The SAMI Galaxy Survey: Quenching of star formation in clusters I.
  Transition galaxies
  We use integral field spectroscopy from the SAMI Galaxy Survey to identify
galaxies that show evidence for recent quenching of star formation. The
galaxies exhibit strong Balmer absorption in the absence of ongoing star
formation in more than 10% of their spectra within the SAMI field of view.
These $\rm{H}{\delta}$-strong galaxies (HDSGs) are rare, making up only $\sim
2$% (25/1220) of galaxies with stellar mass ${\rm log(}M_*/M_{\odot})>10$. The
HDSGs make up a significant fraction of non-passive cluster galaxies (15%;
17/115) and a smaller fraction (2.0%; 8/387) of the non-passive population in
low-density environments. The majority (9/17) of cluster HDSGs show evidence
for star formation at their centers, with the HDS regions found in the outer
parts of the galaxy. Conversely, the $\rm{H}{\delta}$-strong signal is more
evenly spread across the galaxy for the majority (6/8) of HDSGs in low-density
environments, and is often associated with emission lines that are not due to
star formation. We investigate the location of the HDSGs in the clusters,
finding that they are exclusively within 0.6$R_{200}$ of the cluster centre,
and have a significantly higher velocity dispersion relative to the cluster
population. Comparing their distribution in projected-phase-space to those
derived from cosmological simulations indicates that the cluster HDSGs are
consistent with an infalling population that have entered the central
0.5$r_{200, 3D}$ cluster region within the last $\sim 1\,$Gyr. In the 8/9
cluster HDSGs with central star formation, the extent of star formation is
consistent with that expected of outside-in quenching by ram-pressure
stripping. Our results indicate that the cluster HDSGs are currently being
quenched by ram-pressure stripping on their first passage through the cluster.
