arXiv subjects

E. B. Jenkins

Publications and source records attributed to E. B. Jenkins.

47 records · Page 3Linked to original sources

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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arXiv2000

FUSE Observations of the Low-Redshift Lyman-beta Forest

We describe a moderate-resolution (20-25 km/s) FUSE study of the low-redshift intergalactic medium. We report on studies of 7 extragalactic sightlines and 12 Ly-beta absorbers that correspond to Ly-alpha lines detected by HST/GHRS and STIS. These absorbers appear to contain a significant fraction of the low-z baryons and were a major discovery of the HST spectrographs. Using FUSE data, with 40 mA (4-sigma) Lyb detection limits, we have employed the equivalent width ratio of Lyb/Lya and occasionally higher Lyman lines, to determine the doppler parameter, b, and accurate column densities, N(HI), for moderately saturated lines. We detect Lyb absorption corresponding to all Lya lines with EW > 200 mA. The Lyb/Lya ratios yield a preliminary distribution function of doppler parameters, with mean = 31.4 +/- 7.4 km/s and median b = 28 km/s, comparable to values at redshifts z = 2.0-2.5. If thermal, these b-values correspond to T(HI) ~ 50,000 K, although the inferred doppler parameters are considerably less than the widths derived from Lya profile fitting, = 0.52. The typical increase in column density over that derived from profile fitting is Delta[log N(HI)] = 0.3, but ranges up to 1.0 dex. Our data suggest that the low-z Lya absorbers contain sizable non-thermal motions or velocity components in the line profile, perhaps arising from cosmological expansion and infall.

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arXiv2000

The Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of N, O and Ar toward White Dwarf Stars

FUSE spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495 and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I and Ar I in the far ultraviolet, providing new insights on the origin of the partial ionization of the Local Interstellar Medium (LISM), and for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N whose ionizations are coupled to that of H via charge exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium is maintained by a strong EUV flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.

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arXiv2000

Intervening O VI Quasar Absorption Systems at Low Redshift: A Significant Baryon Reservoir

FUV echelle spectroscopy of the radio-quiet QSO H1821+643 (z = 0.297), obtained with the Space Telescope Imaging Spectrograph (STIS) at 7 km/s resolution, reveals 4 definite O VI absorption line systems and one probable O VI absorber at 0.15 < z(abs) < 0.27. The four definite O VI absorbers are located near galaxies and are highly displaced from the quasar in redshift; these are likely intervening systems unrelated to the background QSO. In the case of the strong O VI system at z(abs) = 0.22497, multiple components are detected in Si III and O VI as well as H I Lyman series lines, and the differing component velocity centroids and b-values firmly establish that this is a multiphase absorption system. A weak O VI absorber is detected at z(abs) = 0.22637, i.e., offset by ~340 km/s from the z(abs) = 0.22497 system. Lya absorption is detected at z(abs) = 0.22613, but no Lya absorption is significantly detected at 0.22637. Other weak O VI absorbers at z(abs) = 0.24531 and 0.26659 and the probable O VI system at 0.21326 have widely diverse O VI/H I column density ratios with N(O VI)/N(H I) ranging from <=0.14+/-0.03 to 5.2+/-1.2. The number density of O VI absorbers with rest equivalent width > 30 mA in the H1821+643 spectrum is remarkably high, dN/dz ~48, with a high (90%) confidence that it is greater than 17. We conservatively estimate that the cosmological mass density of the O VI systems Omega_b(O VI) > 0.0008/h_75. With an assumed metallicity of 1/10 solar and a conservative assumption that the fraction of oxygen in the O VI ionization stage is 0.2, we obtain Omega_b(O VI) > 0.004/h_75. This is comparable to the combined cosmological mass density of stars and cool gas in galaxies and X-ray emitting gas in galaxy clusters at low redshift.

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arXiv1999

STIS Observations of He II Gunn-Peterson Absorption Toward Q0302-003

The ultraviolet spectrum (1145--1720A) of the distant quasar Q 0302--003 (z=3.286) was observed at 1.8A resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson HeII absorption trough, produced by HeII Lya, along the line of sight over the redshift range z=2.78-3.28. Its interpretation was facilitated by modeling based on Keck HIRES spectra of the HI Lya forest (provided by A. Songaila and by M. Rauch and W. Sargent). We find that near the quasar HeII Lya absorption is produced by discrete clouds, with no significant diffuse gas; this is attributed to a HeII "proximity effect" in which the quasar fully ionizes He in the diffuse intergalactic medium, but not the He in denser clouds. By two different methods we calculate that the average HeII Lya opacity at z~3.15 is tau >= 4.8. In the Dobrzycki-Bechtold void in the HI Lya forest near z=3.18, the average HeII opacity tau=4.47^{+0.48}_{-0.33}. Such large opacities require the presence of a diffuse gas component as well as a soft UV background spectrum, whose softness parameter, defined as the ratio of the photo-ionization rate in HI over the one in HeII, S=Gamma^J_HI/Gamma^J_HeII~=800, indicating a significant stellar contribution. At z=3.05, there is a distinct region of high HeII Lya transmission which most likely arises in a region where helium is doubly ionized by a discrete local source, quite possibly an AGN. At redshifts z<2.87, the HeII Lya opacity detected by STIS, tau=1.88, is significantly lower than at z>3. Such a reduction in opacity is consistent with Songaila's (1998) report that the hardness of the UV background spectrum increases rapidly from z=3 to z=2.9.

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arXiv1999

The Emergence of the Modern Universe: Tracing the Cosmic Web

This is the report of the Ultraviolet-Optical Working Group (UVOWG) commissioned by NASA to study the scientific rationale for new missions in ultraviolet/optical space astronomy approximately ten years from now, when the Hubble Space Telescope (HST) is de-orbited. The UVOWG focused on a scientific theme, The Emergence of the Modern Universe, the period from redshifts z = 3 to 0, occupying over 80% of cosmic time and beginning after the first galaxies, quasars, and stars emerged into their present form. We considered high-throughput UV spectroscopy (10-50x throughput of HST/COS) and wide-field optical imaging (at least 10 arcmin square). The exciting science to be addressed in the post-HST era includes studies of dark matter and baryons, the origin and evolution of the elements, and the major construction phase of galaxies and quasars. Key unanswered questions include: Where is the rest of the unseen universe? What is the interplay of the dark and luminous universe? How did the IGM collapse to form the galaxies and clusters? When were galaxies, clusters, and stellar populations assembled into their current form? What is the history of star formation and chemical evolution? Are massive black holes a natural part of most galaxies? A large-aperture UV/O telescope in space (ST-2010) will provide a major facility in the 21st century for solving these scientific problems. The UVOWG recommends that the first mission be a 4m aperture, SIRTF-class mission that focuses on UV spectroscopy and wide-field imaging. In the coming decade, NASA should investigate the feasibility of an 8m telescope, by 2010, with deployable optics similar to NGST. No high-throughput UV/Optical mission will be possible without significant NASA investments in technology, including UV detectors, gratings, mirrors, and imagers.

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arXiv1999

Deuterium abundances

We discuss the measurements of deuterium abundances in high redshift quasar absorbers, in the solar system and in the interstellar medium. We present new results that indicate spatial variations of the deuterium abundance in the interstellar medium at the level of 50% over scales possibly as small as 10 pc, and discuss plausible causes for the origin of these variations.

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arXiv1997

UV Absorption Lines from High-Velocity Gas in the Vela Supernova Remnant: New insights from STIS Echelle Observations of HD72089

The star HD72089 is located behind the Vela supernova remnant and shows a complex array of high and low velocity interstellar absorption features arising from shocked clouds. A spectrum of this star was recorded over the wavelength range 1196.4 to 1397.2 Angstroms at a resolving power lambda/Delta lambda = 110,000 and signal-to-noise ratio of 32 by STIS on the Hubble Space Telescope. We have identified 7 narrow components of C I and have measured their relative populations in excited fine-structure levels. Broader features at heliocentric velocities ranging from -70 to +130 km/s are seen in C II, N I, O I, Si II, S II and Ni II. In the high-velocity components, the unusually low abundances of N I and O I, relative to S II and Si II, suggest that these elements may be preferentially ionized to higher stages by radiation from hot gas immediately behind the shock fronts.

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