The Surprisingly Constant Strength of O Vi Absorbers Over Cosmic Time

Abstract

O VI absorption is observed in a wide range of astrophysical environments, including the Local ISM, the disk and halo of the Milky Way, high-velocity clouds, the Magellanic clouds, starburst galaxies, the intergalactic medium, damped Lyman-α systems, and gamma-ray-burst host galaxies. Here a new compilation of 775 O VI absorbers drawn from the literature is presented, all observed at high resolution (instrumental FWHM≤20 km s −1 ) and covering the redshift range z=0-3. In galactic environments [log N (H I) 20], the mean O VI column density is shown to be insensitive to metallicity, taking a value log N (O VI)≈14.5 for galaxies covering the range -1.6 [O/H] 0. In intergalactic environments [log N (H I)<17], the mean O VI component column density measured in datasets of similar sensitivity shows only weak evolution between z=0.2 and z=2.3, but IGM O VI components are on average twice as broad at z=0.2 than at z=2.3. The implications of these results on the origin of O VI are discussed. The existence of a characteristic value of log N (O VI) for galactic O VI absorbers, and the lack of evolution in log N (O VI) for intergalactic absorbers, lend support to the "cooling-flow" model of Heckman et al. (2002), in which all O VI absorbers are created in regions of initially-hot shock-heated plasma that are radiatively cooling through coronal temperatures. These regions could take several forms, including conductive, turbulent, or shocked boundary layers between warm (∼10 4 K) clouds and hot (∼10 6 K) plasma, although many such layers would have to be intersected by a typical galaxy-halo sightline to build up the characteristic galactic N (O VI). The alternative, widely-used model of single-phase photoionization for intergalactic O VI is ruled out by kinematic evidence in the majority of IGM O VI components at low and high redshift.

A high-resolution study of intergalactic O vi absorbers at z∼ 2.3

“…The b (O vi ) distribution at high z is very different from that at low z as measured by Tripp et al (2008). Recently, Fox (2011) has drawn a similar conclusion using the high‐ z Voigt profile fitting results of Bergeron & Herbert‐Fort (2005).…”

High-ion absorption in the proximate damped Ly-αsystem toward Q0841+129

“…The boundary-layer scenario, in which the high ions are cooling rather than photoionized, has other advantages: it explains the metallicity-independence of the strength of O vi absorption in galaxy halos (Heckman et al 2002;Fox 2011), and fits naturally into the established picture of galaxy formation where cold streams pass through hot plasma, since boundary layers will naturally arise between the two phases. Indeed, in this picture, the O vi and C iv detections in DLAs indicate (without tracing directly) the presence of a hot (∼10 6 K) phase of surrounding plasma in the host galaxies.…”

High‐resolution absorption spectroscopy of the circumgalactic medium of the Milky Way

“…The O VI absorption possibly indicates transitiontemperature gas in the interfaces between cooler gas clouds and a surrounding hot coronal gas (see also Fox 2011).…”

“…Using high-resolution (R > 40, 000) optical spectra of QSOs and other extragalactic background sources obtained with the Ultraviolet and Visible Echelle Spectrograph (UVES) installed on the Very Large Telescope (VLT), we have studied the distribution of neutral gas and its kinematics in the Milky Way halo along a large number of sightlines (Richter et al 2005;Ben Bekhti et al 2008,2011. As an example for these data, we show in Fig.…”

“…In an on-going long-term project that aims at studying optical absorption features in circumgalactic gas of the Milky Way and other galaxies using archival VLT/UVES data (Richter et al 2005;Ben Bekhti et al 2008,2011Richter et al 2011), we have revisited the issue of [Fe X] and [Fe XIV] absorption from a possibly existing hot gas component in the CGM. While this project is on-going, we here present some preliminary results and shortly outline our analysis strategy.…”