The Lyman-alpha Halos of Galaxies at z=2-3 in the Keck Baryonic Structure Survey

We present the large-scale spatial Lya profiles of galaxies from the Keck Baryonic Structure Survey (KBSS) at 2<z<3. This work also describes the Lya imaging for the KBSS-Lya survey for the first time. Our sample includes 734 Lya-selected galaxies and 119 continuum-selected galaxies with Lya narrow-band imaging, and we measure the spatial morphology of Lya and continuum emission for stacked subsamples of these two populations. These samples allow us to study the variation in Lya emission profiles over a broad range of UV continuum luminosities and Lya equivalent widths (EW_Lya), including systems with net Lya absorption in slit spectroscopy. We characterize the spatial profiles using two techniques: directly fitting an exponential function to the stacked profile, and a multi-component forward-modeling technique using the empirical large-scale PSF. We find that both methods yield similar results and that the forward-modeling technique self-consistently fits profiles exhibiting central Lya emission or Lya absorption, with the spatial scale of central Lya approximately matching that of the continuum emission. We also find extended Lya emission such that all our subsamples -- including central Lya absorbers -- are net Lya emitters on scales comparable to the circumgalactic medium (R > 50 kpc, theta > 6''). We find that the scale length of the Lya halo is not strongly dependent on the properties of the central galaxy, including its net continuum luminosity or EW_Lya, although we find a possible weak tendency of continuum-faint, high-EW_Lya galaxies to exhibit larger Lya halos in contrast with previous work.