A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z~2: A Mix of Nuclear Activity and Low-Metallicity Star Formation

TL;DR

This study uses HST WFC3 grism spectroscopy to analyze low-mass, emission-line galaxies at z~2, revealing high OIII/Hb ratios, spatial gradients in emission lines, and evidence of weak active galactic nuclei in these galaxies.

Contribution

First detailed spatial analysis of emission lines in low-mass z~2 galaxies using HST grism spectroscopy, uncovering potential weak AGN activity.

Findings

Low-mass galaxies have high OIII/Hb ratios similar to more massive z~2 galaxies.

Stacked data shows OIII emission is more spatially concentrated than Hb.

X-ray stacking indicates some galaxies host weak active galactic nuclei.

Abstract

We present Hubble Space Telescope Wide Field Camera 3 slitless grism spectroscopy of 28 emission-line galaxies at z~2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The high sensitivity of these grism observations, with 1-sigma detections of emission lines to f > 2.5x10^{-18} erg/s/cm^2, means that the galaxies in the sample are typically ~7 times less massive (median M_* = 10^{9.5} M_sun) than previously studied z~2 emission-line galaxies. Despite their lower mass, the galaxies have OIII/Hb ratios which are very similar to previously studied z~2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the OIII emission line is more spatially concentrated than the Hb emission line with 98.1 confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L(OIII)/L(0.5-10 keV) ratio is intermediate between typical z~0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked OIII spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei.