The Nature of Extreme Emission Line Galaxies at z = 1-2: Kinematics and Metallicities from Near-infrared Spectroscopy
Bell, Eric F.; Ferguson, Henry C.; Koekemoer, Anton M.; Trump, Jonathan R.; Maseda, Michael V.; Franx, Marijn; van Dokkum, Pieter; van der Wel, Arjen; Pacifici, Camilla; Momcheva, Ivelina; Meidt, Sharon E.; Rix, Hans-Walter; Patel, Shannon G.; Marchesini, Danilo; Nelson, Erica J.; Whitaker, Katherine E.; Skelton, Rosalind E.; Fumagalli, Mattia; Brammer, Gabriel B.; Lundgren, Britt F.; Straughn, Amber N.; Koo, David C.; da Cunha, Elisabete; Förster-Schreiber, Natascha M.
Germany, South Korea, United States, Netherlands, South Africa
Abstract
We present near-infrared spectroscopy of a sample of 22 Extreme Emission Line Galaxies at redshifts 1.3 < z < 2.3, confirming that these are low-mass (M sstarf = 108-109 M ⊙) galaxies undergoing intense starburst episodes (M sstarf/SFR ~ 10-100 Myr). The sample is selected by [O III] or Hα emission line flux and equivalent width using near-infrared grism spectroscopy from the 3D-HST survey. High-resolution NIR spectroscopy is obtained with LBT/LUCI and VLT/X-SHOOTER. The [O III]/Hβ line ratio is high (gsim 5) and [N II]/Hα is always significantly below unity, which suggests a low gas-phase metallicity. We are able to determine gas-phase metallicities for seven of our objects using various strong-line methods, with values in the range 0.05-0.30 Z ⊙ and with a median of 0.15 Z ⊙ for three of these objects we detect [O III] λ4363, which allows for a direct constraint on the metallicity. The velocity dispersion, as measured from the nebular emission lines, is typically ~50 km s-1. Combined with the observed star-forming activity, the Jeans and Toomre stability criteria imply that the gas fraction must be large (f gas >~ 2/3), consistent with the difference between our dynamical and stellar mass estimates. The implied gas depletion timescale (several hundred Myr) is substantially longer than the inferred mass-weighted ages (~50 Myr), which further supports the emerging picture that most stars in low-mass galaxies form in short, intense bursts of star formation.
This work is based on observations taken by the 3D-HST Treasury Program and the CANDELS Multi-Cycle Treasury Program with the NASA/ESA HST, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. X-Shooter observations were performed at the European Southern Observatory, Chile, Program 089.B-0236(A).