A connection between extremely strong damped Lyman-α systems and Lyman-α emitting galaxies at small impact parameters

We present a study of similar to 100 high redshift (z similar to 2-4) extremely strong damped Lyman-alpha systems (ESDLA, with N(H I) >= 0.5 x 10(22) cm(-2)) detected in quasar spectra from the Baryon Oscillation Spectroscopic Survey (BOSS) of the Sloan Digital Sky Survey (SDSS-III) Data Release 11. We study the neutral hydrogen, metal, and dust content of this elusive population of absorbers and confirm our previous finding that the high column density end of the N(H I) frequency distribution has a relatively shallow slope with power-law index -3.6, similar to what is seen from 21-cm maps in nearby galaxies. The stacked absorption spectrum indicates a typical metallicity similar to 1/20th solar, similar to the mean metallicity of the overall DLA population. The relatively small velocity extent of the low-ionisation lines suggests that ESDLAs do not arise from large-scale flows of neutral gas. The high column densities involved are in turn more similar to what is seen in DLAs associated with gamma-ray burst afterglows (GRB-DLAs), which are known to occur close to star-forming regions. This indicates that ESDLAs arise from a line of sight passing at very small impact parameters from the host galaxy, as observed in nearby galaxies. This is also supported by simple theoretical considerations and recent high-z hydrodynamical simulations. We strongly substantiate this picture by the first statistical detection of Ly alpha emission with < L-ESDLA(Ly alpha)> similar or equal to (0.6 +/- 0.2) x 10(42) erg s(-1) in the core of ESDLAs (corresponding to about 0.1 L-star at z similar to 2-3), obtained through stacking the fibre spectra (of radius 1 '' corresponding to similar to 8 kpc at z similar to 2.5). Statistical errors on the Ly alpha luminosity are of the order of 0.1 x 10(42) erg s(-1) but we caution that the measured Ly alpha luminosity may be overestimated by similar to 35% due to sky light residuals and/or FUV emission from the quasar host and that we have neglected flux-calibration uncertainties. We estimate a more conservative uncertainty of 0.2 x 10(42) erg s(-1). The properties of the Ly alpha line (luminosity distribution, velocity width and velocity offset compared to systemic redshift) are very similar to that of the population of Lyman-alpha emitting galaxies (LAEs) with L-LAE(Ly alpha) >= 10(41) erg s(-1) detected in long-slit spectroscopy or narrow-band imaging surveys. By matching the incidence of ESDLAs with that of the LAEs population, we estimate the high column density gas radius to be about r(gas) = 2.5 kpc, i.e., significantly smaller than the radius corresponding to the BOSS fibre aperture, making fibre losses likely negligible. Finally, the average measured Ly alpha luminosity indicates a star-formation rate consistent with the Schmidt-Kennicutt law, SFR (M(circle dot)yr(-1)) approximate to 0.6/f(esc), where f(esc) < 1 is the Ly alpha escape fraction. Assuming the typical escape fraction of LAEs, f(esc) similar to 0.3, the Schmidt-Kennicutt law implies a galaxy radius of about r(gal) approximate to 2.5 kpc. Finally, we note that possible overestimation of the Ly alpha emission would result in both smaller rgas and rgal. Our results support a close association between LAEs and strong DLA host galaxies.