arXiv subjects

B. D. Savage

Publications and source records attributed to B. D. Savage.

At least 37 records · Page 2Linked to original sources

arXiv2002

The FUSE survey of OVI absorption in and near the Galaxy

We present FUSE observations of OVI absorption in a sample of 100 extragalactic targets and 2 distant halo stars. We describe the details of the calibration, alignment in velocity, continuum fitting, and manner in which contaminants were removed (Galactic H2, absorption intrinsic to the background target and intergalactic Ly-beta lines). We searched for OVI absorption in the velocity range -1200 to 1200 km/s. With a few exceptions, we only find OVI between -400 and 400 km/s; the exceptions may be intergalactic OVI. We discuss the separation of the observed OVI absorption into components associated with the Galactic halo and components at high-velocity, which are probably located in the neighborhood of the Galaxy. We describe the measurements of equivalent width and column density, and we analyze the different contributions to the errors. We conclude that low-velocity Galactic OVI absorption occurs along all sightlines - the few non-detections only occur in noisy spectra. We further show that high-velocity OVI is very common, having equivalent width >65 mAA in 50% of the sightlines and >30 mAA in 70% of the high-quality sightlines. The high-velocity OVI absorption has velocities relative to the LSR of +/-(100--330) km/s; there is no correlation between velocity and absorption strength. We present 50 km/s wide OVI channel maps. These show evidence for the imprint of Galactic rotation. They also highlight two known HI high-velocity clouds (complex~C and the Magellanic Stream). The channel maps further show that OVI at velocities <-200 km/s occurs along all sightlines in the region l=20-150, b<-30, while OVI at velocities >200 km/s occurs along all sightlines in the region l=180-300, b>20 (abbreviated).

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arXiv2002

Highly-Ionized High-Velocity Gas in the Vicinity of the Galaxy

We report the results of an extensive FUSE study of high velocity OVI absorption along 102 complete sight lines through the Galactic halo. The high velocity OVI traces a variety of phenomena, including tidal interactions with the Magellanic Clouds, accretion of gas, outflow from the Galactic disk, warm/hot gas interactions in a highly extended Galactic corona, and intergalactic gas in the Local Group. We identify 85 high velocity OVI features at velocities of -500 < v(LSR) < +500 km/s along 59 of the 102 sight lines. Approximately 60% of the sky (and perhaps as much as 85%) is covered by high velocity H+ associated with the high velocity OVI. Some of the OVI is associated with known high velocity HI structures (e.g., the Magellanic Stream, Complexes A and C), while some OVI features have no counterpart in HI 21cm emission. The smaller dispersion in the OVI velocities in the GSR and LGSR reference frames compared to the LSR is necessary (but not conclusive) evidence that some of the clouds are extragalactic. Most of the OVI cannot be produced by photoionization, even if the gas is irradiated by extragalactic background radiation. Collisions in hot gas are the primary OVI ionization mechanism. We favor production of some of the OVI at the boundaries between warm clouds and a highly extended [R > 70 kpc], hot [T > 10^6 K], low-density [n < 10^-4 cm^-3] Galactic corona or Local Group medium. A hot Galactic corona or Local Group medium and the prevalence of high velocity OVI are consistent with predictions of galaxy formation scenarios. Distinguishing between the various phenomena producing high velocity OVI will require continuing studies of the distances, kinematics, elemental abundances, and physical states of the different types of high velocity OVI features found in this study. (abbreviated)

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arXiv2001

Abundances of Deuterium, Oxygen, and Nitrogen in the Local Interstellar Medium: Overview of First Results from the Far Ultraviolet Spectroscopic Explorer Mission

Observations obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) have been used to determine the column densities of D I, O I, and N I along seven sight lines that probe the local interstellar medium (LISM) at distances from 37 pc to 179 pc. Five of the sight lines are within the Local Bubble and two penetrate the surrounding H I wall. Reliable values of N(H I) were determined for five of the sight lines from HST data, IUE data, and published EUVE measurements. The weighted mean of D I/H I for these five sight lines is (1.52 +/- 0.08) x10-5 (1 sigma uncertainty in the mean). It is likely that the D I/H I ratio in the Local Bubble has a single value. The D I/O I ratio for the five sight lines within the Local Bubble is (3.76 +/- 0.20) x10-2. It is likely that the O I column densities can serve as a proxy for H I in the Local Bubble. The weighted mean for O I/H I for the seven FUSE sight lines is (3.03 +/-0.21) x10-4, comparable to the weighted mean (3.43 +/- 0.15) x10-4 reported for 13 sight lines probing larger distances and higher column densities (Meyer et al. 1998, Meyer 2001). The FUSE weighted mean of N I/H I for the five sight lines is half that reported by Meyer et al. (1997) for seven sight lines with larger distances and higher column densities. This result combined with the variability of O I/N I (six sight lines) indicates that at the low column densities found in the LISM, nitrogen ionization balance is important. Thus, unlike O I, N I cannot be used as a proxy for H I or as a metallicity indicator in the LISM. Subject Headings: cosmology: observations- ISM: abundances- ISM: evolution - Galaxy:abundances-Ultraviolet:ISM

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arXiv2001

FUSE Observations of O VI Overlying the Scutum Supershell

We present Far Ultraviolet Spectroscopic Explorer observations of the B0 III star HD 177989 ($l = 17.8^{rm ø}, b = -11.9^{\rm o}, d$ = 4.9 kpc). This line of sight passes through the high latitude outflow from the Scutum Supershell (GS 018-04+44), a structure that lies at a kinematic distance of ~ 3.5 kpc, and spans ~ 5$^{\rm o}$ in diameter. The O VI $λ$1031.93 line is compared with STIS observations of Si IV, C IV, and N V to examine the ionizing mechanisms responsible for producing the high ion absorption, as well as to study the processes by which gas is expelled into the halo. The O VI profile spans a similar velocity range as the other highly ionized atoms, from -70 to +80 km/s. Component fits reveal very broad absorption at the kinematic velocity of the Scutum shell, which differs from the narrow Si IV, C IV, and N V components, suggesting that these other species occupy a more confined region. It is possible that the O VI is present in hot gas from the shell interior that is diffusing into the high latitude region above it. The column density ratios show that while the C IV and Si IV columns are amplified in this region, the enhancement is not reflected in N V or O VI. We suggest that the ionic ratios and column densities could be produced by ~ 150-200 turbulent mixing layers lying in a fragmented medium above the polar cap of the superbubble. We note through a sight line comparison that although these absorption characteristics are similar to those near the center of Radio Loops I and IV, they differ considerably from those near the edges of the loops. It is apparent that the traits of high ion absorption in a supershell, as well as the ionizing mechanisms responsible, can vary among sight lines through the shell.

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arXiv2001

FUSE and STIS Observations of Intervening OVI Absorption Line Systems in the Spectrum of PG0953+415

We present FUSE and STIS observations of the IGM toward PG 0953+415 (z_em = 0.239). An O VI system at z = 0.06807 is detected in H I Lya, Lyb, Lyg, O VI N V, C IV, and C III. The observations are modeled as a low density intervening gas with a metallicity of 0.4 (+0.6, -0.2) times solar in photoionization equilibrium with the ionizing extragalactic radiation. The best fit is achieved with an ionization parameter, logU = -1.35, which implies nH ~ 10-5 cm-3 and a path length of ~ 80 kpc through the absorbing gas. H I Lya absorption at z = 0.14232 spans a velocity range of 410 km s-1 with the strongest components near 0 and 80 km s-1 in the z = 0.14232 rest frame. In this system O VI absorption is strong near 0 km s-1 and not detected at 80 km s-1. The new observations place constraints on the properties of the z = 0.14232 system but do not discriminate between collisional ionization in hot gas versus photoionization in a very low density medium with an ionization parameter logU > -0.74. The two O VI systems occur at redshifts where there are peaks in the number density of intervening galaxies along the line of sight. We update the estimate of the low redshift number density of intervening O VI systems. Over a total deltaz = 0.43, we detect six O VI systems with restframe equivalent widths of the O VI 1031.93 line exceeding 50 mA yielding dN/dz = 14 (+9, -6) for = 0.09. This implies a low redshift value of the baryonic contribution to the closure density of the O VI systems of WB(O VI) > 0.002h75-1 assuming the average metallicity in the O VI systems is 0.1 solar.

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arXiv2001

Far Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption Toward 3C 273

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the molecular, neutral atomic, weakly ionized, and highly ionized components of the interstellar and intergalactic material toward the quasar 3C 273. We identify Ly-beta absorption in 8 of the known intergalactic Ly-alpha absorbers along the sight line with rest-frame equivalent widths greater than 50 mA. Refined estimates of the H I column densities and Doppler parameters (b) of the clouds are presented. We find a range of b = 16-46 km/s. We detect multiple H I lines (Ly-beta - Ly-theta) in the 1590 km/s Virgo absorber and estimate log N(H I) = 15.85, ten times more H I than in all of the other absorbers along the sight line combined. The Doppler width of this absorber, b ~ 16 km/s, implies T < 15,000 K. We detect O VI absorption at 1015 km/s at the 2-3 sigma level that may be associated with hot, X-ray emitting gas in the Virgo Cluster. We detect weak C III and O VI absorption in the IGM at z=0.12007; this absorber is predominantly ionized and has N(H+)/N(H I) > 4000/Z, where Z is the metallicity. Strong Galactic interstellar O VI is present between -100 and +100 km/s, with an additional high-velocity wing containing about 13% of the total O VI between +100 and +240 km/s. The broad O VI absorption wing likely traces the expulsion of hot gas out of the Galactic disk into the halo. A limit on the amount of diffuse O VI emission present 3.5 arc minutes off the sight line combined with the observed O VI column density toward 3C 273, implies n_e < 0.02 cm^-3 and P/k < 11,500 cm^-3 K for an assumed temperature of 3x10^5 K. (abbreviated version)

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arXiv2001

The Diversity of High- and Intermediate-Velocity Clouds: Complex C versus IV Arch

We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption lines in the Galactic high-velocity cloud Complex C and the Intermediate Velocity Arch (IV Arch) in direction of the quasar PG 1259+593 (l=120,b=+58 deg). Absorption lines from CII, NI, NII, OI, AlII, SiII, PII, SII, ArI, FeII, and FeIII are used to study the atomic abundances in these two halo clouds at V_LSR=-130 km/s (Complex C) and V_LSR=-55 km/s (IV Arch). The OI/HI ratio provides the best measure of the overall metallicity in the diffuse interstellar medium, because ionization effects do not alter the ratio, and oxygen is at most only lightly depleted from the gas into dust grains. For Complex C, we find an oxygen abundance of 0.093 (+0.125, -0.047) solar, consistent with the idea that Complex C represents the infall of low metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV Arch is 0.98 (+1.21,-0.46) solar, which indicates a Galactic origin. We report the detection of an intermediate- velocity absorption component at +60 km/s that is not seen in HI 21cm emission. The clouds along the PG 1259+593 sight line have a variety of properties, proving that multiple processes are responsible for the creation and circulation of intermediate- and high-velocity gas in the Milky Way halo.

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arXiv2001

Observations of O VI Emission from the Diffuse Interstellar Medium

We report the first Far Ultraviolet Spectroscopic Explorer (FUSE) measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the general diffuse interstellar medium outside of supernova remnants or superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l = 315 and b = -41. From the observed intensities (2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic) photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively), derived equations, and assumptions about the source location, we calculate the intrinsic intensity, electron density, thermal pressure, and emitting depth. The intensities are too large for the emission to originate solely in the Local Bubble. Thus, we conclude that the Galactic thick disk and lower halo also contribute. High velocity clouds are ruled out because there are none near the pointing direction. The calculated emitting depth is small, indicating that the O VI-bearing gas fills a small volume. The observations can also be used to estimate the cooling rate of the hot interstellar medium and constrain models. The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2 P3/2 emission line at 1037 Angstroms and place upper limits on the intensities of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe III.

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arXiv2001

FUSE Observations of Interstellar Gas toward the SMC Star Sk 108

We discuss the interstellar absorption lines found in FUSE spectra of the Wolf-Rayet binary Sk 108, located in the northeastern part of the main ``bar'' of the Small Magellanic Cloud. The spectra cover the wavelength range 988-1187 Angstroms, at a resolution of about 12,000 and S/N of 20--40. We use detailed component information from higher resolution near-UV and optical spectra to model the far-UV lines of similarly distributed species. Both the Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, though a significant fraction of the SMC gas is ionized. The column densities of P II, S II, and Ar I are consistent with essentially solar ratios, relative to N(Zn II), in both the Galactic and SMC gas; the column density of N I remains somewhat uncertain. Molecular hydrogen is present in the Galactic gas, with properties similar to those found in low mean density Galactic lines of sight and in the Galactic gas toward several other LMC and SMC stars. We report a tentative detection of H_2 in the SMC gas for J = 1 and 3, with rotational level populations consistent with an excitation temperature of order 1000 K -- similar to the H_2 found in diffuse Galactic gas toward zeta Pup. Strong absorption from N III, S III, and Fe III has revealed a significant ionized component, particularly in the SMC; O VI is present, but relatively weak, especially in the Galactic gas. The N(C IV)/N(O VI) ratio varies somewhat within the SMC --- suggesting that several processes may contribute to the observed high ion abundances.

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arXiv2000

AB Dor in '94: I. HST/GHRS Observations of the Quiescent Chromosphere of an Active Star

We analyze HST/GHRS spectra of AB Doradus, the prototypical ultra-rapidly rotating K dwarf. We observed chromospheric (Mg II) and transition region (C II, Si IV, C IV, and N V) lines periodically throughout the stellar rotation period, and provide a low dispersion stellar atlas of 78 emission lines. The quiescent line profiles of the chromospheric and transition region lines show narrow cores superposed on very broad wings. The broad wings of the Mg II k & h lines and of the transition region lines can be explained by emission from gas co-rotating with the star and extending out to near the Keplerian co-rotation radius (2.8 stellar radii). While this is not a unique solution, it is consistent with previous studies of H-alpha emission that are naturally explained by large co-rotating prominences. We find no evidence for rotational modulation of the emission line fluxes. The density diagnostics suggest that the transition region is formed at constant pressure, with an electron density 2-3 E12 /cm^3 at a temperature of 30,000 K. The electron pressure is about 100 times larger than that for the quiet Sun. The emission measure distribution shows a minimum between log(T) = 5 and 5.5. The Mg II line exhibits three interstellar absorption components along the 15 pc line of sight. We identify the lowest velocity component with the G cloud, but the other components are not identified with any interstellar clouds previously detected from other lines of sight.

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arXiv2000

The FUSE Spectrum of PG0804+761: A Study of Atomic and MolecularGas in the Lower Galactic Halo and Beyond

We present an analysis of interstellar and intergalactic absorption lines in the FUSE spectrum of the low-redshift quasar PG0804+761 (z=0.100) at intermediate resolution (FWHM=25 km/s) in the direction l=138.3, b=31.0. With a good signal-to-noise ratio and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG0804+761 is dominated by strong absorption from local Galactic gas, we concentrate our study on absorption by molecular hydrogen and neutral neutral and ionized metals related to an intermediate-velocity cloud in the lower Galactic halo at -55 km/s, and on absorption from OVI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in 5 lines for rotational levels 0 and 1, leading to a total H_2 column density of log N = 14.71(+-0.30). We derive an OI gas-phase abundance for the IVC of 1.03(+0.71-0.42) solar. Absorption by OVI is found at velocities as negative as -110 km/s, but no absorption from any species is found at velocities of -180 km/ where absorption from the nearby high-velocity Complex A would be expected. We suggest that the extended OVI absorption traces hot gas above the Perseus spiral arm. Finally, we find intergalactic absorption by an intervening HI Ly betax absorber at z=0.019 and absorption by HI, CIII and OVI in an associated system at z=0.102. No intervening OVI absorbers are seen in the spectrum of PG0804+761.

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arXiv2000

FUSE Observations of Interstellar Gas Towards the LMC Star Sk -67 05

We report on measurements of interstellar O VI, H2, P II, Si II, Ar I, and Fe II absorption along the line of sight to Sk -67 05, a B0 Ia star in a diffuse H II region in the western edge of the Large Magellanic Cloud (LMC). We find log N(O VI) = 14.40 +/- 0.04 in the Milky Way (MW) component and, using the C IV column density from previous IUE observations, N(C IV) / N(O VI) = 1.00 +/- 0.16, a value similar to other halo measurements made with FUSE. In the LMC component, log N(O VI) = 13.89 +/- 0.05, and N(C IV) / N(O VI) < 0.4 (3 sigma), since only an upper limit on N(C IV) is available. Along this sightline the LMC is rich in molecular hydrogen, log N(H2) = 19.50 +/- 0.08; in the MW log N(H2) = 14.95 +/- 0.08. A two-component fit for the excitation temperature of the molecular gas in the LMC gives T_01 = 59 +/- 5 K for J=0,1 and T_ex = 800 +/- 330 K for J=3,4,5. For the MW, T_01 = 99 (+30/-20) K; no excitation temperature could be determined for the higher rotational states. The MW and LMC gas-phase [Fe/P] abundances are ~0.6 and ~0.7 dex lower, respectively, than solar system abundances. These values are similar to [Fe/Zn] measurements for the MW and LMC towards SN 1987A.

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arXiv2000

FUSE Spectroscopy of High Velocity Cloud Complex C

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the sightline toward the Seyfert 1 galaxy Markarian 876, which passes through high velocity cloud (HVC) complex C. This sight line demonstrates the ability of FUSE to measure ionic absorption lines in Galactic HVCs. High velocity absorption is clearly seen in both members of the O VI doublet. This is the first detection of O VI in a neutral hydrogen HVC. One component of HVC complex C is resolved in multiple Fe II lines from which we derive N(Fe II)/N(H I)=0.48 (Fe/H)_solar. This value of N(Fe II)/N(H I) implies that the metallicity of complex C along this sightline may be higher than that along the Mrk 290 sightline (0.1 solar) found by Wakker et al. (1999). On the other hand, if the metallicity of complex C is also 0.1 solar along this line of sight, the observed value of N(Fe II)/(N(H I) suggests there may be a significan t amount of H+ along the line of sight. In any case, little, if any, iron can be depleted into dust grains if the intrinsic metallicity of complex C is subsolar. Absorption from complex C is also seen in C II, N I, and N II, and upper limits based on non-detections can be determined for Ar I, P II, and Fe III. Although molecular hydrogen in the Milky Way is obvious in the FUSE data, no H_2 absorption is seen in the high velocity cloud to a limit N(H_2)<2.0x10^14 cm^-2. Future FUSE observations of extragalactic objects behind Galactic high velocity clouds will allow us to better constrain models of HVC origins.

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arXiv2000

FUSE Observations of the Galactic and Intergalactic Medium Towards H1821+643

We have obtained moderate resolution spectra of the bright QSO H1821+643 (z=0.297) in the wavelength interval 990-1185A with FUSE. Strong, complex OVI 1032 and FeII 1145 absorption arising in gas above several spiral arms and the outer warp of our Galaxy is detected. We have identified absorption by a high negative-velocity (-215 km/s) component of OVI, which corresponds to the limiting velocity of v_LSR = 190+/-20 km/s for Milky Way gas along this line of sight, assuming corotation of disk and halo gas and a flat rotation curve for the outer galaxy. We report the detection of four absorbers in the intergalactic medium, through detections of Lyman beta at z=0.2454 and z=0.1213, Lyman delta, epsilon, zeta in a system at z=0.2249, CIII 977 at z=0.1705, and OVI 1032 at z=0.12137. The FUSE data show that the Lyman absorbers at z=0.225 are composed of 2 components separated by 70 km/s. Finally, we have observed associated absorption from OIII 832, OIV 787, and possibly SV 786 at the redshift of the QSO (z= 0.297). NeVIII 770,780 is not detected in this associated system. When combined with the previous detection of associated OVI 1032 (Savage et al, 1998), we conclude that this system is a multiphase absorber, and we discuss the origin of the absorption in this context.

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arXiv2000

FUSE Observations of Molecular Hydrogen in Translucent Interstellar Clouds: The Line of Sight Toward HD 73882

We report the results of initial FUSE observations of molecular hydrogen (H2) in translucent clouds. These clouds have greater optical depth than any of the diffuse clouds previously observed for far-UV H2 absorption, and provide new insights into the physics and chemistry of such regions. Our initial results involve observations of HD 73882, a well-studied southern hemisphere star lying behind substantial interstellar material (E(B-V) = 0.72; A_V = 2.44). We find a total H2 column density, N(H2) = 1.2 x 10^{21} cm^{-2}, about three times larger than the values for diffuse clouds previously measured in the far-UV. The gas kinetic temperature indicated by the ratio N(J=1)/N(J=0) is 58 +/- 10 K. With the aid of ground-based data to calculate an appropriate multi-component curve of growth, we have determined column densities for all rotational levels up to J = 7. The J >= 2 states can be reasonably fitted with a rotational excitation temperature of 307 +/- 23 K. Both the kinetic and rotational temperatures are similar to those found in previous investigations of diffuse clouds. The ratios of carbonaceous molecules to hydrogen molecules are also similar to ratios in diffuse clouds, suggesting a similar chemistry for this line of sight.

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arXiv2000

FUSE Observations of O VI Absorption in the Galactic Halo

Far-ultraviolet spectra of 11 AGNs observed by FUSE are analyzed to obtain measures of O VI 1031.93 A absorption occurring over very long paths through Milky Way halo gas. Strong O VI absorption is detected along 10 of 11 sight lines. Values of log [N(O VI) sin|b|] range from 13.80 to 14.64 with a median value of 14.21. The observations reveal the existence of a widespread but irregular distribution of O VI in the Milky Way halo. Combined with estimates of the O VI mid-plane density, n0 = 2x10^(-8) cm^(-3), from the Copernicus satellite, the FUSE observations imply an O VI exponential scale height of 2.7+/-0.4 kpc. We find that N(C IV)/N(O VI) ranges from 0.15 in the disk to 0.6 along four extragalactic sight lines. The changing ionization state of the gas from the disk to the halo is consistent with a systematic decrease in the scale heights of Si IV, C IV, N V, to O VI from ~5.1 to ~2.7 kpc. While conductive heating models can account for the highly ionized atoms at low |z|, a combination of models (and processes) appears to be required to explain the highly ionized atoms found in the halo. The greater scale heights of Si IV and C IV compared to O VI suggest that some of the Si IV and C IV in the halo is produced in turbulent mixing layers or by photoionization by hot halo stars or the extragalactic background.

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arXiv2000

FUSE Observations of O VI in High Velocity Clouds

We have used moderate-resolution (FWHM ~ 25 km/sec) spectra of AGNs and QSOs observed by the Far Ultraviolet Spectroscopic Explorer (FUSE) to make the first definitive measurements of absorption by hot gas in high velocity clouds (HVCs) at large distances from the Galactic plane. Seven of the 11 sight lines studied exhibit high velocity (|V(LSR)| > 100 km/sec) O VI 1031.93 A absorption with log N(O VI) = 13.79-14.62. High velocity O VI absorption is detected in the distant gas of H I HVC Complex C, the Magellanic Stream, several HVCs believed to be in the Local Group, and the outer Galaxy. The fraction of O VI in HVCs along the seven sight lines containing high velocity O VI averages about 30 percent, with a full range of 10-70 percent. The O VI detections imply that hot (log T(K) = 5-6) collisionally-ionized gas is an important constituent of the HVCs since O VI is difficult to produce by photoionization unless the path lengths over which the absorption occurs are very large (>100 kpc). The association of O VI with H I HVCs in many cases suggests that the O VI may be produced at interfaces or mixing layers between the H I clouds and hot, low density gas in the Galactic corona or Local Group. Alternatively, the O VI may originate within cooling regions of hot gas clouds as they are accreted onto the Galaxy.

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arXiv2000

FUSE Observations of Diffuse Interstellar Molecular Hydrogen

We describe a moderate-resolution FUSE mini-survey of H2 in the Milky Way and Magellanic Clouds, using four hot stars and four AGN as background sources. FUSE spectra of nearly every stellar and extragalactic source exhibit numerous absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. One extragalactic sightline, PKS 2155-304, with low N(HI) shows no detectable H2 and could be the "Lockman Hole" of molecular gas, of importance for QSO absorption-line studies. We measure H2 column densities in low rotational states (J = 0 and 1) to derive rotational and/or kinetic temperatures of diffuse interstellar gas. The higher-J abundances can constrain models of the UV radiation fields and gas densities. In three optically thick clouds toward extragalactic sources, we find n(H) ~ 30-50 cm(-3) and cloud thicknesses of 2-3 pc. The rotational temperatures for H2 at high Galactic latitude, = 107 +/- 17 K (seven sightlines) and 120 +/- 13 K (three optically thick clouds), are higher than those in the Copernicus sample composed primarily of targets in the disk. We find no evidence for great differences in the abundance or state of excitation of H2 between sight lines in the Galaxy and those in the SMC and LMC. In the future, we will probe the distribution and physical parameters of diffuse molecular gas in the disk and halo and in the lower-metallicity environs of the LMC and SMC.

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