2021MNRAS.508...19C

We present the first statistical analysis of kinematically resolved, spatially extended Lyαemission around z = 2-3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (z_med = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse Lyαemission with respect to the stellar continuum within projected galactocentric distances of <=30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged Lyαspectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Lyαresonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Lyαemission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of <=2 per cent (∼2σ) of the integrated Lyαemission. The symmetry implies that the Lyαscattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Lyαemission is marginally stronger along galaxy minor axes for galaxies with relatively weak Lyαemission. We speculate that this weak directional dependence of Lyαemission becomes discernible only when the Lyαescape fraction is low. These discoveries highlight the need for similar analyses in simulations with Lyαradiative transfer modelling.