Project Title: An Integral Field Spectrograph Optimized for Adaptive Optics

Principal Investigator: James Larkin (Assistant Professor, UCLA)

Co-Investigators: Andreas Quirrenbach (Associate Professor, UCSD), James Graham (Associate Professor, UCB)

The UCLA infrared laboratory is currently designing an integral field infrared spectrograph (IFS) for the Keck AO system. The goal of the current instrument design is to produce spectral data cubes with near diffraction limited spatial resolution. The concept uses a cryogenic fiber optic bundle to obtain infrared spectra at over 1000 locations simultaneously. Very faint imaging and low resolution spectroscopy can be achieved through spectral binning and software OH-suppression. We believe that NSF center funding is crucial in optimizing the instrument's performance and in helping to secure primary construction costs from CARA. We also hope to develop general purpose reduction and analysis software that will be available for other integral field instruments. Our long term plans include investigating new instrument concepts built around advanced AO systems. We envision a much more integrated relationship between future instruments and their AO systems.

First-year project plan: During the first year, our center activities would concentrate on optimizing our integral field spectrograph design. There are a large number of options and tradeoffs to be made with this instrument. In particular, spectral coverage must be traded against spectral resolution, and high resolution spatial sampling requires reducing the field of view. Below we list our current design and some of the scientific motivations; but this concept will change as we continue to analyze both the scientific requirements and the physical and financial limitations of the project. We are currently scheduled to make a formal instrument proposal to the Keck Science Steering Committee before June, 1999 with a preliminary design phase lasting roughly 9 months. The goal of the current instrument design is to use the Keck AO system to produce spectral images at the highest spatial resolution ever achieved. The instrument uses an array of 1024 fibers to sample the field at plate scales of 0.05, 0.10 and 0.20 arcseconds per fiber. The total field of view given about 30x30 fibers is then 1.5x1.5, 3.0x3.0 and 6.0x6.0 arcseconds. Each fiber spectrum contains 1024 spectral channels (wavelengths) producing broad band coverage at a spectral resolution of ~5000 (lambda / delta lambda). Emission lines from OH molecules in the atmosphere are the dominant background source from 1 to 2.2 microns. The IFS has sufficient resolution to separate these lines so that over 90% of the pixels have no OH emission and therefore have much lower backgrounds. By rebinning only these OH free channels, a very low background image or lower resolution spectrum can be produced with detection limits 1 to 1.5 magnitudes better that with a direct imaging camera. By combining deep imaging and spectral imaging at close to the diffraction limit of the Keck Telescope, the IFS opens up extremely exciting scientific possiblities. Among these is the kinematic study of faint galaxies in the process of formation and early evolution. The Hubble Space Telescope has shown that with a factor of 10 or so in spatial resolution many faint galaxies show highly distorted morphologies indicating that they are undergoing major tidal distortions and episodes of rapid star formation. These faint galaxies are often seen as they were in the very early Universe and therefore are examples of more normal galaxies like our own during the early formation period of galaxies. These objects are too faint and compact for the current generation of spectrographs and ground based cameras to image in detail or to obtain spectral and kinematic information. The IFS with a field of view around 1.5 arcseconds can image an entire distant galaxy and determine its redshift, metallicity, star formation rate and measure internal motions of material. With this capability, many galaxies at a range of ages and sizes can be examined to determine how galaxies form and evolve. Today this remains among astronomy's greatest unsolved problems. Another area where the IFS will have a major impact is in the study of faint non-stellar objects (Brown Dwarfs). Due to its extremely deep imaging and spectroscopic capability, it is ideally suited for taken spectra of extremely faint objects. With the spatial resolution of the AO system, this capability is extended to the regions around bright stars where the glare of the central star is the dominant source of background. With its spectral imaging capability, the IFS will allow good measurement and rejection of the stellar spectrum from that of the brown dwarf. Brown Dwarfs offer important insights into stellar formation and into the formation of Jovian planets.

First-year milestones:
Make a formal proposal to the Keck SSC for primary funding. Develop a complete preliminary instrument design. This will include prototyping fiber bundles, and software packages.

Second through fifth-year plan:
Starting in the second year of the center, we plan to study new instrument concepts, in particular instruments that interact with their AO systems. A particularly important example of this is an infrared camera or spectrograph that can use phase diversity measurements to optimize AO performance and reduce non-common path errors. Several current AO systems are struggling not because the AO system is inadequate but because their scientific camera can not make the most of the image quality provided. Instruments may also be able to alter the control programs in order to optimize a particular observing mode such as trading off spatial resolution vs. encircled energy in order to reduce slit losses. We believe these studies are virtually impossible without a long term funding source such as an NSF center. We also think that this work requires multicampus cooperation due to the varying AO concepts and implementations.