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We present the Phase A Science Case for the Multi-conjugate Adaptive-optics Visible Imager-Spectrograph (MAVIS), planned for the Adaptive Optics Facility (AOF) of the Very Large Telescope (VLT). MAVIS is a general-purpose instrument for exploiting the highest possible angular resolution of any single optical telescope available in the next decade, either on Earth or in space, and with sensitivity comparable to (or better than) larger aperture facilities. MAVIS uses two deformable mirrors in addition to the deformable secondary mirror of the AOF, providing a mean V-band Strehl ratio of >10% (goal >15%) across a relatively large (30 arc second) science field. This equates to a resolution of, 141 pages, 79 figures, 10Mb. Modified version of the Phase A Science Case submitted to ESO in April 2020 as part of the MAVIS Phase A design study. Full resolution version available upon request

This white paper summarizes those conclusions of the Snowmass White Paper "Spectroscopic Needs for Imaging Dark Energy Experiments" (arXiv:1309.5384) which are relevant to the training of LSST photometric redshifts; i.e., the use of spectroscopic redshifts to improve algorithms and reduce photo-z errors. The larger and more complete the available training set is, the smaller the RMS error in photo-z estimates should be, increasing LSST's constraining power. Among the better US-based options for this work are the proposed MANIFEST fiber feed for the Giant Magellan Telescope or (with lower survey speed) the WFOS spectrograph on the Thirty Meter Telescope (TMT). Due to its larger field of view and higher multiplexing, the PFS spectrograph on Subaru would be able to obtain a baseline training sample faster than TMT; comparable performance could be achieved with a highly-multiplexed spectrograph on Gemini with at least a 20 arcmin diameter field of view., White paper submitted to the National Research Council Committee on a Strategy Optimize the U.S. O/IR System in the Era of the LSST, 5 pages

This white paper summarizes the conclusions of the Snowmass White Paper "Spectroscopic Needs for Imaging Dark Energy Experiments" (arXiv:1309.5384) which are relevant to the calibration of LSST photometric redshifts; i.e., the accurate characterization of biases and uncertainties in photo-z's. Any significant miscalibration will lead to systematic errors in photo-z's, impacting nearly all extragalactic science with LSST. As existing deep redshift samples have failed to yield highly-secure redshifts for a systematic 20%-60% of their targets, it is a strong possibility that future deep spectroscopic samples will not solve the calibration problem on their own. The best options in this scenario are provided by cross-correlation methods that utilize clustering with objects from spectroscopic surveys (which need not be fully representative) to trace the redshift distribution of the full sample. For spectroscopy, the eBOSS survey would enable a basic calibration of LSST photometric redshifts, while the expected LSST-DESI overlap would be more than sufficient for an accurate calibration at z>0.2. A DESI survey of nearby galaxies conducted in bright time would enable accurate calibration down to z~0. The expanded areal coverage provided by the transfer of the DESI instrument (or duplication of it at the Blanco Telescope) would enable the best possible calibration from cross-correlations, in addition to other science gains., White paper submitted to the National Research Council Committee on a Strategy Optimize the U.S. O/IR System in the Era of the LSST, 4 pages

Deep imaging of the diffuse light emitted by the stellar fine structures and outer halos around galaxies is now often used to probe their past mass assembly. Because the extended halos survive longer than the relatively fragile tidal features, they trace more ancient mergers. We use images reaching surface brightness limits as low as 28.5-29 mag.arcsec-2 (g-band) to obtain light and color profiles up to 5-10 effective radii of a sample of nearby early-type galaxies. They were acquired with MegaCam as part of the CFHT MATLAS large programme. These profiles may be compared to those produced by simulations of galaxy formation and evolution, once corrected for instrumental effects. Indeed they can be heavily contaminated by the scattered light caused by internal reflections within the instrument. In particular, the nucleus of galaxies generates artificial flux in the outer halo, which has to be precisely subtracted. We present a deconvolution technique to remove the artificial halos that makes use of very large kernels. The technique based on PyOperators is more time efficient than the model-convolution methods also used for that purpose. This is especially the case for galaxies with complex structures that are hard to model. Having a good knowledge of the Point Spread Function (PSF), including its outer wings, is critical for the method. A database of MegaCam PSF models corresponding to different seeing conditions and bands was generated directly from the deep images. It is shown that the difference in the PSFs in different bands causes artificial changes in the color profiles, in particular a reddening of the outskirts of galaxies having a bright nucleus. The method is validated with a set of simulated images and applied to three representative test cases: NGC 3599, NGC 3489, and NGC 4274, and exhibiting for two of them a prominent ghost halo. The method successfully removes it., Comment: 10 pages, 11 figures, accepted for publication in A&A