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Resolved Star Formation on Sub-galactic Scales in a Merger at z=1.7

, , , , , , and .
(2014)cite arxiv:1405.0490Comment: 14 pages, 8 figures. Re-submitted to ApJ.

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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