Small-scale structure at high redshift. I. Glimpses of the interstellar medium at redshift similar to 3.5

We obtained Keck High Resolution Echelle Spectrometer (FWHM = 4.4 km s(-1)) spectra of images A and C of the gravitationally lensed quasar Q1422+231 ( z(em) = 3.628). The images are separated by 1."3 on the sky. In an absorpti on system at z(abs) = 3.538, gas column density variations by an order of m agnitude and velocity shear on the order of 10 km s(-1) are observed in the low-ionization (Si Ir and C II) lines. The transverse separation in the ab sorbing cloud is estimated to be as small as 26 h(50)(-1) PC, corresponding to an effective angular resolution of 4 mas as seen from the Earth. In con trast, the high-ionization (C Iv) gas appears mostly featureless and thus m ust be considerably more extended. The abundances of the elements carbon, s ilicon, and oxygen appear to be close to the solar values. The observation provides the first spatially and kinematically resolved probe of the inters tellar medium at high redshift on scales small enough to be influenced by i ndividual stars or star clusters. The mass associated with the low-ionizati on "cloudlets" is likely to be less than about 3000 M. and possibly less th an 1 M.. The velocity shear that is seen across the lines of sight is too l arge to be caused by galactic bulk motion, so the velocity field of the low -ionization gas must be strongly influenced by small-scale local gasdynamic s. While presently it cannot be excluded that the disturbances of the gas a re due to high-velocity outflows from the background quasar, the observed v elocity and density structure of the z = 3.538 absorption is consistent wit h our line of sight running through an expanding shell of gas, possibly a s upernova bubble or a stellar wind.