arXiv subjects

David V. Bowen

Publications and source records attributed to David V. Bowen.

At least 37 records · Page 2Linked to original sources

arXiv2006

QSO Absorption Lines from QSOs

We present the results of a search for metal absorption lines in the spectra of background QSOs whose sightlines pass close to foreground QSOs. We detect MgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of four z>1.5 QSOs whose lines of sight pass within 26-98 kpc of lower redshift (z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOs is clearly at odds with the incidence of associated (z_abs ~ z_em) systems -- absorbers which exist towards only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen towards one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central AGN. We discuss possible origins for the MgII lines, including: absorption by gas from the foreground QSO host galaxy; companion galaxies fuelling the QSO through gravitational interactions; and tidal debris left by galaxy mergers or interactions which initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena.

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arXiv2006

The O VI Absorbers Toward PG0953+415: High Metallicity, Cosmic-Web Gas Far From Luminous Galaxies

The spectrum of the low-redshift QSO PG0953+415 shows two strong, intervening O VI absorption systems. To study the nature of these absorbers, we have used the Gemini Multiobject Spectrograph to conduct a deep spectroscopic galaxy redshift survey in the 5' x 5' field centered on the QSO. This survey is fully complete for r' < 19.7 and is 73% complete for r' < 21.0. We find three galaxies at the redshift of the higher-z O VI system (z = 0.14232) including a galaxy at projected distance rho = 155 kpc. We find no galaxies in the Gemini field at the redshift of the lower-z O VI absorber (z = 0.06807), which indicates that the nearest galaxy is more than 195 kpc away or has L < 0.04 L*. Previous shallower surveys covering a larger field have shown that the z = 0.06807 O VI absorber is affiliated with a group/filament of galaxies, but the nearest known galaxy has rho = 736 kpc. The z = 0.06807 absorber is notable for several reasons. The absorption profiles reveal simple kinematics indicative of quiescent material. The H I line widths and good alignment of the H I and metal lines favor photoionization and, moreover, the column density ratios imply a high metallicity: [M/H] = -0.3 +/- 0.12. The z = 0.14232 O VI system is more complex and less constrained but also indicates a relatively high metallicity. Using galaxy redshifts from SDSS, we show that both of the PG0953+415 O VI absorbers are located in large-scale filaments of the cosmic web. Evidently, some regions of the web filaments are highly metal enriched. We discuss the origin of the high-metallicity gas and suggest that the enrichment might have occurred long ago (at high z).

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arXiv2005

The Connections between QSO Absorption Systems and Galaxies: Low-Redshift Observations

Quasar absorption lines have long been recognized to be a sensitive probe of the abundances, physical conditions, and kinematics of gas in a wide variety of environments including low-density intergalactic regions that probably cannot be studied by any other means. While some pre-Hubble Space Telescope (HST) observations indicated that Mg II absorption lines arise in gaseous galactic halos with a large covering factor, many early QSO absorber studies were hampered by a lack of information about the context of the absorbers and their connections with galaxies. By providing access to crucial ultraviolet resonance lines at low redshifts, deployment of HST and the Far Ultraviolet Spectroscopic Explorer enabled detailed studies of the relationships between QSO absorbers and galaxies. The advent of large surveys such as the Sloan Digital Sky Survey (SDSS) has also advanced the topic by greatly improving the size of absorber and galaxy samples. This paper briefly reviews some observational results on absorber-galaxy connections that have been obtained in the HST/SDSS era, including Mg II absorbers, the low-z Lyman alpha forest, Lyman limit and damped Lyman alpha absorbers, and O VI systems.

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arXiv2005

A Comparison of Absorption and Emission Line Abundances in the Nearby Damped Lyman-alpha Galaxy SBS 1543+593

We have used the Space Telescope Imaging Spectrograph (STIS) aboard HST to measure a sulfur abundance of [S/H] = -0.41 +/-0.06 in the interstellar medium (ISM) of the nearby damped Lyman-alpha (DLA) absorbing galaxy SBS 1543+593. A direct comparison between this QSO absorption line abundance on the one hand, and abundances measured from HII region emission line diagnostics on the other, yield the same result: the abundance of sulfur in the neutral ISM is in good agreement with that of oxygen measured in an HII region 3 kpc away. Our result contrasts with those of other recent studies which have claimed order-of-magnitude differences between HI (absorption) and HII (emission) region abundances. We also derive a nickel abundance of [Ni/H] < -0.81, some three times less than that of sulfur, and suggest that the depletion is due to dust, although we cannot rule out an over-abundance of alpha-elements as the cause of the lower metallicity. It is possible that our measure of [S/H] is over-estimated if some SII arises in ionized gas; adopting a plausible star formation rate for the galaxy along the line of sight, and a measurement of the CII* 1335.7 absorption line detected from SBS 1543+593, we determine that the metallicity is unlikely to be smaller than we derive by more than 0.25 dex. We estimate that the cooling rate of the cool neutral medium is log [l_c (ergs s^{-1} H atom^{-1})] = -27.0, the same value as that seen in the high redshift DLA population.

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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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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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arXiv2002

Lyman-alpha absorption around nearby galaxies

We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption lines in the outer regions of eight nearby galaxies using background QSOs and AGN as probes. Lya lines are detected within a few hundred km/s of the systemic velocity of the galaxy in all cases. We conclude that a background line-of-sight which passes within 26-200 h-1 kpc of a foreground galaxy is likely to intercept low column density neutral hydrogen with log N(HI) >~ 13.0. The ubiquity of detections implies a covering factor of ~ 100% for low N(HI) gas around galaxies within 200 h-1 kpc. We discuss the difficulty in trying to associate individual absorption components with the selected galaxies and their neighbors, but show that by degrading our STIS data to lower resolutions, we are able to reproduce the anti-correlation of Lya equivalent width and impact parameter found at higher redshift. We also show that the equivalent width and column density of Lya complexes (when individual components are summed over ~ 1000 km/s) correlate well with a simple estimate of the volume density of galaxies brighter than M(B) = -17.5 at the same redshift as a Lya complex. We do not reject the hypothesis that the selected galaxies are directly responsible for the observed Lya lines, but our analysis indicates that absorption by clumpy intragroup gas is an equally likely explanation. (Abriged)

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arXiv2002

The Redshift of a Lensing Galaxy in PMN J0134-0931

The Sloan Digital Sky Survey (SDSS) automatically targeted as a quasar candidate the recently discovered, gravitationally lensed, extremely reddened z=2.2 quasar PMN J0134-0931. The SDSS spectrum exhibits Ca II absorption at z=0.76451, which we identify as the redshift of a lensing galaxy. Hubble Space Telescope imaging shows that components CDE of the system are significantly redder than components A or B and detects faint galaxy emission between D and A+B. The redshift of the dust responsible for the reddening remains unconstrained with current data. However, we outline a model wherein lensing and differential reddening by a z=0.76451 galaxy pair can entirely explain this system. Detecting mm-wave molecular line absorption from the lensing galaxy or galaxies may be possible in PMN J0134-0931, just as in the lenses PKS1830-211 and B0218+357. Well-constructed optical quasar surveys like the SDSS can contribute to the detection and study of reddened quasars. (Expanded)

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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

The Detection of Lyman-alpha Absorption from Nine Nearby Galaxies

We have used STIS aboard HST to search for Lyman-alpha (Lya) absorption in the outer regions of nine nearby (cz<6000 km/s) galaxies using background QSOs and AGN as probes. The foreground galaxies are intercepted between 26 and 199 h-1 kpc from their centers, and in all cases we detect Lya within +/-500 km/s of the galaxies' systemic velocities. The intervening galaxies have a wide range of luminosities, from M_B = -17.1 to -20.0, and reside in various environments: half the galaxies are relatively isolated, the remainder form parts of groups or clusters of varying richness. The equivalent widths of the Lya lines range from 0.08 - 0.68 A and, with the notable exception of absorption from one pair, crudely correlate with sightline separation in a way consistent with previously published data, though the column densities derived from the lines do not. The lack of correlation between line strength and galaxy luminosity or, in particular, the environment of the galaxy, suggests that the absorption is not related to any individual galaxy, but arises in gas which follows the same dark-matter structures that the galaxies inhabit.

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arXiv2001

A New Measurement of the Primordial Abundance of Deuterium: Toward Convergence with the Baryon Density from the CMB?

From the analysis of the near-UV spectrum of the QSO 2206-199, obtained with a long series of exposures with STIS on the HST, we deduce a value (D/H) = (1.65 +/- 0.35) x 10(-5) (1 sigma error) for the abundance of deuterium in the z(abs) = 2.0762 damped Lyman alpha system (DLA) along this sight-line. The velocity structure of this absorber is very simple and its neutral hydrogen column density, N(H I), is accurately known; the error in D/H is mostly due to the limited signal-to-noise ratio of the spectrum. Since this is also one of the most metal-poor DLAs, with metal abundances of about 1/200 of solar, the correction due to astration of D is expected to be insignificant and the value we deduce should be essentially the primordial abundance of deuterium. When all (six) available measurements of D/H in high redshift QSO absorbers are considered, we find that the three DLAs---where N(H I) is measured most reliably---give consistently lower values than the three Lyman limit systems. We point out that the weighted mean of the DLA measurements, D/H = (2.2 +/- 0.2) x 10(-5), yields a baryon density Omega_B h^2 = 0.025 +/- 0.001 which is within 1 sigma of the value deduced from the analysis of the CMB angular power spectrum, and is still consistent with the present-day D/H and models of Galactic chemical evolution. Future observations of D I absorption in other DLAs are needed to establish whether our finding reflects a real advantage of DLAs over other classes of QSO absorbers for the measurement of D, or is just a statistical fluctuation.

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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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arXiv2001

Optical Spectroscopy Of The X-Ray Transient XTE J1118+480 In Outburst

We report on optical spectroscopic observations of the X-ray Transient XTE J1118+480 covering the period from April 7, 2000 to July 4, 2000. The spectrum is characterized by weak, broad, double-peaked Balmer and He lines on top of a blue continuum of slope p ~ 1/3, as expected for an optically-thick accretion disk. The weak Bowen blend seen in our spectra may indicate a low intrinsic metallicity for the source. The presence of a partial S-wave pattern in the HeII lambda-4686 line appears consistent with the reported photometric orbital period Porb = 4.1 hr for XTE J1118+480. By using a combination of Doppler mapping and various theoretical arguments, we constrain plausible orbital parameters for the system: a mass ratio 0.02 <~ q <~ 0.1, an inclination i >~ 70 deg for a neutron star primary, or 30 deg <~ i <~ 50 deg for a black hole primary with a mass between 4 and 10 Msun. CaII absorption features observed at very high resolution constrain the interstellar hydrogen absorption column log[N_HI (cm^{-2})] ~ 20.45 +/- 0.2 and the identification of three absorbing clouds indicate a distance to the source <~ 1 kpc, assuming the line-of-sight to XTE J1118+480 has average high-latitude properties. These results are discussed in the context of previous multiwavelength observations of this unusual system.

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arXiv2000

Where are the absorbers towards Q2302+029?

We present images and spectroscopy of objects close to the sightline of Q2302+029 in order to search for galaxies responsible for the remarkable z=0.7 high-ionization absorption line system found by Jannuzi et al (1996). This system shows `normal' narrow OVI, NV, and CIV lines superimposed on broader (3,000-5,000 km/s wide), unsaturated absorption troughs some 56,000 km/s away from the QSO emission redshift (z=1.052). Despite reaching sensitivities sufficient to detect 1/10 L* galaxies in the optical and 1/20 L* in the infra-red, we are unable to detect any obvious bright galaxies which might be responsible for the absorption beyond ~ 6 h-1 kpc of the sightline. This supports the hypotheses that the absorption is either intrinsic to the QSO, or arises in intracluster gas. Adopting either explanation is problematic: in the first case, `associated' absorption at such high ejection velocities is hard to understand, and challenges the conventional discrimination between intrinsic and intervening absorbers; in the second case, the gas must reside in a ~ 40 h-1 Mpc long filament aligned along the line of sight in order to reproduce the broad absorption. Spectroscopy of objects beyond the immediate vicinity of the QSO sightline reveals a galaxy cluster at z=0.59, which coincides with strong Lya and more narrow high ionization lines in the quasar spectrum. Here too, the lack of galaxies at distances comparable to those found for, e.g., Lya-absorbing galaxies, suggests that the absorption may arise from intracluster gas unassociated with any individual galaxies.

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arXiv1999

Hubble Space Telescope Observations of the Associated Absorption Systems in Q0122+0338

We have studied a spectrum of Q0122+0338 (z = 1.202) obtained by the FOS on board HST. We present the analysis of three associated absorption systems at z = 1.207, 1.199 and 1.166. The most complex of these at z = 1.207 shows strong absorption from the highly ionized transitions of Lyman alpha, Lyman beta, N V, O VI, Si III, Si IV, and possibly P V. We derive minimal ionic column densities and compare them with those predicted from numerical photoionization models. We find that conditions in the absorbing gas are consistent with an absorber with metallicity twice solar and a total absorbing column density of N(H) = 2*10^19 cm^2. The kinematics of the absorption lines in the z = 1.207 system suggest that a correlation exists between the relative velocity and the creation ionization potential energy for each transition. This is evidence that a complex, multi-component absorber exists. Althought the location of the absorber is uncertain, we consider the origin of this absorption system using the available data and discuss how the high-ionization, and high-metallicity indicate that the absorber may be intrinsic to Q0122+0338.

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