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Coupling MOAO with integral field spectroscopy: specifications for the VLT and the E-ELT

By Puech M, Flores H, Lehnert M, Neichel B, Fusco T, Rosati P, Cuby JG and Rousset G


Elucidating the processes that governed the assembly and evolution of galaxies over cosmic time is one of the main objectives of all of the proposed Extremely Large Telescopes (ELT). To make a leap forward in our understanding of these processes, an ELT will want to take advantage of Multi-Objects Adaptive Optics (MOAO) systems, which can substantially improve the natural seeing over a wide field of view. We have developed an end-to-end simulation to specify the science requirements of a MOAO-fed integral field spectrograph on either an 8-m or 42-m telescope. Our simulations rescale observations of local galaxies or results from numerical simulations of disc or interacting galaxies. The code is flexible in that it allows us to explore a wide range of instrumental parameters such as ensquared energy (EE), pixel size, spectral resolution, etc. For the current analysis, we limit ourselves to a local disc galaxy which exhibits simple rotation and a simulation of a merger. While the number of simulations is limited, we have attempted to generalize our results by introducing the simple concepts of 'point spread function (PSF) contrast' which is the amount of light polluting adjacent spectra which we find drives the smallest EE at a given spatial scale. The choice of the spatial sampling is driven by the 'scale-coupling'. By scale-coupling we mean the relationship between the integral field unit (IFU) pixel scale and the size of the features that need to be recovered by 3D spectroscopy in order to understand the nature of the galaxy and its substructure. Because the dynamical nature of galaxies is mostly reflected in their large-scale motions, a relatively coarse spatial resolution is enough to distinguish between a rotating disc and a major merger. Although we used a limited number of morphokinematic cases, our simulations suggest that, on a 42-m telescope, the choice of an IFU pixel scale of 50-75 mas seems to be sufficient. Such a coarse sampling has the benefit of lowering the exposure time to reach a specific signal-to-noise ratio as well as relaxing the performance of the MOAO system. On the other hand, recovering the full 2D kinematics of z similar to 4 galaxies requires high signal-to-noise ratio and at least an EE of 34 per cent in 150 mas (2 pixels of 75 mas). Finally, we carried out a similar study for a hypothetical galaxy/merger at z = 1.6 with a MOAO-fed spectrograph for an 8 m, and find that at least an EE of 30 per cent at 0.25 arcsec spatial sampling is required to understand the nature of discs and mergers

Year: 2008
DOI identifier: 10.1111/j.1365-2966.2008.13808.x
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