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

TL;DR

This study uses HST spectroscopy to analyze star formation, extinction, and ionization in a high-redshift galaxy merger, revealing enhanced star formation due to interaction and insights into the recent star formation history of individual regions.

Contribution

It provides the first detailed spatially-resolved analysis of star formation and stellar content in a z=1.7 galaxy merger using emission line diagnostics and new detections of helium recombination lines.

Findings

Star formation rates are 10-100 times above the typical sequence.

Most regions contain hot O-stars, indicating recent star formation.

Spatial variations in extinction and ionization conditions are characterized.

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 starburst 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 $\mathrm{E(B-V)_{gas}}=0.40\pm0.07$. 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 H$\beta$ line fluxes for the individual star-forming clumps falling >1-2 dex above the star formation sequence. When combining the HST/WFC3 [OIII]$\lambda$5007/H$\beta$ emission line ratio measurements with [NII]/H$\alpha$ line ratios from Wuyts et a. (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 $\lambda$5876 and He II $\lambda$4686 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 $\lambda$5876 to H$\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 ~5 Myr ago. Together, the He I $\lambda$5876 and He II $\lambda$4686 to H$\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.