A major scientific breakthrough of IRIS+NFIRAOS will be the spatial dissection of galaxies in the early universe in the range of z~ 1-5 (look back time of 7.5 - 12.5 Gyr), which is during the the peak of the cosmic star formation and Active Galactic Nuclei (AGN) accretion history of the universe. Observations of these galaxies with the TMT will exploit both the light gathering power and the unique angular resolution at near-infrared wavelengths provided by the adaptive optics system. Large samples of galaxies throughout this redshift range are already known, and the current generation of 8-10 m telescopes has recently provided intriguing evidence for prevalent dynamically ‘hot’ high-velocity dispersion systems that do not fit neatly into our current picture of galaxy formation.
Spatially resolved spectroscopy of emission lines with the light-gathering power and spatial resolution of TMT will allow differences in chemistry, kinematics, and physical conditions to be mapped as a function of spatial position within the galaxies. Such information is required to go beyond measurements of crude global properties, and thereby gain fresh understanding into the physics of galaxy formation. IRIS, with its imaging and IFU capabilities, will provide the crucial first steps in a comprehensive survey of both massive and low mass systems. It will measure velocity widths and global rotation of galaxies, helping to distinguish kinematics associated with ongoing merging signatures from kinematics of rapidly star-forming but otherwise undisturbed galaxies. By measuring emission line ratios as a function of position within the galaxies, IRIS will uncover active galactic nuclei that might otherwise be hidden, and it will probe the spatial evolution of metallicity gradients within galaxies, which are closely tied to their formation mechanisms.
Simulations of the sensitivity of the IRIS imager and spectrograph were performed for both the point source sensitivity and the expected spectroscopy sensitivity to emission lines for high-z galaxies. These simulations are based on expected instrument noise characteristics and typical sky background and AO performance estimates for TMT. In particular, these simulations show that IRIS will be able to detect normal star forming galaxies at high-z, compared the current capabilities of 8-10 meter class telecopes, which can only detect the most massive galaxies with the highest star formation rates. Star forming galaxies currently being studied on 8-10m telescopes typically have total integrated star formation rates of 10-100 Msun/yr, stellar masses of 1e9-1e11 Msun, and dark matter halo masses of 1e11-1e13 Msun. In comparison, the 0.05" scale of IRIS will allow observations of z=1.5 star forming galaxies with integrated star formation rates of 0.1 - 10 Msun/yr. The figures below highlight the capabilities of IRIS on a z~1.0 galaxy and z~5.0 galaxy, illustrating the signal-to-noise achieved given a SFR and surface brightness profile of these example galaxies. In addition we show the signal-to-noise achieved for a range of surface brightness profiles and velocity dispersions observed with both the 0.004" and 0.05" spatial scale.
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