Alexander Hobbs - March 17, 2017

Positive feedback in galaxy formation

Milky Way-type galaxies require a sufficient amount of cold gas to sustain
near-constant star formation across a Hubble time. The means by which this cold gas is
acquired is still an open question. Cosmological accretion is largely ineffective
after z ~ 2, as are mergers, yet the cold gas available for star formation seems to be
continually replenished over the lifetime of the galaxy. The means by which this
occurs has profound implications for the role of the galactic disk in the larger,
galaxy formation history. With positive feedback models, we have begun to see evidence
for a picture whereby the galactic disk can act as a refrigerator of its hotter,
larger-scale corona, cooling the hot gas through feedback from star formation that
makes it out of the disk and interacts with the hot halo. I will summarise the
(relatively new) field of positive feedback in galaxy formation and present two
promising modes by which positive feedback from star formation might occur; (i) an
entrainment of hot coronal gas by the cooler, ejected gas that causes a larger
fraction of cold gas to fall back onto the galaxy, and (ii) a non-linear thermal
instability excited by large superbubbles that funnels gas back down onto the disk as
it loses pressure support. Quantifying the impact of these modes of positive feedback
will shed light on their importance to galaxy formation and the role of internal
galactic processes on influencing the larger IGM.