A wavelet analysis of the spectra of quasi-stellar objects

The temperature of the intergalactic medium (IGM) is an important factor in determining the linewidths of the absorption lines in the Ly α forest. We present a method to characterize the linewidth distribution using a decomposition of an Ly α spectrum in terms of discrete wavelets. Such wavelets form an orthogonal basis, so the decomposition is unique. We demonstrate using hydrodynamic simulations that the mean and dispersion of the wavelet amplitudes are strongly correlated with both the temperature of the absorbing gas and its dependence on the gas density. Since wavelets are also localized in space, we are able to analyse the temperature distribution as a function of position along the spectrum. We illustrate how this method could be used to identify fluctuations in the IGM temperature that might result from late reionization or local effects.     

The thermal history of the intergalactic medium

At redshifts z ≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Ly α forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Ly α absorption lines ( b -parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z ∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of He  ii .     

The role of non-thermal electrons in the hydrogen and calcium lines of stellar flares

There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca  ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca  ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca  ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy.     

The proximity effect in a close group of QSOs

We present an analysis of the proximity effect in a sample of 10 2-Å-resolution QSO spectra of the Ly α forest at     . Rather than investigating variations in the number density of individual absorption lines, we employ a novel technique that is based on the statistics of the transmitted flux itself. We confirm the existence of the proximity effect at the > 99 per cent confidence level. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of     . This value assumes that QSO redshifts measured from high-ionization lines differ from the true systemic redshifts by     . We find evidence at a level of 2.6 σ that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. Allowing for known QSO variability, the significance of the correlation reduces to 2.1 σ .
The QSOs form a close group on the sky and the sample is thus well suited for an investigation of the foreground proximity effect, where the Ly α forest of a background QSO is influenced by the UV radiation from a nearby foreground QSO. From the complete sample we find no evidence for the existence of this effect, implying either that     or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example. Our sample includes the fortunate constellation of a foreground QSO surrounded by four nearby background QSOs. These four spectra all show underdense absorption within ±3000 km s⁻¹ of the redshift of the foreground QSO.     

An astrophysical oscillator strength for the S ii 94.7-nm resonance line and S abundances in DLAs

By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle spectrograph, we determine an 'astrophysical' f value for the S  ii   λ 94.7-nm line: f =0.00498_−0.00138^+0.00172 , error at 1 σ level. This is almost a factor of 30 smaller than the guessed value found in the Kurucz data base (  f =0.1472) , which was until now the only one available for this transition. We use our 'astrophysical' f to investigate the S abundance in two damped Ly α absorption systems (DLAs) observed with the UV–Visual Echelle Spectrograph (UVES) at the European Southern Observatory's 8.2-m Kueyen telescope. In the case of the absorber at z _abs=3.02486 towards QSO 0347-3819, we find a sulphur column density which is consistent, within errors, with that determined by Centurión et al. by means of the λ 125.9-nm line, thus providing an external check on the accuracy of our f value. For the damped absorber at z _abs=4.4680 towards BR J0307-4945, we determine a high value of the S abundance, which, however, is probably the result of blending with Ly α forest lines.     

The evolution of superbubbles and the detection of Lyα in star-forming galaxies

The detection of Ly α emission in star-forming galaxies in different shapes and intensities (always smaller than predicted for case B recombination) has puzzled the astronomical community for more than a decade. Here we use two-dimensional calculations to follow the evolution of superbubbles and of the H  ii regions generated by the output of UV photons from massive stars. We show the impact caused by massive star formation in the ISM of different galaxies and we look at the conditions required to detect Ly α emission from a nuclear H  ii region, and the variety of profiles that may be expected as a function of time.     

The Second APM UKST Colour Survey for z > 4 quasars

We present the spectra, positions, and finding charts for 31 bright ( R <19.3) colour-selected quasars covering the redshift range z =3.85–4.78, with four having redshifts z >4.5. The majority are in the southern sky ( δ <−25°). The quasar candidates were selected for their red ( B _J− R ≳2.5) colours from UK or POSSII Schmidt Plates scanned at the Automated Plate Measuring (APM) facility in Cambridge. Low-resolution (≳ 10 Å) spectra were obtained to identify the quasars, primarily at the Las Campanas Observatory. The highest redshift quasar in our survey is at z ≈4.8 ( R =18.7) and its spectrum shows a damped Ly α absorption system at z =4.46. This is currently the highest redshift damped Ly α absorber detected. Five of these quasars exhibit intrinsic broad absorption line features. Combined with the previously published results from the first part of the APM United Kingdom Schmidt Telescope (UKST) survey we have now surveyed a total of ∼8000 deg² of sky i.e. 40 per cent of the high galactic latitude (| b |>30°) sky, resulting in 59 optically selected quasars in the redshift range 3.85 to 4.78; 49 of which have z ≥4.00.     

Extended line emission around seven radio-loud quasars at redshift z ∼ 2

We present near-infrared spectra of seven radio-loud quasars with a median redshift of 2.1, five of which were previously known to have Ly α nebulae. Extended [O  iii ] λ 5007 and H α emission are evident around six objects, at the level of a few times 10⁻¹⁶ erg cm⁻² arcsec⁻² s⁻¹ within ≃2 arcsec of the nucleus (≡16 kpc in the adopted cosmology). Nuclear [O  ii ] λ 3727 is detected in three of the five quasars studied at this wavelength and clearly extended in one of them.
The extended [O  iii ] tends to be brighter on the side of the nucleus with the stronger, jet-like radio emission, indicating at least that the extranuclear gas is distributed anisotropically. It is also typically redshifted by several hundred km s⁻¹ from the nuclear [O  iii ], perhaps because of the latter being blueshifted from the host galaxy's systemic velocity. Alternatively, the velocity shifts could be due to infall (which is suggested by linewidths ∼1000 km s⁻¹ FWHM) in combination with a suitable dust geometry. Ly α /H α ratios well below the case B value suggest that some dust is present.
Photoionization modelling of the [O  iii ]/[O  ii ] ratios in the extended gas suggests that its pressure is around or less than a few times 10⁷ cm⁻³ K; any confining intracluster medium is thus likely to host a strong cooling flow. A comparison with lower redshift work suggests that there has been little evolution in the nuclear emission-line properties of radio-loud quasars between redshifts 1 and 2.     

Numerical simulations of the Lyman α forest – a comparison of gadget-2 and enzo

We compare simulations of the Lyman α forest performed with two different hydrodynamical codes, gadget-2 and enzo . A comparison of the dark matter power spectrum for simulations run with identical initial conditions show differences of 1–3 per cent at the scales relevant for quantitative studies of the Lyman α forest. This allows a meaningful comparison of the effect of the different implementations of the hydrodynamic part of the two codes. Using the same cooling and heating algorithm in both codes, the differences in the temperature and the density probability distribution function are of the order of 10 per cent. The differences are comparable to the effects of box size and resolution on these statistics. When self-converged results for each code are taken into account, the differences in the flux power spectrum – the statistics most widely used for estimating the matter power spectrum and cosmological parameters from Lyman α forest data – are about 5 per cent. This is again comparable to the effects of box size and resolution. Numerical uncertainties due to a particular implementation of solving the hydrodynamic or gravitational equations appear therefore to contribute only moderately to the error budget in estimates of the flux power spectrum from numerical simulations. We further find that the differences in the flux power spectrum for enzo simulations run with and without adaptive mesh refinement are also of the order of 5 per cent or smaller. The latter require 10 times less CPU time making the CPU time requirement similar to that of a version of gadget-2 that is optimized for Lyman α forest simulations.     

The Solar Spectral Irradiance as a Function of the Mg <Emphasis Type="SmallCaps">ii</Emphasis> Index for Atmosphere and Climate Modelling

We present a new method to reconstruct the solar spectrum irradiance in the Ly α – 400 nm region, and its variability, based on the Mg ii index and neutron-monitor measurements. Measurements of the solar spectral irradiance available in the literature have been made with different instruments at different times and different spectral ranges. However, climate studies require harmonised data sets. This new approach has the advantage of being independent of the absolute calibration and aging of the instruments. First, the Mg ii index is derived using solar spectra from Ly α (121 nm) to 410 nm measured from 1978 to 2010 by several space missions. The variability of the spectra with respect to a chosen reference spectrum as a function of time and wavelength is scaled to the derived Mg ii index. The set of coefficients expressing the spectral variability can be applied to the chosen reference spectrum to reconstruct the solar spectra within a given time frame or Mg ii index values. The accuracy of this method is estimated using two approaches: direct comparison with particular cases where solar spectra are available from independent measurements, and calculating the standard deviation between the measured spectra and their reconstruction. From direct comparisons with measurements we obtain an accuracy of about 1 to 2%, which degrades towards Ly α. In a further step, we extend our solar spectral-irradiance reconstruction back to the Maunder Minimum introducing the relationship between the Mg ii index and the neutron-monitor data. Consistent measurements of the Mg ii index are not available prior to 1978. However, we remark that over the last three solar cycles, the Mg ii index shows strong correlation with the modulation potential determined from the neutron-monitor data. Assuming that this correlation can be applied to the past, we reconstruct the Mg ii index from the modulation potential back to the Maunder Minimum, and obtain the corresponding solar spectral-irradiance reconstruction back to that period. As there is no direct measurement of the spectral irradiance for this period we discuss this methodology in light of the other proposed approaches available in the literature. The use of the cosmogenic-isotope data provides a major advantage: it provides information about solar activity over several thousands years. Using technology of today, we can calibrate the solar irradiance against activity and thus reconstruct it for the times when cosmogenic-isotope data are available. This calibration can be re-assessed at any time, if necessary.     

Applications of wavelets to the analysis of cosmic microwave background maps

We consider wavelets as a tool to perform a variety of tasks in the context of analysing cosmic microwave background (CMB) maps. Using spherical Haar wavelets, we define a position and angular-scale-dependent measure of power that can be used to assess the existence of spatial structure. We apply planar Daubechies wavelets for the identification and removal of point sources from small sections of sky maps. Our technique can successfully identify virtually all point sources that are above 3 and more than 80 per cent of those above 1 . We discuss the trade-offs between the levels of correct and false detections. We denoise and compress a 100 000-pixel CMB map by a factor of 10 in 5 s, achieving a noise reduction of about 35 per cent. In contrast to Wiener filtering, the compression process is model-independent and very fast. We discuss the usefulness of wavelets for power spectrum and cosmological-parameter estimation. We conclude that at present wavelet functions are most suitable for identifying localized sources.     

Radial velocity study of the roAp star α Circinus

We present results of 548 high-dispersion spectra of the roAp star α Circinus over a five-night period. The pulsational radial velocities measured from the rare-earth elements, Nd iii , Hα and Hβ are easily measured and occur at the photometric period. The amplitude is largest in Nd iii and Hα, is lower in the rare earths and Hβ, and cannot be measured in other metal lines. This behaviour can be understood in terms of an increase of pulsational amplitude with height in the atmosphere coupled with abundance stratification. The radial velocities show a significant variation at the rotational period of 4.463 d and a marginally significant periodicity at a frequency of 8.16 cycle d⁻¹. The latter may be a very low-amplitude δ Scuti pulsation.     

Evidence for an ionized disc in the narrow-line Seyfert 1 galaxy Ark 564

We present simultaneous ASCA and RXTE observations of Ark 564, the brightest known 'narrow-line' Seyfert 1 in the 2–10 keV band. The measured X-ray spectrum is dominated by a steep (Γ≈2.7) power-law continuum extending to at least 20 keV, with imprinted Fe K-line and edge features and an additional 'soft excess' below ∼1.5 keV. The energy of the iron K-edge indicates the presence of highly ionized material, which we identify in terms of reflection from a strongly irradiated accretion disc. The high reflectivity of this putative disc, together with its strong intrinsic O  viii Ly α and O  viii recombination emission, can also explain much of the observed soft excess flux. Furthermore, the same spectral model also provides a reasonable match to the very steep 0.1–2 keV spectrum deduced from ROSAT data. The source is much more rapidly variable than 'normal' Seyfert 1s of comparable luminosity, increasing by a factor of ∼50 per cent in 1.6 h, with no measurable lag between the 0.5–2 keV and 3–12 keV bands, consistent with much of the soft excess flux arising from reprocessing of the primary power-law component in the inner region of the accretion disc. We note, finally, that if the unusually steep power-law component is a result of Compton cooling of a disc corona by an intense soft photon flux, then the implication is that the bulk of these soft photons lie in the unobserved extreme ultraviolet.     

The probability distribution function of the Lyman α transmitted flux from a sample of Sloan Digital Sky Survey quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lyman α (Lyα) forest from a sample of 3492 quasars included in the Sloan Digital Sky Survey data release 3 (SDSS DR3). Our intention is to investigate the sensitivity of the Lyα flux PDF as measured from low-resolution and low signal-to-noise ratio data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. The quasar continuum is described by the superposition of a power law and emission lines. We perform a power-law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of  α_ν= 0.59 ± 0.36  in the redshift range  2.5 < z < 3.5  . We take into account that the variation in the continuum indices increases the mean flux by 3 and 7 per cent at   z = 3  and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lyα flux PDF and power spectrum of high-resolution data. Using our power-law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of ∼10–15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than ≲10 per cent.     

The growth of structure in the intergalactic medium

A 'stochastic adhesion' model is introduced, with the purpose of describing the formation and evolution of mildly non-linear structures, such as sheets and filaments, in the intergalactic medium (IGM), after hydrogen reionization. The model is based on replacing the overall force acting on the baryon fluid – which results from the combination of local gravity, pressure gradients and Hubble drag – by a mock external force, self-consistently calculated from first-order perturbation theory. A small kinematic viscosity term prevents shell-crossing on small scales (which arises because of the approximate treatment of pressure gradients). The emerging scheme is an extension of the well-known adhesion approximation for the dark matter dynamics, from which it differs only by the presence of a small-scale 'random' force, characterizing the IGM. Our algorithm is the ideal tool to obtain the skeleton of the IGM distribution, which is responsible for the structure observed in the low column density Ly α forest in the absorption spectra of distant quasars.     

Composite spectra Paper 11: α Equulei, an astrometric binary with an Am secondary

The spectrum of the secondary component of the bright composite-binary system α Equ, whose visual orbit is already known accurately, is isolated by the method of spectrum subtraction and classified accurately for the first time. The primary is a normal giant of type ∼G7, while the secondary is an Am star of type ∼kA3hA4mA9. The system's mass ratio, q , is determined to be  1.15±0.03  from measurements of the relative radial-velocity displacements between the components. Random and systematic errors in q are evaluated on the basis of the scatter of results derived from sets of spectra obtained from three different sources, and from tests conducted on independent versions of the secondary's spectrum. A spectroscopic analysis of a composite system such as α Equ is strongly challenged by the blending of a great many lines that are common to both spectra. Even when the primary spectrum is thought to have been subtracted adequately, a seemingly unavoidable ghost spectrum of faint residuals can bias wavelength measurements of the secondary's lines. That blending was the principal cause of a history of puzzling and discrepant measurements of q in α Equ. The derived masses of  M₁=2.3 M_⊙  ,  M₂=2.0 M_⊙  for the giant and dwarf, respectively, constrain the choice of models for fitting evolutionary tracks in the (log  T _eff, log  L ) plane; the stellar points fit a single isochrone (for 0.74 Gyr). Both components are found to be slightly over-luminous compared to normal for their supposed luminosity classes. The giant appears to be commencing its first ascent of the red-giant branch. The dwarf has started to evolve away from the main sequence; its M _V is similar to that of a sub-giant.     

The Gunn–Peterson effect and the Lyman alpha forest

We show that spatial correlations in a stochastic large-scale velocity field in an otherwise smooth intergalactic medium (homogeneous comoving density) superposed on the general Hubble flow may cause a 'line-like' structure in QSO spectra similar to the population of unsaturated Lyα forest lines which usually are attributed to individual clouds with 10¹¹ ≲ N _{H i}  5 × 10¹³ cm⁻². Therefore there is no clear observational distinction between a diffuse intergalactic medium and discrete intergalactic clouds. It follows that the H  i density in the diffuse intergalactic medium might be substantially underestimated if it is determined from the observed intensity distribution near the apparent continuum in high-resolution spectra of QSOs. Our tentative estimate implies a diffuse neutral hydrogen opacity τ_GP ∼ 0.3 at z  ∼ 3 and a current baryon density Ω_IGM ≃ 0.08, assuming a Hubble constant H ₀ = 70 km s⁻¹ Mpc⁻¹.     

On the small-scale clustering of the Lyα forest clouds

Recent measurements of the autocorrelation function of the Ly α clouds are analysed from the point of view of a simple model with strong clustering on the small scales. It is shown that this toy model reproduces fairly well the important linear relation between amplitude of the absorber autocorrelation function and neutral hydrogen column density. In addition, it predicts a correct evolutionary trend of correlation amplitudes. Some possible ramifications of these results are discussed.     

Predictions for the clustering properties of the Lyman-alpha forest – I. One-point statistics

We present predictions for the one-point probability distribution and cumulants of the transmitted QSO flux in the high redshift Lyman- α forest. We make use of the correlation between the Lyman- α optical depth and the underlying matter density predicted by gravitational instability theory and seen in numerical hydrodynamic simulations. We have modelled the growth of matter fluctuations using the non-linear shear‐free dynamics, an approximation which reproduces well the results of perturbation theory for the cumulants in the linear and weakly non-linear clustering regime. As high matter overdensities tend to saturate in spectra, the statistics of the flux distribution are dominated by weakly non-linear overdensities. As a result, our analytic approach can produce accurate predictions, when tested against N -body simulation results, even when the underlying matter field has root-mean-square fluctuations larger than unity. Our treatment can be applied to either Gaussian or non-Gaussian initial conditions. Here we concentrate on the former case, but also include a study of a specific non-Gaussian model. We discuss how the methods and predictions we present can be used as a tool to study the generic clustering properties of the Lyman- α forest at high redshift. With such an approach, rather than concentrating on simulating specific cosmological models, we may be in a position to directly test our assumptions for the Gaussian nature of the initial conditions, and the gravitational instability origin of structure itself. In a separate paper we present results for two-point statistics.