Deep spectroscopy of systematically surveyed extended Lyα sources at z∼3-5

Spatially extended Ly alpha sources that are faint and/or compact in continuum are candidates for extremely young (less than or similar to 10(7) yr) galaxies at high redshifts. We present medium-resolution (R similar to 2000) VLT/VIMOS spectroscopy of 18 such extended Ly alpha sources found in our previous study at z similar to 3-5. The deep spectroscopy showed that all 18 objects have large equivalent widths (EWs), exceeding 100 angstrom in the rest frame. For about 30% of our sample (five objects), we identified conspicuous asymmetry in the profile of the Ly alpha line. They show broad wing emission components on the red side, and a sharp cutoff on the blue side of the Ly alpha line. Such asymmetry is often seen in superwind galaxies, and is also consistent with the theoretical prediction of superwinds. In fact, one of them shows systematic velocity structure in the two-dimensional spectrum, suggesting the existence of superwind activity. There are eight objects (8/18 similar to 40%) that have very large EWs, exceeding 200 angstrom (rest frame), and no clear signature of superwind activities. Such large EWs cannot be explained in terms of photoionization by a moderately old (> 10(7) yr) 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 line luminosity and the velocity width. This suggests that these eight objects are good candidates for forming galaxies in a gas-cooling phase (cold accretion).