We present the results of a search for damped Lyman-α (DLA) systems in the Sloan Digital Sky Survey II (SDSS), Data Release 7. We use a fully automatic procedure to identify DLAs and derive their ...column densities. The procedure is checked against the results of previous searches for DLAs in SDSS. We discuss the agreements and differences and show the robustness of our procedure. For each system, we obtain an accurate measurement of the absorber's redshift, the H I column density and the equivalent width of associated metal absorption lines, without any human intervention. We find 1426 absorbers with $2.15 < z < 5.2$ with log N(H I) ≥ 20, out of which 937 systems have log N(H I) ≥ 20.3. This is the largest DLA sample ever built, made available to the scientific community through the electronic version of this paper. In the course of the survey, we discovered the intervening DLA with highest H I column density known to date with log N(H I) = 22.0±0.1. This single system provides a strong constraint on the high-end of the N(H I) frequency distribution now measured with high accuracy. We show that the presence of a DLA at the blue end of a QSO spectrum can lead to important systematic errors and propose a method to avoid them. This has important consequences for the measurement of the cosmological mass density of neutral gas at z ~ 2.2 and therefore on our understanding of galaxy evolution over the past 10 billion years. We find a significant decrease of the cosmological mass density of neutral gas in DLAs, $\Omega_{\rm g}^{\rm DLA}$, from $z = 4$ to $z = 2.2$, consistent with the result of previous SDSS studies. However, and contrary to other SDSS studies, we find that $\Omega_{\rm g}^{\rm DLA}$(z = 2.2) is about twice the value at $z = 0$. This implies that $\Omega_{\rm g}^{\rm DLA}$ keeps decreasing at $z < 2.2$.
We present the results of a search for cold gas at high redshift along quasar lines of sight carried out without any a priori assumption on the neutral atomic-hydrogen content of the absorption-line ...systems. To do this, we systematically looked for neutral- carbon (C I) AA1560, 1656 transition lines in 41696 low-resolution quasar spectra from the SDSS-II - Data Release 7 - database. C I absorption lines should indeed probe the shielded gas in the neutral interstellar medium of galaxies more efficiently than traditional tracers such as neutral atomic-hydrogen (H I) damped Lyman-a (DLA) and/or Mg II systems. We show that, at equal amount of reddening, the 2175 A feature is weak compared to Galactic lines of sight. This is probably the consequence of current or past star formation in the vicinity of the C I systems. The C I-absorber sample presented here hence provides ideal targets for detailed studies of the dust composition and molecular species at high redshift.
We present the first results from an ongoing survey for damped Lyman-α systems (DLAs) in the spectra of z > 2 quasars observed in the course of the Baryon Oscillation Spectroscopic Survey (BOSS), ...which is part of the Sloan Digital Sky Survey (SDSS) III. Our full (non-statistical) sample, based on Data Release 9, comprises 12 081 systems with log N(H i) ≥ 20, out of which 6839 have log N(H i) ≥ 20.3. This is the largest DLA sample ever compiled, superseding that from SDSS-II by a factor of seven. Using a statistical sub-sample and estimating systematics from realistic mock data, we probe the N(H i) distribution at ⟨z⟩ = 2.5. Contrary to what is generally believed, the distribution extends beyond 1022 cm-2 with a moderate slope of index ≈−3.5. This result matches the opacity-corrected distribution observed at z = 0 surprisingly well. The cosmological mass density of neutral gas in DLAs is found to be \hbox{$\omegagdla \approx 10^{-3}$}ΩgDLA≈10-3, evolving only mildly over the past 12 billion years.
ABSTRACT
We probe the small-scale absorption line variability using absorption depth based analysis of a sample of 64 ultra-fast outflow (UFO) C iv broad absorption line (BAL) quasars monitored using ...the Southern African Large Telescope. We confirm the strong monotonic increase in the strength of variability with increasing outflow velocity. We identify regions inside the BAL trough for each source where the normalized flux difference between two epochs is >0.1 for a velocity width ≥500 km s−1 (called ‘variable regions’). We find that the total number of variable regions increases with the time interval probed and the number of BALs showing variable regions almost doubles from short (<2 yr) to long (>2 yr) time-scales. We study the distributions of variable region properties such as its velocity width, depth, and location. These regions typically occupy a few-tenths of the entire width of the BAL. Their widths are found to increase with increasing time-scales having typical widths of ∼2000 km s−1 for Δt > 2 yr. However, their absolute velocity with respect to zem and their relative position within the BAL profile remain random irrespective of the time-scale probed. The equivalent width variations of the BALs are strongly dependent on the size and depth of the variable regions but are little dependent on their total number. Finally, we find that ∼17 per cent of the UFO BALs show uncorrelated variability within the BAL trough.
ABSTRACT
We study the time variability (over ≤7.3 yr) of ultra fast outflows (UFOs) detected in a sample of 64 C iv broad absorption line (BAL) quasars (with 80 distinct BAL components) monitored ...using the Southern African Large Telescope. By comparing the properties of the quasar in our sample with those of a control sample of non-BAL quasars, we show that the distributions of black hole mass are different and the bolometric luminosities and optical photometric variations of UFO BAL quasars are slightly smaller compared to that of non-BAL quasars. The detection fraction of C iv equivalent width (W) variability (∼95 per cent), the fractional variability amplitude $\left(\frac{\Delta W}{W}\right)$ and the fraction of ‘highly variable’ BAL (i.e. $\big|$$\frac{\Delta W}{W}$$\big|$ > 0.67) components (∼33 per cent) are higher in our sample compared to the general BAL population. The scatter in $\frac{\Delta W}{W}$ and the fraction of ‘highly variable’ BALs increase with the time-scale probed. The $\frac{\Delta W}{W}$ distribution is asymmetric at large time scales. We attribute this to the BAL strengthening time-scales being shorter than the weakening time-scales. The BAL variability amplitude correlates strongly with the BAL properties compared to the quasar properties. BALs with low W, high-velocity, shallow profiles, and low-velocity width tend to show more variability. When multiple BAL components are present, a correlated variability is seen between low- and high-velocity components with the latter showing a larger amplitude variations. We find an anticorrelation between the fractional variations in the continuum flux and W. While this suggests photoionization induced variability, the scatter in continuum flux is much smaller than that of W.
This paper presents a survey of X-ray-selected active galactic nuclei (AGNs) with optical spectroscopic follow-up in a ∼ 18 deg2 area of the equatorial XMM-XXL north field. A sample of 8445 ...point-like X-ray sources detected by XMM–Newton above a limiting flux of
$F_{\rm 0.5{\rm -}10\, keV} > 10^{-15} \rm \,erg\, cm^{-2}\, s^{-1}$
was matched to optical (Sloan Digital Sky Survey, SDSS) and infrared (IR; WISE) counterparts. We followed up 3042 sources brighter than r = 22.5 mag with the SDSS Baryon Oscillation Spectroscopic Survey (BOSS) spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGNs in the redshift range z = 0.02–5.0, with 0.5-2 keV luminosities ranging from 1039-1046 erg s− 1. This is currently the largest published spectroscopic sample of X-ray-selected AGNs in a contiguous area. The BOSS spectra of AGN candidates show a distribution of optical line widths which is clearly bimodal, allowing an efficient separation between broad- and narrow-emission line AGNs. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow-emission line objects (22 per cent of the full sample) using standard optical emission line diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of the full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes ‘elusive’ AGN, which would not be easy to identify correctly without their X-ray emission. We also compare X-ray (XMM–Newton), optical colour (SDSS) and and IR (WISE) AGN selections in this field. X-ray observations reveal, by far, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGNs would not be selectable via optical or IR colours due to host galaxy contamination. A substantial fraction of AGN may therefore be missed by these longer wavelength selection methods.
The extended Baryon Oscillation Spectroscopic Survey of the Sloan Digital Sky Survey (SDSS-IV/eBOSS) has an extensive quasar program that combines several selection methods. Among these, the ...photometric variability technique provides highly uniform samples, which are unaffected by the redshift bias of traditional optical-color selections, when z = 2.7−3.5 quasars cross the stellar locus or when host galaxy light affects quasar colors at z< 0.9. We present the variability selection of quasars in eBOSS, focusing on a specific program that led to a sample of 13 876 quasars to gdered = 22.5 over a 94.5 deg2 region in Stripe 82, which has an areal density 1.5 times higher than over the rest of the eBOSS footprint. We use these variability-selected data to provide a new measurement of the quasar luminosity function (QLF) in the redshift range of 0.68 <z< 4.0. Our sample is denser and reaches more deeply than those used in previous studies of the QLF, and it is among the largest ones. At the faint end, our QLF extends to Mg(z = 2) = −21.80 at low redshift and to Mg(z = 2) = −26.20 at z ~ 4. We fit the QLF using two independent double-power-law models with ten free parameters each. The first model is a pure luminosity-function evolution (PLE) with bright-end and faint-end slopes allowed to be different on either side of z = 2.2. The other is a simple PLE at z< 2.2, combined with a model that comprises both luminosity and density evolution (LEDE) at z> 2.2. Both models are constrained to be continuous at z = 2.2. They present a flattening of the bright-end slope at high redshift. The LEDE model indicates a reduction of the break density with increasing redshift, but the evolution of the break magnitude depends on the parameterization. The models are in excellent accord, predicting quasar counts that agree within 0.3% (resp., 1.1%) to g< 22.5 (resp., g< 23). The models are also in good agreement over the entire redshift range with models from previous studies.
ABSTRACT
We present a time variability analysis of broad absorption lines (BAL; spread over the velocity range of 5800–29 000 km s−1) seen in the spectrum of J132216.25 + 052446.3 (zem = 2.04806) at ...ten different epochs spanning over 19 yr. The strongest absorption component (BAL-A; spread over 5800–9900 km s−1) is made up of several narrow components having velocity separations close to C iv doublet splitting. The C iv, N v, and Si iv absorption from BAL-A show correlated optical depth variability without major changes in the velocity structure. A very broad and shallow absorption (BAL-C; spread over the velocity range 15 000–29 000 km s−1) emerged during our monitoring period coinciding with a dimming episode of J1322 + 0524. All the identified absorption lines show correlated variability with the equivalent widths increasing with decreasing flux. This together with the C iv emission line variability is consistent with ionization being the main driver of the correlated variability. The observed UV-continuum variations are weaker than what is required by the photoionization models. This together with a scatter in the C iv equivalent width at a given continuum flux can be understood if variations of the C iv ionizing photons are much larger than that of the UV continuum, the variations in the ionizing photon and UV fluxes are not correlated and/or the covering factor of the flow varies continuously. We suggest BAL-A is produced by a stable clumpy outflow located beyond the broad emission line region and BAL-C is a newly formed wind component located near the accretion disc and both respond to changes in the ionizing continuum.
ABSTRACT
We present the detection of excited fine-structure energy levels of singly ionized silicon and neutral carbon associated with the proximate damped Lyman-α system at zabs = 2.811 towards Q ...0528−250. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e. zabs > zem. We measure the metallicity of the system to be Zn/H = −0.68 ± 0.02. Using the relative populations of the fine-structure levels of Si ii and C i, as well as the populations of H2 rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm−3 and $260^{+30}_{-20}$ cm−3 in two velocity components where both C i and H2 are detected. Taking into account the kinetic temperature in each component, ∼150 K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine’s unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is ∼104 au. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be ∼150−200 kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.
Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < z_{\rm abs} \le 4.2) Damped Lyman- \alpha systems (DLAs) capitalising on observations performed ...with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N (H I) \ge 20 and z_{\rm abs} >1.8, which have data that include redshifted H sub(2) Lyman and/or Werner-band absorption lines. This sample of H I, H sub(2) and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H sub(2) could be completed so far, including all non-detections. Results. H sub(2) is detected in thirteen of the systems, which have molecular fractions of values between f \simeq 5\times10 super(- 7) and f \simeq 0.1, where f = 2 N (H sub(2)) /(2 N (H sub(2)) +N(H I)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N (H sub(2)) similar to 14.3, corresponding to log f <-5. We find that about 35% of the DLAs with metallicities relative to solar X/H \ge-1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f >-4.5, while H sub(2) is detected-regardless of the molecular fraction-in \sim 50% of them. In contrast, only about 4% of the X/H <-1.3 DLAs have log f >-4.5. We show that the presence of H sub(2) does not strongly depend on the total neutral hydrogen column density, although the probability of finding log f >-4.5 is higher for log N (H I) \ge 20.8 than below this limit (19% and 7% respectively). The overall H sub(2) detection rate in log N (H I) \ge 20 DLAs is found to be about 16% (10% considering only log f >-4.5 detections) after correction for a slight bias towards large N (H I). There is a strong preference for H sub(2)-bearing DLAs to have significant depletion factors, X/Fe > 0.4. In addition, all H sub(2)-bearing DLAs have column densities of iron into dust grains larger than log N({\rm Fe}) _{\rm dust} similar to 14.7, and about 40% of the DLAs above this limit have detected H sub(2) lines with log f >-4.5. This demonstrates the importance of dust in governing the detectability of H sub(2) in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H sub(2)-bearing DLAs nor that of the molecular fraction in systems with H sub(2) detections over the redshift range 1.8 < z_{\rm abs} \le 3.
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