Science Case Study

The prime driver for the ARGOS ground layer adaptive optics system is to greatly enhance the science that can be done on existing facility instruments, and in particular with LUCI. It will enable this by improving the resolution and sensitivity for both imaging and multi-object spectroscopy over a very wide field of view. In particular, the combination of GLAO with a wide field MOS will be a unique facility.
The direct benefits to the LBT are afforded by the factor of 2-3 improvement in the spatial resolution. Indeed, ARGOS can be considered as a ‘seeing enhancer’ for the existing facility instruments that enables one to address much more of the primary science that has been identified in their respective science cases.

This is because:

  • observations can be done much faster, saving a significant amount of observing time.
  • demanding science programmes, that would normally require the best seeing conditions, can instead be carried out during most nights.
The science case study addresses these issues in greater detail. It describes the gains that can be achieved with GLAO; and the requirements on the design are justified. It summarises the broad spectrum of science that it enables one to address. A specific detailed comparison is given for a highlight science case: the Dynamics and Stellar populations in high redshift galaxies.

Questions ranked around this topic are:

  • How did galaxies assemble over time, and what is the role of mergers?
  • How did galaxies grow their stellar mass?
  • How did galaxies acquire their morphology and how did the hubble sequence arise?
  • How did galaxies get their angular momentum?

K-band image of NGC4945 taken with LGSAO at the VLT. This galaxy is at a distance of about 4Mpc and yet in this image it is possible to resolve individual stars. The data have a resolution of about 0.14”, similar to that which will be achievable with LGS-GLAO on the LBT in good conditions; but the field is much smaller, only 37”x37” in comparison to the 240”x240” available with LUCI.
In order to answer these fundamental questions, robust measures are needed for mass, age, star formation rate, gas-phase metallicity and ionization state, dust obscuration, sizes, and morphologies for complete samples of z ~ 1 – 4 galaxies. This epoch is crucial as it corresponds to the peak of (dust-enshrouded) star formation and quasar activity, as well as the assembly of a significant fraction of the present-day galaxies. Spectroscopic investigations at z ~ 1 – 4 are however still challenging since the key spectral diagnostics (Hα, Hβ, [NII], [OIII], [OII], [SII] emission lines, continuum emission, stellar absorption features, and Balmer/4000Å breaks) that are emitted in the rest-frame optical are redshifted to the near-IR, between 1 and 2.5μm. Due to the technological challenges to build multiplexed near-IR cryogenic spectrographs there is a lack of such capabilities on 8m-class telescopes. LUCI will therefore play a very important role in answering the above key scientific questions. This highlight science case is accomplished by a numb er of short science cases contributed by the LBT community illustrating the breadth of science that can be addressed with the GLAO system. These include a mixture of cases, some of which require specifically GLAO for the science itself; and others which require AO over a smaller field, but still make use of the wide field GLAO capability in order to measure the corrected PSF. In all cases, the improved resolution enables a better scientific analysis and interpretation, and yields gains in observing time of a factor of 4-9.

Additionally the science case study outlines the gains in science capability with GLAO, as:
  • Increased point source sensitivity
  • Increased slit coupling efficiency
  • Reduced crowding noise
  • Enhanced spatial resolution
The science case study as well contains a detailed comparison with the spectroscopic capabilities of JWST and ground based facilities. While ARGOS and LUCI are not expected to compete effectively with JWST, either in terms of resolution or sensitivity, the simulation results show that ARGOS will make the LBT spectroscopically competitive with JWST between the OH lines and at wavelength shorter 2.2 μm. In comparison with other existing or planned facilities, there are several strong competitors to LUCI and its wide field MOS. ARGOS therefore will be a crucial enhancement to LUCI to give it the edge over other instruments.
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