Highly-ionized Absorber Research Articles

New quasar proximity zone size measurements at z ∼ 6 using the enlarged XQR-30 sample

ABSTRACT Proximity zones of high-redshift quasars are unique probes of their central supermassive black holes as well as the intergalactic medium (IGM) in the last stages of reionization. We present 22 new measurements of proximity zones of quasars with redshifts between 5.8 and 6.6, using the enlarged XQR-30 sample of high-resolution, high-SNR quasar spectra. The quasars in our sample have ultraviolet magnitudes of M1450 ∼ −27 and black hole masses of 109–1010 M⊙. Our inferred proximity zone sizes are 2–7 physical Mpc, with a typical uncertainty of less than 0.5 physical Mpc, which, for the first time, also includes uncertainty in the quasar continuum. We find that the correlation between proximity zone sizes and the quasar redshift, luminosity, or black hole mass, indicates a large diversity of quasar lifetimes. Two of our proximity zone sizes are exceptionally small. The spectrum of one of these quasars, with z = 6.02, displays, unusually for this redshift, damping wing absorption without any detectable metal lines, which could potentially originate from the IGM. The other quasar has a high-ionization absorber ∼0.5 pMpc from the edge of the proximity zone. This work increases the number of proximity zone measurements available in the last stages of cosmic reionization to 87. This data will lead to better constraints on quasar lifetimes and obscuration fractions at high redshift, that in turn will help probe the seed mass and formation redshift of supermassive black holes.

Fast Outflowing Warm Absorbers in Narrow-line Seyfert 1 Galaxy PG 1001+054 Revealed by HST/COS Spectra

Narrow-line Seyfert 1 (NLS1) galaxies are an important type of active galactic nucleus (AGN), generally expected to be accreting at a high Eddington rate. The properties of their outflows and importance of AGN feedback remain intriguing. We report on the discovery of fast outflowing warm absorbers (WAs) in the NLS1 PG 1001+054 with velocities in the range of ∼7000–9000 km s−1. They are identified with blueshifted Lyα, N v, and Si iv lines in the high-resolution ultraviolet (UV) spectra taken with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. We perform photoionization modeling using XSTAR with three WAs. The derived physical properties are typical of WAs in terms of ionization and column density, whereas the outflow velocities are significantly higher. The estimated location of these WAs ranges from 1 to 73 parsecs away from the AGN. Together with previous detection of high ionization absorbers in the X-ray for PG 1001+054, we suggest that fast outflowing UV absorbers are probably part of a multiphase outflow. Such a structure is likely produced by the outflow launched from the AGN at an accretion disk scale, which shocks the ambient interstellar medium (ISM), producing stratified absorbers. Assuming contribution from the three WAs at tens of parsecs, the estimated ratio between the kinetic power of the outflow and AGN Eddington luminosity could reach 1.7%, raising the possibility of sufficient influence on the host galaxy when compared to some theoretical models for efficient AGN feedback.

The environments and hosts of metal absorption at z > 5

ABSTRACT A growing population of metal absorbers are observed at z > 5, many showing strong evolution in incidence approaching the epoch of hydrogen reionization. Follow-up surveys examining fields around these metals have resulted in galaxy detections but the direct physical relationship between the detected galaxies and absorbers is unclear. Upcoming observations will illuminate this galaxy–absorber relationship, but the theoretical framework for interpreting these observations is lacking. To inform future z > 5 studies, we define the expected relationship between metals and galaxies using the Technicolor Dawn simulation to model metal absorption from z = 5 to 7, encompassing the end of reionization. We find that metal absorber types and strengths are slightly better associated with their environment than with the traits of their host galaxies, as absorption system strengths are more strongly correlated with the local galaxy overdensity than the stellar mass of their host galaxy. For redshifts prior to the end of the epoch of reionization, strong high-ionization transitions like C iv are more spatially correlated with brighter galaxies on scales of a few hundred proper kpc than are low-ionization systems, due to the former’s preference for environments with higher ultraviolet background amplitudes and those ions’ relative rarity at z > 6. Post-reionization, the galaxy counts near these high-ionization ions are reduced, and increase surrounding certain low-ionization ions due to a combination of their relative abundances and preferred denser gas phase. We conclude that galaxy–absorber relationships are expected to evolve rapidly such that high-ionization absorbers are better tracers of galaxies pre-reionization, while low-ionization absorbers are better post-reionization.

The faint host galaxies of C IV absorbers at z > 5

ABSTRACT We explore the expected galaxy environments of $\mathrm{C\, \small {IV}}$ absorbers at z > 5 using the Technicolor Dawn simulations. These simulations reproduce the observed history of reionization, the z ∼ 6 galaxy stellar mass function, the Ly α forest transmission at z > 5, and the $\mathrm{Si\, \small {IV}}$ column density distribution (CDD) at z ≈ 5.5. None the less, the $\mathrm{C\, \small {IV}}$ CDD remains underproduced. Comparison with observed $\mathrm{C\, \small {II}}/\mathrm{Si\, \small {II}}$ equivalent width ratios and the $\mathrm{C\, \small {II}}$ line incidence suggests that a low carbon yield accounts for some, but not all, of the $\mathrm{C\, \small {IV}}$ discrepancy. Alternatively, a density-bounded escape scenario could harden the metagalactic ionizing background more dramatically even than binary stellar evolution, boosting the $\mathrm{C\, \small {IV}}$ CDD into near agreement with observations. In this case, galaxies ionize more efficiently and fewer are required to host a given high-ionization absorber. Absorbers’ environments therefore constrain ionizing escape. Regardless of the escape scenario, galaxies correlate with $\mathrm{C\, \small {IV}}$ absorbers out to 300 proper kpc (pkpc). The correlation strengthens independently with galaxy luminosity and $\mathrm{C\, \small {IV}}$ column density. Around strong systems ($\log (N_\mathrm{C\, \small {IV}}/{\rm cm}^{-2})\gt 14)$), the overdensity of galaxies with MUV < −18 or log (LLy α/erg s−1) > 41.9 declines from 200–300 within 100 pkpc to 40–60 within 250 pkpc. The previously suggested association between strong $\mathrm{C\, \small {IV}}$ absorbers and Ly α emitters at z > 5 is not expected. It may arise if both populations inhabit large-scale voids, but for different reasons. Although most neighbouring galaxies are too faint for HST, JWST will, with a single pointing, identify ∼10 neighbouring galaxies per strong $\mathrm{C\, \small {IV}}$ absorber at z > 5. Ground-based tests of these predictions are possible via deep surveys for Ly α emission using integral field units.

Aligned metal absorbers and the ultraviolet background at the end of reionization

We use observations of spatially-aligned C II, C IV, Si II, Si IV, and O I absorbers to probe the slope and intensity of the ultraviolet background (UVB) at $z \sim 6$. We accomplish this by comparing observations with predictions from a cosmological hydrodynamic simulation using three trial UVBs applied in post-processing: a spectrally soft, fluctuating UVB calculated using multi-frequency radiative transfer; a soft, spatially-uniform UVB; and a hard, spatially-uniform "quasars-only" model. When considering our paired high-ionization absorbers (C IV/Si IV), the observed statistics strongly prefer the hard, spatially-uniform UVB. This echoes recent findings that cosmological simulations generically underproduce strong C IV absorbers at $z>5$. A single low/high ionization pair (Si II/Si IV), by contrast, shows a preference for the HM12 UVB, while two more (C II/C IV and O I/C IV) show no preference for any of the three UVBs. Despite this, future observations of specific absorbers, particularly Si IV/C IV, with next-generation telescopes probing to lower column densities should yield tighter constraints on the UVB.

Resolving the Internal Structure of Circumgalactic Medium Using Gravitationally Lensed Quasars* , †

Abstract We study the internal structure of the circumgalactic medium (CGM), using 29 spectra of 13 gravitationally lensed quasars with image separation angles of a few arcseconds, which correspond to 100 pc to 10 kpc in physical distances. After separating metal absorption lines detected in the spectra into high ions with ionization parameter (IP) > 40 eV and low ions with IP < 20 eV, we find that (i) the fraction of absorption lines that are detected in only one of the lensed images is larger for low ions (∼16%) than high ions (∼2%), (ii) the fractional difference of equivalent widths ( ) between the lensed images is almost the same ( ∼ 0.2) for both groups although the low ions have a slightly larger variation, and (iii) weak low-ion absorbers tend to have larger compared to weak high-ion absorbers. We construct simple models to reproduce these observed properties and investigate the distribution of physical quantities such as size and location of absorbers, using some free parameters. Our best models for absorbers with high ions and low ions suggest that (i) an overall size of the CGM is at least ∼500 kpc, (ii) a size of spherical clumpy cloud is ∼1 kpc or smaller, and (iii) only high-ion absorbers can have a diffusely distributed homogeneous component throughout the CGM. We infer that a high ionization absorber distributes almost homogeneously with a small-scale internal fluctuation, while a low ionization absorber consists of a large number of small-scale clouds in the diffusely distributed higher ionized region. This is the first result to investigate the internal small-scale structure of the CGM, based on the large number of gravitationally lensed quasar spectra.

The Soft, Fluctuating UVB atz∼ 6 as Traced by C IV, Si IV, and C II

The sources that drove cosmological reionization left clues regarding their identity in the slope and inhomogeneity of the ultraviolet ionizing background (UVB): Bright quasars (QSOs) generate a hard UVB with predominantly large-scale fluctuations while Population II stars generate a softer one with smaller-scale fluctuations. Metal absorbers probe the UVB's slope because different ions are sensitive to different energies. Likewise, they probe spatial fluctuations because they originate in regions where a galaxy-driven UVB is harder and more intense. We take a first step towards studying the reionization-epoch UVB's slope and inhomogeneity by comparing observations of 12 metal absorbers at $z\sim6$ versus predictions from a cosmological hydrodynamic simulation using three different UVBs: a soft, spatially-inhomogeneous "galaxies+QSOs" UVB; a homogeneous "galaxies+QSOs" UVB (Haardt & Madau 2012); and a QSOs-only model. All UVBs reproduce the observed column density distributions of CII, SiIV, and CIV reasonably well although high-column, high-ionization absorbers are underproduced, reflecting numerical limitations. With upper limits treated as detections, only a soft, fluctuating UVB reproduces both the observed SiIV/CIV and CII/CIV distributions. The QSOs-only UVB overpredicts both CIV/CII and CIV/SiIV, indicating that it is too hard. The Haardt & Madau (2012) UVB underpredicts CIV/SiIV, suggesting that it lacks amplifications near galaxies. Hence current observations prefer a soft, fluctuating UVB as expected from a predominantly Population II background although they cannot rule out a harder one. Future observations probing a factor of two deeper in metal column density will distinguish between the soft, fluctuating and QSOs-only UVBs.

High-resolution observations of two O vi absorbers atz ≈ 2 toward PKS 1448−232

To explore the ionization conditions in highly-ionized absorbers at high redshift we have studied in detail two intervening OVI absorbers at z=2 towards the quasar PKS1448-232, based on high (R=75,000) and intermediate (R=45,000) resolution optical VLT/UVES spectra. We find that both absorption systems are composed of several narrow subcomponents with CIV/OVI Doppler-parameters b<10 km/s, typically. This implies that the gas temperatures are T<10^5 K and that the absorbers are photoionized by the UV background. The system at z=2.1098 represents a simple, isolated OVI absorber that has only two absorption components and that is relatively metal-rich (Z\sim 0.6 solar). Ioinization modeling implies that the system is photoionized with OVI, CIV, and HI coexisting in the same gas phase. The second system at z=2.1660 represents a complicated, multi-component absorption system with eight OVI components spanning almost 300 km/s in radial velocity. The photoionization modeling implies that the metallicity is non-uniform and relatively low (<= 0.1 solar) and that the OVI absorption must arise in a gas phase different from that traced by CIV, CIII, and HI. Our detailed study of the two OVI systems towards PKS1448-232 shows that multi-phase, multi-component high-ion absorbers like the one at z=2.1660 require a detailed ionization modeling of the various subcomponents to obtain reliable results on the physical conditions and metal-abundances in the gas.

An XMM-Newton view of the dipping low-mass X-ray binary XTE J1710-281

(abridged) We analyzed the archived XMM-Newton observation of the poorly studied low-mass X-ray binary XTE J1710-281 performed in 2004 that covered one orbital period of the system (3.8 hr). The source shows dips as well as eclipses, hence it is viewed close to edge-on. We modeled the spectral changes between persistent and dips in the framework of the partial covering model and the ionized absorber approach. The persistent spectrum can be fit by a power law with a photon index of 1.94(+-0.02) affected by absorption from cool material with a hydrogen column density of 0.401(+-0.007)*10^22 cm^-2. The spectral changes from persistent to deep-dipping intervals are consistent with the partial covering of the power-law emission, with the covering fraction increasing from 26% during shallow dipping to 78% during deep dipping. We do not detect any absorption lines from highly ionized species such as FeXXV. The upper-limits we derive on their equivalent width (EW) are not constraining. Despite not detecting any signatures of a warm absorber, we show that the spectral changes are consistent with an increase in column density (4.3(-0.5;+0.4)*10^22 cm^-2 during shallow dipping to 11.6(-0.6;+0.4)*10^22 cm^-2 during deep dipping) and a decrease in ionization state of a highly-ionized absorber (10^2.52 during shallow dipping to 10^2.29 erg.s^-1.cm during deep dipping), associated with a slight increase in the column density of a neutral absorber. The parameters of the ionized absorber are not constrained during persistent emission. The warm absorber model better accounts for the ~1 keV depression visible in the pn dipping spectra, and naturally explains it as a blend of lines and edges unresolved by pn. A deeper observation of XTE J1710-281 would enable this interpretation to be confirmed.

Probing the Ionization Structure of the Narrow‐Line Region in the Seyfert 1 Galaxy NGC 4151

We present a study of the distribution of [O III] λ5007 and [O II] λ3727 emission in the narrow-line region (NLR) of the Seyfert 1 galaxy NGC 4151. While the NLR of NGC 4151 exhibits an overall structure consistent with the unified model of Seyfert galaxies, narrowband [O III] and [O II] images obtained with the Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope reveal significant emission from outside the emission-line bicone. The [O III]/[O II] ratios are lower in these regions, consistent with a weaker ionizing flux. We performed a photoionization modeling analysis of the emission-line gas within a series of annuli, centered on the central continuum source, with inner radii from 13 to 90 pc. The gas is ionized by radiation that has been attenuated by a relatively highly ionized absorber (HABS), which completely covers the central source, and a lower ionization absorber (LABS), which has a covering factor ranging from 0 to 1. We found that the [O III]/[O II] ratios are well fit by assuming that, within each segment of an annulus, some fraction of the NLR gas is completely within the shadow of LABS, while the rest is irradiated by the continuum filtered only by HABS. This suggests that the structure of the NLR is due to filtering of the ionizing radiation by ionized gas, consistent with disk-wind models. One possible scenario is that the low-ionization absorbers are dense knots of gas swept up by a wind.

Simultaneous Ultraviolet and X‐Ray Observations of the Seyfert Galaxy NGC 4151. II. Physical Conditions in the UV Absorbers

We present a detailed analysis, including photoionization modeling, of the intrinsic absorption in the Seyfert 1 galaxy NGC 4151 using ultraviolet (UV) spectra from the Hubble Space Telescope Space Telescope Imaging Spectrograph and the Far Ultraviolet Spectrographic Explorer obtained 2002 May as part of a set of contemporaneous observations that included Chandra High Energy Transmission Grating Spectrometer spectra. In our analysis of the Chandra spectra, we determined that the X-ray absorption was dominated by two components: a high-ionization absorber, revealed by the presence of H-like and He-like lines of Mg, Si, and S, and a lower ionization absorber, in which inner shell absorption lines from lower ionization species of these elements formed. We identified the latter as the source of the saturated UV lines of O VI, C IV, and N V associated with the absorption feature at a radial velocity of ~-500 km s-1, which we referred to as component D+E. In the present work, we have derived tighter constrains on the line-of-sight covering factors, densities, and radial distances of the absorbers. We confirm the presence of the three subcomponents of D+E described in our previous paper, with line-of-sight covering factors (Clos) ranging from ~0.5 to 0.9, and find evidence for a fourth component, D+Ed, characterized by low ionization and a Clos ~ 0.2. The complexity of the UV absorption in NGC 4151 may be a consequence of the fact that we are viewing the black hole/accretion disk system at a relatively high inclination and, therefore, may be detecting the densest part of the flow. Our deconvolution of the underlying C IV emission indicates that D+E must lie outside the intermediate line region (ILR), hence at a radial distance of ~0.1 pc. We find that the equivalent widths (EWs) of the low-ionization lines associated with D+E varied over the period from 1999 July to 2002 May. Although over part of this time, the variations were correlated with changes in the UV continuum, the drop in the EWs of these lines between 2001 April and 2002 May are suggestive of bulk motion of gas out of our line of sight. Over this period, Clos for the low-ionization absorption lines dropped from ~0.7 to ~0.2. If these lines from these two epochs arose in the same subcomponent, the transverse velocity of the gas is ≈2100 km s-1. This is similar to the constraint on transverse velocity derived from the drop in the X-ray absorbing column between 2000 March and 2002 May. Transverse velocities of this order are consistent with an origin in a rotating disk, at the roughly radial distance we derived for D+E. As we suggested in our previous study, it is likely that the absorption arises in a disk-driven wind.

A highly-ionized absorber as a new explanation for the spectral changes during dips from X-ray binaries

We report the XMM-Newton detection of narrow Fe xxv and Fe xxvi X-ray absorption lines at 7 keV in the persistent emission of the dipping low-mass X-ray binary (LMXB) 4U 1323−62. Such features have now been reported in a growing number of LMXBs seen almost edge-on, indicating that the highly-ionized plasma probably originates in an accretion disk atmosphere or wind. During dipping intervals of 4U 1323−62, the strength of the Fe xxv feature increases while that of the Fe xxvi feature decreases, consistent with the presence of less strongly ionized material in the line-of-sight. As observed previously, the changes in the X-ray spectrum during dips are inconsistent with a simple increase in absorption by cool material. However, we show that the changes in both the narrow absorption features and the continuum can be modeled self-consistently by variations in the properties of an ionized absorber. From persistent to deep dipping the photo-ionization parameter decreases while the equivalent hydrogen column density of the ionized absorber increases. No partial covering of any component of the spectrum, and hence no extended corona, are required. Since highly-ionized absorption features are seen from many other dip sources, this mechanism may also explain the overall changes in X-ray spectrum observed during dipping intervals from these systems.

Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers

X-ray observations have revealed that many microquasars and low-mass X-ray binaries (LMXBs) exhibit narrow absorption features identified with resonant absorption from Fe XXV and Fe XXVI and other abundant ions. In many well studied systems there is evidence for blue-shifts, indicating outflowing plasmas. We succesfully model the changes in both the X-ray continuum and the Fe absorption features during dips from all the bright dipping LMXBs observed by XMM-Newton (EXO 0748-676, XB 1254-690, X 1624-490, MXB 1659-298, 4U 1746-371 and XB 1916-053) as resulting primarily from an increase in column density and a decrease in the ionization state of a highly-ionized absorber in a similar way as was done for 4U 1323-62. This implies that the complex spectral changes in the X-ray continua observed from the dip sources as a class can be most simply explained primarily by changes in the highly ionized absorbers present in these systems. There is no need to invoke unusual abundances or partial covering of extended emission regions. Outside of the dips, the absorption line properties do not vary strongly with orbital phase. This implies that the ionized plasma has a cylindrical geometry with a maximum column density close to the plane of the accretion disk. Since dipping sources are simply normal LMXBs viewed from close to the orbital plane this implies that ionized plasmas are a common feature of LMXBs.

A highly-ionized absorber in the X-ray binary 4U 1323-62: A new explanation for the dipping phenomenon

We report the detection of narrow Fe XXV and Fe XXVI X-ray absorption lines at 6.68 +/- 0.04 keV and 6.97 +/- 0.05 keV in the persistent emission of the dipping low-mass X-ray binary 4U 1323-62 during a 2003 January XMM-Newton observation. These features are superposed on a broad emission feature centered on 6.6 {+0.1}{-0.2} keV. During dipping intervals the equivalent width of the Fe XXV feature increases while that of the Fe XXVI feature decreases, consistent with the presence of less strongly ionized material in the line-of-sight. As observed previously, the changes in the 1.0-10 keV spectrum during dips are inconsistent with a simple increase in absorption by cool material. However, the changes in both the absorption features and the continuum can be modeled by variations in the properties of an ionized absorber. No partial covering of any component of the spectrum, and hence no extended corona, are required. From persistent to deep dipping the photo-ionization parameter, Xi, expressed in erg cm s^{-1}, decreases from log(Xi) of 3.9 +/- 0.1 to log(Xi) of 3.13 +/- 0.07, while the equivalent hydrogen column density of the ionized absorber increases from (3.8 +/- 0.4) 10^{22} atoms cm^{-2} to (37 +/- 2) 10^{22} atoms cm^{-2}. Since highly-ionized absorption features are seen from many other dip sources, this mechanism may also explain the overall changes in X-ray spectrum observed during dipping intervals from these systems.

The Structure of High Redshift Galactic Halos

Observations of multiple or lensed QSOs at high redshift can be used to probe transverse structure in intervening absorption systems. Here we present results obtained from STIS spectroscopy of the unique triply imaged QSO APM 08279+5255 at zem = 3.9 and study the coherence scales of intervening low and high ionization absorbers.

X-ray absorption in the strong Fe II narrow-line Seyfert 1 galaxy Markarian 507

We present results from spectral analysis of ASCA data on the NLS1 Mrk 507. This galaxy was found to have an exceptionally flat ROSAT spectrum among the NLS1s. The ASCA spectrum however shows a clear absorption feature in the energy band below 2 keV, which partly accounts for the flat spectrum observed with the ROSAT PSPC. The absorption is mainly due to cold gas with a column density of (2-3)E21 cm-2. A reanalysis of the PSPC data indicates that the absorber is slightly ionized, covers only part of the central source, or there is extra soft thermal emission from an extended region. There is also evidence that the X-ray absorption is complex; an edge feature marginally detected at 0.84 keV suggests the presence of an additional high ionization absorber which imposes a strong OVIII edge on the spectrum. After correction for the absorption, the photon index of the intrinsic continuum, 1.8, obtained from the ASCA data is quite similar to that of ordinary Seyfert 1 galaxies. Mrk 507 still has one of the flattest continuum slopes among NLS1, but is no longer exceptional. The strong optical FeII emission remains unusual in the light of the correlation between FeII strengths and steepness of soft X-ray slope.