Exploring Cosmic Noon and Cosmic Dawn with JWST Guaranteed Time Observations

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When it launches in 2021, the James Webb Space Telescope (JWST) is going to transform our understanding of the distant Universe. Since 2003 I have been anticipating this day as core & founding team member of the JWST NIRISS team (NIRISS = Near-IR Imager and Slitless Spectrograph), Canada's contribution to JWST. We will use 200 hours of our team's Guaranteed Time Observer (GTO) allocation to carry out a dedicated program to study the evolution of low-mass galaxies across cosmic time. At nearly half of Canada's GTO time, these data will represent a once-in-a-career opportunity for my group. During the term of this grant we will also finish a number of ongoing key projects that use the massive CLAUDS+HSC imaging dataset that probes the distant universe to an unprecedented combination of depth and area, and will then publicly release this fantastic dataset to the community. We will also continue development of the GIRMOS instrument for Gemini, building up to first light in mid-2024, to be followed by its scientific exploitation during the next (2025+) term. However, the main focus over this term, 2020-2025, will be the scientific exploitation of the JWST GTO dataset. This work will focus on the evolution and role of low-mass galaxies, which are the most numerous galaxies in the Universe. The primary goal of this program is to understand their properties, impact, and evolution over cosmic time. We will use our JWST GTO data to study distant low-mass galaxies, many of them gravitationally lensed, and some spectacularly so, behind five massive galaxy clusters. This will yield spectra of 10,000 and images of 100,000 distant galaxies, from the Epoch of Reionization ("cosmic dawn"), through "cosmic noon" at z~2-3, to the galaxy populations within the massive z~0.5 lensing clusters. The spectra and images will have unprecedented spatial resolution and sensitivity, both due to JWST's intrinsic sensitivity and - for many of our galaxies - because of the additional boost due to the gravitational lensing. The data will yield emission line maps and line ratios for many galaxies, some at spatial scales of ~100pc, that will let us determine the spatial distribution of star formation, dust, and metals in z~2-5 galaxies and trace how they evolve across cosmic time. We will also detect and characterize galaxies in the Epoch of Reionization, use multiply-imaged lensed galaxies to constrain cluster mass distributions and dark matter substructure, and study the quenching of star formation in the most massive galaxy clusters. The words "treasure-trove" are often used to describe data from large surveys, but our 200-hr JWST GTO program goes beyond this. It is a once-in-a-career opportunity for me, my students, postdocs, and collaborators. It will also provide a rich resource for the world-wide astronomy community as we will publicly release the processed data products and NIRISS data analysis tools that we will develop.