THE NATURE OF EXTREME EMISSION LINE GALAXIES AT z = 1-2: KINEMATICS AND METALLICITIES FROM NEAR-INFRARED SPECTROSCOPY

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 sub([sstarf]) = 10 super(8)-10 super(9) M sub([odot])) galaxies undergoing intense starburst episodes (M sub([sstarf])/SFR ~ 10-100 Myr). The sample is selected by [OIII] or H alpha 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 [OIII]/H beta line ratio is high ([gap]5) and [NuII]/H alpha 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 sub([odot]) and with a median of 0.15 Z sub([odot]); for three of these objects we detect [OIII] [lambda]4363, which allows for a direct constraint on the metallicity. The velocity dispersion, as measured from the nebular emission lines, is typically ~50 km s super(-1). Combined with the observed star-forming activity, the Jeans and Toomre stability criteria imply that the gas fraction must be large (f sub(gas) [gap] 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.