Measuring Emission Lines with JWST MegaScience Medium Bands: A New Window into Dust and Star Formation at Cosmic Noon

We demonstrate the power of JWST-NIRCam medium-band photometry to measure emission line fluxes and study the dust and star formation properties of galaxies at cosmic noon. In this work, we present photometric emission line measurements and spatially resolved maps of Hα and Paβ for 14 galaxies at 1.3 ≤ z ≤ 2.4, observed by the MegaScience medium-band survey and the UNCOVER spectroscopic survey. We measure line fluxes directly from medium-band photometry and compare with spectroscopic measurements from UNCOVER. We find reasonable agreement between the photometric and spectroscopic emission line fluxes for both Hα and Paβ, with scatter <0.15 dex down to emission line equivalent widths of 10 Å. We also make a nebular dust measurement from the ratio Paβ/Hα, finding an average nebular attenuation in the V band, AV, of 1.4 with a standard deviation of 0.8. Our photometric AV measurements show a slightly larger scatter of 0.5 mag when compared to spectroscopic measurements; however, this scatter may be partially caused by aperture effects. Finally, we produce spatially resolved maps of Hα emission, Paβ emission, and the stellar continuum. We find offsets between Hα and Paβ emission, especially for galaxies with high AV, indicating dusty substructures. Furthermore, the correlation between Hα and continuum emission decreases with increasing AV, suggesting that the dustiest objects have clumpy dust and star formation distributions. Our study demonstrates the power of medium-band photometry to directly probe emission line strengths, star formation, and dust attenuation for hundreds of galaxies in UNCOVER and thousands of galaxies in upcoming JWST medium-band surveys.