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Register a new userRecent FUSE Observations of Diffuse O VI Emission from the Interstellar Medium
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We present new results from our survey of diffuse O VI-emitting gas in the interstellar medium with the Far Ultraviolet Spectroscopic Explorer (FUSE). Background observations obtained since 2005 have yielded eleven new O VI detections of 3-sigma significance, and archival searches have revealed two more. An additional 15 sight lines yield interesting upper limits. Combined with previous results, these observations reveal the large-scale structure of the O VI-bearing gas in the quadrant of the sky centered on the Magellanic Clouds. The most prominent feature is a layer of low-velocity O VI emission extending more than 70 degrees from the Galactic plane. At low latitudes (|b| < 30 degrees), the emission comes from narrow, high-density conductive interfaces in the local ISM. At high latitudes, the emission is from extended, low-density regions in the Galactic halo. We also detect O VI emission from the interface region of the Magellanic System, a structure recently identified from H I observations. These are the first detections of emission from high-ionization species in the Magellanic System outside of the Clouds themselves.
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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.
We present a survey of diffuse O VI emission in the interstellar medium obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). Spanning 5.5 years of FUSE observations, from launch through 2004 December, our data set consists of 2925 exposures along 183 sight lines, including all of those with previously-published O VI detections. The data were processed using an implementation of CalFUSE v3.1 modified to optimize the signal-to-noise ratio and velocity scale of spectra from an aperture-filling source. Of our 183 sight lines, 73 show O VI 1032 emission, 29 at > 3-sigma significance. Six of the 3-sigma features have velocities |v_LSR| > 120 km/s, while the others have |v_LSR| < 50 km/s. Measured intensities range from 1800 to 9100 LU, with a median of 3300 LU. Combining our results with published O VI absorption data, we find that an O VI-bearing interface in the local ISM yields an electron density n_e = 0.2--0.3 cm^-3^ and a path length of 0.1 pc, while O VI-emitting regions associated with high-velocity clouds in the Galactic halo have densities an order of magnitude lower and path lengths two orders of magnitude longer. Though the O VI intensities along these sight lines are similar, the emission is produced by gas with very different properties.
NGC 6543 is one of the few planetary nebulae (PNe) whose X-ray emission has been shown to be extended and originate from hot interior gas. Using FUSE observations we have now detected nebular O VI emission from NGC 6543. Its central star, with an effective temperature of ~50,000 K, is too cool to photoionize O V, so the O VI ions must have been produced by thermal collisions at the interface between the hot interior gas and the cool nebular shell. We modeled the O VI emission incorporating thermal conduction, but find that simplistic assumptions for the AGB and fast wind mass loss rates overproduce X-ray emission and O VI emission. We have therefore adopted the pressure of the interior hot gas for the interface layer and find that expected O VI emission to be comparable to the observations.
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, <T_01> = 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.
A significant fraction of baryons in galaxies are in the form of diffuse gas of the circumgalactic medium (CGM). One critical component of the multi-phases of CGM, the so-called coronal warm-hot phase gas ($rm 10^{5}-10^{6}$ K) traced by O VI 1031.93, 1037.62 r{A} resonance lines, has rarely been detected in emission from galaxy halos other than Milky Way. Here we report four additional detections of O VI emission gas in the halos of nearby edge-on galaxies, NGC 4631 and NGC 891, using archival Far Ultraviolet Spectroscopic Explorer data and an updated data pipeline. We find the most intense O VI emission to be from fields forming a vertical line near the center of NGC 4631, despite the close proximity to the disk of two other fields. The detected O VI emission surface brightness are about 1.1$pm 0.3$ $times$ $10^{-18}$ to 3.9$pm0.8$ $times$ $10^{-18}$ ergs s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. The spatial distribution of the five 30 $times$ 30 O VI detection fields in NGC 4631 can be interpreted as the existence of filamentary structures of more intense O VI emission superimposed within a diffuse and faint O VI halo in star-forming galaxies. Volume-filled O VI emission mapping is greatly needed to determine the structure and prevalence of warm-hot gas and the role it plays in the cycling of gas between the galaxy disk and the halo. Finally, we present the sensitivity of future funded and proposed UV missions (LUVOIR-A, LUVOIR-B, CETUS, and Aspera) to the detection of diffuse and faint O VI emission in nearby galaxy halos.
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