Dynamics of Galactic Disks and Mergers at z ~ 1.6: Spatially Resolved Spectroscopy with Keck Laser Guide Star Adaptive Optics

Abstract

We present 0".2 resolution near-infrared integral field spectroscopy of Hα emission from six star-forming galaxies at z ~ 1.6 (a look-back time of ~9.6 Gyr). These observations were obtained with OSIRIS using the Keck Laser Guide Star Adaptive Optics system. All sources have a compact spatial extent of ~1", with an average half-light radius of r_(1/2) = 2.9 kpc and an average dereddened star formation rate of 22 M⊙ yr^(–1). Based on Hα kinematics we find that these six galaxies are dynamically distinguishable, and we classify them as either irregular/merger or disk candidate systems. We find three systems (HDF-BX1287, HDF-BX1315, and Q1623-BX491) with varying geometries and dynamical properties. Three galaxies (HDF-BMZ1299, Q2343-BX344, and Q2343-BM145) are well fitted by an inclined-disk model with low-velocity residuals (20 to 46 km s^(–1)). An average plateau velocity of ν(p) = 185 km s^(–1) is achieved within 1.0 kpc. The majority of observed velocity dispersions (σ ~ 88 km s^(–1)) can be explained by the residual seeing halo, and are not intrinsic to our sources. However, one irregular and one disk candidate have high-velocity dispersions (σ_(obs) ≳ 200 km s^(–1)) that cannot be solely explained by beam smearing. For two disk candidates, we detect [N II] emission and are able to map the [N II]/Hα ratio on kiloparsec scales. In both cases, [N II] emission is more concentrated than Hα emission (≾0".2), and peak ratios are best explained by the presence of an active galactic nucleus (AGN). These are among the weakest known AGNs at high redshift; however their emission is strong enough to impact high-redshift metallicity studies that use nebular ratios. All disk candidates have likely to be completed only a few orbital periods, and if left unperturbed are excellent candidates to become present-day spiral galaxies.