Spatially extended Lya sources that are faint and/or compact in continuum are candidates for extremely young (≲ 107 yr) galaxies at high redshifts. We present medium-resolution (R ∼ 2000) VLT/VIMOS spectroscopy of 18 such extended Lyα sources found in our previous study at z ∼ 3-5. The deep spectroscopy showed that all 18 objects have large equivalent widths (EWs), exceeding 100 Åin the rest frame. For about 30% of our sample (five objects), we identified conspicuous asymmetry in the profile of the Lyα line. They show broad wing emission components on the red side, and a sharp cutoff on the blue side of the Lyα 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 ∼ 40%) that have very large EWs, exceeding 200 Å(rest frame), and no clear signature of superwind activities. Such large EWs cannot be explained in terms of photoionization by a moderately old (> 107 yr) stellar population, even with a top-heavy IMF or an extremely low metallicity. These eight objects clearly show a positive correlation between the Lyα 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).
- Galaxies: formation
- Galaxies: high-redshift