Environmental Regulation of Dust and Star Formation Unveiled by Subaru Dual Narrowband Imaging: Degree-scale Balmer Decrement Mapping Across a <i>z</i> = 0.9 Supercluster

Abstract We present results from a dual narrowband imaging survey targeting the CL1604 supercluster at z = 0.9 using the Subaru Telescope. By combining the NB921 filter on Hyper Suprime-Cam and the NB1244 filter on SWIMS, we can detect redshifted H α and H β emission lines from the supercluster. This unique technique allows us to measure both star formation rates and dust extinction for a sample of 94 emission-line galaxies across the supercluster. We find that dust extinction, estimated from the Balmer decrement (H α /H β ratio), increases with stellar mass in star-forming galaxies, whereas relatively quiescent systems exhibit comparatively low extinction. Among galaxies with intermediate masses (10 8.5 &lt; M * &lt; 10 10.5 M ⊙ ), the dust-corrected H α -based star formation rates align with the main sequence at this epoch. More massive galaxies, however, deviate from this relation, exhibit redder colors, and reside predominantly in higher-density environments. Although stellar mass, star formation rate, and galaxy color are clearly influenced by environment, we detect no strong, systematic environmental dependence of dust extinction for the whole sample. However, a trend of increasing dust extinction toward denser regions becomes evident only after excluding quiescent galaxies, as the massive quiescent populations in cluster cores contain little dust and thereby mask this relationship.