Hot halos around high redshift protogalaxies - Observations of and  absorption in damped Lyman- systems

A. J. Fox; P. Petitjean; C. Ledoux; R. Srianand

doi:10.1051/0004-6361:20066157

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Volume 465 / No 1 (April I 2007)

A&A, 465 1 (2007) 171-184

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Issue		A&A Volume 465, Number 1, April I 2007


Page(s)		171 - 184
Section		Extragalactic astronomy
DOI		https://doi.org/10.1051/0004-6361:20066157
Published online		22 January 2007

A&A 465, 171-184 (2007)

Hot halos around high redshift protogalaxies^*

Observations of $\ion{O}{vi}$ and $\ion{N}{v}$ absorption in damped Lyman- $\mathsf{\alpha}$ systems

A. J. Fox¹, P. Petitjean¹^,2, C. Ledoux³ and R. Srianand⁴

¹ Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris, France e-mail: fox@iap.fr
² LERMA, Observatoire de Paris, 61 Avenue de l'Observatoire, 75014 Paris, France
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
⁴ IUCAA, Post Bag 4, Ganesh Khind, Pune 411 007, India

Received: 1 August 2006
Accepted: 12 January 2007

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 $\ion{O}{vi}$ absorption and corresponding $\ion{Si}{iv}$ , $\ion{C}{iv}$ , and $\ion{N}{v}$ in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering $\ion{O}{vi}$ 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 $\ion{O}{vi}$ absorption. There are no clear $\ion{O}{vi}$ non-detections, so the incidence of $\ion{O}{vi}$ in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, $\ion{C}{iv}$ and $\ion{Si}{iv}$ are seen whenever data is available, and $\ion{N}{v}$ 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 $\ion{O}{vi}$ components), and a warm, photoionized phase (seen just in narrow $\ion{C}{iv}$ and $\ion{Si}{iv}$ components). The presence of inflows and/or outflows is indicated by individual $\ion{O}{vi}$ and $\ion{C}{iv}$ 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 ( $\ion{O}{vi}$ -bearing) and warm ( $\ion{C}{iv}$ -bearing) phases in the range log $N_{\rm \ion{H}{ii}}^{\rm Hot}$ >19.5 to >21.1, and log $N_{\rm \ion{H}{ii}}^{\rm Warm}$ >19.4 to >20.9. On average, the hot and warm phases thus contain $\ga$ 40% and $\ga$ 20% of the baryonic mass of the neutral phase in DLAs, respectively.

Conclusions. If the temperature in the $\ion{O}{vi}$ phase is ≈10⁶ K and so $f_{\rm \ion{O}{vi}}={\rm \ion{O}{vi}/O}\ll0.2$ , the plasma can make a significant contribution to the metal budget at high redshift. Additional searches for $\ion{O}{vi}$ in Lyman Limit Systems (QSO absorbers with $17.0<N_{\rm \ion{H}{i}}<20.3$ ) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at $z\approx2$ .

Key words: quasars: absorption lines / cosmology: observations / galaxies: high-redshift / galaxies: halos / galaxies: ISM

^*

Based on observations taken with the Ultraviolet and Visible Echelle Spectrograph (UVES) on the Very Large Telescope (VLT) Unit 2 (Kueyen) at Paranal, Chile, operated by ESO.

© ESO, 2007

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Hot halos around high redshift protogalaxies*

Observations of and absorption in damped Lyman- systems

Hot halos around high redshift protogalaxies^*

Observations of $\ion{O}{vi}$ and $\ion{N}{v}$ absorption in damped Lyman- $\mathsf{\alpha}$ systems