Survey of O vi absorption in the Large Magellanic Cloud

Abstract

We present a survey of interstellar O VI absorption in the Large Magellanic Cloud (LMC) towards 70 lines of sight based on Far Ultraviolet Spectroscopic Explorer (FUSE) observations. The survey covers O VI absorption in a large number of objects in different environmental conditions of the LMC. Overall, a high abundance of O VI is present in active and inactive regions of the LMC with mean log N(O VI) = 14.23 atoms cm −2 . There is no correlation observed between O VI absorption and emissions from the hot gas (X-ray surface brightness) or the warm gas (Hα surface brightness). O VI absorption in the LMC is patchy and the properties are similar to that of the Milky Way (MW). In comparison to the Small Magellanic Cloud (SMC), O VI is lower in abundance even though SMC has a lower metallicity compared to the LMC and the MW. We present observations in 10 superbubbles of the LMC of which we detect O VI absorption in five superbubbles for the first time and the superbubbles show an excess O VI absorption of about 40 per cent compared to non-superbubble lines of sight. We have also studied the properties of O VI absorption in the 30 Doradus region. Even though O VI does not show any correlation with X-ray emission for the LMC, a good correlation between log N(O VI) and X-ray surface brightness for 30 Doradus region is present. We also find that O VI abundance decreases with increasing distance from the star cluster R136.

The distribution and kinematics of interstellar O vi in the Milky Way

“…These were subtracted from the O VI profiles that appear in the same wavelength region. This is not a major problem for the LMC absorption lines which are shifted to a higher velocity (Howk et al (2002a); Pathak et al (2011)) but may affect the Galactic O VI significantly. We have plotted a representative sample of normalized spectra of the O VI absorption at 1032Å in Figure 1.…”

“…Sk-65D21, Sk-67D69, Sk-65D105, HV2543, Sk-66D100, HV5936, Sk-67D211, Sk-69D220, Sk-66D172, Sk-68D137 and D301-1005. For overlapping lines, the lower velocity limit of the LMC O VI absorption from Pathak et al (2011) It is difficult to directly compare the kinematics of O VI in the MW and the LMC based only on the absorption profiles because the O VI profiles are broad (Figure 2) suggesting that they arise in an extended halo of hot gas. A better probe of the kinematics of the gas is the optically thin Fe II absorption line at 1125.448 (Howk et al 2002b).…”

“…We obtained the calibrated data for the 69 sightlines analysed by Pathak et al (2011) (see Table 1) from the Multimission Archive at STScI (MAST). These were processed by the latest FUSE data reduction pipeline CALFUSE V3.2 ).…”

“…FUSE had a far-ultraviolet (FUV) wavelength coverage between 905 -1187Å with a resolution of ∼ 15,000 -20,000 near the O VI line absorption wavelengths. The study of O VI absorption in the ISM of galaxies with FUSE has given us information about the formation and distribution of O VI in the MW and the Magellanic Clouds that adds to our knowledge of varying ISM conditions in environments of different metallicities (Howk et al (2002a); Savage et al (2000); Wakker et al (2003); Oegerle et al (2005); Savage & Lehner (2006); Welsh & Lallement (2008); Pathak et al (2011)). Apart from absorption studies, FUSE has also been used to observe the O VI spectra in emission from the diffuse ISM in the MW (Shelton et al 2001;Dixon et al 2006;Dixon & Sankrit 2008), from superbubbles (SBs) and in the LMC (Sankrit & Dixon 2007).…”

“…Bowen et al (2008) studied absorption lines of O VI towards 148 early-type stars situated at distances 1 kpc and found an average O VI midplane density of 1.3×10 −8 cm −3 . Pathak et al (2011) surveyed the O VI absorption in the LMC towards 70 lines of sight using FUSE and noted the presence of a MW component in each of these lines of sight. We have analysed all these sightlines and report our findings here.…”