Abstract
We present a detailed analysis of Hubble Space Telescope (HST), Wide Field
Camera 3 (WFC3) G141 grism spectroscopy for seven star-forming regions of the
highly magnified lensed star- burst galaxy RCSGA 032727-132609 at z = 1.704. We
measure the spatial variations of the extinction in RCS0327 through the
observed H$\gamma$/H$\beta$ emission line ratios, finding a constant average
extinction of $E(B-V)_gas = 0.40\pm0.07$. We infer that the star formation is
enhanced as a result of an ongoing interaction, with measured star formation
rates derived from demagnified, extinction-corrected Hbeta line fluxes for the
individual star-forming clumps falling >1-2 dex above the star formation
sequence. When combining the HST/WFC3 OIII $łambda$5007/H$\beta$ emission
line ratio measurements with NII/H$\alpha$ line ratios from Wuyts et al.
(2014), we find that the majority of the individual star-forming regions fall
along the local "normal" abundance sequence. With the first detections of the
He I $łambda$5876 $\AA$ and He II $łambda$4686 $\AA$ recombination lines in a
distant galaxy, we probe the massive-star content of the star-forming regions
in RCS0327. The majority of the star-forming regions have a He I $łambda$5876
to H$\beta$ ratio consistent with the saturated maximum value, which is only
possible if they still contain hot O-stars. Two regions have lower ratios,
implying that their last burst of new star formation ended $\sim5$ Myr ago.
Together, the He I $łambda$5876 $\AA$ and He II $łambda$4686 $\AA$ to
H$\beta$ line ratios provide indirect evidence for the order in which star
formation is stopping in individual star-forming knots of this high redshift
merger. We place the spatial variations of the extinction, star formation rate
and ionization conditions in the context of the star formation history of
RCS0327.
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