The Diversity of Diffuse Lyα Nebulae around Star-forming Galaxies at High Redshift

We report the detection of diffuse Ly alpha emission, or Lya halos (LAHs), around star-forming galaxies at z approximate to 3.78 and 2.66 in the NOAO Deep Wide-Field Survey Bootes field. Our samples consist of a total of similar to 1400 galaxies, within two separate regions containing spectroscopically confirmed galaxy overdensities. They provide a unique opportunity to investigate how the LAH characteristics vary with host galaxy large-scale environment and physical properties. We stack Ly alpha images of different samples defined by these properties and measure their median LAH sizes by decomposing the stacked Ly alpha radial profile into a compact galaxy-like and an extended halo-like component. We find that the exponential scale-length of LAHs depends on UV continuum and Ly alpha luminosities, but not on Ly alpha equivalent widths or galaxy overdensity parameters. The full samples, which are dominated by low UV-continuum luminosity Lya emitters (M-UV greater than or similar to -21), exhibit LAH sizes of 5-6 kpc. However, the most UV- or Ly alpha-luminous galaxies have more extended halos with scale-lengths of 7-9 kpc. The stacked Ly alpha radial profiles decline more steeply than recent theoretical predictions that include the contributions from gravitational cooling of infalling gas and from low-level star formation in satellites. However, the LAH extent matches what one would expect for photons produced in the galaxy and then resonantly scattered by gas in an outflowing envelope. The observed trends of LAH sizes with host galaxy properties suggest that the physical conditions of the circumgalactic medium (covering fraction, H I column density, and outflow velocity) change with halo mass and/or star formation rates.