Hot halos around high redshift protogalaxies : Observations of O VI and N V absorption in damped Lyman-α systems

Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z = 2.1−3.1. Methods. We search for O vi absorption and corresponding Si iv ,C iv ,a nd Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering O vi at S /N > 10. We then use optical depth profile comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at 19. 5t o>21.1, and logN Warmii > 19. 4t o>20.9. On average, the hot and warm phases thus contain >40% and >20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the O vi phase is ≈10 6 Ka nd sofOvi = Ovi/O � 0. 2, the plasma can make as ignif icant contribution to the metal budget at high redshift. Additional searches for O vi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.