2007A&A...465..171F

We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1-3.1. We search for OVI absorption and corresponding SiIV, CIV, and NV in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering OVI 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. We report twelve DLAs (nine intervening and three at <5000km/s from the QSO redshift) with detections of OVI absorption. There are no clear OVI non-detections, so the incidence of OVI in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, CIV and SiIV are seen whenever data is available, and NV 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 CIV and SiIV components). The presence of inflows and/or outflows is indicated by individual OVI and CIV components displaced from the neutral gas (either blueshifted or redshifted) by up to 400km/s. 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 (OVI-bearing) and warm (CIV-bearing) phases in the range logN_HII^Hot>19.5 to >21.1, and logN_H II_^Warm>19.4 to >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. If the temperature in the OVI phase is ≃10⁶K and so f_OVI=OVI/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≃2.