We exploit wide-field Ly$alpha$ imaging with Subaru to probe the environment around TN J1338-1942, a powerful radio galaxy with a >100 kpc Ly$alpha$ halo at z=4.11. We used a sample of Ly$alpha$ emitters (LAEs) down to $log(L_{rm Lyalpha} [erg, s^{-1
}])sim 42.8$ to measure the galaxy density around TNJ1338, compared to a control sample from a blank field taken with the same instrument. We found that TNJ1338 resides in a region with a peak overdensity of $delta_{rm LAE}=2.8pm 0.5$ on scales of $8, h^{-1}rm Mpc$ (on the sky) and $112, h^{-1}rm Mpc$ (line of sight) in comoving coordinates. Adjacent to this overdensity, we found a strong underdensity where virtually no LAEs are detected. We used a semi-analytical model of LAEs derived from the Millennium Simulation to compare our results with theoretical predictions. While the theoretical density distribution is consistent with the blank field, overdense regions such as that around TNJ1338 are very rare, with a number density of $6.4times 10^{-8}rm Mpc^{-3}$ (comoving), corresponding to the densest < 0.4 percentile at $zsimeq 4.1$. We also found that the Ly$alpha$ luminosity function in the TNJ1338 field differs from that in the blank field: the number of bright LAEs ($log(L_{rm Lyalpha}[erg,s^{-1}]) gtrsim 43.3$) is enhanced, while the number of fainter LAEs is relatively suppressed. These results suggest that some powerful radio galaxies associated with Ly$alpha$ nebulae reside in extreme overdensities on $sim 3$--$6, rm Mpc$ scales, where star-formation and AGN activity may be enhanced via frequent galaxy mergers or high rates of gas accretion from the surroundings.
Spatially extended Ly-alpha sources that are faint and/or compact in coninuum are candidates for extremely young (~< 10^7 yrs) galaxies at high redshifts. We present medium-resolution (R~2000) spectroscopy of such extended Ly-alpha sources found in o
ur previous study at z~3-5, using VLT/VIMOS. The deep spectroscopy showed that all 18 objects we observed have large equivalent widths (EWs) exceeding 100 A. For about 30% of our sample (five objects), we identified conspicuous asymmetry on the profiles of the Ly-alpha line. They show broad wing emission components on the red side, and sharp cut-off on the blue side of the Ly-alpha line. Such asymmetry is often seen in superwind galaxies known to date, and also consistent with a theoretical prediction of superwind activity. There are eight objects (8/18 ~ 40%) that have large EWs exceeding 200 A, and no clear signature of superwind activities. Such large EWs cannot be explained in terms of photo-ionization by a moderately old (>10^7 yrs) stellar population, even with a top-heavy IMF or an extremely low metallicity. These eight objects clearly show a positive correlation between the Ly-alpha luminosity and the velocity width. This suggests that these eight objects are good candidates for forming-galaxies in a gas-cooling phase.
