The diversity of quasars unified by accretion and orientation
Nature 513, 7517 (2014). doi:10.1038/nature13712
Authors: Yue Shen & Luis C. Ho
Quasars are rapidly accreting supermassive black holes at the centres 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’, in which many properties correlate with the strength of optical iron and [O iii] 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’), which is an important parameter of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that the Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas in the broad-line region, implying a flattened, disk-like geometry for the fast-moving clouds close to the black hole. Our results show that most of the diversity of quasar phenomenology can be unified using two simple quantities: Eddington ratio and orientation.
Astrophysics: Quasar complexity simplified
Nature 513, 7517 (2014). doi:10.1038/513181a
Authors: Michael S. Brotherton
An analysis of a sample comprising some 20,000 mass-accreting supermassive black holes, known as quasars, shows that most of the diverse properties of these cosmic beacons are explained by only two quantities. See Letter p.210
For over two decades astronomers have been patiently monitoring the fading glow of a supernova in a nearby galaxy. They've been looking for a suspected companion star that pulled off almost all of the hydrogen from the doomed star that exploded. At last Hubble's ultraviolet-light sensitivity pulled out the blue glow of the star from the cluttered starlight in the disk of the galaxy. This observation confirms the theory that the supernova originated in a double-star system where one star fueled the mass-loss from the aging primary star. The surviving star's brightness and estimated mass provide insight into the conditions that preceded the 1993 explosion.
Plate tectonics found on Europa
Nature 513, 7517 (2014). http://www.nature.com/doifinder/10.1038/513153a
Author: Alexandra Witze
Discovery buoys bid for mission to Jovian moon.
Nature 513, 7516 (2014). doi:10.1038/513006a
The discovery of our Galaxy’s place in the Universe adds detail to our address.
Cosmology: Meet the Laniakea supercluster
Nature 513, 7516 (2014). doi:10.1038/513041a
Authors: Elmo Tempel
An analysis of a three-dimensional map of galaxies and their velocities reveals the hitherto unknown edges of the large system of galaxies in which we live — dubbed the Laniakea supercluster. See Letter p.71
The Laniakea supercluster of galaxies
Nature 513, 7516 (2014). doi:10.1038/nature13674
Authors: R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède
Galaxies congregate in clusters and along filaments, and are missing from large regions referred to as voids. These structures are seen in maps derived from spectroscopic surveys that reveal networks of structure that are interconnected with no clear boundaries. Extended regions with a high concentration of galaxies are called ‘superclusters’, although this term is not precise. There is, however, another way to analyse the structure. If the distance to each galaxy from Earth is directly measured, then the peculiar velocity can be derived from the subtraction of the mean cosmic expansion, the product of distance times the Hubble constant, from observed velocity. The peculiar velocity is the line-of-sight departure from the cosmic expansion and arises from gravitational perturbations; a map of peculiar velocities can be translated into a map of the distribution of matter. Here we report a map of structure made using a catalogue of peculiar velocities. We find locations where peculiar velocity flows diverge, as water does at watershed divides, and we trace the surface of divergent points that surrounds us. Within the volume enclosed by this surface, the motions of galaxies are inward after removal of the mean cosmic expansion and long range flows. We define a supercluster to be the volume within such a surface, and so we are defining the extent of our home supercluster, which we call Laniakea.