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 H alpha and H beta 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 theWide-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 (H alpha/H beta). We present dust extinction as a function of H alpha luminosity (down to 3 x 10(41) erg s(-1)), galaxy stellar mass (reaching 4 x 10(8) M-circle dot), and rest-frame H alpha equivalent width. The faintest galaxies are two times fainter in H alpha luminosity than galaxies previously studied at z similar to 1.5. An evolution is observed where galaxies of the same H alpha luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower H alpha luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the [O III] lambda 5007/H alpha flux ratio as a function of luminosity where galaxies with L-H alpha < 5 x 10(41) erg s(-1) are brighter in [O III] lambda 5007 than H alpha. This trend is evident even after extinction correction, suggesting that the increased [O III] lambda 5007/H alpha ratio in low-luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters.