Quasars: Observed properties of optically-selected objects at large redshifts

Abstract

Abstract I present a description of the recent use of slitless spectroscopy to discover most of the known, high-redshift (z > 2) quasi-stellar objects by direct detection of Ly-α emission and then review the observed properties of these objects in four sections: 1. (a)| Space Distribution and Luminosity Function : If we assume the redshifts are cosmological, the co-moving space density of QSOs probably continues to increase from our locality out to the most distant objects so far observed; there is no evidence for any cutoff in the distribution either near z∼2.5 or z∼3.3. Estimates for the surface density of emission-line QSOs on the sky at B=19.5 magnitude, over all redshifts currently vary from about 2 to 15 deg−2. This surface density increases steeply as a function of magnitude to beyond magnitude 20, which is quite distinct from the observed behaviour of radio-selected quasars. Work is beginning on the degree of isotropy in the distribution of optically-selected QSOs, now that hundreds of high redshift objects are being found on a single Schmidt plate. The luminosity function probably scales by a factor ∼4–8 per magnitude and there is a range of at least 5 magnitudes in the optical continuum flux from QSOs near z=2. The first confirmed, optically selected, high-redshift object with broad absorption and no obvious emission lines has recently been found using objective-prism Schmidt plates. 2. (b)| Emission-Line Spectrum : The widths of the emission lines observed in high-redshift objects are comparable to those of permitted lines in class 1 Seyfert galaxies. It is probable that many aspects of Seyfert galaxy models are directly relevant to the quasar phenomenon. Several recent observational estimates of the ratio I(Ly-α)/I(Hβ) give values ∼2–8 in contradiction to straightforward application of recombination theory (which predicts values ∼22–40); a moderate amount of reddening by dust confined to the line-emitting region may provide a satisfactory explanation of the discrepancy. As in radio-selected quasars, the emitting region is likely to consist of optically thick condensations which do not completely cover the source; of 14 optically selected objects studied to date, none show obvious Ly-β emission, and yet at most one or two show a significant discontinuity in the Lyman continuum that must unambiguously be associated with the emission-line region. The average ratios OVI/Ly-α and CIV/Ly-α agree with the values predicted by simple photoionisation models, but the observed NV/Ly-α and (SiIV+OIV])/Ly-α ratios may be too large by a factor of about 5. The detection of Hell λ1640 in several of the objects argues against a very low helium abundance. Possible differences in the optical properties of radio-loud and radio-quiet QSOs are: (i) Radio-loud QSOs tend to have higher intrinsic polarisation (P 3. (c)| Absorption-Line Spectra : Statistical studies of absorption-line systems in well-defined samples of optically selected QSOs have recently begun, at resolutions ranging from 0.3A to 40A; their purpose is to try to elucidate the nature and location of most of the absorbing material. The density of lines per unit wavelength interval seems to be a steep function of redshift, so a reasonably small number of objects may suffice for a suitable statistical sample. In some extreme cases, there is little doubt as to where the absorption arises. Very wide lines seen in objects such as PHL5200 and Q1246-057 provide information on material most likely to be associated with the QSO. The recently discovered sharp-lined systems with metallic lines and velocities relative to the QSO exceeding 0.6c are perhaps more likely to be associated with intervening galaxies, but the case is by no means clear cut. 4. (d)| Radio Properties : Only ∼7–12% of various samples of optically selected QSOs have been detected as radio emitters (e.g. down to a 6cm flux limit of ∼20mJy). The fraction of radio sources seems to be highest among the brightest optical objects; weak radio sources (e.g. in the 20–100mJy range at 6cm) are surprisingly rare in all radio surveys of optically-selected objects. Samples of optically-selected QSOs allow an estimate of the distribution of spectral indices, free of bias associated with radio selection techniques. I have assumed for most of this article that redshift is an indicator of distance in a zero-pressure cosmology with zero cosmological constant and zero deceleration parameter. Some derived parameters depend strongly on these assumptions. I have not included material which came to my attention after 15th May 1978.