3D-HST Grism Spectroscopy of a Gravitationally Lensed, Low-metallicity Starburst Galaxy at z=1.847

TL;DR

This study uses HST imaging and spectroscopy to analyze a gravitationally lensed, low-metallicity starburst galaxy at z=1.847, revealing its extreme emission lines, compact structure, and similarities to local blue dwarf galaxies.

Contribution

It provides detailed characterization of a high-redshift, low-metallicity starburst galaxy, including spatially resolved emission lines and comparison with local analogs, advancing understanding of early galaxy evolution.

Findings

The galaxy has extremely strong emission lines with high equivalent widths.

It is composed of two spatially resolved clumps with distinct properties.

The galaxy's metallicity is very low, similar to local blue compact dwarfs.

Abstract

We present Hubble Space Telescope (HST) imaging and spectroscopy of the gravitational lens SL2SJ02176-0513, a cusp arc at z=1.847. The UV continuum of the lensed galaxy is very blue, which is seemingly at odds with its redder optical colors. The 3D-HST WFC3/G141 near-infrared spectrum of the lens reveals the source of this discrepancy to be extremely strong [OIII]5007 and H-beta emission lines with rest-frame equivalent widths of 2000 +/- 100 and 520 +/- 40 Angstroms, respectively. The source has a stellar mass ~10^8 Msun, sSFR \sim 100/Gyr, and detection of [OIII]4363 yields a metallicity of 12 + log(O/H) = 7.5 +/- 0.2. We identify local blue compact dwarf analogs to SL2SJ02176-0513, which are among the most metal-poor galaxies in the SDSS. The local analogs resemble the lensed galaxy in many ways, including UV/optical SED, spatial morphology and emission line equivalent widths and ratios. Common to SL2SJ02176-0513 and its local counterparts is an upturn at mid-IR wavelengths likely arising from hot dust heated by starbursts. The emission lines of SL2SJ02176-0513 are spatially resolved owing to the combination of the lens and the high spatial resolution of HST. The lensed galaxy is composed of two clumps with combined size r_e \sim 300 pc, and we resolve significant differences in UV color and emission line equivalent width between them. Though it has characteristics occasionally attributed to active galactic nuclei, we conclude that SL2SJ02176-0513 is a low-metallicity star-bursting dwarf galaxy. Such galaxies will be found in significant numbers in the full 3D-HST grism survey.