Small‐Scale Structure at High Redshift. I. Glimpses of the Interstellar Medium at Redshift ∼3.5

We have obtained high resolution (FWHM = 4.4 km/s) Keck spectra of images A and C of the gravitationally lensed QSO Q1422+231. The images are separated by 1.3" on the sky. In an absorption system at z = 3.538 gas column density variations by an order of magnitude and velocity shear on the order of 10 km/s are observed in the low ionization (SiII, CII) lines. The transverse separation between the lines of sight in the absorbing cloud is estimated to be as small as 26 parsec, corresponding to an effective angular resolution of 4 milliarcseconds as seen from the Earth. In contrast, the high ionization (CIV) gas appears mostly featureless and thus must be considerably more extended. The abundances of the elements carbon, silicon and oxygen appear to be close to the solar values. The observation provides the first spatially and kinematically resolved probe of the interstellar medium at high redshift on scales small enough to be influenced by individual stars or star clusters. The mass associated with the low ionization "cloudlets" is likely to be less than about 3000 M_solar and possibly less than 1 M_solar. The velocity shear seen across the lines of sight is too large to be caused by galactic bulk motion, so the velocity field of the low ionization gas must be strongly influenced by small scale local gasdynamics. While it cannot presently be excluded that the disturbances of the gas are due to high velocity outflows from the background QSO, the observed velocity and density structure of the z=3.538 system is consistent with our line of sight running through an expanding shell of gas, possibly a supernova bubble or a stellar wind.