  Quenched galaxies at z>2 are nearly all very compact relative to z~0,
suggesting a physical connection between high stellar density and efficient,
rapid cessation of star-formation. We present restframe UV spectra of
Lyman-break galaxies (LBGs) at z~3 selected to be candidate progenitors of
quenched galaxies at z~2 based on their compact restframe optical sizes and
high surface density of star-formation. We compare their UV properties to those
of more extended LBGs of similar mass and star formation rate (non-candidates).
We find that candidate progenitors have faster ISM gas velocities and higher
equivalent widths of interstellar absorption lines, implying larger velocity
spread among absorbing clouds. Candidates deviate from the relationship between
equivalent widths of Lyman-alpha and interstellar absorption lines in that
their Lyman-alpha emission remains strong despite high interstellar absorption,
possibly indicating that the neutral HI fraction is patchy such that
Lyman-alpha photons can escape. We detect stronger CIV P-Cygni features
(emission and absorption) and HeII emission in candidates, indicative of larger
populations of metal rich Wolf-Rayet stars compared to non-candidates. The
faster bulk motions, broader spread of gas velocity, and Lyman-alpha properties
of candidates are consistent with their ISM being subject to more energetic
feedback than non-candidates. Together with their larger metallicity (implying
more evolved star-formation activity) this leads us to propose, if
speculatively, that they are likely to quench sooner than non-candidates,
supporting the validity of selection criteria used to identify them as
progenitors of z~2 passive galaxies. We propose that massive, compact galaxies
undergo more rapid growth of stellar mass content, perhaps because the gas
accretion mechanisms are different, and quench sooner than normally-sized LBGs
at these early epochs.
