Evidence for Infalling Gas in a Lyman-$\alpha$ Blob
  Lyman-$\alpha$ blobs (LABs) are spatially extended nebulae of emission in the
Ly$\alpha$ line of hydrogen, seen at high redshifts$^{1,2}$, and most commonly
found in the dense environment of star-forming galaxies$^{3,4}$. The origin of
Ly$\alpha$ emission in the LABs is still unclear and under debate$^{5}$.
Proposed powering sources generally fall into two categories: (1)
photoionization, galactic super-winds/outflows, resonant scattering of
Ly$\alpha$ photons from starbursts or active galactic nuclei
(AGNs)$^{6,7,8,9,10}$ and (2) cooling radiation from cold streams of gas
accreting onto galaxies$^{12}$. Here we analyze the gas kinematics within a LAB
providing rare observational evidence for infalling gas. This is consistent
with the release of gravitational accretion energy as cold streams radiate
Ly$\alpha$ photons. It also provides direct evidence for possible cold streams
feeding the central galaxies. The infalling gas is not important by mass but
hints at more than one mechanism to explain the origin of the extended
Ly$\alpha$ emission around young galaxies. It is also possible that the
infalling gas may represent material falling back to the galaxy from where it
originated, forming a galactic fountain.
