Diffuse Lyman-alpha Halos around ~300 Spectroscopically Confirmed
  Lyman-alpha Emitters at z ~ 5.7
  We report the detection of diffuse Lyman-alpha halos (LAHs) around
star-forming galaxies at $z \sim 5.7$ by stacking 310 spectroscopically
confirmed Lyman-alpha (LAEs). The majority of the LAEs are identified from our
spectroscopic survey of galaxies at $z>5.5$. They are all located in
well-studied fields with deep narrowband and broadband imaging data. We combine
the LAE sample and its subsamples in the narrowband NB816 (i.e., the
Lyman-alpha band) and $z$ band (i.e., the continuum band). By comparing the
stacked objects with their corresponding point spread functions, we clearly
detect extended LAHs around these LAEs. We perform sophisticated simulations
and analyses on statistical and systematic errors, and confirm that the
detected halos are not caused by errors. The scale lengths of the LAHs, when
described by a double-component model, range from 1.2 to 5.3 kpc with a typical
value of $\sim$2 kpc. The LAH sizes from our sample are in agreement with those
of individual LAEs at the similar redshift measured by VLT/MUSE, but are
relatively smaller than those of photometrically selected LAEs in previous
studies. We also find that LAEs with higher Lyman-alpha luminosities, higher UV
continuum luminosities or smaller Lyman-alpha equivalent widths tend to have
larger LAH sizes. Our results are consistent with a scenario that LAHs
originate from the scattered light of the central galaxies by H I gas in the
circumgalactic medium.
