Hot halos around high redshift protogalaxies -: Observations of OVI and NV absorption in damped Lyman-α systems

Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-alpha (DLA) systems at z = 2.1-3.1. Methods. We search for O VI absorption and corresponding Si IV, C IV, and N V 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 < 5000 km s(-1) from the QSO redshift) with detections of O VI absorption. There are no clear O VI non-detections, so the incidence of O VI in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, C IV and Si IV are seen whenever data is available, and N V is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow C IV and Si IV components). The presence of inflows and/or outflows is indicated by individual O VI and C IV components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 km s(-1). We find tentative evidence (98% confidence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outflows rather than accretion produced the high ions. Using conservative ionization corrections, we find lower limits to the total hydrogen column densities in the hot (O VI-bearing) and warm (C IV-bearing) phases in the range log N-HII(Hot) > 19.5 to > 21.1, and log N-NII(Warm) > 19.4 to > 20.9. On average, the hot and warm phases thus contain greater than or similar to 40% and greater than or similar to 20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the O VI phase is approximate to 10(6) K and so f(OVI) = O VI/O << 0.2, the plasma can make a significant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < N-HI < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z approximate to 2.