From large-scale environment to CGM angular momentum to star forming
  activities -- I: star-forming galaxies
  The connection between halo gas acquisition through the circumgalactic medium
(CGM) and galaxy star formation has long been studied. In this series of two
papers, we put this interplay within the context of the galaxy environment on
large scales (several hundreds of kpc), which, to a certain degree, maps out
various paths for galaxy interactions. We use the IllustrisTNG-100 simulation
to demonstrate that the large-scale environment modulates the circumgalactic
gas angular momentum, resulting in either enhanced (Paper I) or suppressed
(Paper II) star formation inside a galaxy. In this paper (Paper I), we show
that the large-scale environment around a star-forming galaxy is often
responsible for triggering new episodes of star formation. Such an episodic
star formation pattern is well synced with a pulsating motion of the
circumgalactic gas, which, on the one hand receives angular momentum
modulations from the large-scale environment, yielding in-spiralling gas to
fuel the star-forming reservoir, while, on the other hand, is affected by the
feedback activities from the galaxy centre. As a result, a present-day
star-forming galaxy may have gone through several cycles of star-forming and
quiescent phases during its evolutionary history, with the circumgalactic gas
carrying out a synchronized cadence of "breathing in and out" motions out to
$\sim 100$ kpc.
