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 similar to 2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. The high sensitivity of these grism observations, with > 1 sigma detections of emission lines to f > 2.5 x 10(-18) erg s(-1) cm(-2), means that the galaxies in the sample are typically similar to 7 times less massive (median M(*) = 10(9.5) M(circle dot)) than previously studied z similar to 2 emission-line galaxies. Despite their lower mass, the galaxies have [O III]/H beta ratios which are very similar to previously studied z similar to 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 H beta 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 similar to 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.