Dust Extinction from Balmer Decrements of Star-forming Galaxies at 0.75 ≤ z ≤ 1.5 with Hubble Space Telescope/Wide-Field-Camera 3 Spectroscopy from the WFC3 Infrared Spectroscopic Parallel Survey

Spectroscopic observations of Hα and Hβ emission lines of 128 star-forming galaxies in the redshift range 0.75 ≤ z ≤ 1.5 are presented. These data were taken with slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (Hα/Hβ). We present dust extinction as a function of Hα luminosity (down to 3 × 10^(41) erg s^(–1)), galaxy stellar mass (reaching 4 × 10^8 M _☉), and rest-frame Hα equivalent width. The faintest galaxies are two times fainter in Hα luminosity than galaxies previously studied at z ~ 1.5. An evolution is observed where galaxies of the same Hα luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower Hα luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the [O III] λ5007/Hα flux ratio as a function of luminosity where galaxies with L _(Hα) < 5 × 10^(41) erg s^(–1) are brighter in [O III] λ5007 than Hα. This trend is evident even after extinction correction, suggesting that the increased [O III] λ5007/Hα ratio in low-luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters