2018ApJ...862L..10E

Lyαphotons are resonantly scattered by neutral hydrogen, and may therefore trace both the spatial extent and the kinematics of the gas surrounding galaxies. We present new observations of the extended Lyαhalo of Q2343-BX418, a low-mass (M_*=5×10⁸M_☉), low-metallicity (Z ≃ 0.25 Z_☉) star-forming galaxy at z = 2.3. Using the Keck Cosmic Web Imager (KCWI), the blue-sensitive optical integral field spectrograph recently installed on the Keck II telescope, we detect Lyαin emission to a radius of 23 kpc, and measure an exponential scale length of 6 kpc in the outer region of the extended halo. We study the double-peaked spectroscopic Lyαprofile in individual spectral pixels ("spaxels") over a ∼25 x 30 kpc region, finding significant variations in the peak ratio and peak separation. The profile is dominated by the red peak in the central regions, while in the outskirts of the extended halo the red and blue peak strengths are roughly equal; these observations are consistent with a model in which the peak ratio is largely determined by the radial component of the outflow velocity. We find a gradient of 300 kms^–1 in the Lyαpeak separation across the extended halo, indicating variations in the column density, covering fraction, or velocity range of the gas. These new observations emphasize the need for realistic, spatially resolved models of Lyαradiative transfer in the halos of galaxies.