  Galaxies grow through both internal and external processes. In about 10% of
nearby red galaxies with little star formation, gas and stars are
counter-rotating, demonstrating the importance of external gas acquisition in
these galaxies. However, systematic studies of such phenomena in blue,
star-forming galaxies are rare, leaving uncertain the role of external gas
acquisition in driving evolution of blue galaxies. Based on new measurements
with integral field spectroscopy of a large representative galaxy sample, we
find an appreciable fraction of counter-rotators among blue galaxies (9 out of
489 galaxies). The central regions of blue counter-rotators show younger
stellar populations and more intense, ongoing star formation than their outer
parts, indicating ongoing growth of the central regions. The result offers
observational evidence that the acquisition of external gas in blue galaxies is
possible; the interaction with pre-existing gas funnels the gas into nuclear
regions (< 1 kpc) to form new stars.
