A CANDELS WFC3 Grism Study of Emission-line Galaxies at z ~ 2: A Mix of Nuclear Activity and Low-metallicity Star Formation
Finkelstein, Steven L.; Bell, Eric F.; Dickinson, Mark; Ferguson, Henry C.; Grogin, Norman A.; Hathi, Nimish P.; Kartaltepe, Jeyhan S.; Kocevski, Dale D.; Koekemoer, Anton M.; Trump, Jonathan R.; Weiner, Benjamin J.; Teplitz, Harry I.; Nandra, Kirpal; Scarlata, Claudia; Juneau, Stéphanie; Faber, S. M.; McGrath, Elizabeth J.; Atek, Hakim; Straughn, Amber N.; Yan, Renbin; Koo, David C.; Dunlop, James S.; Newman, Jeffrey A.; Rodney, Steven A.; Laird, Elise S.; Mozena, Mark; Donley, Jennifer L.; Rangel, Cyprian; Yesuf, Hassen
United States, United Kingdom, Germany
Abstract
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.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc., under NASA contract NAS 5-26555.