Interstellar absorptions towards the LMC:

Abstract

Observations show that the interstellar medium (ISM) contains sub-structure on scales less than 1 pc, detected in the form of spatial and temporal variations in column densities or optical depth. Despite the number of detections, the nature and ubiquity of the smallscale structure in the ISM is not yet fully understood. We use UV absorption data mainly from the Far Ultraviolet Spectroscopic Explorer (FUSE) and partly from the Space Telescope Imaging Spectrograph (STIS) of six Large Magellanic Cloud (LMC) stars (Sk -67 • 111, LH 54-425, Sk -67 • 107, Sk -67 • 106, Sk -67 • 104, and Sk -67 • 101) that are all located within 5 of each other, and analyse the physical properties of the Galactic disc gas in front of the LMC on sub-pc scales. We analyse absorption lines of a number of ions within the UV spectral range. Most importantly, interstellar molecular hydrogen, neutral oxygen, and fine-structure levels of neutral carbon have been used in order to study changes in the density and the physical properties of the Galactic disc gas over small angular scales. At an assumed distance of 1 kpc, the 5 separation between Sk -67 • 111 and Sk -67 • 101 implies a linear extent of 1.5 pc.We report on column densities of H 2 , C i, N i, O i, Al ii, Si ii, P ii, S iii, Ar i, and Fe ii in our six lines of sight, as well as C i*, C i**, Mg ii, Si iv, S ii, Mn ii, and Ni ii for four of them. While most species do not show any significant variation in their column densities, we find an enhancement of almost 2 dex for H 2 from Sk -67 • 111 to Sk -67 • 101, accompanied by only a small variation in the O i column density. Based on the formation-dissociation equilibrium, we trace these variations to the actual density variations in the molecular gas. On the smallest spatial scale of <0.08 pc, between Sk -67 • 107 and LH 54-425, we find a gas density variation of a factor of 1.8. The line of sight towards LH 54-425 does not follow the relatively smooth change seen from Sk -67 • 101 to Sk -67 • 111, suggesting that sub-structure might exist on a smaller spatial scale than the linear extent of our sight-lines. The results show that we sample a mix of both neutral and ionised gas in our six lines of sight. Towards Sk -67 • 101 to Sk -67 • 107, we derive the temperature T exc 70 K for the inner self-shielded part of the gas based on the rotational excitation levels of H 2 , and an average density of n H 60 cm −3 , typical of that for CNM. The gas towards LH 54-425 and Sk -67 • 111 shows different properties, and T exc 200 K. Our observations suggest that the detected H 2 in these six lines of sight (with the extent of <1.5 pc) is not necessarily physically connected, but that we are sampling molecular cloudlets with pathlengths <0.1−1.8 pc and possibly different densities.

Narrow absorption line variability in repeat quasar observations from the Sloan Digital Sky Survey

“…However, all of these factors are, at most, of the same order as the uncertainty in the size of the quasar emission region, so we can safely assume that they are incorporated into the size ranges given in equation 3. These results suggest cloud sizes ranging from ∼1 to 100 au in accordance with the findings from small-scale structures in the Milky Way interstellar medium (Lauroesch et al 2000;Lauroesch & Meyer 2003;Welty 2007) and halo (Meyer & Lauroesch 1999;Richter, Sembach, & Howk 2003;Nasoudi-Shoar et al 2012). Outside our Galaxy, the only known discovery of short time-scale variations attributed to small-scale structures occurred in the 21-cm absorption line profile of a damped Lyα quasar absorption system at high redshift (Kanekar & Chengalur 2001).…”

Galaxies Probing Galaxies in PRIMUS. II. The Coherence Scale of the Cool Circumgalactic Medium

“…Furthermore, the H I velocities of HVCs typically vary by < 5 km s −1 over much of their projected surface (Wakker et al 2008), suggesting that if the associated low-ionization absorption were probed along multiple close sightlines it would exhibit minimal velocity shear. In principle, analysis of the coherence of low-ion absorption toward halo stars or AGN (e.g., André et al 2004;van Loon et al 2009;Nasoudi-Shoar et al 2010;van Loon et al 2013;Smoker et al 2015) has the greatest potential of any of the techniques discussed here to reveal the detailed den-sity structure of circumgalactic clouds, and such efforts are ongoing (J. K. Werk et al, in preparation).…”

The VLT-FLAMES Tarantula Survey

“…Nasoudi-Shoar et al (2010) analyzed molecular hydrogen (H 2 ) absorption in the local foreground gas along several closely spaced sightlines towards early-type stars in the N 51 D superbubble in the LMC. Based on the analysis of the spatially varying H 2 absorption pattern and the physical conditions in the gas, these authors conclude that the local cold neutral medium towards the LMC consists of small-scale structure on linear scales <0.1 pc.…”