ORFEUS echelle spectra: Molecular hydrogen in disk, IVC, and HVC gas in front of the LMC

Abstract

Abstract. In front of the LMC molecular hydrogen is found in absorption near 0 km s −1 , being local disk gas, near +60 km s −1 in an intermediate velocity cloud, and near +120 km s −1 , being a high velocity halo cloud. The nature of the gas is discussed based on four Orfeus far UV spectra of LMC stars and including data from the ground and from the IUE satellite. The local gas is cool and, given a span of sight lines of only 2.5• , rather fluffy. The fractional abundance of H2 varies from log f = log[N (H2)/(2 · N(H2) + N(H i))] = −5.4 to −3.3. Metal depletions (up to −1.7 dex for Fe) are typical for galactic disk gas. In the intermediate and high velocity gas an apparent underabundance of neutral oxygen points to an ionization level of the gas of about 90 %. H2 is detected in intermediate and high velocity gas towards HD 269546. In the intermediate velocity gas we find an H2 column density of log (N ) 15.6. The H2 excitation indicates that the line of sight samples a cloud at a temperature below 150 K. Column densities are too small to detect the higher UV pumped excitation levels. The high velocity H2 (log(N ) 15.6) is highly excited and probably exposed to a strong radiation field. Its excitation temperature exceeds 1000 K. Due to the radial velocity difference between the halo gas and the Milky Way disk, the unattenuated disk radiation is available for H2 excitation in the halo. We do not find evidence for an intergalactic origin of this gas; a galactic as well as a Magellanic Cloud origin is possible.

H2absorption in a dense interstellar filament in the Milky Way halo

“…Due to the probable existence of unresolved sub-structure and the uncertain H  column density (see Sect. 3 and Appendix), we do not derive gas-phase abundances for this suggest a high degree of ionization, similar to what is found for the IVC gas (see also Bluhm et al 2001).…”

“…One special case that we want to highlight is the spectrum of Sk -68 82 (HD 269546), the sight line where the phenomenon of H 2 absorption in intermediate-and high-velocity gas was found for the first time in low S /N ORFEUS data Bluhm et al 2001). The ORFEUS H 2 findings in the IVC/HVC gas in front of the LMC were coincidental detections during a project searching for H 2 absorption in the LMC (Richter 2000).…”

“…The presence of IVC/HVC H 2 is evident at a 6σ level in the composite velocity profile of H 2 for which we had co-added various H 2 transitions for J ≤ 6 to study the general velocity distribution of the H 2 towards Sk -68 82 in the ORFEUS data. In individual lines, however, H 2 is detected at low significance (1−4σ; see Bluhm et al 2001) due to the low S /N in the data, so that the H 2 column densities, b values, and excitation temperatures derived for the IVC and HVC gas are quite uncertain. The strong Fe  line (upper panel) suggests a very similar component strucutre as for Sk -68 80.…”

An HST/COS legacy survey of high-velocity ultraviolet absorption in the Milky Way’s circumgalactic medium and the Local Group

“…As can be concluded from these figures, the overall velocity structure in the HVC absorption patterns remains mostly unchanged along the different sightlines at the (limited) velocity resolution of the COS instrument (∼ 20 km s −1 ). So far, the information on small-scale structure in HVC gas from absorption-line studies is limited to only a few cloud complexes (e.g., the HVC towards the LMC; Danforth et al 2003;Bluhm et al 2001;Lehner et al 2009;Smoker et al 2014). In contrast, the presence of pc-scale substructure in HVCs in H i is well established from high-resolution 21 cm observations (e.g., Wakker et al 2002;Sembach et al 2004).…”

Low and high velocity clouds produced by young stellar clusters

“…The general characteristics of HI clouds in the Galaxy were discussed by Dickey & Lockman (1990) and Wakker & van Woerden (1997). Their general chemical properties were described by Lu et al (1998), Wakker et al (1999), Murphy et al (2000), Sembach et al (2000), and Bluhm et al (2001).…”