the $LBG_N$ spectrum the interstellar absorption lines are more prominent than in the  spectrum: this is more evident for the low ionization lines (SII1260, OI+SiII1303, CII1334, SiII1536) while for the high ionization lines the difference is not as marked. This was already observed in more details by Shapley et al. (2003), who found that the kinematic offset between the Ly$\alpha$ line and the interstellar absorption lines also depends on Ly$\alpha$ emission strength. A detailed analysis of the spectral stacks as Shapley et al. is not allowed by the S/N of our composite spectra. Our aim here was only to check the resulting spectral stacks for consistency with previous work.

X-ray emission
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The field has been observed by Chandra in the 2Msec exposure (Luo et al. 2008). We therefore searched for signatures of X-ray emission from our sample of $LBG_L$ and $LBG_N$.\
To start we remind that, in the initial sample selection, we had excluded spectroscopically confirmed broad line and narrow line AGNs (BLAGNs and NLAGNs respectively). The first (BLAGNs) are easily spotted since they usually show a broad Ly$\alpha$ line, with typical FWHM of several thousands km/s, clearly resolved even in our low resolution spectra. The NLAGNs are recognized 