DIFFUSE Lyα EMITTING HALOS: A GENERIC PROPERTY OF HIGH-REDSHIFT STAR-FORMING GALAXIES

Using a sample of 92 UV continuum-selected, spectroscopically identified galaxies with (z) = 2.65, all of which have been imaged in the Ly alpha line with extremely deep narrow-band imaging, we examine galaxy Ly alpha emission profiles to very faint surface brightness limits. The galaxy sample is representative of spectroscopic samples of Lyman break galaxies (LBGs) at similar redshifts in terms of apparent magnitude, UV luminosity, inferred extinction, and star formation rate and was assembled without regard to Ly alpha emission properties. Approximately 45% (55%) of the galaxy spectra have Ly alpha appearing in net absorption (emission), with similar or equal to 20% satisfying commonly used criteria for the identification of Ly alpha emitters (LAEs; W-0(Ly alpha) >= 20 angstrom). We use extremely deep stacks of rest-UV continuum and continuum-subtracted Ly alpha images to show that all sub-samples exhibit diffuse Ly alpha emission to radii of at least 10''' (similar to 80 physical kpc). The characteristic exponential scale lengths for Ly alpha line emission exceed that of the lambda(0) = 1220 angstrom UV continuum light by factors of similar to 5-10. The surface brightness profiles of Ly alpha emission are strongly suppressed relative to the UV continuum light in the inner few kpc, by amounts that are tightly correlated with the galaxies' observed spectral morphology; however, all galaxy sub-subsamples, including that of galaxies for which Ly alpha appears in net absorption in the spectra, exhibit qualitatively similar diffuse Ly alpha emission halos. Accounting for the extended Ly alpha emission halos, which generally would not be detected in the slit spectra of individual objects or with typical narrow-band Ly alpha imaging, increases the total Lya flux (and rest equivalent width W-0(Ly alpha)) by an average factor of similar to 5, and by a much larger factor for the 80% of LBGs not classified as LAEs. We argue that most, if not all, of the observed Ly alpha emission in the diffuse halos originates in the galaxy HII regions but is scattered in our direction by HI gas in the galaxy's circum-galactic medium. The overall intensity of Ly alpha halos, but not the surface brightness distribution, is strongly correlated with the emission observed in the central similar to 1 ''-more luminous halos are observed for galaxies with stronger central Ly alpha emission. We show that whether or not a galaxy is classified as a giant Ly alpha blob (LAB) depends sensitively on the Ly alpha surface brightness threshold reached by an observation. Accounting for diffuse Ly alpha halos, all LBGs would be LABs if surveys were sensitive to 10 times lower Ly alpha surface brightness thresholds; similarly, essentially all LBGs would qualify as LAEs.