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* = 108-109M(circle dot)) galaxies undergoing intense starburst episodes (M*/SFR similar to 10-100 Myr). The sample is selected by [O iii] or Ha 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 line ratio is high (greater than or similar to 5) and [N ii]/Ha 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 similar to and with a median of 0.15 Z similar to; for three of these objects we detect [O iii].4363, which allows for a direct constraint on the metallicity. The velocity dispersion, as measured from the nebular emission lines, is typically similar to 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 (fgas - 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 (similar to 50 Myr), which further supports the emerging picture that most stars in low-mass galaxies form in short, intense bursts of star formation.