arXiv subjects

Edward B. Jenkins

Publications and source records attributed to Edward B. Jenkins.

At least 73 records · Page 4Linked to original sources

arXiv2005

A Near-Solar Metallicity, Nitrogen-Deficient Lyman Limit Absorber Associated with two S0 Galaxies

From UV spectra of the bright quasar PHL 1811 recorded by FUSE and the E140M configuration on STIS, we have determined the abundances of various atomic species in a Lyman limit system at z = 0.0809 with log N(H I) = 17.98. Considerably more hydrogen may be in ionized form, since the abundances of C II, Si II, S II and Fe II are very large compared to that of O I, when compared to their respective solar abundance ratios. Our determination [O/H] = -0.19 in the H I-bearing gas indicates that the chemical enrichment of the gas is unusually high for an extragalactic QSO absorption system. However, this same material has an unusually low abundance of nitrogen, [N/O] < -0.59, indicating that there may not have been enough time during this enrichment for secondary nitrogen to arise from low and intermediate mass stars. In an earlier investigation we found two galaxies at nearly the same redshift as this absorption system and displaced by 34 and 87 kpc from the line of sight. An r-band image recorded by the ACS on HST indicates these are S0 galaxies. One or both of these galaxies may be the source of the gas, which might have been expelled in a fast wind, by tidal stripping, or by ram-pressure stripping. Subtraction of the ACS point-spread function from the image of the QSO reveals the presence of a face-on spiral galaxy under the glare of the quasar; although it is possible that this galaxy may be responsible for the Lyman limit absorption, the exact alignment of the QSO with the center of the galaxy suggests that the spiral is the quasar host.

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arXiv2004

Discovery of a Primitive Damped Lyman alpha Absorber Near an X-ray Bright Galaxy Group in the Virgo Cluster

We present a new UV echelle spectrum of PG1216+069, obtained with HST+STIS, which reveals damped Lya (DLA) absorption as well as O I, C II, Si II, and Fe II absorption lines at z(abs) = 0.00632 near the NGC4261 group. The absorber shows no evidence of highly-ionized gas, which places constraints on "warm-hot" missing baryons in the NGC4261 group. The well-developed damping wings of the Lya line tightly constrain the H I column density; we find log N(H I) = 19.32+/-0.03. The metallicity of this sub-DLA is remarkably low, [O/H] = -1.60^{+0.09}_{-0.11}, which is comparable to many analogous high-redshift systems, and the iron abundance indicates that this absorber contains little or no dust. Nitrogen is underabundant; we detect neither N I or N II, and we show that this is not due to ionization effects but rather indicates that [N/O] < -0.28 (3sigma). Despite the proximity to NGC4261 group, there are no bright galaxies close to the sight line at the absorption redshift. The nearest known galaxy is a sub-L* galaxy with a projected distance rho = 86 kpc; the closest L* galaxy is NGC4260 at rho = 246 kpc. The low metallicity and [N/O] indicate that this low-z sub-DLA is a relatively primitive gas cloud. We consider the nature and origin of the sub-DLA, and we find several possibilities. The properties of the sub-DLA are similar to those of the interstellar media in blue compact dwarf galaxies and are also reminiscent of Milky Way HVCs. Or, the object could simply be a small dark-matter halo, self-enriched by a small amount of internal star formation but mostly undisturbed since its initial formation. In this case, the small halo would likely be an ancient building block of galaxy formation that formed before the epoch of reionization.

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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

PHL 1811: The Local Prototype of the Lineless High-z SDSS QSOs

Abridged) In the SDSS, several unusual QSOs have been discovered that have very blue rest-frame UV spectra but no discernible emission lines. Their UV spectra strongly resemble that of the newly discovered quasar PHL 1811 ($z=0.192$; $\rm M_{V}=-25.9$). With magnitudes of $B = 14.4$ and $R = 14.1$, PHL 1811 is the second brightest quasar known with $z>0.1$ after 3C 273. Optically it is classified as a Narrow-line Seyfert 1 galaxy (NLS1). Objects of this class are generally strong soft X-ray emitters, but a BeppoSAX observation of PHL 1811 showed that it is anomalously X-ray weak. The inferred $α_{ox}$ was 1.9--2.1, much steeper than the nominal value of 1.6 for quasars of this optical luminosity, and comparable to the X-ray weakest quasars. Follow-up {\it Chandra} observations reveal a variable, unabsorbed X-ray spectrum and confirm that it is intrinsically X-ray weak.

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arXiv2003

Interstellar Atomic Abundances

A broad array of interstellar absorption features that appear in the ultraviolet spectra of bright sources allows us to measure the abundances and ionization states of many important heavy elements that exist as free atoms in the interstellar medium. By comparing these abundances with reference values in the Sun, we find that some elements have abundances relative to hydrogen that are approximately consistent with their respective solar values, while others are depleted by factors that range from a few up to around 1000. These depletions are caused by the atoms condensing into solid form onto dust grains. Their strengths are governed by the volatility of compounds that are produced, together with the densities and velocities of the gas clouds. We may characterize the depletion trends in terms of a limited set of parameters; ones derived here are based on measurements of 15 elements toward 144 stars with known values of N(H I) and N(H2). In turn, these parameters may be applied to studies of the production, destruction, and composition of the dust grains. The interpretations must be done with care, however, since in some cases deviations from the classical assumptions about missing atoms in unseen ionization stages can create significant errors. Our experience with the disk of our Galaxy offers important lessons for properly unravelling results for more distant systems, such as high- and intermediate-velocity clouds in the Galactic halo, material in the Magellanic Stream and Magellanic Clouds, and otherwise invisible gas systems at large redshifts (detected by absorption features in quasar spectra). Inferences about the total (gas plus dust) abundances of such systems offer meaningful information on their origins and/or chemical evolution.

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arXiv2003

Absorption-Line Systems and Galaxies in Front of the Second Brightest Quasar, PHL 1811

The extraordinarily bright quasar PHL 1811 at a redshift z = 0.192 provides an attractive opportunity to use ultraviolet absorption-line spectroscopy to study the properties of gas systems in the local universe. An R = 20,000 far-UV spectrum recorded by FUSE revealed 7 extragalactic absorption systems, one of which is a Lyman limit system at z = 0.08093 accompanied by three systems having redshifts which differ from it by less than 0.008. The abundance of O with respect to Fe in the Lyman limit system is not much different from the solar abundance ratio. Supplementary low resolution spectra recorded by STIS (on HST) at longer wavelengths helped to substantiate our identifications of systems in the FUSE spectrum and suggested the presence of an additional 4 systems that could be detected only through their Ly-alpha features. Spectroscopy at visible wavelengths of 7 galaxies within approximately 2' of PHL 1811 indicated that 2 of them are near the redshift of the quasar and 4 have redshifts within 850 km/s of the extragalactic absorption systems. The Lyman limit system is likely associated with an L* galaxy lying 23" from the sightline. Finally, in addition to prominent features at very low velocities arising from the disk of our Galaxy, the strong resonance transitions of C II and Mg II show evidence for material at v = -200 km/s; the column densities of these two species suggest that 17.7 < log N(H I) < 18.1 if the material has a solar composition.

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arXiv2003

Evidence and Implications of Pressure Fluctuations in the ISM

A recent survey of the fine-structure excitation of neutral carbon reveals that the interstellar medium in the Galactic plane exhibits a thermal pressure, nT/k, that ranges from about 10^3 to 10^4 cm{-3}K from one location to the next, with occasional excursions in excess of about 10^5 cm^{-3}K. The large excitations for small amounts of gas indicate that some regions are either subjected to shocks or must be pressurized within time scales much shorter than the time needed to reach thermal equilibrium. These rapid fluctuations probably arise from the cascade of macroscopic mechanical energy to small scales through a turbulent cascade. One consequence of this effect is that changes in gas temperature can arise from near adiabatic compressions and expansions, and this may explain why investigations of 21-cm emission and absorption reveal the presence of hydrogen at temperatures well below the expected values derived from the balance of various known heating and cooling processes.

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arXiv2002

Comparative Absorption and Emission Abundance Analyses of Nebulae: Ion Emission Densities for IC 418

Recent analyses of nebular spectra have resulted in discrepant abundances from CNO forbidden and recombination lines. We consider independent methods of determining ion abundances for emission nebulae, comparing ion emission measures with column densities derived from resonance absorption lines viewed against the central star continuum. Separate analyses of the nebular emission lines and the stellar UV absorption lines yield independent abundances for ions, and their ratio can be expressed in terms of a parameter n_e_{em}, the ``emission density'' for each ion. Adequate data for this technique are still scarce, but separate analyses of spectra of the planetary nebula and central star of IC 418 do show discrepant abundances for several ions, especially Fe II. The discrepancies are probably due to the presence of absorbing gas which does not emit and/or to uncertain atomic data and excitation processes, and they demonstrate the importance of applying the technique of combining emission- and absorption-line data in deriving abundances for nebulae.

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arXiv2002

Thermal Pressures in Neutral Clouds inside the Local Bubble, as Determined from C I Fine-Structure Excitations

High-resolution spectra covering absorption features from interstellar C I were recorded for four early-type stars with spectrographs on HST, in a program to measure the fine-structure excitation of this atom within neutral clouds inside or near the edge of the Local Bubble, a volume of hot (T ~ 10^6 K) gas that emits soft x-rays and extends out to about 100 pc away from the Sun. The excited levels of C I are populated by collisions, and the ratio of excited atoms to those in the ground level give a measure of the local thermal pressure. Absorptions from the two lowest levels of C I were detected toward alpha Del and delta Cyg, while only marginal indications of excited C I were obtained for gamma Ori, and lambda Lup. Along with temperature limits derived by other means, the C I fine-structure populations indicate that for the clouds in front of gamma Ori, delta Cyg and alpha Del, 10^3 < p/k < 10^4 cm^{-3}K at about the +-1 sigma confidence level in each case. The results for lambda Lup are not as well constrained, but still consistent with the other three stars. The results indicate that the thermal pressures are below generally accepted estimates p/k > 10^4 cm^{-3}K for the Local Bubble, based on the strength of x-ray and EUV emission from the hot gas. This inequality of pressure for these neutral clouds and their surroundings duplicates a condition that exists for the local, partly-ionized cloud that surrounds the Sun. An appendix in the paper describes a direct method for determining and eliminating small spectral artifacts arising from variations of detector sensitivity with position.

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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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arXiv2001

A FUSE Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution FUSE survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) >= 10^14 cm^-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 (+0.005, -0.002) for the SMC and = 0.012 (+0.006, -0.003) for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lya in pencil-beam sight lines as measures of N(HI). These results imply that the diffuse H2 masses of the LMC and SMC are 8 x 10^6 Msun and 2 x 10^6 Msun, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 x 10^-18 cm^3 s^-1, and incident radiation fields ranging from 10 - 100 times the Galactic mean value. We find that these high-radiation, low-formation-rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive a mean kinetic and/or rotational temperature = 82 +/- 21 K for clouds with N(H2) >= 10^16 cm^-2, similar to Galactic gas. We discuss the implications of this work for theories of star formation in low-metallicity environments. [Abstract abridged]

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arXiv2001

The Distribution of Thermal Pressures in the Interstellar Medium

It is generally recognized that the interstellar medium has a vast range of densities and temperatures. While these two properties are usually anticorrelated with each other, there are nevertheless variations in their product, i.e., the thermal gas pressure divided by the Boltzmann constant k. In neutral gas, the relative populations of neutral carbon atoms in the excited fine-structure states can give a direct measure of a local thermal pressure. A picture of the distribution function for thermal pressures in H I regions is now arising from a survey of interstellar C I absorption features in the UV spectra of 21 early-type stars, observed with a wavelength resolving power of 200,000 by the STIS instrument on the Hubble Space Telescope. Most of the gas is within the range 1000 < p/k < 10,000 cm^{-3}K, but there is also evidence for some of the material being at much higher pressures, i.e., p/k > 10^5 cm^{-3}K. While the fraction of gas at these elevated pressures is quite small, it seems nearly ubiquitous. This phenomenon may arise from small-scale, short-lived density enhancements that are produced by converging flows of material in supersonic turbulence.

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arXiv2001

The Hot Phase in the Galactic Disk

Spectra of about 150 early-type stars in the disk of the Milky Way recorded by the Far Ultraviolet Spectroscopic Explorer (FUSE) reveal absorption features from interstellar O VI, a tracer of collisionally ionized gases at temperatures near 300,000 K. (Hotter material is better sensed by its diffuse x-ray emission.) The FUSE survey indicates that hot gas in the plane of the Galaxy yields an average density n(O VI) = 1.7 X 10^{-8} cm^{-3}, a value consistent with earlier results from the Copernicus satellite, but at great distances there is a large dispersion in the measurements from one line of sight to another. This dispersion indicates that O VI absorbing regions probably have a broad distribution of sizes, perhaps giving a power-law behavior for column densities. Comparisons of O VI with other Li-like ions should give some guidance on the nature and origin of the O VI-absorbing regions, but poor correspondences in the velocity profile shapes will complicate the interpretations.

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arXiv2001

HST/STIS observations of the HeII Gunn-Peterson effect towards HE 2347-4342

We present an HST STIS spectrum of the HeII Gunn-Peterson effect towards HE2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al. (1997), the STIS spectrum has a much improved resolution. The 2-D detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the HeII G-P opacity in the high opacity regions. We use the STIS spectrum together with a Keck--HIRES spectrum covering the corresponding HI Lya forest to calculate a 1-D map of the softness S of the ionization radiation along the line of sight towards HE 2347-4342, where S is the ratio of the HI to HeII photoionization rates. We find that S is generally large but presents important variations, from S ~ 30 in the opacity gaps to a 1 sigma lower limit of 2300 at z~2.86, in a region which shows an extremely low HI opacity over a 6.5 A range. We note that a large S naturally accounts for most of the large SiIV to CIV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. (2000) to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al. (1997), we propose that the large softness observed at z~2.86 is due to the presence of bright soft sources close to the line of sight, i.e. for which the ratio between the number of HI to HeII ionizing photons reaching the IGM is large. We discuss these two models and suggest ways to discriminate between them.

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arXiv2001

Revealing the Warm-Hot Intergalactic Medium with OVI Absorption

Hydrodynamic simulations of growth of cosmic structure suggest that 30-50% of the total baryons at z=0 may be in a warm-hot intergalactic medium (WHIM) with temperatures ~10^5-10^7K. The O VI λλ1032, 1038 absorption line doublet in the FUV portion of QSO spectra provides an important probe of this gas. Utilizing recent hydrodynamic simulations, it is found that there should be ~5 O VI absorption lines per unit redshift with equivalent widths >= 35 mA, decreasing rapidly to ~0.5 per unit redshift at >= 350 mA. About 10% of the total baryonic matter or 20-30% of the WHIM is expected to be in the O VI absorption line systems with equivalent width >= 20 mA; the remaining WHIM gas may be too hot or have too low metallicity to be detected in O VI. We find that the simulation results agree well with observations with regard to the line abundance and total mass contained in these systems. Some of the O VI systems are collisionally ionized and some are photoionized, but most of the mass is in the collisionally ionized systems. We show that the gas that produces the O VI absorption lines does not reside in virialized regions such as galaxies, groups, or clusters of galaxies, but rather has an overdensity of 10-40 times the average density. These regions form a somewhat connected network of filaments. The typical metallicity of these regions is 0.1-0.3Zsun.

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arXiv2001

The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation

We used the smallest entrance aperture (0.03 arc-sec wide slit) and highest resolution echelle gratings (E140H and E230H) of STIS on HST to record the interstellar absorption features for 10 different multiplets of neutral carbon at a wavelength resolving power of 200,000 in the UV spectra of 21 early-type stars. We measured the amount of C I in each of its three fine-structure levels, so that we could determine nT of the absorbing gas and how much it varies in different regions. To obtain internally consistent results for all multiplets, we had to modify the transition f-values so that generally weak transitions were stronger than their published values. The fine-structure populations indicate that the median thermal pressure for our entire sample is p/k=2240 cm^-3 K, or slightly higher for temperatures that differ appreciably from 40 K. About 15% of the gas moving at peculiar velocities shows p/k > 10^4 cm^-3 K, and this fraction drops to 1.5% for quiescent gas. Often, there are small amounts of gas at p/k > 10^5 cm^-3 K. These ubiquitous wisps of high pressure material probably arise from small-scale density enhancements created by converging flows in a turbulent medium. The C I excitations indicate a barytropic index gamma_eff > 0.90 when the gas is compressed, which is larger than the value 0.72 expected for material in thermal equilibrium. This indicates that the regions have a dimension r < 0.01 pc which allows them to compress very quickly and nearly adiabatically. Appendices of this paper present evidence that STIS can indeed achieve R = 200,000, and that telluric oxygen absorption lines are broadened by macroscopic motions.

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arXiv2001

21-cm H I emission from the Damped Lyman-alpha absorber SBS 1543+593

We detect 21-cm emission from the Low Surface Brightness (LSB) galaxy SBS 1543+593, which gives rise to a Damped Lyman-alpha (DLA) absorption line in the spectrum of the background QSO HS 1543+5921 (z=0.807). We obtain an accurate measure of the velocity of the H I gas in the LSB galaxy, v=2868 km/s, and derive a mass of 1.3e9 solar masses. We compare this value with limits obtained towards two other z~0.1 DLA systems, and show that SBS 1543+593 would not have been detected. Hence LSB galaxies similar to SBS 1543+593 can be responsible for DLA systems at even modest redshifts without being detectable from their 21-cm emission.

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arXiv2000

Local clouds: ionization, temperatures, electron densities and interfaces, from GHRS and IMAPS spectra of epsilon Canis Majoris

The composition and physical properties of several local clouds, including the Local Interstellar Cloud (LIC) in which the Sun is embedded, are derived from absorption features in the UV spectrum of the star epsilon CMa. We derive temperatures and densities for three components by combining our interpretations of the ionization balance of magnesium and the relative population of C II in an excited fine-structure level. We find that for the LIC n(e) = 0.12 +/-0.05 cm-3 and T = 7000 +/-1200 K. We derive the ionization fractions of hydrogen and discuss the ionizing processes. In particular the hydrogen and helium ionizations in the LIC are compatible with photoionization by the local EUV radiation fields from the hot stars and the cloud interface with the hot gas. We confirm the detection of high ionization species : Si III is detected in all clouds and C IV in two of them, including the LIC, suggesting the presence of ionized interfaces around the local clouds.

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