A candels WFC3 grism study of emission-line galaxies at z ∼ 2: A mix of nuclear activity and low-metallicity star formation

We present Hubble Space Telescope Wide Field Camera 3 (WFC3) 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. The high sensitivity of these grism observations, with >1σ detections of emission lines to f > 2.5 × 10^-18ergs^-1cm ^-2, means that the galaxies in the sample are typically 7 times less massive (median M _* = 10^9.5 M than previously studied z 2 emission-line galaxies. Despite their lower mass, the galaxies have [O III]/Hβ 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 [O III] emission line is more spatially concentrated than the Hβ emission line with 98.1% confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L _{[O III]}/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 [O III] 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.