Spatial Variation in Strong Line Ratios and Physical Conditions in Two Strongly-Lensed Galaxies at z~1.4

TL;DR

This study investigates spatial variations in emission line ratios and physical conditions within two strongly-lensed galaxies at z~1.4 using HST and ground-based spectroscopy, highlighting the importance of spatially-resolved data for understanding galaxy evolution.

Contribution

It demonstrates the significance of spatially-resolved spectroscopy in analyzing ionization and metallicity variations in distant galaxies, emphasizing the need for space-based IFUs for future research.

Findings

Significant spatial variation in reddening and line ratios within the galaxies.

Evidence of negative radial star formation gradients, with asymmetry.

Spatial reddening differences necessitate resolved reddening corrections.

Abstract

For studies of galaxy formation and evolution, one of the major benefits of the James Webb Space Telescope is that space-based IFUs like those on its NIRSpec and MIRI instruments will enable spatially resolved spectroscopy of distant galaxies, including spectroscopy at the scale of individual star-forming regions in galaxies that have been gravitationally lensed. In the meantime, there is only a very small subset of lensed sources where work like this is possible even with the Hubble Space Telescope's Wide Field Camera 3 infrared channel grisms. We examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially-unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission line ratios like O32, R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement as an indicator of reddening. We find significant spatial variation in the reddening and the reddening-corrected O32 and R23 values which correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially-resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based IFUs for future work on larger, more statistically robust samples.