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Quasars are rapidly accreting supermassive black holes at the center of massive galaxies. They display a broad range of properties across all wavelengths, reflecting the diversity in the physical conditions of the regions close to the central engine. These properties, however, are not random, but form well-defined trends. The dominant trend is known as Eigenvector 1, where many properties correlate with the strength of optical iron and [OIII] emission. The main physical driver of Eigenvector 1 has long been suspected to be the quasar luminosity normalized by the mass of the hole (the Eddington ratio), an important quantity of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas, implying a flattened, disklike geometry for the fast-moving clouds close to the hole. Our results show that most of the diversity of quasar phenomenology can be unified with two simple quantities, Eddington ratio and orientation., Comment: This is the author's version of the work; 18 pages including Supplementary Information; to appear in the 11 September 2014 issue of Nature at http://dx.doi.org/10.1038/nature13712

UNIFIED models1-3 of radio-loud quasars and powerful radio galaxies suggest that they are intrinsically similar objects observed from different angles. This can be tested by comparing the isotropically emitted radiation from the spatially extended nebulae surrounding the nuclei; the unified models predict that the intensities of these emissions should be comparable for the two classes of object. But when this prediction was examined4 for the [O III] 5,007-A emission line, it was found that quasar [O III] luminosities significantly exceed those of otherwise similar radio galaxies. We have measured the spatially integrated [O II] 3,727-A emission-line luminosities for a number of quasars and radio galaxies taken from the 3C catalogue. Supplementing our data with values from the literature, we find no systematic difference in the [O II] luminosities. We argue that this emission is indeed isotropic, and that our results are consistent with the unification hypothesis; the [O III] line, on the other hand, may still have a significant component from the nuclear region, and thus be subject to pronounced anisotropic obscuration.

FOLLOWING an initial radio outburst1 which decayed on a time-scale of a few weeks, supernova 1987A2 was undetectable at radio wavelengths until recently. In mid-1990, 1,200 days after the explosion, radio emission was once again detected3 with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array. Our observations since then show that the source has increased in strength at all monitored frequencies between 843 MHz and 8.6 GHz, with considerable variations in spectral index. The extended radio emission is centred within 0.5 arcsec of the optical supernova, and probably lies within the [O III] ring imaged by the Bubble Space Telescope. We interpret the emission as optically thin synchrotron emission from shock–accelerated electrons. This is the first time the birth of a nearby radio supernova remnant has been witnessed, and future observations will allow the structure of the remnant to be compared with the many other known radio remnants.

IT has been suggested recently1–4 that two classes of radio-loud extragalactic object, narrow-line radio galaxies and quasars, are intrinsically similar, differing only in the angle from which they are seen by the observer. In the simplest version of such models, one would expect that the [O III] 5,007-A line luminosities of the two types of object would be similar for a given extended radio luminosity and redshift. In spectroscopic observations of a sample of radio galaxies and quasars selected so as to be otherwise comparable, we find that this is not the case: the quasar [O III] luminosities exceed those of radio galaxies by factors of 5–10. Extinction by at least two magnitudes as a result of orientation differences is necessary for these observations to remain consistent with the radio galaxy/quasar unification hypothesis; we suggest that evidence for such extinction may exist in the form of high Hα/Hβ ratios in radio galaxies5–8.

IT is of fundamental importance in the study of quasars to understand whether or not their continuum emission is isotropic, because this affects the overall energy budget and can also help to distinguish between the different models proposed for their central regions1. The reported small range of emission-line equivalent widths in quasars have earlier been used to argue against a relativistically beamed or anisotropic continuum2,3. Here we use recent spectroscopic observations of 46 radio-loud quasars to show that the equivalent width of the [O III] 5,007-A emission line is strongly anticorrelated with the degree of radio core prominence R, which has been found to be a statistical indicator of orientation relative to the line of sight4–6. This observed anticorrelation shows that the quasar continuum is anisotropic and provides important evidence in favour of relativistic-beaming unified schemes4,7,8. We also show that the strength of the [O III] emission line is independent of R, consistent with unified schemes.

THE spectrum of CH Cygni usually has the appearance of a normal M6 III star, but at intervals of several years, it has outbursts during which it shows the characteristics of a symbiotic star, that is a blue continuum extending shortward of the B almer discontinuity, and emission lines of He I, Fe II, [Fe II], [S II], [O III]; the Balmer lines and the H and K lines of Ca II present emission wings. The observations of CH Cygni reported here were made to determine whether a symbiotic star is a binary system composed of an M6 giant and a hot subdwarf, or whether it is a cool star surrounded by a thick corona.

Evidence that the galaxy A0945-30 is a new extragalactic X-ray source was found from an analysis of rotating modulation collimator quick-look data collected from the SAS 3 X-ray observatory. A strong piece of evidence was the hydrogen column density of 3 plus or minus 1 times 10 to the 22nd power per sq cm for a power law spectrum extending to about 35 keV inferred from HEAO 1 data. Observations indicate that the galaxy is an early type spiral with a bright nucleus. A faint star alongside the galaxy within the 90% confidence X-ray source error circle is an F dwarf with no indication of emission features. Spectra of the galaxy in the red and blue show sharp, intermediate strength emission lines of forbidden O III, forbidden N II, and H-alpha together with strong Mg, Fe, Ca absorption features characteristic of a normal stellar continuum in the galactic nucleus.

THE different kinds of active galactic nuclei (AGNs) and quasars may be fundamentally similar, and owe their observed differences to anisotropy in the distribution of their emitted radiation: apparently different objects may then be the same thing seen from different viewpoints. In particular, it has been proposed that narrow-line Seyfert 2 galaxies are Seyfert 1 galaxies for which the broad-line and continuum emitting regions are hidden by obscuring material along our line of sight1,2 Here we present narrow-band imaging observations of the Seyfert 2 galaxy NGC5252 which strongly support such unifying hypotheses. Images in the light of the [O III] line at λ = 5,007 A reveal a sharply defined bi-conical structure extending to a maximum radius of ∼33 kpc (H0 = 50 km s−1 Mpc−11) along the radio-emission axis. This is just the type of structure expected if the ionizing-radiation field from the nucleus is anisotropic. A map of the ratio of the [O III] and Hα lines shows a central band perpendicular to the cones that we identify as the disk or torus responsible for collimating the radiation field.

THE observation of forbidden transitions in the emission spectra of quasi-stellar objects1,2 has led previous authors to deduce limiting values for the electron density in relevant regions of these sources (for example, ref. 1). In this report I shall show that significant limits must be set on the ultraviolet radiation intensity in the emitting region, and thus on the size of the region, if the observations of certain [O II] transitions are to be explained even at low electron densities.

The contribution of line emission to the diffuse 'cirrus' radiation components observed by IRAS at 60 and 100 microns, respectively, is evaluated. The cirrus radiation was observed straddling the galactic plane to latitudes of + or - 80 degrees. Comparison of the observed brightness from cirrus clouds with forbidden line emission of O I and O III at 63 and 88 microns showed that the flux levels observed by IRAS are of the same order as the fine-structure line emission predicted by Stacey et al. (1985). It is argued that the fine-structure line emission from neutral gas at 63 microns as well as the line emission from diffuse ionized clouds at 88 microns, are significant components in most clouds, in some clouds they are the prime source of cirrus radiation.

The radio source PKS2155–304 has recently been identified1 with a HEAO 1 X-ray source and a ∼14 mag object that has a featureless spectrum and is interpreted as a member of the BL Lac class. We describe here our optical spectroscopy of this object which reveals two weak emission features at λ 5,820 and 5,870 A. We suggest that these lines are [O III] λ 4,959, 5,007 redshifted by z = 0.17. This result supports the interpretation of H2155–304 as a highly luminous BL Lac object.

The jet-like feature in the Crab Nebula discovered by van den Bergh1 shows a tubular structure of 75 × 45 arc s with strikingly well-collimated sharp parallel boundaries in the recent deep O III image obtained by Gull and Fesen2. I now report detection of radio emission at 20cm from this faint optical jet, in the Very Large Array (VLA) map obtained with a resolution of 15 × 15 arc s. The radio emission is extremely faint at less than 1% of the peak brightness within the nebula, and its detection was possible because of the high dynamic range in this VLA map. Its positional coincidence and structural similarity to the optical jet are remarkable. The emission at 20 cm appears to be nonthermal, with strong linear polarization of ∼30%.

RESEARCH in the field of high-current gas discharges is now being pursued vigorously1. In these discharges, where the gas is highly ionized, higher spectra can be predominantly excited (for example, O III, A II) and the pinch effect reduces the loss of ions to the walls. It is thus interesting to consider the possibility of detecting in the laboratory those forbidden lines which are of astrophysical importance.

THE suggestions made by Bowen in NATURE of Oct. 1 as to the probable origin of the well-known ‘nebulium’ doublet λλ5007, 4959, and of the strong radiations sometimes occurring at λλ3727, 9, have been confirmed by Prof. A. Fowler (NATURE, Oct. 29), and so generally accepted that it may not be premature to discuss the actual distribution of these radiations in the gaseous nebulae on the assumption that the first doublet represents O III (twice ionised oxygen) and the second O II (once ionised oxygen). It is first of all important to note that there is no recorded evidence of the existence of the neutral atom of oxygen in the gaseous nebulae, although some of the strongest lines, being in the extreme red, are beyond the reach of the apparatus usually employed. Neither can I find any references which support Bowen's statement that “the [gaseous] nebulae are known to emit the well-known spectra of highly ionised nitrogen and oxygen.” The most exhaustive study yet published of the nebular lines is that by W. H. Wright in Lick. Obs. Pub., vol. 13, but although one line each is ascribed to neutral carbon and nitrogen, there is no identification with oxygen. On the other hand, the doublet λλ5007, 4959 is present in all cases considered, a total of 47 planetaries and the Orion nebula.

IN his letter to NATURE of Oct. 1, p. 473. Mr. I. S. Bowen has made the important suggestion that several of the chief lines in the spectra of gaseous nebulae may be due to what spectroscopists have called ‘forbidden’ combinations of terms in the spectra of ionised nitrogen (N II), ionised oxygen (O II), and doubly-ionised oxygen (O III). It is, of course, no longer permissible to suppose the existence of hypothetical elements to account for the long-standing mysteries of nebular spectra, and we must accordingly regard the nebular lines as being produced by known elements under conditions of excitation which have not yet been imitated in the laboratory. It has seemed natural enough to look to the lighter elements, and those who, like myself, have given special attention to the spectra of these elements under widely varied conditions, have doubtless kept in view the possibility of finding some indications of nebular lines in the course of their observations. Extensive experiments, however, have failed to reveal any traces of them.

WE have obtained a widened spectrogram (April 22, 1968, dispersion 180 A mm−1) with an image tube spectrograph of the intense ultraviolet object identified with the X-ray source GX3 + 1. In this communication we refer to this object as GX3 + 1. Its magnitude and colours are V=13.4, B−V= +1.0, U−B= −0.9. Very broad emission lines in its spectrum have been reported previously (communications from V. Blanco et al.). Fig. 1 is a linear microphotometer record of the spectrum of GX3 + 1. It shows many very broad emission bands (average width 130 A at half-intensity); these come from high excitation lines of He II, O III–VI, and C III–IV. Table 1 gives their measured wavelengths, identification, widths and equivalent widths. There is evidence for a mean velocity of about −1,000 km s−1, but this is difficult to estimate because the emission features are so broad and are mostly blends of several lines. There are no broad Balmer lines; the sharp lines of [O III] and Hα come from nebulosity in which the object is embedded, and give some idea of the instrumental profile.

ALTHOUGH the two well-known lines of [O III] λ=5007 A. (3P2–1D2) and λ = 4959 A. (3P1–1D2) are the most prominent features in the spectra of planetary nebulae and novae at the nebular stage, the third line of the triplet, corresponding to the3P0–1D2 transition, for which the calculated wave-length is λ= 4931·8 A., has not been generally observed. Recently, however, Bowen and Wyse1 attributed to the3P0–1D2 transition the faint line which they observed in the spectra of the nebulae N.G.C. 7027 and N.G.C. 6572.

THE chief lines of the nebular spectrum have been identified by I. S. Bowen with forbidden transitions in the atoms of singly ionised nitrogen and singly and doubly ionised oxygen (NATURE, 120, 473; Oct. 1, 1927). The life of an atom in the metastable state is very long compared to that of the other excited states, and the transfer to another state is accomplished by collisions of the second kind. Bowen argues that in the nebula, the density being extremely low and hence a long interval of time between impacts, the atoms in metastable states would return to normal states spontaneously with the emission of radiation. From spectroscopic data he has computed the frequency and frequency differences for N II, O II, and O III, and finds agreements with the chief nebular lines. A. Fowler has since computed the frequency differences, using data not available to Bowen, and finds even better agreement with the nebular lines (NATURE, 120, 582, 617; 1927).

AFTER dropping in brightness at the end of April to about mag. 13, this star has brightened markedly again. A spectrogram secured by an 8-hours exposure spread over two nights, May 17–19 (with the star's magnitude at 10·6m–10·2m) shows a complete change of spectrum. The lines of Fe II and [Fe II] have vanished, and the lines of [O I] are much weaker. The hydrogen lines are stronger again and N II, [N II] and He II are all represented. The strongest lines are now the well-known nebular lines of [O III] at 5007, 4959 and 4363. At an earlier stage (late in March) bright bands appearing simultaneously at 4360, 5006 were identified as [O III] lines. As the changing spectrum developed, it became clear that these bands were due to [Fe II] and N II respectively. The presence of a bright line at 4959 and the absence of [Fe II] show that the [O III] lines are now present. This transformation to the spectrum of a planetary nebula and nucleus is typical of novae.