  Currently-proposed galaxy quenching mechanisms predict very different
behaviours during major halo mergers, ranging from significant quenching
enhancement (e.g., clump-induced gravitational heating models) to significant
star formation enhancement (e.g., gas starvation models). To test real
galaxies' behaviour, we present an observational galaxy pair method for
selecting galaxies whose host haloes are preferentially undergoing major
mergers. Applying the method to central L* (10^10 Msun < M_* < 10^10.5 Msun)
galaxies in the Sloan Digital Sky Survey (SDSS) at z<0.06, we find that major
halo mergers can at most modestly reduce the star-forming fraction, from 59% to
47%. Consistent with past research, however, mergers accompany enhanced
specific star formation rates for star-forming L* centrals: ~10% when a paired
galaxy is within 200 kpc (approximately the host halo's virial radius),
climbing to ~70% when a paired galaxy is within 30 kpc. No evidence is seen for
even extremely close pairs (<30 kpc separation) rejuvenating star formation in
quenched galaxies. For galaxy formation models, our results suggest: (1)
quenching in L* galaxies likely begins due to decoupling of the galaxy from
existing hot and cold gas reservoirs, rather than a lack of available gas or
gravitational heating from infalling clumps, (2) state-of-the-art semi-analytic
models currently over-predict the effect of major halo mergers on quenching,
and (3) major halo mergers can trigger enhanced star formation in non-quenched
central galaxies.
