  The InfraRed Imaging Spectrograph (IRIS) is a first-light instrument being
designed for the Thirty Meter Telescope (TMT). IRIS is a combination of an
imager that will cover a 16.4" field of view at the diffraction limit of TMT (4
mas sampling), and an integral field unit spectrograph that will sample objects
at 4-50 mas scales. IRIS will open up new areas of observational parameter
space, allowing major progress in diverse fields of astronomy. We present the
science case and resulting requirements for the performance of IRIS.
Ultimately, the spectrograph will enable very well-resolved and sensitive
studies of the kinematics and internal chemical abundances of high-redshift
galaxies, shedding light on many scenarios for the evolution of galaxies at
early times. With unprecedented imaging and spectroscopy of exoplanets, IRIS
will allow detailed exploration of a range of planetary systems that are
inaccessible with current technology. By revealing details about resolved
stellar populations in nearby galaxies, it will directly probe the formation of
systems like our own Milky Way. Because it will be possible to directly
characterize the stellar initial mass function in many environments and in
galaxies outside of the the Milky Way, IRIS will enable a greater understanding
of whether stars form differently in diverse conditions. IRIS will reveal
detailed kinematics in the centers of low-mass galaxies, allowing a test of
black hole formation scenarios. Finally, it will revolutionize the
characterization of reionization and the first galaxies to form in the
universe.
