Spectra Of Quasi-stellar Objects Research Articles

The rate of extreme coronal line emitting galaxies in the Sloan Digital Sky Survey and their relation to tidal disruption events

ABSTRACT High-ionization iron coronal lines (CLs) are a rare phenomenon observed in galaxy and quasi-stellar object spectra that are thought to be created by high-energy emission from active galactic nuclei and certain types of transients. In cases known as extreme coronal line emitting galaxies (ECLEs), these CLs are strong and fade away on a time-scale of years. The most likely progenitors of these variable CLs are tidal disruption events (TDEs), which produce sufficient high-energy emission to create and sustain the CLs over these time-scales. To test the possible connection between ECLEs and TDEs, we present the most complete variable ECLE rate calculation to date and compare the results to TDE rates from the literature. To achieve this, we search for ECLEs in the Sloan Digital Sky Survey (SDSS). We detect sufficiently strong CLs in 16 galaxies, more than doubling the number previously found in SDSS. Using follow-up spectra from the Dark Energy Spectroscopic Instrument and Gemini Multi-Object Spectrograph, Wide-field Infrared Survey Explorer mid-infrared observations, and Liverpool Telescope optical photometry, we find that none of the nine new ECLEs evolve in a manner consistent with that of the five previously discovered variable ECLEs. Using this sample of five variable ECLEs, we calculate the galaxy-normalized rate of variable ECLEs in SDSS to be $R_\mathrm{G}=3.6~^{+2.6}_{-1.8}~(\mathrm{statistical})~^{+5.1}_{-0.0}~(\mathrm{systematic})\times 10^{-6}~\mathrm{galaxy}^{-1}~\mathrm{yr}^{-1}$. The mass-normalized rate is $R_\mathrm{M}=3.1~^{+2.3}_{-1.5}~(\mathrm{statistical})~^{+4.4}_{-0.0}~(\mathrm{systematic})\times 10^{-17}~\mathrm{M_\odot ^{-1}}~\mathrm{yr}^{-1}$ and the volumetric rate is $R_\mathrm{V}=7~^{+20}_{-5}~(\mathrm{statistical})~^{+10}_{-0.0}~(\mathrm{systematic})\times 10^{-9}~\mathrm{Mpc}^{-3}~\mathrm{yr}^{-1}$. Our rates are one to two orders of magnitude lower than TDE rates from the literature, which suggests that only 10–40 per cent of all TDEs produce variable ECLEs. Additional uncertainties in the rates arising from the structure of the interstellar medium have yet to be included.

Time variation of fine-structure constant constrained by [O iii] emission-lines at 1.1 < z < 3.7

ABSTRACT [O iii] λλ4960,5008 doublet are often the strongest narrow emission lines in starburst galaxies and quasi-stellar objects (QSOs), and thus are a promising probe to possible variation of the fine-structure constant α over cosmic time. Previous such studies using QSOs optical spectra were limited to z < 1. In this work, we constructed a sample of 40 spectra of Ly α emitting galaxies (LAEs) and a sample of 46 spectra of QSOs at 1.09 < z < 3.73 using the VLT/X-Shooter near-infrared spectra publicly available. We measured the wavelength ratios of the two components of the spin-orbit doublet and accordingly calculated α(z) using two methods. Analysis on all of the 86 spectra yielded Δα/α = (− 3 ± 6) × 10−5 with respect to the laboratory α measurements, consistent with no variation over the explored time interval. If assuming a uniform variation rate, we obtained α−1dα/dt = (− 3 ± 6) × 10−15 yr−1 within the last 12 Gyr. Extensive tests indicate that α variation could be better constrained using starburst galaxies’ spectra than using QSO spectra in future studies.

Probing the parameters of the intergalactic medium using quasars

ABSTRACT We continue our series of papers on intergalactic medium (IGM) tracers using quasi-stellar objects (QSOs), having examined gamma-ray bursts (GRBs) and blazars in earlier studies. We have estimated the IGM properties of hydrogen column density ($\mathit {N}\small {\rm HXIGM}$), temperature, and metallicity using XMM–Newton QSO spectra over a large redshift range, with a collisional ionization equilibrium model for the ionized plasma. The $\mathit {N}\small {\rm HXIGM}$ parameter results were robust with respect to intrinsic power laws, spectral counts, reflection hump, and soft excess features. There is scope for a luminosity bias given both luminosity and $\mathit {N}\small {\rm HXIGM}$ scale with redshift, but we find this unlikely given the consistent IGM parameter results across the other tracer types reviewed. The impact of intervening high-column density absorbers was found to be minimal. The $\mathit {N}\small {\rm HXIGM}$ from the QSO sample scales as (1 + z)1.5 ± 0.2. The mean hydrogen density at z = 0 is n0 = (2.8 ± 0.3) × 10−7 cm−3, the mean IGM temperature over the full redshift range is log(T/K) =6.5 ± 0.1, and the mean metallicity is [X/H] = −1.3 ± 0.1(Z ∼ 0.05). Aggregating with our previous GRB and blazar tracers, we conclude that we have provided evidence of the IGM contributing substantially and consistently to the total X-ray absorption seen in the spectra. These results are based on the necessarily simplistic slab model used for the IGM, due to the inability of current X-ray data to constrain the IGM redshift distribution.

The influence of metagalactic ultraviolet background fluctuations on the high-redshift Lyα forest

ABSTRACT Under the assumption that galaxies and quasi-stellar objects (QSOs) dominate the metagalactic ultraviolet (UV) background, it is shown that at high redshifts, fluctuations in the UV background are dominated by QSO shot noise and have an autocorrelation length of a few to several comoving Mpcs, depending on the bright end of the QSO luminosity function. The correlations create long-range spatial coherence in the neutral hydrogen fraction. Using a semi-analytic model, it is demonstrated that the coherence may account for the broad distribution in effective optical depths measured in the Lyα forest spectra of background QSOs, for line-of-sight segments of comoving length $50\, h^{-1}$ Mpc at redshifts 5 < z < 6. Capturing the fluctuations in a numerical simulation requires a comoving box size of ∼1 Gpc, although a box half this size may be adequate if sufficient random realizations of the QSO population are performed.

A Giant Intragroup Nebula Hosting a Damped Absorber at z = 0.313

Abstract This Letter reports the discovery of spatially extended line-emitting nebula, reaching to ≈100 physical kpc (pkpc) from a damped absorber (DLA) at z DLA = 0.313 along the sightline toward quasi-stellar object (QSO) PKS 1127−145 (z QSO = 1.188). This DLA was known to be associated with a galaxy group of dynamical mass M group ∼ 3 × 1012 M ⊙, but its physical origin remained ambiguous. New wide-field integral field observations revealed a giant nebula detected in [O ii], Hβ, [O iii], Hα, and [N ii] emission, with the line-emitting gas following closely the motions of group galaxies. One of the denser streams passes directly in front of the QSO with kinematics that are consistent with the absorption profiles recorded in the QSO echelle spectra. The emission morphology, kinematics, and line ratios of the nebula suggest that shocks and turbulent mixing layers, produced as a result of stripped gaseous streams moving at supersonic speed across the ambient hot medium, contribute significantly to the ionization of the gas. While the DLA may not be associated with any specific detected member of the group, both the kinematic and dust properties are consistent with the DLA originating in streams of gas stripped from sub-L * group members at ≲25 pkpc from the QSO sightline. This study demonstrates that gas stripping in low-mass galaxy groups is effective in releasing metal-enriched gas from star-forming regions, producing absorption systems in QSO spectra, and that combining absorption and emission-line observations provides an exciting new opportunity for studying gas and galaxy co-evolution.

An Updated Constraint on Variations of the Fine-Structure Constant Using Wavelengths of Fe II Absorption Line Multiplets

A new stringent limit relating to the variation of the fine-structure constant ( α = e 2 4 π ε 0 ℏ c ) has been extracted from Ritz wavelengths of 27 quasi_stellar object (QSO) absorption spectra lines of Fe II. The calculation was combined with laboratory wavelengths and QSO spectra to obtain the result Δ α / α = ( 0.027 ± 0.832 ) × 10 − 6 . This result suggests how dedicated astrophysical estimations can improve these limits in the future and can also constrain space_time variations.

The role of galaxies and AGNs in reionizing the IGM – II. Metal-tracing the faint sources of reionization at 5 ≲z≲ 6

We present a new method to study the contribution of faint sources to the UV background using the 1D correlation of metal absorbers with the intergalactic medium (IGM) transmission in a Quasi Stellar Object (QSO) sightline. We take advantage of a sample of $25$ high signal-to-noise ratio QSO spectra to retrieve $150$ triply-ionised carbon (\cfour) absorbers at $4.5\lesssim z\lesssim 6.2$, of which $37$ systems whose expected H{~\small I} absorption lie in the Lyman-$\alpha$ forest. We derive improved constraints on the cosmic density of \cfour \,at $4.3< z < 6.2$ and infer from abundance-matching that \cfour \,absorbers trace $M_{\text{UV}}\lesssim -16$ galaxies. Correlation with the Lyman-$\alpha$ forest of the QSOs indicates that these objects are surrounded by a highly opaque region at $r\lesssim 5 $ cMpc/h followed by an excess of transmission at $r\gtrsim 10$ cMpc/h detected at $2.7\sigma$. This is in contrast to equivalent measurements at lower redshifts where only the opaque trough is detected. We interpret this excess as a statistical enhancement of the local photoionisation rate due to clustered faint galaxies around the \cfour \,absorbers. Using the analytical framework described in Paper I of this series, we derive a constraint on the average product of the escape fraction and the Lyman continuum photon production efficiency of the galaxy population clustered around the \cfour \,absorbers, $\log \langle f_{\text{esc}}\xi_{\text{ion}}\rangle /[{\rm erg^{-1}~Hz}] = 25.01^{+0.30}_{-0.19}$. This implies that faint galaxies beyond the reach of current facilities may have harder radiation fields and/or larger escape fractions than currently detected objects at the end of the reionisation epoch.

C IV broad absorption line disappearance in a large SDSS QSO sample

Context. Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs) originate from outflowing winds along our line of sight; winds are thought to originate from the inner regions of the QSO accretion disk, close to the central supermassive black hole (SMBH). These winds likely play a role in galaxy evolution and are responsible for aiding the accretion mechanism onto the SMBH. Several works have shown that BAL equivalent widths can change on typical timescales from months to years; such variability is generally attributed to changes in the covering factor (due to rotation and/or changes in the wind structure) and/or in the ionization level of the gas. Aims. We investigate BAL variability, focusing on BAL disappearance. Methods. We analyze multi-epoch spectra of more than 1500 QSOs – the largest sample ever used for such a study – observed by different programs from the Sloan Digital Sky Survey-I/II/III (SDSS-I/II/III), and search for disappearing C IV BALs. The spectra cover a rest-frame time baseline ranging from 0.28 to 4.9 yr; the source redshifts range from 1.68 to 4.27. Results. We detect 73 disappearing BALs in the spectra of 67 unique sources. This corresponds to 3.9% of BALs disappearing within 4.9 yr (rest frame), and 5.1% of our BAL QSOs exhibit at least one disappearing BAL within 4.9 yr (rest frame). We estimate the average lifetime of a BAL along our line of sight (≈80–100 yr), which appears consistent with the accretion disk orbital time at distances where winds are thought to originate. We inspect various properties of the disappearing BAL sample and compare them to the corresponding properties of our main sample. We also investigate the existence of a correlation in the variability of multiple troughs in the same spectrum, and find it persistent at large velocity offsets between BAL pairs, suggesting that a mechanism extending on a global scale is necessary to explain the phenomenon. We select a more reliable sample of disappearing BALs on the basis of some criteria adopted in a previous publication, where a subset of our current sample was analyzed, and compare the findings from the two works, obtaining generally consistent results.

Color Variabilities of Spectrally Defined Red QSOs at z = 0.3–1.2

Abstract We study the brightness and the color variabilities of 34 red and 122 typical quasi-stellar objects (QSOs) at z = 0.3–1.2 using data from the Pan-STARRS Medium Deep Survey. The red and the typical QSOs are selected based on the ratios of the flux densities at 3000 Å to those at 4000 Å in the rest frame. We find that 16 out of 34 red QSOs are identified as extended sources, which exhibit strong brightness and color variabilities at shorter wavelengths due to the contamination of the emission from their host galaxies. Some point-like QSOs with significant color variabilities are able to change their color classification according to our spectral definition. The timescales of the color variabilities for these point-like QSOs are within 4 years, suggesting that the size scales of the mechanisms producing the color variabilities are less than a few light years. The spectra of some extended and point-like red QSOs can be well fitted with the dust-reddened spectra of a typical QSO, while others are difficult to explain with dust reddening.

C IV Broad Absorption Line Variability in QSO Spectra from SDSS Surveys

Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs) are thought to arise from outflowing winds along our line of sight; winds, in turn, are thought to originate from the accretion disk, in the very surroundings of the central supermassive black hole (SMBH), and they likely affect the accretion process onto the SMBH, as well as galaxy evolution. BALs can exhibit variability on timescales typically ranging from months to years. We analyze such variability and, in particular, BAL disappearance, with the aim of investigating QSO physics and structure. We search for disappearing C {\scriptsize{IV}} BALs in the spectra of 1319 QSOs from different programs from the Sloan Digital Sky Survey (SDSS); the analyzed time span covers $0.28-4.9$ yr (rest frame), and the source redshifts are in the range $1.68-4.27$. This is to date the largest sample ever used for such a study. We find 67 sources ($5.1_{-0.6}^{+0.7}$\% of the sample) with 73 disappearing BALs in total ($3.9_{-0.5}^{+0.5}$\% of the total number of C {\scriptsize{IV}} BALs detected; \textbf{some sources have more than one BAL that disappears}). We compare the sample of disappearing BALs to the whole sample of BALs, and investigate the correlation in the variability of multiple troughs in the same spectrum. We also derive estimates of the average lifetime of a BAL trough and of the BAL phase along our line of sight.

VoIgt profile Parameter Estimation Routine (viper): H i photoionization rate at z &lt; 0.5

We have developed a parallel code called the VoIgt profile Parameter Estimation Routine (viper) for automatically fitting the H i Lyα forest seen in the spectra of quasi-stellar objects (QSOs). We obtained the H i column density distribution function (CDDF) and linewidth (b) parameter distribution for z < 0.45 using spectra of 82 QSOs obtained using the Cosmic Origins Spectrograph and viper. The consistency of these with existing measurements in the literature validates our code. By comparing this CDDF with those obtained from hydrodynamical simulations, we constrain the H i photoionization rate (⁠|$\Gamma _{\rm H\,\small {I}}$|⁠) at z < 0.45 in four redshift bins. viper, together with the Code for Ionization and Temperature Evolution (cite), which we developed for gadget-2, allows us to explore parameter space and perform χ2 minimization to obtain |$\Gamma _{\rm H\,\small {I}}$|⁠. We notice that the b parameters from the simulations are smaller than those derived from observations. We show that the observed b parameter distribution and b versus |$\log {N_{\rm H\,\small {I}}}$| scatter can be reproduced in simulations by introducing subgrid-scale turbulence. However, it has very little influence on the derived |$\Gamma _{\rm H\,\small {I}}$|⁠. The |$\Gamma _{\rm H\,\small {I}}(z)$| obtained here, (3.9 ± 0.1) × 10−14 (1 + z)4.98 ± 0.11 s−1, is in good agreement with the values derived by us using flux-based statistics in a previous article. These values are consistent with the hydrogen ionizing ultraviolet (UV) background being dominated mainly by QSOs, without needing any contribution from non-standard sources of UV photons.

The physics of the intergalactic medium

Intergalactic space is filled with a pervasive medium of ionized gas, the Intergalactic Medium (IGM). A residual neutral fraction is detected in the spectra of Quasi-Stellar Objects at both low and high redshifts, revealing a highly fluctuating medium with temperatures characteristic of photoionized gas. The statistics of the fluctuations are well-reproduced by numerical gravity-hydrodynamics simulations within the context of standard cosmological structure formation scenarios. As such, the study of the IGM offers an opportunity to probe the nature of the primordial density fluctuations on scales unavailable to other methods. The simulations also suggest the IGM is the dominant reservoir of baryons produced by the Big Bang, and so the principal source of the matter from which galaxies formed. The detection of metal systems within the IGM shows that it was enriched by evolved stars early in its history, demonstrating an intimate connection between galaxy formation and the IGM. The author presents a comprehensive review of the current understanding of the structure and physical properties of the IGM and its relation to galaxies, concluding with comments on prospects for furthering the study of the IGM using future ground-based facilities and space-based experiments.

Fe ii emission spectra in AGN: observations and theoretical interpretation

The enrichment of Fe, relative to alpha elements such as O and Mg, represents a potential means to measure the ages of quasi-stellar object (QSO) host galaxies and probe nucleosynthesis in the early universe. QSOs exhibit prominent Fe ii features and Mg ii 2800 Å resonance doublet emission in the ultraviolet. Although chemical evolutionary models predict that the Fe/Mg abundance ratio decreases with increasing redshift, measurements of QSO Fe ii (UV)/Mg ii emission line ratios show large scatter from 1 to 20, with no redshift dependency up to z∼6.4. Before using Fe ii emission as an abundance indicator, one must ascertain how Fe ii emission varies with physical conditions. We have constructed an 830-level model atom for Fe ii and used it in a photoionization code to calculate Fe ii emission. This model is more sophisticated than previous efforts, and uses the most recent laboratory atomic data and includes the numerous transitions that are sensitive to the strong radiation field in QSOs. Predicted Fe ii(UV)/Mg ii ratios and fluxes strongly depend on non-abundance factors such as microturbulence, ionizing flux, and hydrogen density; all must be taken into account before any accurate abundance can be derived. Our calculations show that Fe ii is the dominant coolant at densities found in active galactic nucleus (AGN) broad emission line regions (BLRs), and must be included in photoionization modelling. Our close collaboration with spectroscopists at Lund University has been highly beneficial for further development of our Fe ii model, most importantly through atomic data studies that link high-energy levels in Fe ii. Additional studies of the atomic structure of Fe ii are necessary to improve our understanding of the AGN continua by accounting for the effects of the Fe ii pseudo-continuum, which blanket QSO spectra from 1000 to 10 000 Å. Predicted Fe ii emission spectra, suitable for BLRs in AGN, are available at http://iacs.cua.edu/people/verner/Feii.

New Magellan Inamori Kyocera Echelle Observations ofz&lt; 1.5 sub-damped Lyman α systems

The damped Lyman a (DLA) and sub-damped Lyman a (sub-DLA) systems seen in the spectra of quasi-stellar objects (QSOs) offer a unique way to study the interstellar medium (ISM) of high-redshift galaxies. In this paper, we report on new abundance determinations in a sample of 10 new systems, nine of the lesser studied sub-DLAs and one DLA along the line of sight to seven QSOs from spectra taken with the Magellan Inamori Kyocera Echelle spectrograph. Lines of Mg i, Mg II, Al II, Al III, Can, Mn II, Fe II and Zn II were detected. Here, we give the column densities and equivalent widths of the observed absorption lines, as well as the abundances determined for these systems. Zn, a relatively undepleted element in the local ISM is detected in one system with a high metallicity of [Zn/H] = +0.27 ± 0.18. In another system, a high abundance based on the more depleted element Fe is seen with [Fe/H] = -0.37 ± 0.13, although Zn is not detected. The N H1 -weighted mean metallicity of these sub-DLA systems based on Fe is ([Fe/H]) = -0.76 ± 0.11, nearly ∼0.7 dex higher (a factor of 5) than what is seen in DLAs in this redshift range. The relative abundance of [Mn/Fe] is also investigated. A clear trend is visible for these systems as well as systems from the literature, with [Mn/Fe] increasing with increasing metallicity in good agreement with Milky Way stellar abundances.

On the Incidence of CivAbsorbers Along the Sight Lines to Gamma‐Ray Bursts

We report on the statistics of strong (Wr > 0.15 A) C IV absorbers at z = 1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a recent survey for strong Mg II absorption systems at z < 2, we find that the number of C IV absorbers per unit redshift dN/dz does not show a significant deviation from previous surveys using quasi-stellar objects (QSOs) as background sources. We find that the number density of C IV toward GRBs is dN/dz|GRB(z ~ 1.5) = 2.2, dN/dz|GRB(z ~ 2.5) = 2.3 and dN/dz|GRB(z ~ 3.5) = 1.1. These numbers are consistent with previous C IV surveys using QSO spectra. Binning the entire data set, we set a 95% c.l. upper limit to the excess of C IV absorbers along GRB sight lines at twice the incidence observed along QSO sight lines. Furthermore, the distribution of equivalent widths of the GRB and QSO samples are consistent with being drawn from the same parent population. Although the results for Mg II and C IV absorbers along GRB sight lines appear to contradict one another, we note that the surveys are nearly disjoint: the C IV survey corresponds to higher redshift and more highly ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical samples may constrain properties of the galaxies hosting these metals (e.g., mass, dust content) and/or the coherence-length of the gas giving rise to the metal-line absorption.

Identification and redshift determination of quasi-stellar objects with medium-band photometry: application to Gaia

All-sky, multicolour, medium deep (V≃ 20) surveys have the potentiality of detecting several hundred thousands of quasi-stellar objects (QSOs). Spectroscopic confirmation is not possible for such a large number of objects, so that secure photometric identification and precise photometric determination of redshifts (and other spectral features) become mandatory. This is especially the case for the Gaia mission, in which QSOs play the crucial role of fixing the celestial referential frame, and in which more than 900 gravitationally lensed QSOs should be identified. We first built two independent libraries of synthetic QSO spectra reflecting the most important variations in the spectra of these objects. These libraries are publicly available for simulations with any instrument and photometric system. Traditional template fitting and artificial neural networks (ANNs) are compared to identify QSOs among the population of stars using broad- and medium-band photometry (BBP and MBP, respectively). Besides those two methods, a new one, based on the spectral principal components (SPCs), is also introduced to estimate the photometric redshifts. Generic trends as well as results specifically related to Gaia observations are given. We found that (i) ANNs can provide clean, uncontaminated QSO samples suitable for the determination of the reference frame, but with a level of completeness decreasing from ≃50 per cent at the Galactic pole at V= 18 to ≃ 16 per cent at V= 20; (ii) the χ2 approach identifies about 90 per cent (60 per cent) of the observed QSOs at V= 18 (V= 20), at the expense of a higher stellar contamination rate, reaching ≃95 per cent in the galactic plane at V= 20. Extinction is a source of confusion and makes difficult the identification of QSOs in the galactic plane and (iii) the χ2 method is better than ANNs to estimate the photometric redshifts. Due to colour degeneracies, the largest median absolute error (|Δz|Median≃ 0.2) is predicted in the range 0.5 < zspec < 2. The method based on the SPCs is promisingly good at recovering the redshift, in particular for V < 19 and z < 2.5 QSOs. For bright (V≲ 18) QSOs, SPCs are also able to recover the spectral shape from the BBP and MBP data.

Cosmological and astrophysical parameters from the Sloan Digital Sky Survey flux power spectrum and hydrodynamical simulations of the Lyman α forest

The flux power spectrum of the Lyman α forest in quasar [quasi-stellar object (QSO)] absorption spectra is sensitive to a wide range of cosmological and astrophysical parameters and instrumental effects. Modelling the flux power spectrum in this large parameter space to an accuracy comparable to the statistical uncertainty of large samples of QSO spectra is very challenging. We use here a coarse grid of hydrodynamical simulations run with gadget-2 to obtain a ‘best-guess’ model around which we calculate a finer grid of flux power spectra using a Taylor expansion of the flux power spectrum to first order. In this way, we investigate how the interplay between astrophysical and cosmological parameters affects their measurements using the recently published flux power spectrum obtained from 3035 Sloan Digital Sky Survey (SDSS) QSOs. We find that the SDSS flux power spectrum alone is able to constrain a wide range of parameters including the amplitude of the matter power spectrum σ8, the matter density Ωm, the spectral index of primordial density fluctuations n, the effective optical depth τeff and its evolution. The thermal history of the intergalactic medium (IGM) is, however, poorly constrained and the SDSS data favour either an unplausibly large temperature or an unplausibly steep temperature-density relation. By enforcing a thermal history of the IGM consistent with that inferred from high-resolution QSO spectra, we find the following values for the best-fitting model (assuming a flat universe with a cosmological constant and zero neutrino mass): Ωm= 0.28 ± 0.03, n= 0.95 ± 0.04 and σ8= 0.91 ± 0.07 (1σ error bars). The values for σ8 and n are consistent with those obtained by McDonald et al. with different simulations for similar assumptions. We argue, however, that the major uncertainties in this measurement are still systematic rather than statistical.

The correlation of line strength with luminosity and redshift from composite quasi-stellar object spectra

We have generated a series of composite quasi-stellar object (QSO) spectra using over 22 000 individual low-resolution (∼8-Å) QSO spectra obtained from the 2dF (18.25 bJ 20.85) and 6dF (16bJ≤18.25) QSO Redshift Surveys. The large size of the catalogue has enabled us to construct composite spectra in relatively narrow redshift (Δz = 0.25) and absolute magnitude (ΔMB = 0.5) bins. The median number of QSOs in each composite spectrum is ∼200, yielding typical signal-to-noise ratios of ∼100. For a given redshift interval, the composite spectra cover a factor of over 25 in luminosity. For a given luminosity, many of the major QSO emission lines (e.g. Mg iiλ2798, [Oii]λ3727) can be observed over a redshift range of 1 or greater. Using the composite spectra we have measured the line strengths (equivalent widths) of the major broad and narrow emission lines. We have also measured the equivalent width of the Ca iiλ3933 K absorption feature caused by the host galaxy of the active galactic nuclei (AGN). Under the assumption of a fixed host galaxy spectral energy distribution (SED), the correlation seen between Ca ii K equivalent width and source luminosity implies Lgal∝L0.42±0.05QSO. We find strong anticorrelations with luminosity for the equivalent widths of [O ii]λ3727 and [Ne v]λ3426. These provide hints to the general fading of the NLR in high-luminosity sources, which we attribute to the NLR dimensions becoming larger than the host galaxy. This could have important implications for the search for type 2 AGN at high redshifts. If average AGN host galaxies have SEDs similar to average galaxies, then the observed narrow [O ii] emission could be solely a result of the host galaxy at low luminosities (MB∼−20). This suggests that the [O ii] line observed in high-luminosity AGN may be emitted, to a large part, by intense star-forming regions. The AGN contribution to this line could be weaker than previously assumed. We measure highly significant Baldwin effects for most broad emission lines (C ivλ1549, C iii]λ1909, Mg iiλ2798, Hγ, Hβ) and show that they are predominantly caused by correlations with luminosity, not redshift. We find that the Hβ and Hγ Balmer lines show an inverse Baldwin effect and are positively correlated with luminosity, unlike the broad ultraviolet lines. We postulate that this previously unknown effect is caused by a luminosity-dependent change in the ratio of disc to non-disc continuum components.

Hydrodynamical simulations of the Lyαforest: data comparisons

Numerical hydrodynamical simulations are used to predict the expected absorption properties of the Lyα forest for a variety of cold dark matter (CDM) dominated cosmological scenarios: CHDM, OCDM, ΛCDM, SCDM, and tCDM. Synthetic spectra are constructed duplicating the resolution, signal-to-noise ratio and wavelength coverage of several published high resolution spectra, and their statistical properties are compared on the basis of the flux distribution of the spectra, the distribution of coefficients in a wavelet decomposition of the spectra and the distributions of absorption line profile parameters. Agreement between the measured and predicted cumulative distributions is found at the few to several per cent level. The best-fitting models to the flux distribution correspond to normalizations on the scale of the cosmological Jeans length of at . No single model provides a statistically acceptable match to all the distributions. Significantly larger median Doppler parameters are found in the measured spectra than predicted by all but the lowest normalization models (CHDM and tCDM), which provide poor fits to the flux distributions. The discrepancy in Doppler parameters is particularly large for absorption systems optically thin at the Lyα line centre. This may indicate a need to introduce additional energy injection throughout the intergalactic medium, as may be provided by late He ii reionization or supernovae-driven winds from young galaxies, and/or a larger baryon fraction than given by recent determinations of the deuterium abundance within the context of standard Big Bang nucleosynthesis. The models require a hydrogen ionization rate at redshifts within a factor of 2 of that predicted from quasi-stellar objects (QSOs) alone as the sources of the UV photoionization background, although with a slower rate of decline with redshift at than predicted from current QSO counts. Principal systematic uncertainties in comparing the models with the observations are the setting of the continuum level of the QSO spectra and the prevalence of metal absorption lines, particularly at .

Blueshifts in Emission Line Spectra of Quasi Stellar Objects

A thorough search for blueshifts in Quasi Stellar Objects (QSOs) has not yet been made, although possibilities have been suggested. The main reason appears to be the usual practice of identifying observed lines always with search lines in the UV/blue region, and ignoring the red/IR region. We demonstrate that sufficient search lines are available for blueshift measurements in QSOs, and show that within observational errors observed emission lines can be identified with alternative lines at higher wavelengths, that are blueshifted to the observing window. Blueshifts are calculated with the same or higher levels of accuracy as redshifts. Consideration of equivalent widths demonstrates further that blueshift identifications of observed lines are more convincing than redshift identifications.