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Register a new userThe FUSE survey of OVI absorption in and near the Galaxy
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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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We outline the results from a FUSE Team program designed to characterize OVI absorption in the disk of the Milky Way. We find that OVI absorption occurs throughout most of the Galactic plane, at least out to several kpc from the Sun, and that it is distributed smoothly enough for the column density to decline with height above the disk and with distance in the plane. However, the OVI absorbing gas is clumpy, and moves at peculiar velocities relative to that expected from Galactic rotation. We conclude that the observed absorption is likely to be a direct indicator of the structures formed when violent, dynamical processes heat the ISM, such as blowout from multiple supernovae events.
We report the results of a survey of OVI 1032 absorption along the lines of sight to 25 white dwarfs in the local interstellar medium (LISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). We find that interstellar OVI absorption along all sightlines is generally weak, and in a number of cases, completely absent. No OVI absorption was detected with significance greater than 2 sigma for 12 of the 25 stars, where the 1 sigma uncertainty is 4 mA, equivalent to an OVI column density of ~3 x 10^12 cm^-2. Of the remaining stars, most have column densities N(OVI) < 10^13 cm^-2 and no column densities exceed 1.7 x 10^13 cm^-2. For lines of sight to hot (T_eff > 40,000 K) white dwarfs, there is some evidence that the OVI absorption may be at least partially photospheric or circumstellar in origin. We interpret the patchy distribution of OVI absorption in terms of a model where OVI is formed in evaporative interfaces between cool clouds and the hot, diffuse gas in the Local Bubble (LB). If the clouds contain tangled or tangential magnetic fields, then thermal conduction will be quenched over most of the cloud surface, and OVI will be formed only in local patches where conduction is allowed to operate. We find an average OVI space density in the LISM of 2.4 x 10^-8 cm^-3, which is similar to, or slightly larger than, the value in the Galactic disk over kpc scales. This local density implies an average OVI column density of ~7 x 10^12 cm^-2 over a path length of 100 pc within the LB. The OVI data presented here appears to be inconsistent with the model proposed by Breitschwerdt & Schmutzler (1994), in which highly ionized gas at low kinetic temperature (~50,000 K) permeates the LB. Our survey results are consistent with the supernova-driven cavity picture of Cox & Smith (1974).
We describe the survey for galaxies in the fields surrounding 9 sightlines to far-UV bright, z~1 quasars that define the COS Absorption Survey of Baryon Harbors (CASBaH) program. The photometry and spectroscopy that comprise the dataset come from a mixture of public surveys (SDSS, DECaLS) and our dedicated efforts on private facilities (Keck, MMT, LBT). We report the redshifts and stellar masses for 5902 galaxies within ~10 comoving-Mpc (cMpc) of the sightlines with a median of z=0.28 and M_* ~ 10^(10.1) Msun. This dataset, publicly available as the CASBaH specDB, forms the basis of several recent and ongoing CASBaH analyses. Here, we perform a clustering analysis of the galaxy sample with itself (auto-correlation) and against the set of OVI absorption systems (cross-correlation) discovered in the CASBaH quasar spectra with column densities N(O^+5) >= 10^(13.5)/cm^2. For each, we describe the measured clustering signal with a power-law correlation function xi(r) = (r/r_0)^(-gamma) and find that (r_0,gamma) = (5.48 +/- 0.07 h_100^-1 Mpc, 1.33 +/- 0.04) for the auto-correlation and (6.00 +/- 1 h^-1 Mpc, 1.25 +/- 0.18) for galaxy-OVI cross-correlation. We further estimate a bias factor of b_gg = 1.3 +/- 0.1 from the galaxy-galaxy auto-correlation indicating the galaxies are hosted by halos with mass M_halo ~ 10^(12.1 +/- 0.05) Msun. Finally, we estimate an OVI-galaxy bias factor b_OVI = 1.0 +/- 0.1 from the cross-correlation which is consistent with OVI absorbers being hosted by dark matter halos with typical mass M_halo ~ 10^(11) Msun. Future works with upcoming datasets (e.g., CGM^2) will improve upon these results and will assess whether any of the detected OVI arises in the intergalactic medium.
We present far-ultraviolet spectra of the Seyfert 1 galaxy Mrk 509 obtained in 1999 November with the Far Ultraviolet Spectroscopic Explorer (FUSE). Our data span the observed wavelength range 915-1185 A at a resolution of ~20 km/s. The spectrum shows a blue continuum, broad OVI 1032,1038 emission, and a broad CIII 977 emission line. Superposed on these emission components, we resolve associated absorption lines of OVI 1032,1038, CIII 977, and Lyman lines through Lzeta. Seven distinct kinematic components are present, spanning a velocity range of -440 to +170 km/s relative to the systemic velocity. The absorption is clustered in two groups, one centered at -370m km/s and another at the systemic velocity. The blue-shifted cluster may be associated with the extended line emission visible in deep images of Mrk 509 obtained by Phillips et al. Although several components appear to be saturated, they are not black at their centers. Partial covering or scattering permits ~7% of the broad-line or continuum flux to be unaffected by absorption. Of the multiple components, only one has the same ionization state and column density as highly ionized gas that produces the OVII and OVIII ionization edges in X-ray spectra of Mrk 509. This paper will appear in a special issue of Astrophysical Journal Letters devoted to the first scientific results from the FUSE mission.
To probe the distribution and physical characteristics of interstellar gas at temperatures T ~ 3e5 K in the disk of the Milky Way, we have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to observe absorption lines of OVI toward 148 early-type stars situated at distances 1 kpc. After subtracting off a mild excess of OVI arising from the Local Bubble, combining our new results with earlier surveys of OVI, and eliminating stars that show conspicuous localized X-ray emission, we find an average OVI mid-plane density n_0 = 1.3e-8 cm^-3. The density decreases away from the plane of the Galaxy in a way that is consistent with an exponential scale height of 3.2 kpc at negative latitudes or 4.6 kpc at positive latitudes. Average volume densities of OVI along different sight lines exhibit a dispersion of about 0.26 dex, irrespective of the distances to the target stars. This indicates that OVI does not arise in randomly situated clouds of a fixed size and density, but instead is distributed in regions that have a very broad range of column densities, with the more strongly absorbing clouds having a lower space density. Line widths and centroid velocities are much larger than those expected from differential Galactic rotation, but they are nevertheless correlated with distance and N(OVI), which reinforces our picture of a diverse population of hot plasma regions that are ubiquitous over the entire Galactic disk. The velocity extremes of the OVI profiles show a loose correlation with those of very strong lines of less ionized species, supporting a picture of a turbulent, multiphase medium churned by shock-heated gas from multiple supernova explosions.
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