A CANDELS WFC3 GRISM STUDY OF EMISSION-LINE GALAXIES AT z ～ 2: A MIX OF NUCLEAR ACTIVITY AND LOW-METALLICITY STAR FORMATION*

摘要

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 × 10–18?erg?s–1?cm–2, means that the galaxies in the sample are typically ~7 times less massive (median M * = 109.5 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.