Absorption Line Systems Research Articles

Exploring the physical properties of the cool circumgalactic medium with a semi-analytic model

We develop a semi-analytic model to explore the physical properties of cool pressure-confined circumgalactic clouds with mass ranging from $10$ to $10^{8} \, \rm M_{\odot}$ in a hot diffuse halo. We consider physical effects that control the motion and mass loss of the clouds, and estimate the lifetime and the observed properties of individual cool gas clouds inferred from the CLOUDY simulation. Our results show that the cool pressure-confined gas clouds have physical properties consistent with absorption line systems with neutral hydrogen column densities $N_{\rm HI}\geq10^{18.5} \rm cm^{-2}$ such as strong metal absorbers, sub-DLAs, and DLAs. The cool circumgalactic clouds are transient due to evaporation and recycling and therefore a constant replenishment is needed to maintain the cool CGM. We further model the ensemble properties of the cool CGM with clouds originated from outflows, inflows, or/and in-situ formation with a range of initial cloud mass function and velocity distribution. We find that only with a certain combination of parameters, an outflow model can broadly reproduce three cool gas properties around star-forming galaxies simultaneously: the spatial distribution, down-the-barrel outflow absorption, and gas velocity dispersion. Both a constant insitu model and gas inflow model can reproduce the observed covering fractions of high $\rm N_{HI}$ gas around passive galaxies but they fail to reproduce sufficient number of low $\rm N_{HI}$ systems. The limitations and the failures of the current models are discussed. Our results illustrate that semi-analytic modeling is a promising tool to understand the physics of the cool CGM which is usually unresolved by state-of-the-art cosmological hydrodynamic simulations.

Evidence for diffuse molecular gas and dust in the hearts of gamma-ray burst host galaxies

Context. Damped Lyman-α (DLA) absorption-line systems at the redshifts of gamma-ray burst (GRB) afterglows offer a unique way to probe the physical conditions within star-forming galaxies in the early Universe. Aims. Here we built up a large sample of 22 GRBs at redshifts z > 2 observed with VLT/X-shooter in order to determine the abundances of hydrogen, metals, dust, and molecular species. This allows us to study the metallicity and dust depletion effects in the neutral interstellar medium at high redshift and to answer the question of whether (and why) there might be a lack of H2 in GRB-DLAs. Methods. We developed new methods based on the Bayesian inference package, PyMC, to FIT absorption lines and measure the column densities of different metal species as well as atomic and molecular hydrogen. The derived relative abundances are used to FIT dust depletion sequences and determine the dust-to-metals ratio and the host-galaxy intrinsic visual extinction. Additionally, we searched for the absorption signatures of vibrationally-excited H2 and carbon monoxide. Results. We find that there is no lack of H2-bearing GRB-DLAs. We detect absorption lines from molecular hydrogen in 6 out of 22 GRB afterglow spectra, with molecular fractions ranging between f ≃ 5 × 10−5 and f ≃ 0.04, and claim tentative detections in three additional cases. For the remainder of the sample, we measure, depending on S/N, spectral coverage and instrumental resolution, more or less stringent upper limits. The GRB-DLAs in our sample have on average low metallicities, [X/H]¯ ≈ −1.3, comparable to the population of extremely-strong QSO-DLAs (log N(H I) > 21.5). Furthermore, H2-bearing GRB-DLAs are found to be associated with significant dust extinction, AV > 0.1 mag, and dust-to-metals ratios DTM > 0.4, confirming the importance of dust grains for the production of molecules. All these systems exhibit neutral hydrogen column densities log N(H I) > 21.7. The overall fraction of H2 detections in GRB-DLAs is ≥ 27% (41% including tentative detections), which is three to four times larger than in the general QSO-DLA population. For 2 < z < 4, and considering column densities log N(H I) > 21.7, the H2 detection fraction is 60–80% in GRB-DLAs and in extremely strong QSO-DLAs. This is likely due to the fact that both GRB- and QSO-DLAs with high neutral hydrogen column densities are probed by sight-lines with small impact parameters, indicating that the absorbing gas is associated with the inner regions of the absorbing galaxy, where the gas pressure is higher and the conversion of H I to H2 takes place. In the case of GRB hosts, this diffuse molecular gas is located at distances ≳ 500 pc from the GRB and hence is unrelated to the star-forming region where the event occurred.

A Sub-damped Lyα Absorber with Unusual Abundances: Evidence of Gas Recycling in a Low-redshift Galaxy Group

Abstract Using Hubble Space Telescope/Space Telescope Imaging Spectrograph G140M spectroscopy, we investigate an absorption-line system at z = 0.07489 in the spectrum of the quasi-stellar object PG 1543+489 (z QSO = 0.401). The sightline passes within ρ = 66 kpc of an edge-on disk galaxy at a similar redshift, but the galaxy belongs to a group with four other galaxies within ρ = 160 kpc. We detect H i [log N(H i/cm−2) = 19.12 ± 0.04] as well as N i, Mg ii, Si ii, and Si iii, from which we measure a gas-phase abundance of [N/H] = −1.0 ± 0.1. Photoionization models indicate that the nitrogen-to-silicon relative abundance is solar, yet magnesium is underabundant by a factor of ≈2. We also report spatially resolved emission-line spectroscopy of the nearby galaxy, and we extract its rotation curve. The galaxy’s metallicity is ≈8× higher than [N/H] in the absorber, and interestingly, the absorber velocities suggest that the gas at ρ = 66 kpc is corotating with the galaxy’s stellar disk, possibly with an inflow component. These characteristics could indicate that this sub-damped Lyα absorber system arises in a “cold-accretion” flow. However, the absorber abundance patterns are peculiar. We hypothesize that the gas was ejected from its galaxy of origin (or perhaps is a result of tidal debris from interactions between the group galaxies) with a solar nitrogen abundance, but that subsequently mixed with (and was diluted by) gas in the circumgalactic medium or group. If the gas is bound to the nearby galaxy, this system may be an example of the gas “recycling” predicted by theoretical galaxy simulations. Our hypothesis is testable with future observations.

Detection of the Missing Baryons toward the Sightline of H1821+643

Abstract Based on constraints from Big Bang nucleosynthesis and the cosmic microwave background, the baryon content of the high-redshift Universe can be precisely determined. However, at low redshift, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). In this work, we employ a novel stacking approach to explore the hot phases of the WHIM. Specifically, we utilize the 470 ks Chandra LETG data of the luminous quasar, H 1821+643, along with previous measurements of UV absorption line systems and spectroscopic redshift measurements of galaxies toward the quasar’s sightline. We repeatedly blueshift and stack the X-ray spectrum of the quasar corresponding to the redshifts of the 17 absorption line systems. Thus, we obtain a stacked spectrum with 8.0 Ms total exposure, which allows us to probe X-ray absorption lines with unparalleled sensitivity. Based on the stacked data, we detect an O vii absorption line that exhibits a Gaussian line profile and is statistically significant at the 3.3σ level. Since the redshifts of the UV absorption line systems were known a priori, this is the first definitive detection of an X-ray absorption line originating from the WHIM. The equivalent width of the O vii line is (4.1 ± 1.3) mÅ, which corresponds to an O vii column density of . We constrain the absorbing gas to have a density of for a single WHIM filament. We derive for the cosmological mass density of O vii, assuming that all 17 systems contribute equally.

Narrow C iv Absorption Lines with Multi-epoch Observations

Abstract Using the quasar spectra from the Sloan Digital Sky Survey, all with two-epoch to six-epoch observations, we search for C IV λλ1548,1551 narrow absorption doublets in spectral data from the blue wing υ r = 30,000 km s−1 until the red wing of C IV λ1549 emission lines. We have obtained 21,239 C IV narrow absorption line (NAL) systems, of which 647 systems were significantly changed on timescales from ΔMJD = 0.24 to 1842.38 days at rest-frame. Both samples of the C IV NAL systems with multi-epoch observations and the variable C IV NAL systems are the largest ones to date. The offset velocity distribution of the variable C IV NALs from quasars has a peak at υ r ≈ 2000 km s−1 and gently extends to a larger offset velocity. There are 110 variable C IV NALs with υ r > 12,000 km s−1. In addition, about 10% intrinsic C IV NALs show time variability on a timescale of <2000 days. We find that the variations of C IV NALs are positively or negatively correlated with the alternations of quasar continua. The two kinds of correlations suggest that the variable C IV NALs included in our absorber catalog can be classified into at least two populations: (1) highly ionized systems that are dominated by highly ionized absorbing gas and negatively respond to the fluctuations of quasar emissions; and (2) lowly ionized systems that are dominated by lowly ionized absorbing gas and positively respond to fluctuations of quasar emissions. We also find that the absolute fractional changes in the absorption strengths of C IV NALs are inversely correlated with absorption strengths.

Neutral-gas-phase metal abundances in Zn ii-selected quasar absorption line systems near redshift z = 1.2★

We present ten metallicity measurements for quasar absorbers near $z=1.2$ that were selected for having unusually significant ZnII absorption in their SDSS spectra. Follow-up UV space spectroscopy of the Ly$\alpha$ region shows that all ten have damped Ly$\alpha$ (DLA) absorption, corresponding to neutral hydrogen column densities in the range $2.4\times 10^{20}$ $\le$ N(HI) $\le$ $2.5\times 10^{21}$ atoms cm$^{-2}$, and indicating that the gas is very optically thick and essentially neutral. The sample is a very small subset of systems compiled by searching the University of Pittsburgh catalog of $\approx 30,000$ intervening MgII absorption line systems in SDSS quasar spectra up to DR7. We started by isolating $\approx 3,000$ that had strong MgII absorption in the redshift interval $1.0 < z < 1.5$ in brighter background quasars. Of these, 36 exhibited significant absorption near ZnII. Space UV spectroscopy was then obtained for nine of these (25% of the total), and a tenth system in a fainter quasar was found in the HST archives. The result is a representative sample of the highest Zn$^+$ columns of gas within the DLA population. These ZnII-selected systems define the upper envelope of DLA metallicities near $z=1.2$. They show a tight anti-correlation between N(HI) and [Zn/H], with the higher metallicity systems clearly exhibiting more depletion based on [Cr/Zn] values. The various metal-line measurements (Zn, Cr, Si, Fe, Mn) indicate evolved neutral gas with $-0.9 \le$ [Zn/H] $\le +0.4$.

The CGM and IGM at z ∼ 5: metal budget and physical connection

We present further results of a survey for absorption line systems in the spectra of four high redshift quasars (5.79 $\le$ z$_{\textrm{em}}$ $\le$ 6.13) obtained with the ESO Very Large Telescope X-Shooter. We identify 36 $\textrm{CIV}$ and 7 $\textrm{SiIV}$ systems with a $\ge$ 5$\sigma$ significance. The highest redshift $\textrm{CIV}$ and $\textrm{SiIV}$ absorbers identified in this work are at z = 5.80738 $\pm$ 0.00017 and z = 5.77495 $\pm$ 0.00038, respectively. We compute the comoving mass density of $\textrm{SiIV}$ ($\Omega_{\textrm{SiIV}}$) and find that it evolves from $\Omega_{\textrm{SiIV}}$ = 4.3$^{+2.1}_{-2.1}$ $\times$10$^{-9}$ at <z> = 5.05 to $\Omega_{\textrm{SiIV}}$ = 1.4$^{+0.6}_{-0.4}$ $\times$10$^{-9}$ at <z> = 5.66. We also measure $\Omega_{\textrm{CIV}}$ = 1.6$^{+0.4}_{-0.1}$ $\times$10$^{-8}$ at <z> = 4.77 and $\Omega_{\textrm{CIV}}$ = 3.4$^{+1.6}_{-1.1}$ $\times$10$^{-9}$ at <z> = 5.66. We classify our $\textrm{CIV}$ absorber population by the presence of associated $\textit{low}$ and/or $\textit{high ionisation}$ systems and compute their velocity width ($\Delta$v$_{90}$). We find that all $\textrm{CIV}$ systems with $\Delta$v$_{90}$ > 200 kms$^{-1}$ have associated $\textit{low ionisation}$ systems. We investigate two such systems, separated by 550 physical kpc along a line of sight, and find it likely that they are both tracing a multi-phase medium where hot and cold gas is mixing at the interface between the CGM and IGM. We further discuss the \textrm{MgII} systems presented in a previous work and we identify 5 $\textrm{SiII}$, 10 $\textrm{AlII}$, 12 $\textrm{FeII}$, 1 $\textrm{CII}$, 7 $\textrm{MgI}$ and 1 $\textrm{CaII}$ associated transitions. We compute the respective comoving mass densities in the redshift range 2 to 6, as allowed by the wavelength coverage.

ALMA Observations of Molecular Absorption in the Gravitational Lens PMN 0134−0931 at z = 0.7645

Abstract We report the detection of molecular absorption lines at z = 0.7645 toward the radio-loud quasi-stellar object (QSO) PMN 0134−0931. The CO J = 2–1 and HCO+ J = 2–1 lines are seen in absorption along two different lines of sight to lensed images of the background QSO. The lines of sight are separated by ∼0.″7, corresponding to 5 kpc in the lens plane. PMN 0134−0931 represents one out of only five known molecular absorption line systems at cosmologically significant distances. Moreover, it is also one of three such systems where the absorption occurs in a galaxy acting as a gravitational lens. The absorption lines through the two lines of sight are shifted by 215 ± 8 km s−1, possibly representing rotational motion in one of the lensing galaxies. The absorption profiles are wide, ∼200 km s−1, suggesting that the absorption occurs in a highly inclined disk galaxy with a flat rotation curve and a cloud–cloud velocity dispersion ∼30 km s−1. Gravitational lens models require two equal mass galaxies to account for the observed configuration of lensed images. The presence of two galaxies in close proximity means that they might be interacting and potentially merging and the kinematics of the molecular gas may not reflect ordered rotational motion. Compared with other high-redshift molecular absorption systems, the column densities of both CO and HCO+ are normal for diffuse molecular gas toward one of the lensed images, but significantly higher toward the other. Also, the abundance ratio is 2 − 3 times higher than in typical diffuse molecular gas. It is plausible that the second line of sight probes denser molecular gas than what is normally the case for absorption.

Saturation Effect on Photoionization-driven Broad Absorption Line Variability

Abstract We study the saturation effect on broad absorption line (BAL) variability through a variation phenomenon, which shows significant variation in Si iv BAL but no, or only small, change in C iv BAL (hereafter Phenomenon I). First, we explore a typical case showing Phenomenon I, quasar SDSS J153715.74+582933.9 (hereafter J1537+5829). We identify four narrow absorption line (NAL) systems within its Si iv BAL and two additional NAL systems within its C iv BAL, and confirm their coordinated weakening. Combining with the obvious strengthening of the ionizing continuum, we attribute the BAL variability in J1537+5829 to the ionization changes caused by the continuum variations. Second, a statistical study based on multiobserved quasars from SDSS-I/II/III are presented. We confirm that (1) the moderate anticorrelation between the fractional variations of Si iv BALs and the continuum in 74 quasars show Phenomenon I and (2) the sample showing BAL variations tends to have larger ionizing continuum variations. These results reveal the ubiquitous effect of the continuum variability on Phenomenon I and BAL variation. We attribute the relative lack of variation of C iv BALs in Phenomenon I to the saturation effects. Nonetheless, these absorbers are not very optically thick in Si iv and the ionization changes in response to continuum variations could be the main driver of their variations. Finally, we find that the saturation effect on BAL variability can explain many phenomena of BAL variations that have been reported before.

The Correlated Variations of Narrow Absorption Lines and Quasar Continuum

Abstract We assemble 207 variable quasars from the Sloan Digital Sky Survey, all with at least 3 observations, to analyze C iv narrow absorption doublets, and obtain 328 C iv narrow absorption line systems. We find that 19 out of 328 C iv narrow absorption line systems were changed by on timescales from 15.9 to 1477 days at rest-frame. Among the 19 obviously variable C iv systems, we find that (1) 14 systems have relative velocities and 4 systems have , where c is the speed of light; (2) 13 systems are accompanied by other variable C iv systems; (3) 9 systems were changed continuously during multiple observations; and (4) 1 system with 16,862 km s−1 was enhanced by in 0.67 day at rest-frame. The variations of absorption lines are inversely correlated with the changes in the ionizing continuum. We also find that large variations of C iv narrow absorption lines are form differently over a short timescale.

Statistical properties of Faraday rotation measure in external galaxies – I. Intervening disc galaxies

Deriving the Faraday rotation measure (RM) of quasar absorption line systems, which are tracers of high-redshift galaxies intervening background quasars, is a powerful tool for probing magnetic fields in distant galaxies. Statistically comparing the RM distributions of two quasar samples, with and without absorption line systems, allows one to infer magnetic field properties of the intervening galaxy population. Here, we have derived the analytical form of the probability distribution function (PDF) of RM produced by a single galaxy with an axisymmetric large-scale magnetic field. We then further determine the PDF of RM for one random sight line traversing each galaxy in a population with a large-scale magnetic field prescription. We find that the resulting PDF of RM is dominated by a Lorentzian with a width that is directly related to the mean axisymmetric large-scale field strength $\langle B_0 \rangle$ of the galaxy population if the dispersion of $B_0$ within the population is smaller than $\langle B_0 \rangle$. Provided that RMs produced by the intervening galaxies have been successfully isolated from other RM contributions along the line of sight, our simple model suggests that $\langle B_0 \rangle$ in galaxies probed by quasar absorption line systems can be measured within $\approx50$ per cent accuracy without additional constraints on the magneto-ionic medium properties of the galaxies. Finally, we discuss quasar sample selection criteria that are crucial to reliably interpret observations, and argue that within the limitations of the current database of absorption line systems, high-metallicity damped Lyman-$\alpha$ absorbers are best suited to study galactic dynamo action in distant disc galaxies.

Circum-Galactic Medium in the Halo of Quasars

The properties of circum-galactic gas in the halo of quasar host galaxies are investigated analyzing Mg II 2800 and C IV 1540 absorption-line systems along the line of sight close to quasars. We used optical spectroscopy of closely aligned pairs of quasars (projected distance $\leq$ 200 kpc, but at very different redshift) obtained at the VLT and Gran Telescopio Canarias to investigate the distribution of the absorbing gas for a sample of quasars at z$\sim$1. Absorption systems of EW $\geq$ 0.3 $\rm{\AA}$ associated with the foreground quasars are revealed up to 200 kpc from the centre of the host galaxy, showing that the structure of the absorbing gas is patchy with a covering fraction quickly decreasing beyond 100 kpc. In this contribution we use optical and near-IR images obtained at VLT to investigate the relations between the properties of the circum-galactic medium of the host galaxies and of the large scale galaxy environments of the foreground quasars.

ALMACAL – III. A combined ALMA and MUSE survey for neutral, molecular, and ionized gas in an H i-absorption-selected system

Studying the flow of baryons into and out of galaxies is an important part of understanding the evolution of galaxies over time. We present a detailed case study of the environment around an intervening Ly $\alpha$ absorption line system at $z_{\rm abs} = 0.633$, seen towards the quasar J0423$-$0130 ($z_{\rm QSO} = 0.915$). We detect with ALMA the $^{12}$CO(2--1), $^{12}$CO(3--2) and $1.2$~mm continuum emission from a galaxy at the redshift of the Ly $\alpha$ absorber at a projected distance of $135$ kpc. From the ALMA detections, we infer ISM conditions similar to those in low redshift Luminous Infrared Galaxies. DDT MUSE integral field unit observations reveal the optical counterpart of the $^{12}$CO emission line source and three additional emission line galaxies at the absorber redshift, which together form a galaxy group. The $^{12}$CO emission line detections originate from the most massive galaxy in this group. While we cannot exclude that we miss a fainter host, we reach a dust-uncorrected star-formation rate (SFR) limit of > $0.3 \text{M}_{\odot} \text{ yr}^{-1}$ within $100$ kpc from the sightline to the background quasar. We measure the dust-corrected SFR (ranging from $3$ to $50$ M$_{\odot}$ yr$^{-1}$), the morpho-kinematics and the metallicities of the four group galaxies to understand the relation between the group and the neutral gas probed in absorption. We find that the Ly $\alpha$ absorber traces either an outflow from the most massive galaxy or intra-group gas. This case study illustrates the power of combining ALMA and MUSE to obtain a census of the cool baryons in a bounded structure at intermediate redshift.

Quasar 2175 Å dust absorbers – II. Correlation analysis and relationship with other absorption line systems

We present the cold neutral content (H I and C I gas) of 13 quasar 2175 \AA$ $ dust absorbers (2DAs) at $z$ = 1.6 - 2.5 to investigate the correlation between the presence of the UV extinction bump with other physical characteristics. These 2DAs were initially selected from the Sloan Digital Sky Surveys I - III and followed up with the Keck-II telescope and the Multiple Mirror Telescope as detailed in our Paper I. We perform a correlation analysis between metallicity, redshift, depletion level, velocity width, and explore relationships between 2DAs and other absorption line systems. The 2DAs on average have higher metallicity, higher depletion levels, and larger velocity widths than Damped Lyman-$\alpha$ absorbers (DLAs) or subDLAs. The correlation between [Zn/H] and [Fe/Zn] or [Zn/H] and log$\Delta$V$_{90}$ can be used as alternative stellar mass estimators based on the well-established mass-metallicity relation. The estimated stellar masses of the 2DAs in this sample are in the range of $\sim$ 10$^9$ to $\sim$2 $\times$ 10$^{11}$ $M_{\odot}$ with a median value of $\sim$2 $\times$ 10$^{10}$ $M_{\odot}$. The relationship with other quasar absorption line systems can be described as (1) 2DAs are a subset of Mg II and Fe II absorbers, (2) 2DAs are preferentially metal-strong DLAs/subDLAs, (3) More importantly, all of the 2DAs show C I detections with logN(C I) $>$ 14.0 cm$^{-2}$, (4) 2DAs can be used as molecular gas tracers. Their host galaxies are likely to be chemically enriched, evolved, massive (more massive than typical DLA/subDLA galaxies), and presumably star-forming galaxies.

Mg ii Absorbers: Metallicity Evolution and Cloud Morphology

Abstract Metal abundance and its evolution are studied for Mg ii quasar absorption line systems from their weak, unsaturated spectral lines using stacked spectra from the archived data of the Sloan Digital Sky Survey. They show an abundance pattern that resembles that of the Galactic halo or Small Magellanic Cloud, with metallicity [Z/H] showing an evolution from redshift z = 2 to 0.5: metallicity becomes approximately solar or even larger at . We show that the evolution of the metal abundance traces the cumulative amount of the hydrogen fuel consumed in star formation in galaxies. With the aid of a spectroscopic simulation code, we infer the median gas density of the cloud to be roughly 0.3 , with which the elemental abundance in various ionization stages, in particular C i, is consistently explained. This gas density implies that the size of the Mg ii clouds is of the order of 0.03 kpc, which suggests that individual Mg ii clouds around a galaxy are of a baryonic mass typically . This means that Mg ii clouds are numerous and “foamy,” rather than a large entity that covers a sizable fraction of galaxies with a single cloud.

The Formation and Physical Origin of Highly Ionized Cooling Gas

Abstract We present a simple model that explains the origin of warm, diffuse gas seen primarily as highly ionized absorption-line systems in the spectra of background sources. We predict the observed column densities of several highly ionized transitions such as O vi, O vii, Ne viii, N v, and Mg x, and we present a unified comparison of the model predictions with absorption lines seen in the Milky Way disk, Milky Way halo, starburst galaxies, the circumgalactic medium, and the intergalactic medium at low and high redshifts. We show that diffuse gas seen in such diverse environments can be simultaneously explained by a simple model of radiatively cooling gas. We show that most such absorption-line systems are consistent with being collisionally ionized, and we estimate the maximum-likelihood temperature of the gas in each observation. This model satisfactorily explains why O vi is regularly observed around star-forming low-z L* galaxies, and why N v is rarely seen around the same galaxies. We further present some consequences of this model in quantifying the dynamics of the cooling gas around galaxies and predict the shock velocities associated with such flows. A unique strength of this model is that while it has only one free (but physically well-constrained) parameter, it nevertheless successfully reproduces the available data on O vi absorbers in the interstellar, circumgalactic, intragroup, and intergalactic media, as well as the available data on other absorption lines from highly ionized species.

Discovery of Variable Hydrogen Balmer Absorption Lines with Inverse Decrement in PG 1411+442

Abstract We present new optical spectra of the well-known broad absorption line (BAL) quasar PG 1411+442, using the DBSP spectrograph at the Palomar 200 inch telescope in 2014 and 2017 and the YFOSC spectrograph at the Lijiang 2.4 m telescope in 2015. A blueshifted narrow absorption line system is clearly revealed in 2014 and 2015 consisting of hydrogen Balmer series and metastable He i lines. The velocity of these lines is similar to the centroid velocity of the UV BALs, suggesting that both originate from the outflow. The Balmer lines vary significantly between the two observations and vanished in 2017. They were also absent in the archived spectra obtained before 2001. The variation is thought to be driven by photoionization change. Besides, the absorption lines show inversed Balmer decrement, i.e., the apparent optical depths of higher-order Balmer absorption lines are larger than those of lower-order lines, which is inconsistent with the oscillator strengths of the transitions. We suggest that such anomalous line ratios can be naturally explained by the thermal structure of a background accretion disk, which allows the obscured part of the disk to contribute differently to the continuum flux at different wavelengths. High-resolution spectroscopic and photometric monitoring would be very useful to probe the structure of the accretion disk as well as the geometry and physical conditions of the outflow.

A deep search for metals near redshift 7: the line of sight towards ULAS J1120+0641

We present a search for metal absorption line systems at the highest redshifts to date using a deep (30h) VLT/X-Shooter spectrum of the z = 7.084 quasi-stellar object (QSO) ULAS J1120+0641. We detect seven intervening systems at z > 5.5, with the highest-redshift system being a C IV absorber at z = 6.51. We find tentative evidence that the mass density of C IV remains flat or declines with redshift at z < 6, while the number density of C II systems remains relatively flat over 5 < z < 7. These trends are broadly consistent with models of chemical enrichment by star formation-driven winds that include a softening of the ultraviolet background towards higher redshifts. We find a larger number of weak ( W_rest < 0.3A ) Mg II systems over 5.9 < z < 7.0 than predicted by a power-law fit to the number density of stronger systems. This is consistent with trends in the number density of weak Mg II systems at z = 2.5, and suggests that the mechanisms that create these absorbers are already in place at z = 7. Finally, we investigate the associated narrow Si IV, C IV, and N V absorbers located near the QSO redshift, and find that at least one component shows evidence of partial covering of the continuum source.

Implications of z ∼ 6 Quasar Proximity Zones for the Epoch of Reionization and Quasar Lifetimes

Abstract We study quasar proximity zones in the redshift range 5.77 ≤ z ≤ 6.54 by homogeneously analyzing 34 medium-resolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude measurements. Whereas previous studies found strong evolution of proximity zone sizes with redshift and argued that this provides evidence for a rapidly evolving intergalactic medium (IGM) neutral fraction during reionization, we measure a much shallower trend ∝ ( 1 + z ) − 1.44 . We compare our measured proximity zone sizes to predictions from hydrodynamical simulations post-processed with one-dimensional radiative transfer and find good agreement between observations and theory, irrespective of the ionization state of the ambient IGM. This insensitivity to IGM ionization state has been previously noted, and results from the fact that the definition of proximity zone size as the first drop of the smoothed quasar spectrum below the 10% flux transmission level probes locations where the ionizing radiation from the quasar is an order of magnitude larger than the expected ultraviolet ionizing background that sets the neutral fraction of the IGM. Our analysis also uncovered three objects with exceptionally small proximity zones (two have R p &lt; 1 proper Mpc), which constitute outliers from the observed distribution and are challenging to explain with our radiative transfer simulations. We consider various explanations for their origin, such as strong absorption line systems associated with the quasar or patchy reionization, but find that the most compelling scenario is that these quasars have been shining for ≲105 years.

The Galaxy’s veil of excited hydrogen

Many of the baryons in our Galaxy probably lie outside the well-known disk and bulge components. Despite a wealth of evidence for the presence of some gas in galactic halos—including absorption line systems in the spectra of quasars, high-velocity neutral hydrogen clouds in our Galaxy halo, line-emitting ionized hydrogen originating from galactic winds in nearby starburst galaxies and the X-ray coronas surrounding the most massive galaxies—accounting for the gas in the halo of any galaxy has been observationally challenging, primarily because of the low density in these expansive regions. The most sensitive measurements come from detecting absorption due to the intervening gas in the spectra of distant objects, such as quasars or distant halo stars, but these have typically been limited to a few lines of sight to sufficiently bright objects. Extensive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present evidence for a newly discovered, widely distributed, neutral, excited hydrogen component of the Galaxy’s halo. It is observed as the slight (0.779 ± 0.006%) absorption of flux near the rest wavelength of Hα in the combined spectra of hundreds of thousands of galaxy spectra and is ubiquitous in high-latitude lines of sight. This observation provides an avenue to tracing, both spatially and kinematically, the majority of the gas in the halo of our Galaxy. The stacking of nearly three-quarters of a million spectra has unearthed a previously unknown component of the Galactic halo: a widely distributed, neutral, excited hydrogen layer that could harbour a sizeable proportion of the Milky Way’s baryons.