Morphological Properties of Lyman Alpha Emitters at Redshift 4.86 in the
  COSMOS Field: Clumpy Star Formation or Merger?
  We investigate morphological properties of 61 Lyman-alpha emitters (LAEs) at
z = 4.86 identified in the COSMOS field, based on Hubble Space Telescope
Advanced Camera for Surveys (ACS) imaging data in the F814W-band. Out of the 61
LAEs, we find the ACS counterparts for the 54 LAEs. Eight LAEs show
double-component structures with a mean projected separation of 0."63 (~ 4.0
kpc at z = 4.86). Considering the faintness of these ACS sources, we carefully
evaluate their morphological properties, that is, size and ellipticity. While
some of them are compact and indistinguishable from the PSF half-light radius
of 0."07 (~ 0.45 kpc), the others are clearly larger than the PSF size and
spatially extended up to 0."3 (~ 1.9 kpc). We find that the ACS sources show a
positive correlation between ellipticity and size and that the ACS sources with
large size and round shape are absent. Our Monte Carlo simulation suggests that
the correlation can be explained by (1) the deformation effects via PSF
broadening and shot noise or (2) the source blending in which two or more
sources with small separation are blended in our ACS image and detected as a
single elongated source. Therefore, the 46 single-component LAEs could contain
the sources which consist of double (or multiple) components with small spatial
separation (i.e., < 0."3 or 1.9 kpc). Further observation with high angular
resolution at longer wavelengths (e.g., rest-frame wavelengths of > 4000 A) is
inevitable to decipher which interpretation is adequate for our LAE sample.
