For 15 bright (V 3) quasars, we have obtained infrared spectra and photometry, and optical spectrophotometry and photometry, which we use to construct their spectral energy distributions (SEDs) from ?rest ~ 1285-5100 ?. High-resolution spectroscopy for seven enable measurements of their continua shortward of Ly?, and L' detections of four of these extend their SEDs redward to ?rest ~ 7500 ?. We examine the optical/UV continuum shapes and compare these to those of a set of 27 well-studied low-redshift (z ~ 0.1) quasars which are matched to the high-redshift ones in evolved luminosity. Single power-law fits to the average fluxes within a set of narrow, line-free, windows between 1285 and 5100 ?, but excluding the 2000-4000 ? region of the Fe II+BaC small bump, are adequate for most of the objects. For both the high- and low-redshift samples, the distributions of spectral indices, ?ouv (F? ~ ?) span a wide range, with ??ouv ~ 1. The cause of such diversity is investigated, and our analysis is consistent with the conclusion of Rowan-Robinson: that it arises from differences in both the emitted continua themselves and in the amounts of intrinsic extinction undergone. The mean (median) optical/UV spectral indices for the high- and low-redshift samples are -0.32 (-0.29) and -0.38 (-0.40), respectively. A Student's t-test indicates that these do not differ significantly, and a K-S test shows likewise for the distributions. Assuming the optical/UV continuum derives from accretion, the similarity of the spectral indices at high and low redshift is inconsistent with models which interpret the statistical evolution as resulting from a single generation of slowly dimming quasars and instead favors those involving multiple generations of short-lived quasars formed at successively lower luminosities. A clear difference between the high- and low-redshift samples occurs in the region of small bump. The power-law fit residuals for the low-redshift sample show a systematic excess from ~2200 to 3000 ?; but this feature is weak or absent in the high-redshift sample. Further study is needed to determine what is responsible for this contrast, but it could reflect differences in iron abundance or Fe II energy source, or alternatively, an intrinsic turnover in the continuum itself which is present at low but not at high redshift.

http://iopscience.iop.org/article/10.1086/322535/pdf

High resolution CASPEC observations of the z = 4.11 QSO 0000-26.

A Search for Signatures of Quasar Evolution: Comparison of the Shapes of the Rest-Frame Optical/Ultraviolet Continua of Quasars at z > 3 and z ~ 0.1*

THE quasar absorption lines known as the Lyman-α forest reveal the presence of a population of extragalactic objects, thought to be clouds of gas or gas-rich protogalaxies The gas seems to be highly ionized (NHl/NH11 ≈ 10–4), with column densities (NH1) ranging from 1012 to 1018 cm–2 and Doppler parameters of 15–40 km s–1 (refs 1–3) Different types of measurement have yielded strongly discrepant numbers for the typical diameter, from several tens of kiloparsecs4,5 to only ∼5 parsecs6,7 Nothing is known about the clouds' shape (spheres or sheets), which could provide another clue to their origin8 The observation of huge, luminous Ly-α haloes around some high-redshift radio galaxies9–11 provides a new way of obtaining information about Lyman cloud size and shape: the large sizes (many tens of kiloparsecs) and broad emission-line profiles of these haloes make them ideal light screens against which absorbing clouds at similar redshift might be seen as dark shadows We report here the possible detection of such an absorbing cloud in front of the radio galaxy 4C4117; in the simplest interpretation, the cloud has an elongated shape, and dimensions of ∼40 x 10 kiloparsecs

Imaging of a Lyman-α absorption cloud in front of the radio galaxy 4C41.17

A walk in the Lyman-α forest

High resolution quasar spectra can provide a wealth of detailed information on the Lyα absorbing systems seen at high redshifts. The distribution of the numbers of these systems with redshift, Doppler width, and HI column density, and any relationship between them, derived from such data is discussed. The inferred properties of the clouds derived from such data depend on size estimates, which rely on a single key observation of a gravitationally lensed quasar. The data available provide some constraints for various cloud models, but do not yet allow any to be clearly ruled out.

High resolution CASPEC observations of the z = 4.11 QSO 0000-26.

Citations

A Search for Signatures of Quasar Evolution: Comparison of the Shapes of the Rest-Frame Optical/Ultraviolet Continua of Quasars at z > 3 and z ~ 0.1*

Imaging of a Lyman-α absorption cloud in front of the radio galaxy 4C41.17

A walk in the Lyman-α forest

Lyman Line Absorption Systems

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