CO(2-1)
emission in four galaxies, with the molecular gas spreading over up to $>1"$,
or $\sim10$ kpc, with a mass of up to $\sim2 \times10^{10} M_\odot$. However,
star-forming regions are unresolved by either the slit spectra or 3~GHz
continuum observed by the Very Large Array. Comparisons between the
star-formation rates and gas masses, and the sizes of CO emission and
star-forming regions suggest a low star-forming efficiency. We show that the
star-formation rates derived from IR and radio luminosities with commonly-used
calibrations tend to overestimate the true values because of the prodigious
amount of radiation from old stars and the contribution from AGN, as the
optical spectra reveal weak AGN-driven outflows.
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