Log NH Research Articles

GRB 081203A: Swift UVOT captures the earliest ultraviolet spectrum of a gamma-ray burst

Abstract We present the earliest ultraviolet (UV) spectrum of a gamma-ray burst (GRB) as observed with the Swift Ultra-Violet/Optical Telescope (UVOT). The GRB 081203A spectrum was observed for 50 s with the UV-grism starting 251 s after the Swift-Burst-Alert-Telescope (BAT) trigger. During this time, the GRB was ≈13.4 mag (u filter) and was still rising to its peak optical brightness. In the UV-grism spectrum, we find a damped Lyα line, Lyβ and the Lyman continuum break at a redshift z= 2.05 ± 0.01. A model fit to the Lyman absorption implies a gas column density of log NH i= 22.0 ± 0.1 cm−2, which is typical of GRB host galaxies with damped Lyα absorbers. This observation of GRB 081203A demonstrates that for brighter GRBs (v≈ 14 mag) with moderate redshift (0.5 < z < 3.5) the UVOT is able to provide redshifts, and probe for damped Lyα absorbers within 4–6 min from the time of the Swift-BAT trigger.

SUZAKUOBSERVATIONS OF NEAR-RELATIVISTIC OUTFLOWS IN THE BAL QUASAR APM 08279+5255

We present results from three Suzaku observations of the z=3.91 gravitationally lensed broad absorption line quasar APM 08279+5255. We detect strong and broad absorption at rest-frame energies of <2 keV (low-energy) and 7-12 keV (high-energy). The detection of these features confirms the results of previous long-exposure (80-90 ks) Chandra and XMM-Newton observations. The low and high-energy absorption is detected in both the back-illuminated (BI) and front-illuminated (FI) Suzaku XIS spectra (with an F-test significance of >99%). We interpret the low-energy absorption as arising from a low-ionization absorber with log NH~23 and the high-energy absorption as due to lines arising from highly ionized (2.75 < log xi < 4.0; where xi is the ionization parameter) iron in a near-relativistic outflowing wind. Assuming this interpretation we find that the velocities in the outflow range between 0.1c and 0.6c. We constrain the angle between the outflow direction of the X-ray absorber and our line of sight to be <36 degrees. We also detect likely variability of the absorption lines (at the >99.9% and >98% significance levels in the FI and BI spectra, respectively) with a rest-frame time scale of ~1 month. Assuming that the detected high-energy absorption features arise from Fe xxv, we estimate that the fraction of the total bolometric energy injected over the quasar's lifetime into the intergalactic medium in the form of kinetic energy to be >10%.

XMM-NEWTONVIEW OF THE MULTIPHASE WARM ABSORBER IN SEYFERT 1 GALAXY NGC 985

We present an analysis of a new XMM-Newton observation of the Seyfert 1 galaxy NGC 985. The European Photon Imaging Camera (EPIC) spectra present strong residuals to a single power-law model, indicating the presence of ionized absorbing gas and a soft excess. A broadband fit to the EPIC and RGS spectra shows that the continuum can be well fitted with a power law (Γ ≈ 1.57) and a blackbody component (kT ≈ 0.09 keV). The RGS can be modeled either with two or three absorption components. In the two absorber model the low-ionization one, with log U ≈ .05 and log NH ≈ 21.08, accounts for the presence of the Fe M-shell unresolved transition array (Fe vii–xiii), and the high-ionization component (log U ≈ 1.31 and log NH ≈ 21.99) is required by the presence of several Fe L-shell transitions. The data suggest the presence of a third ionized component with higher ionization, so that the Fe L-shell absorption features are produced by two different components (one producing absorption by Fe xvii–xx, and the other absorption by Fe xx–xxii). However, the presence of the third absorbing component cannot be detected by means of an isolated absorption line in a significant way, so we consider this detection only as tentative. Interestingly, all ionization components have similar kinematics (with outflow velocities ∼ 280 km s−1). In addition, whether two or three absorbers are considered, the components appear to be in pressure balance within 1σ. These results give further support to the idea that warm absorbers in active galactic nuclei consist of a two- or three-phase medium. We note that, while in the model with only two absorbers one of them (the high-ionization component) lies on an unstable branch of the thermal equilibrium curve, in the model with three absorbers all of the components lie on stable branches of the curve. This gives further plausibility to a multiphase absorber.

Emergence of a quasar outflow

Abstract We report the first discovery of the emergence of a high-velocity broad-line outflow in a luminous quasar, J105400.40+034801.2, at redshift z∼ 2.1. The outflow is evident in ultraviolet C iv and Si iv absorption lines with velocity shifts v∼ 26 300 km s−1 and deblended widths FWHM ∼4000 km s−1. These features are marginally strong and broad enough to be considered broad absorption lines (BALs), but their large velocities exclude them from the standard BAL definition. The outflow lines appeared between two observations in the years 2002.18 and 2006.96. A third observation in 2008.48 showed the lines becoming ∼40 per cent weaker and 10 to 15 per cent narrower. There is no evidence for acceleration or for any outflow gas at velocities ≲23 000 km s−1. The lines appear to be optically thick, with the absorber covering just 20 per cent of the quasar continuum source. This indicates a characteristic absorber size of ∼4 × 1015 cm, but with a BAL-like total column density log NH(cm−2) ≳ 21.2 and average space density nH≳ 2 × 105 cm−3. We attribute the emergence of the outflow lines to a substantial flow structure moving across our line of sight, possibly near the ragged edge of the main BAL flow or possibly related to the onset of a BAL evolutionary phase.

Towards understanding the mass–metallicity relation of quasar absorbers: evidence for bimodality and consequences

Abstract One way to characterize and understand H i-selected galaxies is to study their metallicity properties. In particular, we show that the metallicity of absorbers is a bivariate function of the H i column density (NH i) and the Mg ii equivalent width (Wλ2796r). Thus, a selection upon Wλ2796r is not equivalent to a H i selection for intervening absorbers. A direct consequence for damped absorbers with log NH i &amp;gt; 20.3 that falls from the bivariate metallicity distribution is that any correlation between the metallicity [X/H] and velocity width (using Wλ2796r as a proxy) cannot be interpreted as a signature of the mass–metallicity relation akin to normal field galaxies. In other words, damped Lyα absorber (DLA) samples are intrinsically heterogeneous and the [X/H]–Wλ2796r or [X/H]–Δv correlation reported in the literature arises from the H i cut. On the other hand, a sample of Mg ii-selected absorbers, which are statistically dominated by lowest NH i systems (sub-DLAs) at each Wλ2796r, is found to have a more uniform metallicity distribution. We postulate that the bivariate distribution [[X/H](NH i, Wλ2796r)] can be explained by two different physical origins of absorbers, namely sight lines through the interstellar medium of small galaxies and sight lines through the out-flowing material. Several published results follow from the bivariate [X/H] distribution, namely (i) the properties of the two classes of DLAs, reported by Wolfe et al. and (ii) the constant dust-to-gas ratio for Mg ii absorbers.

Physical Conditions in Quasar Outflows: Very Large Telescope Observations of QSO 2359–1241

We analyze the physical conditions of the outflow seen in QSO 2359-1241 (NVSS J235953–124148), based on high-resolution spectroscopic VLT observations. This object was previously studied using Keck HIRES data. The main improvement over the HIRES results is our ability to accurately determine the number density of the outflow. For the major absorption component, the populations from five different Fe II excited levels yield a gas density nH = 104.4 cm−3 with less than 20% scatter. We find that the Fe II absorption arises from a region with roughly constant conditions and temperature greater than 9000 K, before the ionization front where temperature and electron density drop. Further, we model the observed spectra and investigate the effects of varying gas metallicities and the spectral energy distribution of the incident ionizing radiation field. The accurately measured column densities allow us to determine the ionization parameter (log UH ≈ − 2.4) and total column density of the outflow [log NH(cm −2) ≈ 20.6]. Combined with the number density finding, these are stepping stones toward determining the mass flux and kinetic luminosity of the outflow, and therefore its importance to AGN feedback processes.

SwiftBAT Survey of AGNs

We present the results of the analysis of the first 9 months of data of the Swift BAT survey of AGNs in the 14-195 keV band. Using archival X-ray data or follow-up Swift XRT observations, we have identified 129 (103 AGNs) of 130 objects detected at | b| > 15° and with significance >4.8 σ. One source remains unidentified. These same X-ray data have allowed measurement of the X-ray properties of the objects. We fit a power law to the log N–log S distribution, and find the slope to be 1.42 ± 0.14. Characterizing the differential luminosity function data as a broken power law, we find a break luminosity log L*(erg s−1) = 43.85 ± 0.26, a low-luminosity power law slope a = 0.84+ 0.16−0.22, and a high-luminosity power law slope b = 2.55+ 0.43−0.30, similar to the values that have been reported based on INTEGRAL data. We obtain a mean photon index 1.98 in the 14-195 keV band, with an rms spread of 0.27. Integration of our luminosity function gives a local volume density of AGNs above 1041 erg s−1 of 2.4 × 10−3 Mpc−3, which is about 10% of the total luminous local galaxy density above M* = − 19.75. We have obtained X-ray spectra from the literature and from Swift XRT follow-up observations. These show that the distribution of log nH is essentially flat from nH = 1020 to 1024 cm−2, with 50% of the objects having column densities of less than 1022 cm−2. BAT Seyfert galaxies have a median redshift of 0.03, a maximum log luminosity of 45.1, and approximately half have log nH > 22.

Characterizing the Low‐Redshift Intergalactic Medium toward PKS 1302−102

We present a detailed analysis of the intergalactic metal-line absorption systems in the archival HST STIS and FUSE ultraviolet spectra of the low-redshift quasar PKS 1302?102 ( -->zQSO = 0.2784). We supplement the archive data with CLOUDY ionization models and a survey of galaxies in the quasar field. There are 15 strong Ly? absorbers with column densities --> NH I > 14. Of these, six are associated with at least C III ?977 absorption [ -->log N(C+ +) > 13]; this implies a redshift density --> dNC III/dz = 36-9+13 (68% confidence limits) for the five detections with rest equivalent width -->Wr > 50 m?. Two systems show O VI ??1031, 1037 absorption in addition to C III [ -->log N(O+ 5) > 14]. One is a partial Lyman limit system (--> log NH I = 17) with associated C III, O VI, and Si III ?1206 absorption. There are three tentative O VI systems that do not have C III detected. For one O VI doublet with both lines detected at 3 ? with -->Wr > 50 m?, --> dNO VI/dz = 7-4+9. We also search for O VI doublets without Ly? absorption but identify none. From CLOUDY modeling, these metal-line systems have metallicities spanning the range ? -->4 [ M/H ] ? 0.3. The two O VI systems with associated C III absorption cannot be single-phase, collisionally ionized media based on the relative abundances of the metals and kinematic arguments. From the galaxy survey, we discover that the absorption systems are in a diverse set of galactic environments. Each metal-line system has at least one galaxy within 500 km s?1 and 600 -->h?175 kpc with -->L > 0.1L*.

Growth of Massive Black Holes during Radiatively Inefficient Accretion Phases

We derive the black hole mass density as a function of redshift from the hard X-ray active galactic nucleus (AGN) luminosity function, assuming that massive black holes grow via accreting circumnuclear gases. The derived black hole mass density matches the measured local black hole mass density at z = 0, which requires the average radiative efficiency of AGNs to be ~0.1-0.17. The massive black holes in most faint AGNs and even normal galaxies are still accreting gases, although their accretion rates are very low. Radiatively inefficient accretion flows (RIAFs) are supposed in these faint sources, which should radiate mostly in the hard X-ray band. We calculate the contribution to the X-ray background (XRB) from both the bright AGNs and the RIAFs in faint AGNs and normal galaxies. Our calculations show that both the observed intensity and spectral shape of the XRB with an energy peak at ~30 keV can be well reproduced without including the emission of Compton-thick AGNs, if the massive black holes in faint AGNs and normal galaxies are spinning rapidly, with a ~ 0.9, and are accreting at rates of ~ (1.0-3.0) × 10-4. It indicates that less than ~5% of the local massive black hole mass density was accreted during radiatively inefficient accretion phases, which is obviously only an upper limit, because Compton-thick AGNs have not been considered. If the same number of Compton-thick AGNs with log NH = 24-25 as those with log NH = 23-24 is considered, the fraction of the local black hole mass density accumulated during inefficient accretion phases should be lower than ~2%. The constraints of the XRB can provide upper limits on the average accretion rate for inactive galaxies.

Unveiling the Cygnus OB2 stellar population with Chandra

Aims. The aim of this work is to identify the so far unknown low mass stellar population of the∼ 2Myr old Cygnus OB2 star forming region, and to investigate the X-ray and near-IR stellar properties of its members. Methods. We analyzed a 97.7 ksec Chandra ACIS-I observation pointed at the core of the Cygnus OB2 region. Sources were detected using the PWDetect code and were then positionally correlated with optical and near-IR catalogs from the literature. Source events were then extracted with the Acis Extract package. X-ray variability was characterized through the Kolmogorov-Smirnov test and spectra were fitted using absorbed thermal plasma models. Results. We detected 1003 X-ray sources. Of these, 775 have near-IR counterparts and are expected to be almost all associated with Cygnus OB2 members. From near-IR color-color and color-magnitude diagrams we estimate a typical absorption toward Cygnus OB2 of Av≈ 7.0 mag. Although the region is young, very few stars (∼4.4%) show disk-induced excesses in the near-IR. X-ray variability is detected in ∼13% of the sources, but this fraction increases, up to 50%, with increasing source statistics. Flares account for at leas t 60% of the variability. Despite being generally bright, all but 2 of the 26 detected O- that early B-type stars are not significantly variable. Typ ical X-ray spectral parameters are log NH∼ 22.25 (cm −2 ) and kT∼ 1.35 keV with 1σ dispersion of 0.2 dex and 0.4 keV, respectively. Variable and flaring sources have harder spectra with median kT=3.3 and 3.8 keV, respectively. OB stars are typically softer (kT∼ 0.75 keV). X-ray luminosities range between 10 30 and 10 31 erg s −1 for intermediate- and low-mass stars, and 2.5×10 30 and between 6.3×10 33 erg s −1 for OB stars. Conclusions. The Cygnus OB2 region has a very rich population of low-mass X-ray emitting stars. Circumstellar disks seem to be very scarce. X-ray variability is essentially related to the magnetic ac tivity of low-mass stars (M/M⊙∼0.5 to 3.0) which display X-ray activity levels comparable to those of Orion Nebular Cluster (ONC) sources in the same mass range.

Hard X-ray luminosity function and absorption distribution of nearby AGN: INTEGRAL all-sky survey

We use the INTEGRAL all-sky hard X-ray survey to perform a statistical study of a representative sample of nearby AGN. Our entire all-sky sample consists of 127 AGN, of which 91 are confidently detected (>5 sigma) on the time-averaged map obtained with the IBIS/ISGRI instrument and 36 are detected only during single observations. Among the former there are 66 non-blazar AGN located at |b|>5 deg, where the survey's identification completeness is ~93%, which we use for calculating the AGN luminosity function and X-ray absorption distribution. In broad agreement with previous studies, we find that the fraction of obscured (log NH>22) objects is much higher (~70%) among the low-luminosity AGN (Lx<10^43.6 erg/s) than among the high-luminosity ones (Lx>10^43.6 erg/s), \~25%, where Lx is the luminosity in the 17-60 keV energy band. We also find that locally the fraction of Compton-thick AGN is less than 20% unless there is a significant population of AGN that are so strongly obscured that their observed hard X-ray luminosities fall below 10^40-10^41 erg/s, the effective limit of our survey. The constructed hard X-ray luminosity function has a canonical, smoothly broken power-law shape in the range 40<log Lx<45.5 with a characteristic luminosity of log L*=43.40+/-0.28. The estimated local luminosity density due to AGN with log Lx>40 is (1.4+/-0.3) 10^39 erg/s/Mpc^3 (17-60 keV). We demonstrate that the spectral shape and amplitude of the CXB are consistent with the simple scenario in which the NH distribution of AGN (for a given Lx/L*(z) ratio has not changed significantly since z~1.5, while the AGN luminosity function has experienced pure luminosity evolution.

New submillimeter diagnostics of physical properties of ISM in high redshift galaxies

AbstractWe present a new diagnosis method for determining physical properties of star-forming gas in high-z galaxies. In this method, we employed three key observational quantities, [CI], CO, and FIR luminosities, including our new detections of CO J = 4–3 emission from the pure-starburst (non-AGN) submm galaxy SMM J14011+0252 (z = 2.6) and the type-2 AGN IRAS FSC 10214+4724 (z = 2.3) obtained with the Nobeyama Millimeter Array (NMA) at the Nobeyama Radio Observatory. These two sources have extremely high star formation rate, and exhibit strong emission of CO and [CI] 609 μm lines. We determined ISM physical conditions for the two objects and another three high-z quasars in order to investigate the relationship between their ISM and power sources (i.e., massive star formation or AGN). A new PDR analysis (Wolfire et al. 2005, private communication) using CO, [CI], and FIR on five high-z sources provides new evidence that AGN host galaxies harbor denser (log nH ~ 5–6) ISM exposed to stronger far-UV fluxes of log G0 ~ 3.5–4 than the non-AGN submm galaxy. Volume filling factors of the star-forming dense gas in the AGN hosts are an order of magnitude smaller than that of the pure-starburst submm galaxy. This suggests that, in these AGN hosts, dense molecular clouds are dominating the central kpc around AGN, triggering extensive circumnuclear starbursts, and possibly feeding their central supermassive black hole simultaneously.

Spectral Statistics and Local Luminosity Function of a Complete Hard X-Ray Sample of the Brightest Active Galactic Nuclei

We have measured the X-ray spectral properties of a complete flux-limited sample of bright active galactic nuclei (AGNs) from the HEAO-1 all-sky catalogs to investigate their statistics and provide greater constraints on the bright end of the hard X-ray luminosity function (HXLF) and the AGN population synthesis model of the X-ray background. Spectral studies using data from ASCA, XMM-Newton, and/or BeppoSAX observations have been made for almost all AGNs in this sample. The spectral measurements enable us to construct the neutral absorbing column density (log NH) distribution and separate HXLFs for absorbed (log NH[cm-2] > 21.5) and unabsorbed AGNs in the local universe. Our results show evidence of a difference in the shapes of the HXLFs of absorbed and unabsorbed AGNs in that the absorbed AGN HXLF drops more rapidly at higher luminosities than that of unabsorbed AGNs, similar to what has been previously reported. In the LX-NH plot we find no AGNs in the high-luminosity, high-intrinsic-absorption regime (log LX[ergs s-1] > 44.5 and log NH[cm-2] > 21.5) in our sample, in which we expect approximately five AGNs if we assume that the absorbed and unabsorbed AGNs have identical HXLF shapes. We also find that the fluxes observed with ASCA or XMM-Newton are smaller than that observed with HEAO-1 by a factor of 0.29 on average, which is expected for reobservation of sources with a factor of ~2.5 variability amplitude scale.

A Rich Population of X-Ray-emitting Wolf-Rayet Stars in the Galactic Starburst Cluster Westerlund 1

Recent optical and infrared studies have revealed that the heavily reddened starburst cluster Westerlund 1 (Wd 1) contains at least 22 Wolf-Rayet (W-R) stars, constituting the richest W-R population of any Galactic cluster. We present results of a sensitive Chandra X-ray observation of Wd 1 that detected 12 of the 22 known W-R stars and the mysterious emission-line star W9. The fraction of detected WN stars is nearly identical to that of WC stars. The WN stars WR-A and WR-B, as well as W9, are exceptionally luminous in X-rays and have similar hard, heavily absorbed X-ray spectra with strong Si XIII and S XV emission lines. The luminous high-temperature X-ray emission of these three stars is characteristic of colliding-wind binary systems, but their binary status remains to be determined. Spectral fits of the X-ray-bright sources WR-A and W9 with isothermal plane-parallel shock models require high absorption column densities, log NH = 22.56 (cm-2), and yield characteristic shock temperatures kTs ≈ 3 keV (Ts ≈ 35 MK).

FUSEObservation of the Narrow‐Line Seyfert 1 Galaxy RE 1034+39: Dependence of Broad Emission Line Strengths on the Shape of the Photoionizing Spectrum

We present an analysis from simultaneous FUSE, ASCA, and EUVE observations, as well as a reanalysis of archival HST spectra, from the extreme narrow-line Seyfert 1 Galaxy RE 1034+39 (KUG 1031+398). RE 1034+39 has an unusually hard spectral energy distribution (SED) that peaks in the soft X-rays. Its emission lines are unusual in that they can all be modeled as a Lorentzian centered at the rest wavelength with only a small range in velocity widths. In order to investigate whether the unusual SED influences the emission-line ratios and equivalent widths, we present three complementary types of photoionization analysis. The FUSE spectrum was particularly important because it includes the high-ionization line O VI. First, we use the photoionization code CLOUDY and the SED developed from the coordinated observations to confirm that the emission lines are consistent with observed hard SED. The best model parameters were an ionization parameter log U ≈ -2 and a hydrogen number density log nH = 9.75 (cm-2). Second, we present a locally optimally emitting cloud model. This model produced enhanced O VI as observed, but also yielded far too strong Mg II. Third, we develop a series of semiempirical SEDs, run CLOUDY models, and compare the results with the measured values using a figure of merit (FOM). The FOM minimum indicates SED and gas properties similar to those inferred from the one-zone model using the RE 1034+39 continuum. Furthermore, the FOM increases sharply toward softer continua, indicating that a hard SED is required by the data in the context of a one-zone model.

AChandraHETGS Spectral Study of the Iron K Bandpass in MCG −6‐30‐15: A Narrow View of the Broad Iron Line

We present a high-resolution X-ray spectrum of the iron K bandpass in MCG -6-30-15 based on a 522 ks observation with Chandra's High Energy Transmission Grating Spectrometer (HETGS). The Chandra spectrum is consistent with the presence of a relativistically broadened, highly redshifted iron Kα emission line with a profile similar to previous observations. A number of narrow features are detected above 2 keV, including a narrow Fe Kα emission line and narrow absorption lines from H- and He-like Fe, H-like S, and H-like Si. This absorption is well described by a photoionized plasma with a column density log NH = 23.2 and an ionization parameter log ξ = 3.6, assuming the iron abundance has the solar value and a velocity dispersion parameter b = 100 km s-1. Applying this absorption model to a high-fidelity XMM-Newton EPIC pn spectrum, we find that a broad iron line is still required with emission extending to within 1.9rg of the black hole. If the iron line comes from an accretion disk truncated at the innermost stable circular orbit, this indicates that the black hole must be spinning rapidly with a > 0.95. Ionized absorption models attempting to explain the 3-6 keV spectral curvature without strong gravity predict absorption lines in the 6.4-6.6 keV range that are inconsistent with the Chandra spectrum. The H- and He-like iron absorption lines in the Chandra spectrum are blueshifted by 2.0 × 103 km s-1 compared to the source frame and may originate in a high-velocity, high-ionization component of the warm absorber outflow. This high-ionization component may dominate the energy budget of the outflow and account for a significant fraction of the outflowing mass. Detailed modeling of the warm absorber below 2 keV will be addressed in a later paper, but our results are robust to the broader details of the warm absorber behavior. The difference spectrum between the high- and low-flux states is well described by a power law, in agreement with previous studies.

HiGas in Higher Density Regions of the Intergalactic Medium

Using H I absorption alone, we attempt to separate H I absorption lines in quasar spectra into two categories: higher density lines (HDLs) and lower density lines (LDLs), and we discuss the difference in their physical properties. We deblend and fit all H I lines with Voigt profiles and make an unbiased sample of H I lines covering a wide column density range (12 < log NH I [cm-2] < 19). To reduce the influence of line blending, we simultaneously fit several Lyman series lines. As a result of a two-point correlation analysis, we found that higher column density H I lines are clustering at Δv < 200 km s-1, whereas lower ones cluster at Δv < 100 km s-1. We define HDLs as H I lines with 15 < log NH I < 19 and all H I lines within 200 km s-1 of a line with log NH I > 15 and LDLs as all others with 12 < log NH I < 15. We found that the HDLs have smaller minimum b-values for a given column density than the LDLs. This difference is successfully reproduced by our hydrodynamic simulation. The LDLs seem to be cool or shock-heated diffuse intergalactic medium gas, whereas the HDLs are likely to be cooler dense gas near to galaxies.

Hubble Space TelescopeSTIS Ultraviolet Spectral Evidence of Outflow in Extreme Narrow‐Line Seyfert 1 Galaxies. II. Modeling and Interpretation

We present modeling to explore the conditions of the broad-line-emitting gas in two extreme narrow-line Seyfert 1 galaxies, using the observational results described in the first paper of this series. Photoionization modeling using CLOUDY was conducted for the broad blueshifted wind lines and the narrow, symmetric, rest-wavelength-centered disk lines separately. A broad range of physical conditions was explored for the wind component, and a figure of merit was used to quantitatively evaluate the simulation results. Of the three minima in the figure-of-merit parameter space, we favor the solution characterized by an X-ray-weak continuum, elevated abundances, a small column density (log NH ? 21.4), a relatively high ionization parameter (log U ? -1.2 to -0.2), a wide range of densities (log n ? 7-11), and a covering fraction of ~0.15. The presence of low-ionization emission lines implies that the disk component is optically thick to the continuum, and the Si III]/C III] ratio implies a density of 1010-1010.25 cm-3. A low ionization parameter (log U = -3) is inferred for the intermediate-ionization lines, unless the continuum is filtered through the wind before illuminating the intermediate-line-emitting gas, in which case log U = -2.1. The location of the emission regions was inferred from the photoionization modeling and a simple toy dynamical model. A large black hole mass (1.3 ? 108 M?) radiating at 11% of the Eddington luminosity is consistent with the kinematics of both the disk and wind lines, and an emission radius of ~104 RS is inferred for both. We compare these results with previous work and discuss implications.

Classical and Mg ii-selected damped Lyman α absorbers: impact on ΩH iatz&lt; 1.7

The damped Lyα systems (DLAs), seen in absorption in the spectrum of background quasars, are believed to contain a large fraction of the neutral gas in the Universe. Paradoxically, these systems are more difficult to observe at zabs < 1.7, since they are rare and their H I feature then falls in UV spectra, thus requiring the use of space-borne facilities. In order to overcome this observational difficulty, Rao & Turnshek pioneered a method based on Mg II-selected DLAs; that is, absorbers discovered thanks to our knowledge of their Mg II feature in optical spectra. In the present work, we use new observations undertaken at the TNG as well as a careful literature and archival search to build samples of low-redshift absorbers classified according to the technique used for their discovery. We successfully recover NH I and equivalent widths of Fe II 2600, Mg II 2796, Mg II 2803 and Mg I 2852 for a sample of 36 absorbers, 21 of which are Mg II-selected. We find that the Mg II-selected sample contains a marginally larger fraction of absorbers with log NH I > 21.0 than seen otherwise at low redshift. If confirmed, this property will in turn affect estimates of ΩH I, which is dominated by the highest H I column densities. We investigate the source of the potential discrepancy and find that log NH I does not correlate significantly with metal equivalent widths. Similarly, we find no evidence that gravitational lensing, the fraction of associated systems or redshift evolution affect the absorber samples in a different way. We conclude that the hint of discrepancies in NH I distributions between the Mg II-selected DLAs and the others probably arises from small-number statistics. Therefore, further observations based on both H I and Mg II selection techniques are required in order to clarify the impact on estimates of ΩH I at low redshifts.

The Heavy‐Element Enrichment of Lyα Clouds in the Virgo Supercluster

Using high signal-to-noise ratio echelle spectra of 3C 273 obtained with the Space Telescope Imaging Spectrograph (resolution of 7 km s-1 FWHM), we constrain the metallicities of two Ly? clouds in the vicinity of the Virgo Cluster. We detect C II, Si II, and Si III absorption lines in the Ly? absorber at zabs = 0.00530. Previous observations with the Far Ultraviolet Spectroscopic Explorer have revealed Ly?-Ly? absorption lines at the same redshift, thereby accurately constraining the H I column density. We model the ionization of the gas and derive [C/H] = -1.2, [Si/C] = 0.2 ? 0.1, and log nH = -2.8 ? 0.3. The model implies a small absorber thickness, ~70 pc, and thermal pressure p/k ? 40 cm-3 K. It is most likely that the absorber is pressure confined by an external medium because gravitational confinement would require a very high ratio of dark matter to baryonic matter. Based on a sample of Milky Way sight lines in which carbon and silicon abundances have been reliably measured in the same interstellar cloud (including new measurements presented herein), we argue that it is unlikely that the overabundance of Si relative to C is due to depletion onto dust grains. Instead, this probably indicates that the gas has been predominately enriched by ejecta from Type II supernovae. Such enrichment is most plausibly provided by an unbound galactic wind, given the absence of known galaxies within a projected distance of 100 kpc and the presence of galaxies capable of driving a wind at larger distances (e.g., H I 1225+01). Such processes have been invoked to explain the observed abundances in the hot, X-ray-emitting gas in Virgo. However, the sight line to 3C 273 is more than 10? away from the X-ray emission region. We also constrain the metallicity and physical conditions of the Virgo absorber at zabs = 0.00337 in the spectrum of 3C 273 based on detections of O VI and H I and an upper limit on C IV. If this absorber is collisionally ionized, the O /C limit requires T 105.3 K in the O VI-bearing gas. For either collisional ionization or photoionization, we find that [O/H] -2.0 at zabs = 0.00337.