Absorption Line Systems Research Articles

The First FIRST Gravitationally Lensed Quasar: FBQ 0951+2635

The V=16.9 quasar FBQ 0951+2635 at redshift z=1.24 appears double on CCD exposures taken in subarcsecond seeing. The two objects are separated by 1.1 and differ in brightness by 0.9 mag. VLA observations show the radio source to be double with the same separation and position angle. Spectra taken with the Keck II telescope show the two components to have nearly identical emission line spectra, but with somewhat different absorption line systems. Subtraction of two stellar point spread functions from the pair of components consistently leaves a residual object. Depending upon whether this third object is extended or a point source it may be as much as 1/10 or as little as 1/100 as bright as the brighter QSO component. The observations leave no doubt that the 2 brighter objects are gravitationally lensed images of the same quasar. The third object might be either the lensing galaxy or a third image of the quasar, but both interpretations have serious shortcomings.

Detection of the Galaxy Lensing the Doubly Imaged Quasar SBS 1520+530

H-band observations with a spatial resolution of 015 carried out with the Canada-France-Hawaii Telescope adaptive optics system show a galaxy between the components of the double broad absorption line QSO SBS 1520+530, thereby confirming this system as a gravitational lens. The galaxy is located 040 from the fainter of the two QSO images and is offset 012 from the line joining them. The H magnitude of the lensing galaxy is ∼1 mag fainter than expected from the velocity dispersion derived for the lensing galaxy were it at z = 0.72 or z = 0.81, the redshifts of the two absorption-line systems.

The Form and Evolution of the Clustering of QSO Heavy‐Element Absorption‐Line Systems

We have analyzed the clustering of C IV and Mg II absorption-line systems on comoving scales r from 1 to 16 h-1 Mpc, using an extensive catalog of heavy-element QSO absorbers with mean redshift zC IV = 2.2 and zMg II = 0.9. We find that, for the C IV sample as a whole, the absorber line-of-sight correlation function is well fitted by a power law of the form ξaa(r) = (r0/r)γ, with maximum-likelihood values of γ=1.75 -->−0.70+0.50 and comoving r -->0=3.4 -->+ 0.7−1.0 h-1 Mpc (q0 = 0.5). The clustering of absorbers at high redshift is thus of a form that is consistent with that found for galaxies and clusters at low redshift, and of amplitude such that absorbers are correlated on scales of galaxy clusters. We also trace the evolution of the mean amplitude ξ0(z) of the correlation function, as a function of redshift, from z = 3 to z = 0.9. We find that, when parameterized in the conventional manner as ξ0(z) (1 + z)-(3+)+γ, the amplitude grows with decreasing redshift, with a maximum-likelihood value for the evolutionary parameter of = 2.05 ± 1.0 (q0 = 0.5). When extrapolated to zero redshift, the amplitude of the correlation function implies that the correlation length r -->0=30 -->−13+22 h-1 Mpc (q0 = 0.5). This suggests that strong C IV and Mg II absorbers, on megaparsec scales, are biased tracers of the higher density regions of space and that agglomerations of strong absorbers along a line of sight are indicators of clusters and superclusters. This is supported by recent observations of Lyman break galaxies. The growth seen in the clustering of absorbers is consistent with gravitationally induced growth of perturbations.

A Subkiloparsec Disk in Markarian 231

We present imaging with the Very Long Baseline Array of the neutral hydrogen 21 cm absorption-line system seen toward the nuclear regions of Mrk 231 at z☉ = 0.04217, and imaging of the radio continuum emission at 1.4 GHz on scales ranging from a few parsecs to a few hundred parsecs. These data indicate the existence of a subkiloparsec gas disk in Mrk 231, as seen in H I 21 cm absorption and in radio continuum emission. The radio continuum morphology is consistent with a disk of maximum radius of 440 mas (260 h-1 pc), at an inclination angle of 45°, with a major axis oriented east-west. The H I 21 cm absorption shows an east-west gradient in position and velocity of about ±110 km s-1 out to radii of 100 mas (60 h-1 pc). We identify this H I and radio continuum disk as the inner part of the molecular disk seen on a factor of 3 larger scale. The physical conditions for the thermal and nonthermal gas in the subkiloparsec disk of Mrk 231 are similar to those proposed for compact nuclear starburst galaxies and, in particular, to the conditions proposed for the subkiloparsec gas disk in Arp 220. From the neutral hydrogen velocity field we derive a gravitational mass enclosed within a 50 h-1 pc radius of 3 × 108 h-1 M☉, and from the radio continuum emission we derive a massive star formation rate in the disk of 60 M☉ yr-1. We also present a search for H I 21 cm absorption associated with the optical broad absorption line (BAL) systems toward Mrk 231. We do not detect H I 21 cm absorption associated with any of the optical BAL systems. These negative results require that the neutral atomic gas in the BAL clouds be fairly warm (spin temperature Ts > 50 K), unless the Na I abundance is higher than solar, or the dust-to-gas ratio is higher than Galactic, or the observed extinction toward the nucleus of Mrk 231 is not due to the BAL gas.

Short-Term Optical Variability in Broad Absorption Line QSO[CLC]s[/CLC

We present the first results from a pilot programme to monitor the short term optical variability in Broad Absorption Line system QSOs. Intra-night optical variations of ~5% were detected on time scales of ~ one hour in QSOs 0846+156 and 0856+172. Further, the mean magnitude level decreased in the two objects by ~0.05 and ~0.15 magnitude respectively during the period of our observations. The observed light curves are quite similar to those previously seem in the flat spectrum radio-loud sources, especially the BL Lacertae objects, and can provide important constraints for the origin of microvariability, and also a possible evolutionary link between the radio-loud and the radio-quiet QSOs.

Redshifted Neutral Hydrogen 21 Centimeter Absorption toward Red Quasars

We have searched for redshifted neutral hydrogen 21 cm absorption toward sources from the Stickel et al. subsample. The red quasar subsample is taken from the 1 Jy sample of flat-spectrum radio sources and is comprised of the 15 sources that are undetected on the POSS. Five of these red quasars have been searched for redshifted H I 21 cm absorption to optical depth levels of a few percent, and four show strong absorption, with neutral hydrogen column densities between 4 and 80 × 1018 × (Ts/f) cm-2. This 80% success rate for the red quasars compares to the much lower success rate of only 11% for detecting H I 21 cm absorption associated with optically selected Mg II absorption line systems. The large neutral hydrogen column densities seen toward the Stickel et al. red quasars provide circumstantial evidence supporting the dust-reddening hypothesis, as opposed to an intrinsically red spectrum for the AGN emission mechanism. The lower limits to rest-frame values of AV are between two and seven, leading to lower limits to the spin temperatures for the neutral hydrogen between 50 K and 1000 K, assuming a Galactic dust-to-gas ratio. We consider the question of biases in optically selected samples of quasars caused by dust obscuration. Overall, the data on the red quasar subsample support the models of Fall & Pei for dust obscuration by damped Lyα absorption line systems and suggest that (1) there may be a significant, but not dominant, population of quasars missing from optically selected samples because of dust obscuration, perhaps as many as 20% at the POSS limit for an optical sample with a redshift distribution similar to the 1 Jy flat-spectrum quasar sample, and (2) optically selected samples may miss about one-half the high column density quasar absorption line systems. The redshifted H I 21 cm absorption line detections presented herein are toward the sources 0108+388 at z = 0.6685, 0500+019 at z = 0.5846, and 1504+377 at z = 0.6733. No absorption is seen toward 2149+056 at z = 0.740 at a level below that seen for the three detections, although there is some uncertainty in this case as to the expected line redshift. In some systems, the absorbing gas is in the vicinity of the AGN, as either circumnuclear material or material in the general ISM of the AGN's host galaxy, as is probably the case for 0108+388 and 1504+377, and in other systems the absorption is by gas associated with galaxies cosmologically distributed along the line of sight to the quasar, as may be the case for 0500+019. The Westerbork Synthesis Radio Telescope spectrum of 1504+377 confirms the lack of H I 21 cm absorption associated with the narrow molecular absorption line system at z = 0.67150.

Spectropolarimetry of 3CR 68.1: A Highly Inclined Quasar

We present Keck spectropolarimetry of the highly polarized radio-loud quasar 3CR 68.1 (z=1.228, V=19). The polarization increases from 5 in the red (4000 A rest-frame) to >10% in the blue (1900 A rest-frame). The broad emission lines are polarized the same as the continuum, which shows that 3CR 68.1 is not a blazar as it has sometimes been regarded in the past. We also present measurements of the emission lines and a strong, blueshifted, associated absorption line system, as well as a detection at the emission-line redshift of Ca II K absorption, presumably from stars in the host galaxy. 3CR 68.1 belongs to an observationally rare class of highly polarized quasars that are neither blazars nor partially obscured radio-quiet QSOs. Taking into account 3CR 68.1's other unusual properties, such as its extremely red spectral energy distribution and its extreme lobe dominance, we explain our spectropolarimetric results in terms of unified models. We argue that we have a dusty, highly inclined view of 3CR 68.1, with reddened scattered (polarized) quasar light diluted by even more dust-reddened quasar light reaching us directly from the nucleus.

The Intervening and Associated O [CSC]vi[/CSC] Absorption-Line Systems in the Ultraviolet Spectrum of H1821+643

GHRS and FOS ultraviolet spectra of the bright QSO H1821+643 (mV = 14.2, zem = 0.297) reveal the presence of strong O VI λλ1031.93, 1037.62 absorption systems at zabs = 0.225 and 0.297, the latter being at the redshift of the QSO itself. Ground-based galaxy redshift measurements by us and others reveal two emission-line galaxies near the redshift of the intervening system at zabs = 0.225, suggesting the existence of a galaxy group at this redshift. The intervening O VI absorption system is also detected in H I but is not detected in the lines of Si II, Si IV, C IV, or N V. These ionization characteristics can be explained by a low-density, extended (~300 kpc) diffuse gas distribution that is photoionized by the metagalactic UV background if the gas has a metallicity of ~0.1 times solar. Such a photoionized gas may be associated with the extended halo of the luminous intervening spiral galaxy at a projected distance of 100 h kpc, or with an intragroup medium. Alternatively, the absorption may be produced in hot collisionally ionized halo gas or in a hot intragroup medium. The associated system with zabs = 0.297 contains narrow and broad O VI absorption. The narrow absorption, which is also detected in H I, C III, C IV, and Si IV, can be modeled as gas photoionized by H1821+643 with roughly solar abundances. This gas is probably situated close to H1821+643. The broad O VI absorption that is centered at the emission redshift of H1821+643 may represent a weak and narrow example of the broad absorption line phenomena. Another possibility is that the broad O VI absorption occurs in 105–106 K gas associated with a cooling flow in the rich, X-ray–luminous cluster surrounding H1821+643 or with a cooling flow in the host elliptical galaxy that H1821+643 resides in. However, the observed O VI column density is 60–120 times smaller than expected for gas in a simple cooling flow passing through the 106–105 K temperature regime. The strong radiative flux from H1821+643 could substantially modify the ionization of the cooling gas in the flow and may help explain the discrepancy.

A Retrospective Introduction

“It takes little talent to see clearly what lies under one’s nose, a good deal of it to know in which direction to point that organ.” W. H. AudenListening to the sequence of fine talks that have made up this Joint Discussion, I have been impressed by the veritable flood of new observations that bear on the question of the abundance ratios in the oldest stars. The flood is marked by depth and diversity, as ‘noses’ are being pointed in highly profitable directions. As an example of depth, I would note the extensive investigations reviewed by Andy McWilliam of the chemical compositions of very metal-poor stars where [Fe/H] ≃ -3 marks the upper end of the range defining the class. To characterize diversity, I draw attention to the analyses of quasar absorption line systems so ably discssed by Limin Lu who showed that the abundance ratios for the gas in such high z systems resemble ratios found from Galactic metal-poor stars. Of course, to many at this Joint Discussion the wealth and diversity did not come as a surprise. Indeed, I suspect that many of us pursue the topic of the composition of the first generations of stars because of the wealth and diversity of observational constraints that may now spawn a unified interpretation. Hopefully, the marvellous talks given here will pull in a few new minds - young and old - to ponder nucleosynthesis in very young galaxies and in our Galaxy in particular.

SOME CONSEQUENCES OF A CLASS OF GASEOUS GALACTIC HALO MODELS

If a significant fraction of QSO absorption line systems arises in halos of normal galaxies, absorber statistical data will tell us a lot about the structure of the gaseous envelopes of galaxies. Comparison of a class of density profile models for the metal-absorbing halos of luminous galaxies with the recent Lyα empirical data is presented. Cosmological mass fraction contained in such gas is also briefly discussed. It is shown that important constraints on the model parameters can be inferred in this way.

Elemental Abundances in Quasar Absorption Line Systems

AbstractWe discuss elemental abundances in quasar absorption line systems and their implications for galaxy formation and chemical evolution.

Interstellar Abundances in the Magellanic Clouds. I. GHRS Observations of the Small Magellanic Cloud Star Sk 108

We present HST GHRS echelle-B and G160M spectra of the Wolf-Rayet binary Sk 108, located in the northeastern part of the main bar of the Small Magellanic Cloud. The spectra show interstellar absorption from C I, O I, Si II, Si II*, S II, P II, Cr II, Mn II, Fe II, Ni II, and Zn II. We find at least 25 interstellar components, which may be grouped via similar kinematics and/or similar relative heavy element abundances into two sets arising in the Galactic disk and halo, plus three sets located in the SMC. The SMC component groups may correspond to large-scale SMC gas complexes identified in H I 21 cm emission surveys. The relative gas phase abundances for Si II, Cr II, Mn II, Fe II, Ni II, and Zn II found for the SMC interstellar components are similar to those found for interstellar clouds in the Galactic halo. Since the relative total abundances for those elements found for F-G supergiants and gaseous nebulae in the SMC are similar to those present in comparable Galactic objects, we conclude that the interstellar depletion patterns are also similar in the SMC and in the Galactic halo clouds, despite the significantly lower metallicity and dust-to-gas ratio in the SMC. We do not see the distinctive depletion patterns commonly found for cold and warm clouds in the Galactic disk for any of the SMC components along the line of sight to Sk 108. We discuss some implications of these results for understanding the abundances found for QSO absorption-line systems and conclude that both nucleosynthetic and depletion effects contribute to the observed abundances—to different degrees for different systems.

Spectroscopy of PKS 0528−250: New Limits on CO Absorption and Emission

We have obtained a moderate-resolution spectrum of the quasar PKS 0528-250 with the Red Channel Spectrograph on the Multiple Mirror Telescope in order to study a damped Lyα absorption-line system at z = 2.8115. We obtain a new upper limit for the CO column density for the z = 2.8108 velocity component in the z = 2.8115 damped Lyα system. The ionization of different species in this component rules out a quasar spectral energy distribution as the ionization field and implies an ultraviolet radiation field intensity a few times that of the Milky Way value. The estimated total number density is n(H) ~ 20 cm-3. The physical size for the z = 2.8108 component implied by these models is about 40 pc. The ionization of different species also suggests a structure with a hot intercloud medium associated with a H I cloud in this component, that is, most low ionized ions are from the cold medium where photoionization and photodissociation dominates. The highly ionized species may be from the intercloud medium where collisional ionization dominates. We also present newly identified Ni II absorption lines in the z = 2.1408 and z = 2.8115 damped Lyα systems. The derived depletion of nickel by dust confirms previous results that the dust-to-gas ratio in these two damped Lyα systems is about 10% of the Milky Way ratio. Millimeter-wavelength observations obtained at the NRAO 12 m telescope provide new upper limits on CO (3-2) emission in the z = 2.8115 damped Lyα system.

The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field

The Hubble Deep Field (HDF) is the deepest set of multicolor optical photometric observations ever undertaken, and it offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometrically estimated redshifts for HDF galaxies with J 2, and they bridge the redshift gap between those two samples. The overall star formation or metal enrichment rate history is consistent with the theoretical models of White and Frenk and the predictions of Pei and Fall based on the evolving H I content of Lyα QSO absorption line systems.

The Primordial Abundance of4He: An Update

We include new data in an updated analysis of helium in low-metallicity extragalactic H II regions with the goal of deriving the primordial abundance of 4He (YP). We show that the new observations of Izotov et al. are consistent with previous data. However, they should not be taken in isolation to determine YP, owing to the lack of sufficiently low-metallicity points. We use the extant data in a semi-empirical approach to bounding the size of possible systematic uncertainties in the determination of YP. Our best estimate for the primordial abundance of 4He assuming a linear relation between 4He and O/H is YP = 0.230 ? 0.003(stat) based on the subset of H II regions with the lowest metallicity; for our full data set we find YP = 0.234 ? 0.002(stat). Both values are entirely consistent with our previous results. We discuss the implications of these values for standard big bang nucleosynthesis (SBBN), particularly in the context of recent measurements of deuterium in high-redshift, low-metallicity QSO absorption-line systems.

A big band nucleosynthesis likelihood analysis of the baryon-to-photon ratio and the number of light particle degrees of freedom

We extend the model-independent likelihood analysis of big bang nucleosynthesis (BBN) based on 4He and 7Li to allow for numbers of degrees of freedom which differ from the standard model value characterized by Nv = 3. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos. The upper limit thus obtained is Nv < 4.0 (at 95% C.L.). We also consider the consequences if recent observations of deuterium in high-redshift QSO absorption-line systems (QSOALS) are confirmed.

The unusual near-infrared morphology of the radio-loud quasar 4C + 09.17

Near-infrared images of the luminous, high redshift (z=2.1108) radio loud quasar 4C+09.17 reveal a complex structure. The quasar (K=15.76 mag) is surrounded by three "companion" objects having 17.9< K < 20.2 mag at radii of 1.7" < r < 2.9", as well as bright, diffuse emission. The brightest companion has a redshift of z=0.8384 (Lehnert & Becker 1997) and its optical-infrared colors (Lehnert et al. 1997) are consistent with a late-type spiral galaxy at this redshift with a luminosity of about 2L*. This object is likely the galaxy responsible for the strongest MgII absorption line system seen in the spectrum of 4C+09.17 by Barthel et al. (1990). Redshifts are not available for the remaining two companions. The red colors of the second brightest companion appear most consistent with a high redshift star-forming galaxy at z > 1.5. If this object is at the redshift of 4C +09.17 it has a luminosity of about 7L*. The faintest companion has colors which are unlike those expected from either a spiral or an E/S0 galaxy at any redshift associated with the 4C+09.17 system. Since this object lies along the same direction as the radio jet/lobe of 4C+09.17, as well as the extended Ly-alpha emission mapped by Heckman et al. (1991) we suggest that this component can be explained as a combination of strong line emission and scattered QSO light. The resolved, diffuse emission surrounding 4C+09.17 is bright, K~17.0 mag, and about one magnitude redder in J-K than the quasar. If this emission is starlight, a very luminous elliptical host galaxy is implied for 4C+09.17. Scattered and reddened AGN light, emission line gas, and flux from absorbing galaxies along the line of sight may all contribute to this emission.

Quasars as the formation sites of high-redshift ellipticals: a signature in the 'associated' absorption-line systems?

Quasars as the formation sites of high-redshift ellipticals: a signature in the 'associated' absorption-line systems?

Time‐variable Intrinsic Absorption Lines in the Quasi‐stellar Object Q2343+125

We discuss high-resolution Keck Observatory spectra of an intrinsic absorption-line system at redshift za ≈ 2.24 in the radio-quiet QSO Q2343+125 (ze = 2.515). The absorber's physical relationship to the QSO is confirmed by time-variable line strengths and partial line-of-sight coverage of the QSO continuum source. The N V, C IV, and Si IV doublets varied in unison by factors of 4 in less than 0.3 yr in the QSO rest frame. The resolved C IV doublet ratios show that the absorber occults 20% of the QSO continuum source; therefore, the absorbing clouds are likely to have characteristic sizes less than 0.01 pc. There is evidence for smaller coverage fractions when the lines were weaker, which could explain the line-strength changes. Lower ionization species such as C II and Si II are not detected. The Lyα line at za ≈ 2.24 is contaminated by the dense forest of cosmologically intervening absorption lines, but also appears to be weak or absent. The large uncertainties in the H I column prevent reliable constraints on the metal abundances. The za ≈ 2.24 lines clearly form in high-velocity ejecta from the QSO. The line profiles are smooth and rounded, with FWHM ~ 400 km s-1 and centroids shifted -24,000 km s-1 from the QSO emission redshift. Several narrow-line systems are present inside the broader absorption profiles, but they did not vary in strength and may be due to intervening gas. The large-amplitude variability, low coverage fractions, and large ratio of centroid to FWHM velocities imply different outflow properties than those inferred from the broad absorption lines (BALs) in other sources. Nonetheless, the ejecta identified by the za ≈ 2.24 absorber could be related to the BAL phenomenon and could be ubiquitous in QSOs if the gas subtends a small fraction of the sky as seen from the emission source(s).

Molecular Hydrogen Absorption in the [CLC][ITAL]z[/ITAL][/CLC] = 1.97 Damped L[CLC]y[/CLC]α Absorption System toward Quasi-Stellar Object Q0013−004

We present a new ultra-violet spectrum of the QSO 0013-004 with 0.9 \AA resolution obtained with the MMT Blue spectrograph. The \upsilon = 0 - 0, 1 - 0, 2 - 0 and 3 - 0 Lyman bands of H_2 associated with the z = 1.9731 damped Ly alpah absorption line system have been detected. The H_2 column density is N(H_2) = 6.9 (\pm 1.6)\times 10^{19} cm^{-2}, and the Doppler parameter b = 15\pm 2 km/s. The populations of different rotational levels are measured and used to derive the excitation temperatures. The estimated kinetic temperature T_K\sim 70 K, and the total particle number density n(H) \sim 300 cm^{-3}. The UV photoabsorption rate $\beta_0 \sim 6.7\times 10^{-9}$ s^{-1}, about a factor of few times greater than that in a typical diffuse Milky Way interstellar cloud. The total hydrogen column density is $N(H) = 6.4(\pm 0.5)\times 10^{20} cm^{-2}$. The fractional H_2 abundance f = 2N(H_2)/(2N(H_2) + N(H I)) \sim 0.22 \pm 0.05 is the highest among all observed damped Ly\al absorbers. The high fractional H_2 abundance is consistent with the inferred presence of dust and strong C I absorption in this absorber.