Alpha Absorption Line Research Articles

The statistical properties of neutral gas at z < 1.65 from UV measurements of Damped Lyman Alpha systems

We derive the statistical properties of neutral gas at redshifts 0.11<z<1.65 from UV measurements of quasar Lyman alpha absorption lines corresponding to 369 MgII systems with $W^{\lambda2796}_{0} \ge 0.3$ \AA. In addition to the 41 damped Lyman alpha (DLA) systems presented in Rao et al. (2006), the current DLA sample includes 29 newly discovered DLAs. Of these, 26 were found in our HST ACS prism survey for DLAs (Turnshek et al. 2015) and three were found in a GALEX archival search. In addition, an HST COS Cycle 19 survey yielded no DLAs that could be used for this study. Formally, this DLA sample includes 70 systems with $N_{\rm HI}\ge 2\times 10^{20}$ atoms cm$^{-2}$. We find that the incidence of DLAs, or the product of their gas cross section and their comoving number density, can be described by $n_{\rm DLA}(z) = (0.027 \pm 0.007) (1+z)^{(1.682 \pm 0.200)}$ over the redshift range 0<z<5. The cosmic mass density of neutral gas can be described by $\Omega_{\rm DLA}(z) = (4.77 \pm 1.60)\times10^{-4} (1 + z)^{(0.64\pm 0.27)}$. The low-redshift column density distribution function is well-fitted by a power law of the form $f(N) \sim N^\beta$ with $\beta = -1.46 \pm 0.20$. It is consistent with the high-redshift as well as z=0 estimates at the high column density end but, lies between them at the low column density end. We discuss possible $N_{\rm HI}$ and metallicity bias in MgII-selected DLA samples and show that such biases do not exist in the current data at z<1.65. Thus, at least at z<1.65, DLAs found through MgII selection statistically represent the true population of DLAs. However, we caution that studies of DLA metallicities should take into the account the relative incidence of DLAs with respect to $W^{\lambda2796}_{0}$ (or gas velocity spread) in order to correctly measure the mean neutral-gas cosmic metallicity of the universe.

An outburst scenario for the X-ray spectral variability in 3C 111

We present a combined Suzaku and Swift BAT broad-band E=0.6-200keV spectral analysis of three 3C 111 observations obtained in 2010. The data are well described with an absorbed power-law continuum and a weak (R~0.2) cold reflection component from distant material. We constrain the continuum cutoff at E_c~150-200keV, which is in accordance with X-ray Comptonization corona models and supports claims that the jet emission is only dominant at much higher energies. Fe XXVI Ly\alpha emission and absorption lines are also present in the first and second observations, respectively. The modelling and interpretation of the emission line is complex and we explore three possibilities. If originating from ionized disc reflection, this should be emitted at r_in> 50r_g or, in the lamp-post configuration, the illuminating source should be at a height of h> 30r_g over the black hole. Alternatively, the line could be modeled with a hot collisionally ionized plasma with temperature kT = 22.0^{+6.1}_{-3.2} keV or a photo-ionized plasma with log\xi=4.52^{+0.10}_{-0.16} erg s^{-1} cm and column density N_H > 3x10^23 cm^{-2}. However, the first and second scenarios are less favored on statistical and physical grounds, respectively. The blue-shifted absorption line in the second observation can be modelled as an ultra-fast outflow (UFO) with ionization parameter log\xi=4.47^{+0.76}_{-0.04} erg s^{-1} cm, column density N_H=(5.3^{+1.8}_{-1.3})x 10^{22} cm^{-2} and outflow velocity v_out = 0.104+/-0.006 c. Interestingly, the parameters of the photo-ionized emission model remarkably match those of the absorbing UFO. We suggest an outburst scenario in which an accretion disc wind, initially lying out of the line of sight and observed in emission, then crosses our view to the source and it is observed in absorption as a mildly-relativistic UFO.

HIGHLY IONIZED Fe-K ABSORPTION LINE FROM CYGNUS X-1 IN THE HIGH/SOFT STATE OBSERVED WITH SUZAKU

We present observations of a transient He-like Fe K alpha absorption line in Suzaku observations of the black hole binary Cygnus X-1 on 2011 October 5 near superior conjunction during the high/soft state, which enable us to map the full evolution from the start and the end of the episodic accretion phenomena or dips for the first time. We model the X-ray spectra during the event and trace their evolution. The absorption line is rather weak in the first half of the observation, but instantly deepens for ~10 ks, and weakens thereafter. The overall change in equivalent width is a factor of ~3, peaking at an orbital phase of ~0.08. This is evidence that the companion stellar wind feeding the black hole is clumpy. By analyzing the line with a Voigt profile, it is found to be consistent with a slightly redshifted Fe XXV transition, or possibly a mixture of several species less ionized than Fe XXV. The data may be explained by a clump located at a distance of ~10^(10-12) cm with a density of ~10^((-13)-(-11)) g cm^-3, which accretes onto and/or transits the line-of-sight to the black hole, causing an instant decrease in the observed degree of the ionization and/or an increase in density of the accreting matter. Continued monitoring for individual events with future X-ray calorimeter missions such as ASTRO-H and AXSIO will allow us to map out the accretion environment in detail and how it changes between the various accretion states.

Variable Lyα sheds light on the environment surrounding GRB 090426

Long duration gamma-ray bursts are commonly associated with the deaths of massive stars. Spectroscopic studies using the afterglow as a light source provide a unique opportunity to unveil the medium surrounding it, probing the densest region of their galaxies. This material is usually in a low ionisation state and at large distances from the burst site, hence representing the normal interstellar medium in the galaxy. Here we present the case of GRB 090426 at z=2.609, whose optical spectrum indicates an almost fully ionised medium together with a low column density of neutral hydrogen. For the first time, we also observe variations in the Ly alpha absorption line. Photoionisation modeling shows that we are probing material from the vicinity of the burst (~80 pc). The host galaxy is a complex of two luminous interacting galaxies, which might suggest that this burst could have occurred in an isolated star-forming region outside its host galaxy created in the interaction of the two galaxies.

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0.6" away. The spectrum shows a depression in flux in between 4000 A and 5500 A. This could be the result of a 2175 A-type extinction feature in the host of GRB991216, but at a rather red wavelength of 2360 A. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB011211 are identified with lines originating in a single absorption system at z=2.142+/-0.002, the redshift of the GRB011211 host galaxy. We also detect the Lyman alpha absorption line in the host, to which we fit a neutral hydrogen column density of log N(HI)=20.4+/-0.2, which indicates that it is a damped Lyman alpha system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB011211 redshift. For GRB021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [OII] 3727 at z=1.006. The corresponding unobscured [OII] star-formation rate is 1.4 Msun/yr.

Probing Halos of Galaxies at Very Large Radii Using Background QSOs

(ABRIDGED) Gaseous halos of nine nearby galaxies (with redshifts cz < 6000 km/s) were probed at large galactocentric radii using background quasars observed with HST GHRS and STIS. The projected quasar-galaxy separations range from 55 to 387 h^{-1}_{75} kpc. Ly\alpha absorption lines were successfully detected in the spectra of five quasars, at impact parameters of up to 170 h^{-1}_{75} kpc from the center of the nearby galaxy, and in each case at wavelengths consistent with the galaxy's redshift. Our observations include the lowest redshift Ly\alpha lines detected to date. HI velocity fields were obtained at the VLA for three of the galaxies in our sample (and in one case was available from the literature), to derive their rotation curves. When comparing the inner rotation curves of the galaxies with the velocity at large radius provided by the Ly\alpha line it is apparent that it is very difficult to explain the observed Ly\alpha velocity as due to gas in an extended rotating disk. In most cases one would need to invoke large warps in the outer gas disks and also thick gas disks in order to reconcile the observed velocities with the predicted ones. In light of these results we conclude that in a typical galaxy there is no longer detectable atomic gas corotating in an extended disk at radii > 35 \alpha ^{-1}, where \alpha^{-1} is the stellar disk exponential scale-length. The cosmic web is the most likely origin for the detected Ly\alpha lines.

Discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 with XMM-Newton

We report the discovery of narrow X-ray absorption features from the dipping low-mass X-ray binary X 1624-490 during an XMM-Newton observation in 2001 February. The features are identified with the K alpha absorption lines of Fe xxv and Fe xxvi and have energies of 6.72 +/- 0.03 keV and 7.00 +/- 0.02 keV and equivalent widths (EWs) of -7.5 +1.7 -6.3 eV and -16.6 +1.9 -5.9 eV, respectively. The EWs show no obvious dependence on orbital phase, except during a dip, and correspond to a column of greater than 10^17.3 Fe atom /cm2. In addition, faint absorption features tentatively identified with Ni xxvii K alpha and Fe xxvi K beta may be present. A broad emission feature at 6.58 +0.07 -0.04 keV with an EW of 78 +19 -6 eV is also evident. This is probably the 6.4 keV feature reported by earlier missions since fitting a single Gaussian to the entire Fe-K region gives an energy of 6.39 +0.03 -0.04 keV. A deep absorption feature is present during the dip with an energy consistent with Fe xxv K alpha. This is the second dipping LMXRB source from which narrow Fe absorption features have been observed. Until recently the only X-ray binaries known to exhibit narrow X-ray absorption lines were two superluminal jet sources and it had been suggested that these features are related to the jet formation mechanism. It now appears likely that ionized absorption features may be common characteristics of accreting systems with accretion disks.

Lyα absorption in the nearby Universe: the sightline to Q1821+643

We present the first results of a survey designed to understand the origin of the Lyman alpha-forest absorption systems at low redshift. Using the WYFFOS and HYDRA multi-fibre spectrographs on the WHT and WIYN telescopes, we have identified 51 galaxies brighter than b_j=18.5 within 30 arcmins of the sightline of the QSO 1821+643. We find three galaxies within 500 h-1 kpc of the QSO sightline; the nearest galaxy is 104 h-1 kpc away from the line of sight, and is at the same redshift as a strong Lyman alpha absorption line. The remaining two galaxies have no corresponding absorption to extremely low equivalent width limits (< 0.05 \AA). Beyond 500 h-1 kpc Lyman-alpha absorption lines are found at similar redshifts as several galaxies, but we show that these coincidences are likely to be accidental. Half of the Lyman alpha systems for which we could have found at least an L^* galaxy have no galaxies at the absorbers' redshifts. For the majority of the remainder, we show that any apparent association with galaxies is probably coincidental. These \lya systems are characterised by their weak equivalent widths (W_r < 0.2 \AA), and we conclude that this population of absorbers is uncorrelated, or at best, weakly correlated, with galaxies.

The Dust‐to‐Gas Ratio in the Damped Lyα Clouds toward the Gravitationally Lensed QSO 0957+561

We present HST/FOS spectra of the two bright images (A and B) of the gravitationally lensed QSO 0957+561 in the wavelength range 2200-3300 A. We find that the absorption system (Z(sub abs)) = 1.3911) near z(sub em) is a weak, damped Ly alpha system with strong Ly alpha absorption lines seen in both images. However, the H(I) column densities are different, with the line of sight to image A intersecting a larger column density. The continuum shapes of the two spectra differ in the sense that the flux level of image A increases more slowly toward shorter wavelengths than that of image B. We explain this as the result of differential reddening by dust grains in the damped Ly alpha absorber. A direct outcome of this explanation is a determination of the dust-to-gas ratio, k, in the damped Ly alpha system. We derive k = 0.55 + 0.18 for a simple 1/lambda extinction law and k = 0.31 + 0.10 for the Galactic extinction curve. For gravitationally lensed systems with damped Ly alpha absorbers, our method is a powerful tool for determining the values and dispersion of k, and the shapes of extinction curves, especially in the FUV and EUV regions. We compare our results with previous work.

Detection of H(alpha) Emission From The Magellanic Stream: Evidence For an Extended Gaseous Galactic Halo

We have detected faint, diffuse H$\alpha emission from several points along the Magellanic Stream, using the Rutgers Fabry--Perot Interferometer at the CTIO 1.5-m telescope. At points on the leading edges of the H I clouds MS II, MS III, and MS IV, we detect H$\alpha emission of surface brightness $0.37 \pm 0.02$ Rayleighs, $0.21 \pm 0.04$ R, and $0.20 \pm 0.02$ R respectively, corresponding to emission measures of 1.0 to 0.5 \cmsixpc. We have observed several positions near the MS IV concentration, and find that the strongest emission is on the sharp leading-edge density gradient. There is less emission at points away from the gradient, and halfway between MS III and MS IV the H$\alpha surface brightness is $< 0.04$ R. We attribute the H$\alpha emission at cloud leading edges to heating of the Stream clouds by ram pressure from ionized gas in the halo of the Galaxy. These observations suggest that ram pressure from halo gas plays a large role in stripping the Stream out of the Magellanic Clouds. They also suggest the presence of a relatively large density of gas, $n_{\rm H} \sim 10^{-4} cm^{-3}$, in the Galactic halo at $\sim 50$ kpc radius, and far above the Galactic plane, $b \sim -80°$. This implies that the Galaxy has a very large baryonic, gaseous extent, and supports models of Lyman-$\alpha and metal-line QSO absorption lines in which the absorption systems reside in extended galactic halos.

The Local LY alpha Forest: Association of Clouds with Superclusters and Voids

The Goddard High Resolution Spectrograph aboard the Hubble Space Telescope was used with the G160M grating to obtain high-resolution (6.2 A) spectra of three very bright active galactic nuclei located behind voids in the nearby distribution of bright galaxies (i.e., CfA and Arecibo redshift survey regions). A total of eight definite (greater than or equal to 4 sigma) Ly(alpha) absorption lines were discovered ranging in equivalent width from 26 to 240 mA at Galactocentric velocities 1740-7740 km/s. Of these eight systems, we locate seven in supercluster structures and one, in the sight line of Mrk 501 at 7740 km/s, in a void. In addition, one of two tentative (3-4 sigma) Ly(alpha) absorption lines are found in voids. Thus, the voids are not entirely devoid of matter, and not all Ly(alpha) clouds are associated with galaxies. Also, since the path lengths through voids and superclusters probed by our observations thus far are nearly equal, there is some statistical evidence that the Ly(alpha) clouds avoid the voids. The nearest galaxy neighbors to these absorbing clouds are 0.45-5.9 Mpc away, too far to be physically associated by most models. The lower equivalent width absorption lines (W(sub lambda) less than or equal to 100 mA) are consistent with random locations with respect to galaxies and may be truly intergalactic, similar to the bulk of the Ly(alpha) forest seen at high z. These results on local Ly(alpha) clouds are in full agreement with those found by Morris et al. (1993) for the 3C 273 sight line but are different from the results for higher equivalent width systems where closer cloud-galaxy associations were found by Lanzetta et al. (1994). Pencil-beam optical and 21 cm radio line observations of the area of sky surrounding Mrk 501 fail to find faint galaxies near the velocities of the Ly(alpha) clouds in that sight line. Specifically, for the 'void absorption' system at 7740 km/s, we find no galaxy at comparable redshift to the absorber within 100 h(sub 75)(sup -1) kpc (H(sub 0) = 75 h(sub 75) km/s Mpc(sup -1)) with an absolute magnitude of B less than or equal to - 16 and no object with H I mass greater than or equal to 7 x 10(exp 8) h(sub 75)(sup -2) M(solar) within 500 h(sub 75)(sup -1) kpc. Thus, neither a faint optical galaxy nor a gas-rich, optically dim or low surface brightness galaxy is present close to this absorber.

A search for H I 21 CM absorption and emission from a damped LY alpha absorption line system at redshift Z approximately equal to 3.06 in the direction of PKS 0336-017

A search for H I 21 CM absorption and emission from a damped LY alpha absorption line system at redshift Z approximately equal to 3.06 in the direction of PKS 0336-017

An IUE survey of interstellar H I LY alpha absorption. 1: Column densities

view Abstract Citations (279) References (77) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An IUE Survey of Interstellar H i LY alpha Absorption. I. Column Densities Diplas, Athanassios ; Savage, Blair D. Abstract We measure Galactic interstellar neutral hydrogen column densities by analyzing archival interstellar Ly alpha absorption line data toward 554 B2 and hotter stars observed at high resolution with the IUE satellite. This study more than doubles the number of lines of sight with measures of N(H I) based on Ly alpha. We have included the scattered light background correction algorithm of Bianchi and Bohlin in our data reduction. We use the correlation between the Balmer discontinuity (c1) index and the stellar Ly alpha absorption in order to assess the effects of stellar Ly alpha contamination. Approximately 40% of the B stars with measured (c1) index, exhibit serious stellar Ly alpha contamination. One table contains the derived values of the interstellar N(H I) for 393 stars with at most small amounts of stellar contamination. Another lists the observed values of total N(H I) for 161 stars with suspected stellar Ly alpha contamination and/or uncertain stellar parameters. Publication: The Astrophysical Journal Supplement Series Pub Date: July 1994 DOI: 10.1086/192052 Bibcode: 1994ApJS...93..211D Keywords: Absorption Spectra; Astronomical Spectroscopy; H I Regions; Interstellar Gas; Lyman Alpha Radiation; Lyman Spectra; Ultraviolet Astronomy; Background Radiation; Gas Density; Hot Stars; Iue; Astrophysics; ISM: ABUNDANCES; STARS: EARLY-TYPE; ULTRAVIOLET: ISM full text sources ADS | data products SIMBAD (554) CDS (2) MAST (1) Related Materials (2) Catalog: 1997yCat..20930211D Part 2: 1994ApJ...427..274D

A search for H I 21 CM emission from a damped Ly(alpha) absorption line system and possible cluster of galaxies at redshift Z approximately = 3.4 in the direction of Q0000-2619

A search for H I 21 CM emission from a damped Ly(alpha) absorption line system and possible cluster of galaxies at redshift Z approximately = 3.4 in the direction of Q0000-2619

An IUE survey of interstellar H I LY alpha absorption. 2: Interpretations

We present an analysis of interstellar neutral hydrogen column densities obtained from archival Ly alpha absorption line data toward the complete sample of B2 and hotter stars observed at high spectral resolution with the IUE satellite. The full sample includes 554 stars and more than doubles the number of lines of sight previously observed. Many of the B2 and B1.5 stars exhibit contamination from stellar Ly alpha absorption, and were excluded from the subsequent analysis. The final working sample includes 393 stars. We present statistical averages for a number of quantities derived for the sample. The stars range in distance from 0.12 to 11 kpc with an average distance of 2.1 kpc. Values for average sight-line density range from 0.017 to 8.62 atoms cm(exp -3), with an overall average of 0.23 atoms cm(exp -3). Neutral hydrogen and dust are well correlated, with N(H I)/E(B - V) = 4.93 x 10(exp 21) cm(exp -2) mag(exp -1), and N(H I)/E(Bump) = 2.17 x 10(exp 21) cm(exp -2) mag(exp -1). There is an increase in the H I to dust ratios for the densest sight lines. Evidently, the H I to dust ratios in dense clouds are higher than in the lower density medium between the clouds and in interarm directions. The effect is probably an indication of dust modification in the dense clouds. The H I to dust ratios are approx. 17% smaller for the sight lines to O stars compared to the ratios found toward B stars. We have identified sight lines with anomalously large and small values of N(H I)/E(B - V) and H(H I)/E(Bump). Some of these sight lines are well-known examples of peculiar extinction (i.e., HD 147933 and HD 37061 (NU Ori)). Others are less well known and may provide important new examples of regions with highly modified dust. The number of stars with anomalously low H I to dust ratios is disproportionately larger than the number of stars with anomalously high H I to dust ratios. The distribution of the gas away form the Galactic plane for the sample of objects is complex. For the subset of 375 stars whose lines of sight have a statistically low probablility of intersecting a large cloud, the distribution may be roughly approximated by an exponential density distribution, with a midplane density of 0.366 atoms cm(exp -3), a scale height of 195 pc, and a random logarithmic scattering parameter sigma (sub p) of 0.159 dex. For the same sample of stars, interstellar dust is found to be confined closer to the plane than the H I, with (E(B - V)/r)(sub 0) = 0.257 mag kpc(exp -1), scale height h = 152 pc, and sigma(sub p) = 0.263 dex. The dust distribution is more inhomogeneous than the gas distribution. The observed distribution of H I away from the Galactic plane can also be fitted with a two-component exponential model including the effects of patchiness. Unfortunately, the foreground contamination produced by the cloudy and more confined component of H I makes it difficult to estimate the scale height of the extended component without invoking an independent estimate of its midplane density. Assuming n(sub 2)(0) = 0.16 cm(exp -3) from the Copernicus satellite H I/H(sub 2) survey, we obtain n(sub 1)(0) = 0.247 cm(exp -3), h(sub 1) = 73 PC, and h(sub 2) + 357 pc.

Stellar spectral reconstruction from speckle spectroscopic data, 2

In the previous paper, we described speckle spectroscopic instrumentation and presented some observational results with data acquired by our speckle spectrometer. The shift-and-add (SAA) method was used for the data reduction in the previous paper. We have revised the speckle spectrometer to augment the performance. The cross-correlation method is newly introduced for data reduction. The spectra of single stars gamma Cas and alpha Per reconstructed in nearly diffraction-limited angular resolution with the resolved H alpha emission and absorption lines, respectively. It is shown that the cross-correlation method is superior to the shift-and-add method in the narrow bandwidth region. The results of alpha Per are compared with the photometric measurement of Procyon's spectrum which has the similar spectral type as alpha Per and they are shown to be fairly consistent. The new system exhibits a sufficient performance to enable us to make the quantitative spectral analysis and the cross-correlation method produces the higher SNR results.

Causes and effects of the first quasars.

The light we observe from the most distant known quasars set out when the Universe was about 200 times denser than it is now and less than one-tenth of its present age. The existence of these objects implies that galaxy formation had already, at that early epoch, proceeded to the stage when massive (>10(8)M[symbol, see text]) objects had accumulated in the centers of at least some young galaxies. A specific model is presented to show that the evolution and luminosity function of quasars are compatible with the cold dark matter cosmogony. Most big galaxies probably passed through a quasar phase; the remnant black holes in nearby galaxies may reveal themselves via the flares that occur whenever a star passes too close to them and gets tidally disrupted. The rich absorption spectra of quasars serve as a probe of the intervening medium. The gas responsible for the Lyman alpha absorption lines may be due to primordial gas gravitationally confined in minihalos of dark matter--shallow potential wells whose evolution and relation to dwarf galaxies are briefly discussed. The patchy heat input into the intergalactic medium from early quasars could modulate the environment in which galaxies form, leading to large-scale spatial correlations in the galaxy distribution. This review concludes with general comments on the prospects for a fully quantitative understanding of galaxy formation.

MULTIWAVELENGTH SPECTROSCOPY OF ACTIVE GALACTIC NUCLEI

ABSTRACT This thesis comprises extensive observational studies of two active galactic nuclei: the bright QSO H1821+643 and the Seyfert 1 galaxy NGC 3516. We report on the first simultaneous spectroscopy of the nearby (z = 0.297) QSO H1821+643 in teh optical, UV, and X-ray bands, in addition to optical, X-ray, and radio mapping. The QSO, which had not been detected at radio frequencies before, exhibits a detailed radio structure in which two separate components are distinguished. In the soft X-ray observations we separated (both spectroscopically and spatially) the emission originating in H1821+643 from that from the nearby hot planetary nucleus K1-16, thus isolating the intrinsic soft X-ray excess in the QSO. In the coadded UV spectrum we detected a Lyman alpha absorption line at z=0.225, which we identify with a foreground galaxy. The combined spectroscopy shows a strong optical/UV bump with a flat slope in the UV (alpha -0.96, fv approaches Valpha) and a steep excess (alpha -4) in the soft X-ray band in addition to a typical hard slope of alpha -0.8 between 0.5 and 10 keV. Standard accretion-disk models can account for the observed optical/UV emission, but fail to fit both the soft X-ray excess and the optica/UV bump. We also report on the first simultaneous UV and hard X-ray observations of the Seyfert 1 galaxy NGC 3516, which were carried out to investigate the connection between the exceptionally strong and variable absorption in the X-ray and ultraviolet bands. The X-ray spectrum is complex, and can be fitted satisfactorily with a warm-absorber model, a partial-covering model, or a cold-reflector model. Lower limits on the column densities of the X-ray and UV absorbers are established at 4 X 1022 and 2 X 1019 cm-2, respectively. Assuming that the variability in the C IV lambda-1550 absorption is due to changes in ionization level, we find that the UV absorbers should be located closer than 9 pc from the ionizing source for log NH 20, and at smaller distances for higher column densities, thus excluding the narrow-line region. Although the absence of short-term variability in absorption during the simultaneous X-ray and UV observations preempted a direct test of the common origin of the UV and X-ray absorption, indirect arguments lead to the conclusion that these absorptions probably do not arise in the same location.

A new spectroscopic survey for damped Ly-alpha absorption lines from high-redshift galaxies

The results of a new survey for distant galaxies identified on the basis of the damped Lyα absorption lines they produce in the spectra of high-redshift background QSOs are presented. These results are based on new, low-resolution spectroscopy of a sample of 57 QSOs, which are combined with similar low-resolution spectroscopy of 53 QSOs, to yield a total of 101 spectra of high-redshift QSOs which are suitable for identifying damped Lyα absorption lines. The spectra are analyzed to generate a complete sample of absorption features which, if the features are assumed to be Lyα lines, have rest-frame equivalent widths satisfying W ≥ 5 A. A total of 84 such damped Lyα absorption-line candidates are detected.

Correlation between the Lyman alpha absorption line density and the emission line redshift of QSOs

A statistical analysis of the absorption line density N( z abs) in the Lαforest of 16 high-redshift quasars shows that the line density is clearly dependent on the emission redshift of the quasar, z em. Not only is the mean density of all the Lα absorption lines larger for larger z em, but more importantly, for each given absorption redshift interval, N( z abs) is also positively correlated with z em. A two-factor variance analysis, with ejection velocity as the second factor, shows that the line density in the Lα forest depends sensitively on z em but not on the ejection velocity. These results are difficult to interpret by either the intervening or the ejecting hypothesis and are unlikely to be due to the selection effects suggested by Carswell et al. [13].