Most stars form in rotationally supported disks, at a rate closely
correlated with a galaxy’s mass. This is as true in the local Universe
as it was at a redshift of 2: However much else has changed. At low
redshift pre-existing stars dominate the local potential, molecular gas
fractions and star formation rates are low, and disks are fairly
stable, modulo long-term instabilities such as bars. At high redshift
gas disks can be equally or more massive than stellar ones, leading to
high star formation rates, strong turbulence and violent instabilities.

In this talk I will focus on the relation of the star forming disk to
the stellar one, tracing ongoing star formation via Halpha emission
line mapping. I will explore the properties of disks in the local
Universe using HAGGIS (H-Alpha Narrow Band Imaging Survey) data: high
quality Halpha narrow band imaging of hundreds of galaxies at
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