Infrared slitless spectroscopy from the Hubble Space Telescope (HST) has become a mainstream method to study distant galaxies. This observing mode provides detailed information of galaxies with high angular resolution (spatially resolved) and at wavelengths unobstructed by the Earth's atmosphere. I will discuss constraints using such data to study distant galaxies based on a large HST program (from the CLEAR [CANDELS Lyman-alpha Emission At Reionization] survey). CLEAR targets roughly 100 square arcminutes in the GOODS-North and -South fields using slitless spectroscopy covering 0.85 to 1.65 micron. CLEAR was designed to study the evolution of Lyman-alpha emission in galaxies at z > 6, but the data also cover important spectral features of galaxies, including rest-frame optical absorption and emission lines for galaxies at redshifts z~1-3. I will discuss results from CLEAR, including (1) the implications for Lyman-alpha emission from galaxies into the epoch of reionization, (2) star-formation histories and chemical evolution of quenched galaxies at z > 1, (2) the evolution of gas metallicity and ionization of galaxies at z ~ 1-3, and (3) results from spatially resolved metallicities and star-formation of galaxies as a constraint on gas accretion and feedback processes. The results from HST/CLEAR bode well for future, and I will discuss forthcoming programs from JWST (in Cycle 1) and beyond (e.g., NGRST) that will employ both deep and/or wide-area space-based infrared slitless spectroscopy to study galaxy properties.
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