The Australian National University

Gemini South Adaptive Optics Imager (GSAOI)

The Formation of the Disks of Disk Galaxies:

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The formation of the disks of disk galaxies remains poorly understood. Cold Dark Matter (CDM) simulations have difficulty in reproducing the observed properties of disks. For example:
  • The disks that are formed in the simulations have significantly smaller scale lengths and less angular momentum than real galactic disks.
  • Many disk galaxies have only the thin disk component, with no bulge or significant thick disk. This means that they formed in a very quiescent way, with no star formation occurring before the gas disk had settled. Furthermore, since the epoch of disk settling, there cannot have been any significant disturbance by the mergers and interactions that are such a feature of the CDM simulations.
Other important problems include:
  • Almost all disk galaxies have an exponential radial surface brightness distribution. The reason for this exponential form is not known; nor is it known when the exponential form is established.
  • The exponential disks are typically truncated radially at a radius of about 3 exponential scale lengths. This trucation is an important and probably fundamental indicator of the nature of the galaxy formation process. At this time, the reason for the truncation is not known. Here are some of the many possibilities:
    • The truncation may reflect the maximum angular momentum of baryons in the protogalaxy, or
    • It may be caused by tidal interactions of lumps of dark + baryonic matter early in the hierarchical aggregation process, or
    • It may be the radius where the gas density goes below the critical density for star formation, or
    • It may be associated with the viscous evolution of the star-forming disk (which itself may also lead to the exponential light distribution)
Almost everything that we know about galactic disks comes from disks at low redshift, so little is known about when the fundamental properties of the disks were established. GSAOI gives us the opportunity to study the detailed structure of galactic disks at earlier epochs out to a redshift of z=1.

The primary goal is to see if the exponential nature of the disks, their characteristic scale lengths, and their truncation are already established at z=1. To achieve this goal, we need the high spatial resolution and high near-infrared sensitivity offered by GSAOI. Observationally, we would aim to study galaxies over a range of redshifts, but at the same rest-frame mean wavelength. We would compare low redshift galaxies observed in the I band with galaxies at z=0.5 in the J band and at z=1 in the H band.