The high redshift galaxy MS1512–cB58 ( z = 2.724 , m _ { V } = 20.64 ) has been observed with the very efficient high resolution echelle spectrograph VLT/UVES .
Although this is a very challenging observational program for a Southern hemisphere telescope ( the galaxy is located at +36 ^ { \circ } declination ) , high resolution spectra ( FWHM \simeq 26 km s ^ { -1 } ) have revealed , with unprecedented detail along a galaxy sight line , the Lyman– \alpha forest due to intervening clouds in the intergalactic medium ( IGM ) .
The mean depression D _ { A } due to IGM absorption blueward of the galaxy Ly \alpha wavelength and the number density dn / dz of Ly \alpha clouds have been compared with equivalent results obtained for QSO sight lines at similar redshifts .
Our results indicate a possible excess of absorption close to the galaxy .
The mean depression at \sim 150 ~ { } h _ { 65 } ^ { -1 } Mpc comoving ( \Omega _ { m } = 0.3 , \Omega _ { \Lambda } = 0.7 ) from the galaxy is D _ { A } = 0.36 \pm 0.03 , to be compared with 0.22 \pm 0.04 , expected from a best fit to QSO sight lines .
In the same region ( z \simeq 2.610 ) , the number density of lines with HI column density in excess of 10 ^ { 14 } atoms cm ^ { -2 } is also \sim 3 \sigma larger than expected .
This high density region is at least 60 h _ { 65 } ^ { -1 } Mpc comoving wide , but the large Ly \alpha absorption of the galaxy itself prevents us from detecting a possible structure extending down to the galaxy .

This excess of Ly \alpha clouds is suggestive of two possible scenarios .
One is the presence of a super cluster of Ly \alpha clouds not associated with cB58 .
The other is a high density of gas associated with the environment of cB58 .
Indeed , a hint of the complexity of cB58 and possibly its environment is given by the huge velocity range ( almost 1000 km s ^ { -1 } ) between the optical emission of star forming regions and UV absorption of its interstellar medium .