Evidence for spatially compact Lyα emission in z = 3.1 Lyα-emitting galaxies

We present the results of a high spatial resolution study of the line emission in a sample of z = 3.1 Lyα-emitting galaxies (LAEs) in the Extended Chandra Deep Field-South. Of the eight objects with coverage in our HST/WFPC2 narrowband imaging, two have clear detections and two are barely detected (∼2 σ). The clear detections are within ∼0.5 kpc of the centroid of the corresponding rest-UV continuum source, suggesting that the line-emitting gas and young stars in LAEs are spatially coincident. The brightest object exhibits extended emission with a half-light radius of ∼1.5 kpc, but a stack of the remaining LAE surface brightness profiles is consistent with the WFPC2 point-spread function. This suggests that the Lyα emission in these objects originates from a compact (≲2 kpc) region and cannot be significantly more extended than the far-UV continuum emission (≲1 kpc). Comparing our WFPC2 photometry to previous ground-based measurements of their monochromatic fluxes, we find at 95% (99.7%) confidence that we cannot be missing more than 22% (32%) of the Lyα emission.