THE FMOS-COSMOS SURVEY OF STAR-FORMING GALAXIES AT z ∼ 1.6. I. Hα-BASED STAR FORMATION RATES AND DUST EXTINCTION

We present the first results from a near-IR spectroscopic survey of the COSMOS field, using the Fiber Multi-Object Spectrograph on the Subaru telescope, designed to characterize the star-forming galaxy population at 1.4 < z < 1.7. The high-resolution mode is implemented to detect H alpha in emission between 1.6-1.8 mu m with f(H alpha) greater than or similar to 4 x 10(-17) erg cm(-2) s(-1). Here, we specifically focus on 271 sBzK-selected galaxies that yield a H alpha detection thus providing a redshift and emission line luminosity to establish the relation between star formation rate and stellar mass. With further J-band spectroscopy for 89 of these, the level of dust extinction is assessed by measuring the Balmer decrement using co-added spectra. We find that the extinction (0.6 less than or similar to A(H alpha) less than or similar to 2.5) rises with stellar mass and is elevated at high masses compared to low-redshift galaxies. Using this subset of the spectroscopic sample, we further find that the differential extinction between stellar and nebular emission E-star(B - V)/E-neb(B - V) is 0.7-0.8, dissimilar to that typically seen at low redshift. After correcting for extinction, we derive an H alpha-based main sequence with a slope (0.81 +/- 0.04) and normalization similar to previous studies at these redshifts.