We present here the current status of a continuing program to investigate the possibility that certain resonance lines in the spectra of QSO's can be used as luminosity calibrators for the spectra.

Spectra of six quasars have been obtained with the International Ultraviolet Explorer Satellite. Five of the six show no evidence for strong Lyα absorption between the redshifted and rest wavelengths, for .23≤ zem ≤1.72. In addition, the quasar PG 1115+080 at z=1.72 shows no evidence for strong He I absorption from the resonance transition at λ584 Å. These results confirm that the intergalactic medium must be both tenuous and hot enough to produce an optical depth <0.1 in neutral hydrogen and helium. In no case was the Lyman edge detected in absorption near zem. Four of the objects produce an average Lyα/Hβ intensity ratio of 6.3, in disagreement with the theoretical prediction for Case B optically thick recombination of 30. Also, two of the objects show Lyγ in emission, a result unexpected from Case B line transfer assumptions. The Lyα emission line in 3C 351 shows the identical sharp core plus 20,000 km s−1 broad wings observed in Hβ and Mg II, implying a common origin in the same dynamical ensemble of emitting regions. These quasars show systematically steeper spectral indices when the energy distributions are fit from the ultraviolet through the visible than those derived from the visible spectra alone. PG 1115+080 shows a featureless continuum down to an observed λ1173 Å. The ionizing spectrum, with fν αν−2.0, therefore persists beyond 2 Rydbergs. The spectrum of PG 1247+268, with z=2.038, contains a strong absorption line at observed λ2697 Å, with no net flux detected from λ2000 Å down to the observed limit at λ1150 Å. This result is interpreted as absorption in Lyα and the Ly edge at z=1.218. Low dispersion optical spectra show no evidence for Mg II or C IV absorption in the same system; the signal to noise ratio is too low in the IUE spectrum to confirm Lyβ. We conclude that the line of sight intersects a metal-poor cloud with τ ≲ 1 in the Ly continuum, at (1+zem)/(1 + zabs) = 1.37.

The evidence for association of high redshift QSOs with low redshift galaxies is subject to controversies. If it can be convincingly argued that the observed associations are accidental and could occur with reasonably high probabilities (say > 5%) then the cosmological interpretation of the redshifts of QSOs remains unaffected by the data. If, however, there is increasing evidence, either statistically or in direct physical terms, for these associations to be real, then the excess redshifts of the QSOs become anomalous. Taking the latter alternative seriously, a suggestion was made by Narlikar (Annals of Physics, 107, 325, 1977) that the anomalous redshift of the QSO in a typical QSO-galaxy pair could arise because the particle masses in the QSO were systematically smaller than those in the companion galaxy, as predicted by a theory of gravitation based on Mach's principle. One astrophysical consequence of this effect is that the QSO should appear younger than the galaxy.

The radio source 1309–216 is identified with an 18m stellar object having a steep non-thermal continuum (α = 2.6, fν α ν−α) with no emission features. High resolution spectra obtained with the AAT reveal strong absorption due to the CIV doublet at redshifts 1.361, 1.489 and 1.491. In the strongest absorption system (za = 1.489) the SiIV doublet and possibly SiII are also detected. No information on the optical polarization is available but spectrophotometry at several epochs shows evidence for optical variability and the object is therefore classified tentatively as a BL Lac object.

The species found in absorption in 1309–216 are consistent either with ejection from a central source or absorption in the haloes of intervening galaxies. The velocity dispersions and column densities derived for the absorbing regions are comparable to those for gas in the LMC and in the halo of the galaxy.

Large scale (19 arc sec/mm), deeply exposed photographs (26.5 mag/sq arc sec) of a sample of 12 QSO's over the redshift range, z = 0.158 to 0.528 have been obtained. Baked IIIa-F plates were used at the prime focus of the ESO 3.6-m telescope. The band pass was 5700–6900A. Logarithmic intensity contour maps and logarithmic image profiles were constructed from PDS microdensitometer scans. Seven of the 12 QSO's show extended (5–10 arc sec), asymmetric structure on the contour maps; while 9 of 12 QSO's show broader image profiles than stars of the same magnitude. The intensity contour maps provide two essential bits of information for follow-up spectroscopy: 1) a guide for locating the slit aperture and 2) a means of estimating integration times.

The ability to identify the many tens of thousands of extragalactic radio sources now known depends upon the precision with which the radio positions and structures are known as well as the availability of high quality plate material. For the brightest radio sources, positions with accuracy better than 1 arcsec are now routinely available and radio structures with angular resolution 1–5 arcsec can be readily measured with instruments such as the Westerbork Synthesis Radio Telescope, the Cambridge 5-km Telescope and the VLA. The sensitivity of the VLA is such that these observations can be extended to sources in the flux density range ~ 1–10 mJy.

How matter distributes in the Universe is still an open question in spite of the many efforts people have done since early 1924 when Hubble first studied the distribution of bright galaxies.

21 cm studies, carried out with the 305 m telescope at Arecibo, of absorption-line clouds in QSOs have resulted in the following discoveries: (a) A search for 21 cm absorption in redshift systems selected for exhibiting Mg II/Fe II absorption in radio-bright QSOs resulted in the detection of a z ≈ 0.4 21 cm line in PKS 1229–02; (b) the detection of 21 cm absorption at z ≈ 1.8 in MC3 1331+170; and (c) the discovery of 21 cm emission from regions ~ 100 kpc distant from the ScI galaxy NGC 628, that occurred during a search for outlying H I in a complete sample of spiral galaxies.

We have recently mapped about a dozen Seyfert galaxies with the Very Large Array at 5 GHz and obtained less complete structural information on about a dozen more. Most sources are heavily resolved at resolutions near or below 1 arc sec, with linear scales in the range several hundred parsecs to a few kiloparsecs. For 3 galaxies (Mark 3, NGC 1068 and NGC 5548) the structure is double with two components more or less symmetrically placed on opposite sides of the optical nucleus. A third component, when present, coincides with the optical nucleus. This result provides strong evidence that “double radio source machines” also reside in the nuclei of active spiral galaxies as well as ellipticals. Other sources show a more diffuse morphology, but usually also possess a compact radio source associated with the optical continuum nucleus. A close relation between the extended radio emission and the thermal gas in the forbidden line region is indicated since (a) they have similar extents, (b) the radio and forbidden line powers are correlated, (c) the relativistic plus magnetic (B2eq/4π) and thermal pressures (nekTe) are similar and (d) the kinetic energy of the thermal gas and the minimum energy for synchrotron radiation are comparable. For the double sources, the radio emitting plasma is probably ejected from the compact nucleus and slowed by the large quantities of thermal gas in the forbidden line region. Alternatively, radio sources with more diffuse morphology may derive their luminosity from cosmic rays accelerated “in situ” by shock waves associated with the high velocity thermal gas and a magnetic field from a compressed (accreted?) interstellar medium.

It is now possible, by means of VLBI hybrid mapping, to make maps of radio sources with a resolution of ~ 1 milliarcsecond. This enables us for the first time to compare the morphologies of the small- and large-scale structures of extragalactic radio sources, and they are strikingly different.

The VLA, now under construction in New Mexico, is an aperture synthesis array of twenty-seven 25-meter diameter antennas, with overall dimension of about 35 km. At 6-cm wavelength it will have resolution of 0.6 arcseconds and sensitivity of 0.1 mJy, and should be a superb instrument for radio studies of objects of large redshift. A more detailed description of the VLA has been given by Heeschen (1975).

The radio source 0957+561 was identified by Walsh et al. (1979, Nature 279, pp.381–384) with a pair of quasars, 6.1 apart on the sky, whose optical emission and absorption spectra are nearly identical. Walsh et al. suggested a gravitational lens interpretation in which a single object is split into two images by an intervening massive object. Using the Very Large Array of the NRAO we have made a 6-cm wavelength radio map of 0957+561. The map shows unresolved sources of 36 and 30 mJy coincident with the optical N and S quasars, and a complex extended source of ~130 mJy (Roberts, Greenfield, and Burke: 1979, Science August 31). The extended emission lies on an arc containing the N quasar, and consists of two resolved sources containing 75 and 28 mJy, located 5.8 and 5.6 NE of the N quasar, a weak source of ~10 mJy about 5.5 SW of the N quasar, and a suggestion of a bridge connecting the NE and SW sources and the N quasar. There is no evidence of radio emission from a massive object between the two quasars. Although the existence of the extended source does not rule out a gravitational lens model for 0957+561, the underlying source required would have an unusual morphology. In addition, if the refracting object is at the redshift of the absorption seen in both quasars, its mass would have to be at least 2 × 1014 solar masses. Further observations at the VLA could rule out the gravitational lens model for 0957+561 if a second image corresponding to the NE extended component is not found.

An analysis of all observations of the “twin” QSOs, 0957+561 A, B, to date does not yet allow us to distinguish between their being two nearly identical QSOs or a single QSO split into two images by an intervening gravitational lens. The more identical the two objects are found to be, the more difficult any explanation which postulates the existence of two distinct QSOs becomes. Jodrell Bank and VLA observations reveal additional radio structure to the northeast of the northern QSO image which, if physically associated with a single QSO doubly imaged by a gravitational lens, would itself be imaged weakly to the southwest. More detailed radio mapping should be able to test the existence of such an image.

The VLBI map of Porcas and his collaborators reveals that the radio images corresponding to the optical ones are point sources separated by 6.175 arcsec having an angular extent to less than 20 milliarcseconds, whereas all further radio structure is resolved out.

Optical spectroscopy of the twins reveals two nearly identical sources with indistinguishable emission line redshifts and with absorption line redshifts identical to within 15 km/sec. It is the identity of these optical characteristics which makes all non-gravitational lens hypotheses most difficult.

The most compelling test of the lens hypothesis is the measurement of time variations of the two images at as many wavelengths as possible. If brightness variations of one image are repeated by the other after a time interval determined by the details of the observerlens-QSO geometry (such an interval could be of the order of many months or years) the lens hypothesis would be confirmed. Several observations indicate prior variations of the images, and programs to monitor their relative brightness in the future will be of great importance.

A recent study of the properties of BL Lacertae Objects at λλ6 and 21 cm by K.J. Johnston and myself has shown:

The launch of the Einstein X-ray Observatory on November 13, 1978 has brought about a qualitative change in the field of X-ray astronomy. A fairly detailed description of the Observatory has appeared in print (Giacconi et al, 1979a) and therefore will not be given here.

This paper will be concerned with three topics relevant to the X-ray background: (i) X-ray emission mechanisms in quasars; (ii) the contributions to the X-ray background from quasars, clusters of galaxies, intercluster gas, young galaxies, etc; and (iii) the use of X-ray background observations as a probe for large-scale density irregularities in the Universe.

The A2 experiment on HEAO-1 was especially developed to make systematics-free measurements of the extragalactic X-ray background (Boldt et al. 1979) over the band (up to 60 keV) of maximum flux. The spectrum observed has a remarkably simple thermal form (Marshall et al. 1980) with a mean photon energy of about 40 keV, an order of magnitude above the high-energy limit of the Einstein Observatory (HEAO-2) telescope. If most of this hard X-ray flux is not diffuse, then the main sources of this background could be 1) unresolved objects of known classification (e.g. BL Lac type, quasars, active galaxies) at high redshift, 2) redshifted (z > 1) gamma-ray bursts and/or 3) a new class of X-ray objects peculiar to high redshifts. If we assume that the number of such sources that are highly variable is less than 106, then our first-cut analysis of the temporal stability measured for the X-ray background indicates that 1) their contribution is less than 15% if they are variable on scales less than 104 seconds, and 2) their contribution is less than 60% if they are variable on scales less than a half-year.

The luminosity function of galaxy systems - from single galaxies and small groups to rich clusters - is determined. I first determine the rich-cluster luminosity function, then a “groupings” luminosity function, which includes all systems from small groups to rich clusters, and finally, I estimate a general luminosity function of all galaxy systems (AGS), from single galaxies to rich clusters. The catalogs of Abell (rich clusters) and Turner and Gott (groups, singles) are used. The AGS luminosity function is found to be a smooth and steeply decreasing function of number density with luminosity; it spans over twelve decades of number densities in the observed five-decade range of luminosities (~109 to 1014 L⊙). The function exhibits an exponential cutoff in the rich-cluster luminosity domain, a power law dependence over most of the intermediate luminosity range, and a flattening of slope below the characteristic galactic luminosity, L*. A smooth transition between the groups and rich-clusters occurs near ~1013L⊙. The groupings LF (i.e., groups to rich-clusters) can be represented well by a Schechter-type analytic form η(L)=ηo (L/Lo)−α exp (-L/Lo), where η(L) is number of systems per unit volume per unit luminosity. The best-fit parameters are α=2.0 ± 0.1, Lo=1.0 ± 0.2×1013 L⊙, and a normalization ηo=1.6 × 10−7 systems Mpc−3 (1012L⊙)−1. The “break” at Lo≃1 × 1013L⊙ occurs at the typical luminosity of rich Abell-type clusters. The slope α is found to depend on the density enhancement factor with which the groups are selected, but the dependence is relatively weak. The exponential decay at the bright end (rich clusters) has an equivalent power-law slope of approximately −5, i.e., η(L)∝L−5. The functional form of the LF is consistent with the Press-Schechter (1974) model of galaxy formation, and with the recent Silk-White (1978) model (predicted slope of −2.0). When single galaxies are added to the groupings luminosity function, the luminosity function of all galaxy systems (AGS) can be roughly described as η(L)=1.6 × 10−7 (L/Lo)−2e-L/Lo (1 + 1.6 × 1010/L)−0.75Mpc−3(1012L⊙)−1 with Lo=1 × 1013L⊙. This function approaches Schechter's field galaxy luminosity function at low luminosities, and the groupings function at high luminosities. Comparisons of the LF with the LF of galaxies and X-ray clusters of galaxies are discussed, and an estimate of is given for the observed luminosity density in the universe.

The above luminosity function of galaxy systems could be compared with high redshift cluster data, when available, and thus shed light on possible evolutionary processes.

The X-ray luminosity function of clusters of galaxies is calculated using the general LF of optical clusters above and a relation between X-ray and optical luminosity based on a thermal bremmstrahlung model. The derived X-ray LF applies to clusters whose X-ray emission is mostly due to thermal bremmstrahlung from a smooth, hot intracluster gas. The main features predicted by the LF are a change of slope in the log ηx -log Lx plane at Lx ~1044 ergs/s, a moderately steep slope of −2.5 at high luminosities (~1044-1046 ergs/s), and a flatter slope of ~-1.3 below 1044 ergs/s (down to ~1042 or 1043 ergs/s, where the smooth thermal bremsstrahlung may not dominate the total X-ray emission). Simple analytic expressions of the LF are given for various X-ray energy bands. X-ray data from Uhuru and Ariel-5, available for the 1044-1046 ergs/s luminosity range (including upper limits in the 1045-1046 range), agree well with the LF calculated here. The present calculations suggest that at least to Lx ~ 1046 ergs/s the X-ray LF does not decay faster than ~Lx−2.5 (for a wide enough energy band). It is also suggested that fewer faint X-ray clusters than previously expected may exist (and be detected by the Einstein satellite).

I predict the evolution of the X-ray luminosity function of clusters of galaxies. Predominantly, I treat the assumption that galaxies form first, then cluster purely due to gravitation. I show that the richness distribution of Abell clusters favors this scenario, rather than the protocluster hypothesis. The luminosity function is produced by combining a generalized (for all Ω) Press-Schechter evolutionary mass function for clusters (derived herein) with a power law X-ray luminosity-mass relation; a power law relation is supported by observations of low-redshift clusters.

I find very steep evolution in the luminosity function, and thus in the source counts, for large Ω, and moderate evolution for small Ω. For a variety of models for the gas supply rate to the intracluster medium, the evolution of the luminosity function does not vary greatly. Thus it appears that the Ω, dependence will dominate and that number counts of X-ray clusters will yield cosmological information. The power of a test of Ω with an evolving luminosity function is considerably enhanced relative to a test which involves solely global cosmological effects on a non-evolving population. This occurs because of the well-known result that, at late times, clustering tends to proceed slowly for universes of small Ω and rapidly for large Ω.

The recent deep X-ray surveys suggest that discrete sources comprise most, if not all, of the energy content of the background (cf. Giacconi, this volume). We have shown that, if sources have flat power law spectra and relatively sharp high-energy cutoffs, their combined emission can also mimic very accurately its extended (2 - 400 keV) spectral shape. A broad distribution of cutoff energies Ec is required, in this case, to model the high-energy (E ≳ 20 - 40 keV) part: a power law envelope of distribution functions of various types of sources can be envisaged; alternatively, the well-known fact that most of the 2 − 10 keV background should be produced by low flux, low Ec sources can suggest that Ec ∝ (1 + z)−η. The quality of the fit turns out to be not very sensitive to the amount of number/luminosity evolution assumed, though the minimum χ2 test slightly favours strongly evolving sources. In the case of differential luminosity ∝ E−γ and evolution ∝ (1 + z)α with 0 ≤ α ≤ 6, the best fits to pre HEAO-1 data are obtained for mean values of the spectral index γ ≈ 0.5 − 0.9 and for dispersions Δγ ≃ 0.5 - 0.7. Rather wide ranges of values of γ and Δγ are, however, still allowed; e.g., for α = 6, the allowed intervals are 0.2 ≤ γ ≤ 1.3, 0 ≤ Δγ ≤ 0.7.