arXiv subjects

Todd M. Tripp

Publications and source records attributed to Todd M. Tripp.

88 records · Page 5Linked to original sources

arXiv2003

The Physical Conditions and Metal Enrichment of Low-Redshift Interstellar and Intergalactic Media: The Benefits of High-Resolution Ultraviolet Spectra

To underscore the value of high spectral resolution for the study of low-z QSO absorption lines, we briefly present Space Telescope Imaging Spectrograph echelle data that demonstrate how increased resolution leads to dramatic improvement in line measurements. We show that even R = lambda/Delta lambda = 20,000 is insufficient for some measurements. The higher the resolution the better, but R ~ 50,000 is adequate for many outstanding questions about the IGM and the ISM of galaxies that can be probed using QSO absorption lines.

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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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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

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

The Ionization and Metallicity of the Intervening O VI Absorber at z = 0.1212 in the Spectrum of H1821+643

We use high-resolution UV spectra of the radio-quiet QSO H1821+643 (z_em = 0.297), obtained with STIS and FUSE, to study the ionization and metallicity of an intervening O VI absorption line system at z(abs) = 0.1212. This absorber has the following notable properties: (1) Several galaxies are close to the sight line at the absorber redshift, including an actively star-forming galaxy at a projected distance of 144 kpc. (2) There is a complex cluster of H I lya absorption lines near the O VI redshift, including at least five components spread over a velocity range of ~700 km/s . (3) The strongest lya line in the cluster appears to be composed of a mildly saturated component with a typical b-value blended with a broad component with b = 85 km/s . (4) The O VI absorption is not aligned with the strongest H I absorption, but instead is well-aligned with the broad component. (5) The only detected species (>4sigma) are O VI and H I despite coverage of strong transitions of other abundant elements. Based on these constraints, we find that the absorption line properties can be produced in collisionally ionized gas with roughly 5.3 < log T < 5.6 and -1.8 < [O/H] < -0.6. However, we find that photoionization is also viable if the pathlength l through the absorbing gas is long enough; simple photoionization models require 85 < l < 1900 kpc and -1.1 < [O/H] < -0.3. We briefly discuss how observations of X-ray absorption lines due to O VII and O VIII could be used, in principle, to break the ionization mechanism degeneracy.

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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

Highly-Ionized Intergalactic Gas at Low Redshifts: Constraints from QSO Absorption Lines

High-resolution UV spectroscopy of low-z QSOs with the Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectroscopic Explorer has indicated that O VI absorption-line systems provide a valuable probe of the low-z intergalactic medium. These observations and their implications are briefly reviewed. Though still uncertain due to the small sample, the number of O VI absorbers per unit redshift is quite high, and these absorbers appear to trace an important baryon reservoir. The O VI systems are intervening; they are highly displaced from the background QSO redshifts and are correlated with foreground luminous galaxies. Their physical conditions are variable and sometimes complicated. In some cases, there is clear evidence that the absorbers have a multiphase nature. Some appear to be photoionized by the UV background from QSOs, while others are probably collisionally ionized. However, the majority of the O VI lines have b-values that suggest an origin in collisionally ionized hot gas. The observations are in agreement with hydrodynamic simulations of cosmological structure growth, which predict substantial quantities of shock-heated, hot intergalactic gas at low z, but more observational and theoretical work is needed to confirm the nature of the O VI systems.

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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

The Statistical and Physical Properties of the Low Redshift Lyman Alpha Forest Observed with HST/STIS

We examine the Ly-alpha absorber population at z<0.3 detected in spectra of the QSOs PG0953+415 and H1821+643 taken with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. We compare their statistical properties to those in carefully-constructed mock quasar spectra drawn from a cosmological hydrodynamic simulation of a Lambda-CDM universe. We find very good agreement in the column density and b-parameter distributions, down to the smallest observable absorbers with N_HI\approx 10^{12.3} cm^-2. The observed absorber population is complete for N_HI\ga 10^{13} cm^-2, with a column density distribution slope of β=2.04\pm 0.23 and a median b-parameter of 21 km/s above this limit. The intergalactic gas giving rise to these weak absorbers is analogous to that at high redshift, located in diffuse large-scale structures that are highly photoionized by the metagalactic UV flux, though a greater number arise within shock-heated warm gas. The density, temperature, and column density of these absorbers follow similar relationships to those at high redshift, though with substantially larger scatter due to the shock-heated gas. The b-parameters typically have a significant contribution from thermal broadening, which facilitates a measurement of the low-z IGM temperature as traced by Ly-alpha absorbers. From our simulation we estimate T_IGM\sim 5000 K, with an upper limit of 10^4 K, at the mean density. The agreement in predicted and observed amplitude of the column density distributions allows us to measure the HI photoionization rate at z=0.17 to be 10^{-13.3\pm 0.7} s^{-1} (estimated modeling uncertainty), close to predictions based on quasar properties.

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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

Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. II. Observations toward Gamma2 Velorum and Zeta Puppis by the Interstellar Medium Absorption Profile Spectrograph

To measure interstellar atomic deuterium abundances, we used the Interstellar Medium Absorption Profile Spectrograph (IMAPS) to obtain spectra of gamma2 Vel and zeta Pup over the wavelength interval 930-1150 A at a resolving power of 80,000. The interstellar D I features are resolved and cleanly separated from interstellar H I in the Ly-delta and Ly-epsilon profiles of both sight lines, and also in the Ly-gamma profile of zeta Pup. The D I profiles were modeled using a velocity template derived from several N I lines in the IMAPS spectra recorded at higher S/N. To find the best D I column density, we minimized chi-squares for model D I profiles that included not only the N(D I) as a free parameter, but also the effects of several potential sources of systematic error which could also be varied. For both stars, H I column densities were measured by analyzing Ly-alpha absorption profiles in a large number of IUE high dispersion spectra. Ultimately we found that D/H = 2.18(+0.36,-0.31)e-5 for gamma2 Vel and 1.42(+0.25,-0.23)e-5 for zeta Pup, values that contrast markedly with D/H derived in Paper I for delta Ori (the stated errors are 90% confidence limits). Evidently, the atomic D/H ratio in the ISM, averaged over path lengths of 250 to 500 pc, exhibits significant spatial variability. Furthermore, variations in D/H do not appear to be anticorrelated with N/H. Within the framework of standard Big Bang Nucleosynthesis, the large value of D/H found toward gamma2 Vel is equivalent to a cosmic baryon density of Omega_B h^2 = 0.023 (+-0.002), which we regard as an upper limit since there is no correction for the destruction of deuterium in stars.

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arXiv2000

O VI and Multicomponent H I Absorption Associated with a Galaxy Group in the Direction of PG0953+415: Physical Conditions and Baryonic Content

We report the discovery of an O VI absorption system at z(abs) = 0.14232 in a high resolution FUV spectrum of PG0953+415 obtained with the Space Telescope Imaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet and multicomponent H I Lya absorption are detected, but the N V doublet and the strong lines of C II and Si III are not apparent. We examine the ionization mechanism of the O VI absorber and find that while theoretical considerations favor collisional ionization, it is difficult to observationally rule out photoionization. If the absorber is collisionally ionized, it may not be in equilibrium due to the rapid cooling of gas in the appropriate temperature range. Non-equilibrium collisionally ionized models are shown to be consistent with the observations. A WIYN survey of galaxy redshifts near the sight line has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/s from this absorber, and three additional galaxies are found within 130 km/s of this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc. Combining the STIS observations of PG0953+415 with previous high S/N GHRS observations of H1821+643, we derive a large number of O VI absorbers per unit redshift, dN/dz ~20. We use this sample to obtain a first estimate of the cosmological mass density of the O VI systems at z ~ 0. If further observations confirm the large dN/dz derived for the O VI systems, then these absorbers trace a significant reservoir of baryonic matter at low redshift.

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arXiv2000

The Properties of Molecular Hydrogen toward the Orion Belt Stars from Observations by the Interstellar Medium Absorption Profile Spectrograph

Absorption features from the Lyman and Werner bands of interstellar molecular hydrogen were recorded by the Interstellar Medium Absorption Profile Spectrograph (IMAPS) at a wavelength resolving power R=80,000 in the spectra of delta Ori and epsilon Ori. The objective was to find and study more examples of an unusual phenomenon found for one of the velocity components of H2 in the spectrum of zeta Ori by Jenkins & Peimbert (1997). Specifically, they detected a gradual shift in velocity and broadening for features arising from progressively higher rotational excitations J. This effect appears to be absent in the spectra of both delta and epsilon Ori, which are only a few degrees away in the sky from zeta Ori. The absence of atomic material at a large negative velocity in the spectra of delta and epsilon Ori (and its presence in zeta Ori) supports a proposal by Jenkins & Peimbert that the line of sight to zeta intercepts a bow shock facing away from us, perhaps created by the collision of wind-like material with some foreground obstruction. For both stars, the H2 absorption features are separated into two velocity components. Total H2 column densities toward delta and epsilon Ori are 5.5e14 and 1.9e16 cm^-2, respectively. The rotation temperatures of the molecules with J > 2 toward epsilon Ori indicate that the gas is in the general vicinity of the stars that emit UV fluxes capable of rotationally pumping the molecules. For the strongest component of H2 toward delta Ori, the pumping rate is lower and consistent with a general UV flux level in the plane of the Galaxy.

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arXiv1999

Spatial Variability in the Ratio of Interstellar Atomic Deuterium to Hydrogen. I. Observations toward delta Orionis by the Interstellar Medium Absorption Profile Spectrograph

Studies of the abundances of deuterium in different astrophysical sites are of fundamental importance to answering the question about how much deuterium was produced during big bang nucleosynthesis and what fraction of it was destroyed later. With this in mind, we used the Interstellar Medium Absorption Profile Spectrograph (IMAPS) on the ORFEUS-SPAS II mission to observe at a wavelength resolution of 4 km/s (FWHM) the L-delta and L-epsilon absorption features produced by interstellar atomic deuterium in the spectrum of delta Ori A. A chi-square analysis indicated that 0.96 < N(D I)< 1.45e15 cm^{-2} at a 90% level of confidence, and the gas is at a temperature of about 6000K. To obtain an accurate value of N(H I) needed for a determination of the atomic ratio of D to H, we measured the L-alpha absorption features in 57 spectra of delta Ori in the IUE archive. From our measurement of N(H I)= 1.56e20 cm^{-2}, we found that N(D I)/N(H I)= 7.4(+1.9,-1.3)e-6 (90% confidence). Our result for D/H contrasts with the more general finding along other lines of sight that D/H is approximately 1.5e-5. The underabundance of D toward delta Ori A is not accompanied by an overabundance of N or O relative to H, as one might expect if the gas were subjected to more stellar processing than usual.

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arXiv1998

The Relationship Between Galaxies and Low Redshift Weak Lyman alpha Absorbers in the Directions of H1821+643 and PG1116+215

To study the nature of low z Lya absorbers in the spectra of QSOs, we have obtained high signal-to-noise UV spectra of H 1821+643 (z = 0.297) and PG 1116+215 (z = 0.177) with the GHRS on the HST. The spectra have minimum S/N of 70-100 and 3 sigma limiting equivalent widths of 50-75 mA. We detect 26 Lya lines with Wr > 50 mA toward H1821+643 and 13 toward PG1116+215, which implies a density of 102+/-16 lines per unit redshift. The two-point correlation function shows marginal evidence of clustering on ~500 km/s scales, but only if the weakest lines are excluded. We have also used the WIYN Observatory to measure galaxy redshifts in the ~1 degree fields centered on each QSO. We find 17 galaxy-absorber pairs within projected distances of 1 Mpc with velocity separations of 350 km/s or less. Monte Carlo simulations show that if the Lya lines are randomly distributed, the probability of observing this many close pairs is 3.6e-5. We find that all galaxies with projected distances of 600 kpc or less have associated Lya absorbers within 1000 km/s, and the majority of these galaxies have absorbers within 350 km/s. We also find that the Lya equivalent width is anticorrelated with the projected distance of the nearest galaxy out to at least 600 kpc, but this should be interpreted cautiously because there are potential selection biases. Statistical tests using the entire sample also indicate that the absorbers are not randomly distributed. We discuss the nature of the Lya absorbers in light of the new data.

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arXiv1997

Echelle Spectroscopy of QSO Absorption Line Systems with Metals in the Direction of HS 1700+6416

We present a high signal-to-noise high resolution spectrum of the radio-quiet QSO HS1700+6416 (z = 2.72). Analysis of the absorption line systems with metals yields the following results. (1) The dense cluster of C IV doublets at 2.432 < z(abs) < 2.441 shows the weak line of one C IV absorber apparently aligned with the strong line of a different C IV doublet, i.e., line locked, for two pairs of C IV absorbers. This is surprising since these absorption lines are displaced by more than 24000 km/s from the QSO redshift, but it is possible that these alignments are coincidental. (2) Physical conditions in the Lyman limit absorber at z(abs) = 2.3150 differ significantly from the Galactic gaseous halo, and photoionization models require [Si/H] > -0.95 and [Al/H] > -0.96 for the strongest component of this absorber. Abundances of three other Lyman limit absorbers are also derived from photoionization models. (3) Unsaturated C IV and rather strong N V are detected in the associated absorber at z(abs) = 2.7125. The apparent column density profiles indicate that N V is affected by unresolved saturation or that the N V absorbing gas does not completely cover the QSO flux source. From photoionization models we derive [N/H] > -0.65 and [C/H] > -0.82. (4) We tentatively conclude that the number of Mg II systems detected per unit redshift is dominated by weak Mg II lines with rest equivalent width less than 0.3 A.

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