arXiv subjects

Blair D. Savage

Publications and source records attributed to Blair D. Savage.

At least 55 records · Page 3Linked to original sources

arXiv2006

Optical Structure and Proper-Motion Age of the Oxygen-rich Supernova Remnant 1E 0102-7219 in the Small Magellanic Cloud

We present new optical emission-line images of the young SNR 1E 0102-7219 (E0102) in the SMC obtained with the HST Advanced Camera for Surveys (ACS). E0102 is a member of the oxygen-rich class of SNRs showing strong oxygen, neon , and other metal-line emissions in its optical and X-ray spectra, and an absence of H and He. The progenitor of E0102 may have been a Wolf-Rayet star that underwent considerable mass loss prior to exploding as a Type Ib/c or IIL/b SN. The ejecta in this SNR are fast-moving (V > 1000 km/s) and emit as they are compressed and heated in the reverse shock. In 2003, we obtained optical [O III], H-alpha, and continuum images with the ACS Wide Field Camera. The [O III] image captures the full velocity range of the ejecta, and shows considerable high-velocity emission projected in the middle of the SNR that was Doppler-shifted out of the narrow F502N bandpass of a previous Wide Field and Planetary Camera 2 image from 1995. Using these two epochs separated by ~8.5 years, we measure the transverse expansion of the ejecta around the outer rim in this SNR for the first time at visible wavelengths. From proper-motion measurements of 12 ejecta filaments, we estimate a mean expansion velocity for the bright ejecta of ~2000 km/s and an inferred kinematic age for the SNR of \~2050 +/- 600 years. The age we derive from HST data is about twice that inferred by Hughes et al.(2000) from X-ray data, though our 1-sigma error bars overlap. Our proper-motion age is consistent with an independent optical kinematic age derived by Eriksen et al.(2003) using spatially resolved [O III] radial-velocity data. We derive an expansion center that lies very close to X-ray and radio hotspots, which could indicate the presence of a compact remnant (neutron star or black hole).

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arXiv2005

Properties of O VI Absorption in the Local Interstellar Medium

We report on the properties of LISM O VI absorption observed with 20 km/s resolution FUSE observations of 39 white dwarfs (WDs) ranging in distance from 37 to 230 pc with a median distance of 109 pc. LISM O VI is detected with >2sigma significance along 24 of 39 lines of sight. The column densities range from log N(O VI) = 12.38 to 13.60 with a median value of 13.10. The line of sight volume density, n(O VI) = N(O VI)/d exhibits a large dispersion ranging from (0.68 to 13.0)x10(-8) cm(-3) with an average value 3.6x10(-8) cm(-3) twice larger than found for more distant sight lines in the Galactic disk. The narrowest profiles are consistent with thermal Doppler broadening of O VI near its temperature of peak abundance, 2.8x10(5) K. Comparison of the average velocities of O VI and C II absorption reveals 10 cases where the O VI absorption is closely aligned with the C II absorption as expected if the O VI is formed in a condensing interface between the cool and warm absorption and a hot exterior gas. The comparison also reveals 13 cases where O VI absorption is displaced to positive velocity by 7 to 29 km/s from the average velocity of C II. The positive velocity O VI appears to be tracing the evaporative flow of O VI from a young interface between warm gas and a hot exterior medium. However, it is possible the positive velocity O VI is instead tracing cooling hot Local Bubble (LB) gas. The properties of the O VI absorption in the LISM are broadly consistent with the expectations of the theory of conductive interfaces caught in the old condensing phase and possibly in the young evaporative phase of their evolution.

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arXiv2005

A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo

We describe a new method for determining total gas-phase abundances for the Galactic ISM with minimal ionization uncertainties. For sight lines toward globular clusters containing both UV-bright stars and radio pulsars, one can measure column densities of HI and several metal ions using UV absorption measurements and of H II using radio dispersion measurements, thereby minimizing ionization uncertainties. We apply this method to the globular cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total along this sight line, the highest measured fraction along a high-latitude pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) = -4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar system abundance, consistent with significant depletion. Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher, respectively, than our measurements. Ionization corrections to the gas-phase abundances measured in the standard way are, therefore, significant compared with the measurement uncertainties along this sight line. The systematic uncertainties associated with the uncertain contribution to the electron column density from ionized helium could raise these abundances by <+0.03 dex (+7%). [Abridged]

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arXiv2005

Measurement of Noisy Absorption Lines using the Apparent Optical Depth Technique

To measure the column densities of interstellar and intergalactic gas clouds using absorption line spectroscopy, the apparent optical depth technique (AOD) of Savage & Sembach (1991) can be used instead of a curve-of-growth analysis or profile fit. We show that the AOD technique, whilst an excellent tool when applied to data with good S/N, will likely overestimate the true column densities when applied to data with low S/N. This overestimation results from the non-linear relationship between the flux falling on a given detector pixel and the apparent optical depth in that pixel. We use Monte Carlo techniques to investigate the amplitude of this overestimation when working with data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS), for a range of values of S/N, line depth, line width, and rebinning. AOD measurements of optimally sampled, resolved lines are accurate to within 10% for FUSE/LiF and STIS/E140M data with S/N>7 per resolution element.

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arXiv2005

Multi-phase High-Velocity Clouds toward HE 0226-4110 and PG 0953+414

We study the physical conditions, elemental abundances, and kinematics of the high-velocity clouds (HVCs) along the sight lines toward active galaxies HE0226-4110 and PG0953+414 using Hubble Space Telescope Imaging Spectrograph and Far Ultraviolet Spectroscopic Explorer data. Our observations reveal multiple components of HVC absorption in lines of HI, CII, CIII, CIV, OVI, SiII, SiIII, and SiIV in both directions. We investigate whether photoionization by the extragalactic background radiation or by escaping Milky Way radiation can explain the observed ionization pattern. We find that photoionization is a good explanation for the CII, CIII, SiII, and SiIII features, but not for the OVI or CIV associated with the HVCs, suggesting that two principal phases exist: a warm (T~10^4K), photoionized phase and a hotter (T=1-3x10^5K), collisionally-ionized phase. The warm HVCs toward HE0226-4110 have high levels of ionization (97-99%), and metallicities ([Z/H] between -0.9 and -0.4) close to those in the Magellanic Stream, which lies eleven degrees away on the sky at similar velocities. These HVCs have thermal pressures that would place them close to equilibrium in a fully ionized 10^6 K Galactic corona with n_H=4-9x10^{-5}cm^{-3} at 50 kpc. A mini-survey of the hot, collisionally ionized HVC components seen here and in five other sight lines finds that in 11/12 cases, the high ions have kinematics and ionic ratios that are consistent with an origin in conductive interfaces. However, the broad absorption wing on the OVI profile toward PG0953+414 is not completely explained by the interface scenario, and may be tracing the outflow of hot gas into the Milky Way halo as part of a Galactic fountain or wind.

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arXiv2005

Detection of Ne VIII in the Low-Redshift Warm-Hot IGM

High resolution FUSE and STIS observations of the bright QSO HE 0226-4110 (zem = 0.495) reveal the presence of a multi-phase absorption line system at zabs(O VI) = 0.20701 containing absorption from H I (Ly alpha to Ly theta), C III, O III, O IV, O VI, N III, Ne VIII, Si III, S VI and possibly S V. Single component fits to the Ne VIII and O VI absorption doublets yield logN(Ne VIII) = 13.89+/-0.11 and logN(O VI) = 14.37+/-0.03. The Ne VIII and O VI doublets are detected at 3.9 sigma and 16 sigma significance levels, respectively. This represents the first detection of intergalactic Ne VIII, a diagnostic of gas with temperature in the range from 5x10(5) to 1x10(6) K. The O VI and Ne VIII are not likely created in a low density medium photoionized solely by the extragalactic background at z = 0.2 since the required path length of ~11 Mpc implies the Hubble flow absorption line broadening would be ~10 times greater than the observed line widths. A collisional ionization origin is therefore more likely. Assuming [Ne/H] and [O/H] = -0.5, the value N(Ne VIII)/N(O VI) = 0.33+/-0.10 is consistent with gas in collisional ionization equilibrium near T=5.4x10(5) K with logN(H)= 19.9 and N(H)/N(H I) = 1.7x10(6). The observations support the basic idea that a substantial fraction of the baryonic matter at low redshift exists in hot very highly ionized gaseous structures.

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arXiv2005

Exploring Hot Gas in the Galactic Halo and High Velocity Clouds

In the five years since its launch, the Far Ultraviolet Spectroscopic Explorer has given the astronomical community a marvelous glimpse into the realm of previously unexplored regions of the Milky Way and its surroundings. Absorption lines produced by gas along entire paths through the nearby universe have now been recorded in the spectra of distant QSOs and active galactic nuclei. Of the lines recorded, the O VI 1031.926, 1037.617 Angstrom doublet is the best tracer of highly ionized regions. Exciting discoveries include a definitive confirmation of the hot Galactic halo postulated by Lyman Spitzer nearly 50 years ago, the discovery of a hot, highly extended Galactic corona enveloping the Milky Way out to distances of several tens of kiloparsecs, and the discovery of an extensive network of highly ionized, high velocity clouds surrounding the Galaxy.

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arXiv2004

Highly Ionized Gas in the Galactic Halo and the High Velocity Clouds Toward PG 1116+215

We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG 1116+215. The semi-continuous coverage of the ultraviolet spectrum over the wavelength range 916-2800 provides detections of Galactic and high velocity cloud (HVC) absorption over a wide range of ionization species over the local standard of rest velocity range -100 - +200 km/s. The high dispersion of these spectra (6.5-20 km/s) reveals that low ionization species consist of five discrete components, three at low-to-intermediate velocities, and two at high velocities (v = +100, +184 km/s). Over the same velocity range, the higher ionization species show continuous absorption with column density peaks at v = +10 km/s +184 km/s. The absorption kinematics of the v=+184 km/s HVC suggest a scenario in which a low-ionization cloud of gas is streaming through a hot external medium that is stripping gas from this cloud. Using the OI and HI column densities, we estimate [O/H]=-0.66, with a substantial uncertainty due to saturation of the HI Lyman series. If the ionization of the cloud core is photonionization by the extragalactic UV background, we estimate the cloud has a density of 10^-2.7 cm^-3. If photons escaping the Galactic disk are also included, the density could be higher by nearly 2 dex. In either case, the relative abundances of O, Si, and Fe in the cloud core are readily explained by a solar pattern. Magellanic Stream gas is a possible origin for this gas and is consistent with the location of the HVC on the sky, as well as its high positive velocity, the ionization, and metallicity. [Abridged]

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arXiv2004

Missing Baryons in the Warm-Hot Intergalactic Medium

We briefly review the use of UV absorption lines in the spectra of low-redshift QSOs for the study of the physical conditions, metallicity, and baryonic content of the low-z IGM, with emphasis on the missing baryons problem. Current results on the statistics and baryonic content of intervening, low-z O VI and Lya absorption-line systems are presented with some comments on overlap between these two classes of absorbers and consequent baryon double-counting problems. From observations of a sample of 16 QSOs observed with the E140M echelle mode of STIS, we find 44 intervening O VI absorbers and 14 associated O VI systems [i.e, z(abs) ~ z(QSO)]. The implied number of intervening O VI absorbers per unit redshift is dN/dz(O VI) = 23+/-4 for rest equivalent width > 30 mA. The intervening O VI systems contain at least 7% of the baryons if their typical metallicity is 1/10 solar and the O VI ion fraction is <0.2. This finding is consistent with predictions made by cosmological simulations of large-scale structure growth. Recently, a population of remarkably broad Lya lines have been recognized in low-z quasar spectra. If these Lya lines are predominantly thermally broadened, then these H I absorbers likely harbor an important fraction of the baryons. We present and discuss some examples of the broad Lya absorbers. Finally, we briefly summarize some findings on the relationships between O VI absorbers and nearby galaxies/large-scale structures.

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arXiv2004

Physical Properties and Baryonic Content of Low-Redshift Intergalactic Ly-alpha and O VI Absorption Systems: The PG1116+215 Sight Line

We present HST and FUSE observations of the intergalactic absorption toward PG1116+215 in the 900-3000 A spectral region. We detect 25 Ly-alpha absorbers at rest-frame equivalent widths W_r > 30 mA, yielding (dN/dz)_Ly-alpha = 154+/-18 over an unblocked redshift path of 0.162. Two additional weak Ly-alpha absorbers with W_r ~ 15-20 mA are also present. Eight of the Ly-alpha absorbers have large line widths (b > 40 km/sec). The detection of narrow OVI in the broad Ly-alpha absorber at z=0.06244 supports the idea that the Ly-alpha profile is thermally broadened in gas with T > 10^5 K. We find dN/dz ~ 50 for broad Ly-alpha absorbers with W_r > 30 mA and b > 40 km/sec. If the broad Ly-alpha lines are dominated by thermal broadening in hot gas, the amount of baryonic material in these absorbers is enormous, perhaps as much as half the baryonic mass in the low-redshift universe. We detect OVI absorption in several of the Ly-alpha clouds along the sight line. Two detections at z=0.13847 and z=0.16548 are confirmed by the presence of other ions at these redshifts, while the detections at z=0.04125, 0.05895, 0.05928, and 0.06244 are based upon the Ly-alpha and OVI detections alone. The information available for 13 low-redshift OVI absorbers with W_r > 50 mA along 5 sight lines yields (dN/dz)_OVI ~ 14 and Omega_b(OVI) > 0.0027/h_75, assuming a metallicity of 0.1 solar and an OVI ionization fraction < 0.2. The properties and prevalence of low-redshift OVI absorbers suggest that they too may be a substantial baryon repository, perhaps containing as much mass as stars and gas inside galaxies. The redshifts of the OVI absorbers are highly correlated with the redshifts of galaxies along the sight line, though few of the absorbers lie closer than 600/h_75 kpc to any single galaxy. [abbreviated]

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arXiv2004

FUSE and STIS Observations of the Warm-Hot Intergalactic Medium towards PG1259+593

We use FUSE and STIS spectra to study intergalactic absorption towards the quasar PG1259+593 (z=0.478). We identify 135 intergalactic absorption lines with equivalent widths >10mA, tracing 78 absorption components in 72 Ly alpha/beta absorption-line systems. We concentrate on the distribution and physical properties of the WHIM as sampled by OVI and intrinsically broad Ly alpha lines. The number of intervening OVI absorbers for equivalent widths W>24 mA is 3-6 over an unobscured redshift path of dz=0.368. This implies a number density of OVI systems, dN/dz, of ~8-16 along this sight line. This range is consistent with estimates from other sight lines, supporting the idea that intervening intergalactic OVI absorbers contain an substantial fraction of the baryonic mass in the low-redshift Universe. We identify a number of broad Ly alpha absorbers with large Doppler parameters (b~40-200 km/s) and low column densities (N(HI)<10^14 cm^-2). For pure thermal broadening, these widths correspond to temperatures of ~1x10^5 to 3x10^6 K. While these broad absorbers could be caused by blends of multiple, unresolved lines, continuum undulations, or by kinematic flows and Hubble broadening, we consider the possibility that some of these features are single-component, thermally broadened Ly alpha lines. These systems could represent WHIM absorbers that are too weak, too metal-poor, and/or too hot to be detected in OVI. If so, their widths and their frequency in the PG1259+593 spectrum imply that these absorbers trace an even larger fraction of the baryons in the low-redshift Universe than the OVI absorbing systems (abridged version).

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arXiv2003

Highly Ionized Gas Surrounding High Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of high, intermediate, and low ion absorption in high-velocity cloud Complex C along the lines of sight toward five active galaxies. Our purpose is to investigate the idea that Complex C is surrounded by an envelope of highly ionized material, arising from the interaction between the cloud and a hot surrounding medium. We measure column densities of high-velocity high ion absorption, and compare the kinematics of low, intermediate, and high ionization gas along the five sight lines. We find that in all five cases, the H I and O VI high-velocity components are centered within 20 km/s of one another, with an average displacement of =3+-12 km/s. In those directions where the H I emission extends to more negative velocities (the so-called high-velocity ridge), so does the O VI absorption. We compare our high ion column density ratios to the predictions of various models, adjusted to account for both recent updates to the solar elemental abundances, and for the relative elemental abundance ratios in Complex C. Our observed ratios are inconsistent with collisional ionization equilibrium at one kinetic temperature. Photoionization by the extragalactic background is ruled out as the source of the high ions, since the path lengths required would make Complex C unreasonably large. We consider the most likely origin for the highly ionized high-velocity gas to be at the conductive or turbulent interfaces between the neutral/warm ionized components of Complex C and a surrounding hot medium. The presence of interfaces on the surface of HVCs provides indirect evidence for the existence of a hot medium in which the HVCs are immersed.

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arXiv2002

Ionized Gas in the First 10 kpc of the Interstellar Galactic Halo

We present FUSE observations of the post-asymptotic giant branch star von Zeipel 1128 (l=42.5, b=+78.7; z=10.0 kpc), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905-1187 Ang at ~20 km/s (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, including absorption from ions associated with the warm neutral, warm ionized, and highly-ionized phases of the interstellar medium along this sight line. Ionized hydrogen represents >12%, most likely ~45%, of the total hydrogen column along this sight line, most of it associated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very similar gas-phase abundances and kinematics: the neutral and ionized gases in this region of the thick disk are closely related. Strong O VI absorption is seen with the same central velocity as the warm ionized gas, though the O VI velocity dispersion is much higher (sigma=32 km/s). Virtually all of the O VI is found at velocities where lower-ionization gas is seen, suggesting the O VI and WNM/WIM probes are tracing different portions of the same structures (e.g., the O VI may reside in interfaces surrounding the WNM/WIM clouds). We see no evidence for interstellar absorption associated with the globular cluster Messier 3 itself nor with the circumstellar environment of von Zeipel 1128. Neither high velocity cloud absorption (with v(LSR)>125 km/s) nor high velocity-dispersion gas (with sigma~60 km/s) is seen toward von Zeipel 1128. [Abridged]

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arXiv2002

Origins of the Highly Ionized Gas along the Line of Sight towards HD 116852

We present Hubble Space Telescope Imaging Spectrograph (HST/STIS) and Far Ultraviolet Spectroscopic Explorer (FUSE) observations of high ion interstellar absorption along the sight line to HD 116852. At a distance of 4.8 kpc, HD 116852 is an O9 III star lying in the low Galactic halo, -1.3 kpc from the plane of the Galaxy in the direction l = 304.9, b = -16.1. The STIS E140H grating observations provide high-resolution (FWHM = 2.7 km/s) spectra of the resonance doublets of Si IV, C IV, and N V. These data are complemented by medium-resolution (FWHM = 20 km/s) FUSE spectra of O VI. We find evidence for three distinct types of highly ionized gas present in the data. First, two narrow absorption components are resolved in the Si IV and C IV profiles, at approximate LSR velocities of -36 and -10 km/s. These narrow components appear to be produced in gas associated with the Norma and Sagittarius spiral arms, at approximate z-distances of -1.0 and -0.5 kpc, respectively. Second, we detect an intermediate-width component in C IV and Si IV, at 17 km/s, which we propose could arise at the conductive interface at the boundary between a low column density neutral or weakly ionized cloud and the surrounding hot medium. Finally, a broad collisionally ionized component of gas responsible for producing the smooth N V and O VI profiles is observed; such absorption is also present to a lesser degree in the profiles of Si IV and C IV. The broad O VI absorption is observed at a velocity displaced from the broad C IV component by almost 20 km/s, an amount large enough to suggest that the two ions may not co-exist in the same physical location.

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arXiv2002

A FUSE Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the FUSE survey of molecular hydrogen in lines of sight with A_V $\gtrsim$ 1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J = 0 and J = 1, which contain $\gtrsim$99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of 10^21 cm^-2, measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN abundance, the steepness of the far-UV extinction rise, and the width of the 2175A bump. Despite the relatively large molecular fractions, we do not see the values greater than 0.8 expected in translucent clouds. With the exception of a few lines of sight, we see little evidence for the presence of individual translucent clouds in our sample. We conclude that most of the lines of sight are actually composed of two or more diffuse clouds similar to those found toward targets like zeta Oph. We suggest a modification in terminology to distinguish between a "translucent line of sight" and a "translucent cloud."

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arXiv2002

FUSE Observations of Degree-Scale Variations in Galactic Halo O VI

We report Far Ultraviolet Spectroscopic Explorer (FUSE) observations of interstellar O VI absorption in the halo of the Milky Way towards 12 early-type stars in the Large Magellanic Cloud (LMC) and 11 in the Small Magellanic Cloud (SMC). The mean column densities of O VI associated with the Galactic halo towards the LMC and SMC are log = 14.52(+0.10,-0.14) and 14.13(+0.14,-0.20), respectively, where the uncertainties represent the standard deviations of the individual measurements about the means. Significant variations in the O VI column densities are observed over all of the angular scales probed by our observations: 0.5-5.0 degrees towards the LMC and 0.05-3.3 degrees towards the SMC. The maximum factors by which the O VI varies between sight lines towards the LMC and SMC are ~2.8 and ~4.2, respectively. Though low-, intermediate-, and high-velocity clouds are present along most of the sight lines towards the LMC, the variations in O VI column densities are found in all of these O VI absorbing structures. The column density variations trace the structure of the hot ionized medium in the Galactic halo on scales of <<80 to 800 pc towards the LMC and <<6 to 400 pc towards the SMC (assuming the absorption arises within the first 5 kpc above the Galactic plane).

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arXiv2002

A FUSE Survey of Interstellar O VI Absorption in the Small Magellanic Cloud

We present the results of a FUSE survey of O VI 1031.93 Angstrom and 1037.62 Angstrom absorption toward 18 OB stars in the Small Magellanic Cloud (SMC). We detect O VI along every sight line. The average value of the O VI column density in the SMC is log =14.53. This value is 1.7 times higher than the average value for the Milky Way halo (perpendicular to the Galactic plane) of log N_perp(O VI)=14.29 found by FUSE, even though the SMC has much lower metallicity than the Galaxy. The column density in the SMC is higher along sight lines that lie close to star-forming regions, in particular NGC 346 in the northern part of the SMC. The standard deviation of the column densities for sight lines outside of NGC 346 is +/- 27%, somewhat lower than the deviation seen in the Milky Way halo. The lowest O VI column densities, log N(O VI)~14.3, occur in the central region and in the southeastern ``Wing'' of the galaxy. Even these low column densities are as high as the Milky Way average, establishing the presence of a substantial, extended component of coronal gas in the SMC. The O VI absorption is always shifted to higher velocities than the main component of lower ionization gas. The O VI line widths are broader than expected for pure thermal broadening. We discuss several mechanisms that may be able to explain the observed properties of the hot gas, including supershells, a galactic fountain, and the infall of gas previously stripped from the SMC by tidal interactions with the Milky Way and the Large Magellanic Cloud. If a fountain produces the hot gas, the mass flux per unit surface area is ~0.02 M_sun/yr/kpc2.

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arXiv2001

The Global Content, Distribution, and Kinematics of Interstellar O VI in the Large Magellanic Cloud

We present FUSE observations of interstellar O VI absorption towards 12 early-type stars in the Large Magellanic Cloud (LMC). O VI 1031.926 Ang absorption at LMC velocities is seen towards all 12 stars. The observed columns are in the range log N(O VI)=13.9 to 14.6, with a mean of 14.37. The observations probe several sight lines projected onto known superbubbles, but these show relatively little (if any) enhancement in O VI column density compared to sight lines towards relatively quiescent regions of the LMC. The observed LMC O VI absorption is broad, with Gaussian dispersions of 30 to 50 km/sec, implying temperatures T<(2-5)x10^6 K. The O VI absorption is typically displaced -30 km/sec from the corresponding low-ionization absorption associated with the bulk of the LMC gas. The properties of the LMC O VI absorption are very similar to those of the Milky Way halo. The average column density of O VI and the dispersion of the individual measurements about the mean are identical to those measured for the halo of the Milky Way, even though the metallicity of the LMC is a factor of ~2.5 lower than the Milky Way. Much of the LMC O VI may arise in a vertically-extended distribution similar to the Galactic halo. If the observed O VI absorption is tracing a radiatively cooling galactic fountain flow, the mass flow rate from one side of the LMC disk is of the order 1 Msun/yr, with a mass flux per unit area of the disk ~0.02 Msun/yr/kpc^2. (abridged)

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