Hot halos around high redshift protogalaxies Observations of Ovi and Nv absorption in damped Lyman-αsystems

Abstract

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 Ovi absorption and corresponding Si iv,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. 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 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 (Civ-bearing) phases in the range log N^Hot _{H II}> 19.5 to > 21.1, and log N^Warm_{H II} > 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. Conclusions. If the temperature in the O vi phase is ≈10⁶ Κ 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 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.