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

TL;DR

This study showcases how JWST-NIRCam medium-band photometry can effectively measure emission lines, dust, and star formation in galaxies at cosmic noon, providing spatially-resolved insights and demonstrating its potential for large surveys.

Contribution

It introduces a method to measure emission line fluxes and dust properties from medium-band photometry, validated against spectroscopic data, and produces spatially-resolved emission maps for high-redshift galaxies.

Findings

Photometric emission line fluxes agree with spectroscopic measurements within 0.15 dex.

Average nebular dust attenuation A_V is 1.4, with some scatter due to aperture effects.

Dusty galaxies show offset and clumpy star formation distributions.

Abstract

We demonstrate the power of JWST-NIRCam medium-band photometry to measure emission line fluxes and study dust and star formation properties of galaxies at cosmic noon. In this work, we present photometric emission line measurements and spatially-resolved maps of H$\alpha$ and Pa$\beta$ for a sample of 14 galaxies at $1.3\leq z\leq 2.4$, observed by the MegaScience medium-band survey and the UNCOVER deep spectroscopic survey. We measure line fluxes directly from the medium-band photometry and compare with spectroscopic measurements from UNCOVER. We find reasonable agreement between the photometric and spectroscopic emission line fluxes for both H$\alpha$ and Pa$\beta$, with scatter $<0.15$ dex down to emission line equivalent widths of $10$\r{A}. We also make a nebular dust measurement from the ratio Pa$\beta$ / H$\alpha$, finding an average nebular A$_\mathrm{V}$ of 1.4. Our photometric A$_\mathrm{V}$ measurements show a slightly larger scatter of $0.5$ magnitudes when compared to spectroscopic measurements; however, this scatter may be partially caused by aperture effects. Finally, we produce spatially resolved maps of H$\alpha$ emission, Pa$\beta$ emission, and stellar continuum. We find that offsets in H$\alpha$ and Pa$\beta$ emission are common, especially for galaxies with the highest A$_\mathrm{V}$, indicating dusty sub-structures. Furthermore, the correlation between H$\alpha$ and continuum emission decreases with increasing A$_\mathrm{V}$, 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.