A population of high-redshift type 2 quasars – I. Selection criteria and optical spectra

We discuss the relative merits of mid-infrared and X-ray selection of type 2 quasars. We describe the mid-infrared, near-infrared and radio selection criteria used to find a population of redshift   z ∼ 2  type 2 quasars which we previously argued suggests that most supermassive black hole growth in the Universe is obscured. We present the optical spectra obtained from the William Herschel Telescope, and we compare the narrow emission-line luminosity, radio luminosity and maximum size of jets to those of objects from radio-selected samples. This analysis suggests that these are genuine radio-quiet type 2 quasars, albeit the radio-bright end of this population. We also discuss the possibility of two different types of quasar obscuration, which could explain how the ∼2–3:1 ratio of type 2 to type 1 quasars preferred by modelling our population can be reconciled with the ∼1:1 ratio predicted by unified schemes.     

Climatology of short-period mesospheric gravity waves over Halley,Antarctica (76°S, 27°W)

We present a first detailed climatological study of individual quasi-monochromatic mesospheric, short-period gravity-wave events observed over Antarctica. The measurements were made using an all-sky airglow imager located at Halley Station (76°S, 27°W) and encompass the 2000 and 2001 austral winter seasons. Distributions of wave parameters were found to be similar to findings at other latitudes. The wave headings exhibited unusually strong anisotropy with a dominant preference for motion towards the Antarctic continent and a rotation from westward during fall, to poleward in mid-winter, to eastward in spring. This rotation was accompanied by a systematic increase of ～50% in the magnitudes of the horizontal wavelengths and observed phase speeds. It is postulated that the observed wave anisotropy was due to a succession of wave sources of different characteristics lying equatorward of Halley, or a dominant source mechanism evolving with time during the winter months.     

Longitudinal variation in E- and F-region ionospheric trends

A novel technique is used to examine northern hemisphere midlatitude longitudinal variations in ionospheric long-term trends. Differences in hour-by-hour monthly median ionospheric parameters between equilatitudinal observatory pairs are analysed for long-term trends, thus eliminating at source the large solar cycle and geomagnetic variability that normally hinders ionospheric trend calculations. The results confirm the finding of Bremer [1998. Trends in the ionsopheric E- and F-regions over Europe. Annales Geophysicae 16, 698–996] that there are longitudinal variations in the F-region altitude trend across Europe, but suggest the influence of a stationary wave-like feature between 3°W and 104°E. Possible causes such as scaling errors, insufficient account of changes in ionisation underlying the F-region, varying gravity wave fluxes, and secular change in the geomagnetic field are ruled out. The data suggest that the longitudinal variation may reflect long-term changes in a large-scale stationary feature induced via non-migrating tides induced by latent heat release in the troposphere.Significant differences in the long-term trend of E-region peak plasma frequency between observatories were also found. These E-region differential trends varied with solar zenith angle reaching over 0.3 MHz per decade between Juliusruh and Moscow at midday in summer.     

Evidence of a link between the evolution of clusters and their AGN fraction

We discuss the optical properties, X-ray detections and active galactic nucleus (AGN) populations of four clusters at   z ∼ 1  in the Subaru–XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialized and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialized clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialized clusters is consistent with an overdensity of a factor of ∼200; the radio AGN appear to be clustered with a factor of 3 to 6 higher. The median K -band luminosities of   L_K = 1.7 ± 0.7 L *  for the X-ray sources and   L_K = 2.3 ± 0.1 L *  for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialization evolutionary stage.     

The properties of 70 μm-selected high-redshift galaxies in the Extended Groth Strip

We examine the infrared properties of 43 high-redshift (0.1 < z < 1.2), infrared-luminous galaxies in the Extended Groth Strip (EGS), selected by a deep 70 μm survey with the Multiband Imaging Photometer on Spitzer (MIPS). In addition and with reference to starburst-type spectral energy distributions (SEDs), we derive a set of equations for estimating the total infrared luminosity ( L _IR) in the range 8–1000 μm using photometry from at least one MIPS band. 42 out of 43 of our sources' optical/infrared SEDs (λ_observed < 160 μm) are starburst type, with only one object displaying a prominent power-law near-infrared continuum. For a quantitative analysis, models of radiation transfer in dusty media are fit on to the infrared photometry, revealing that the majority of galaxies are represented by high extinction, A _v > 35, and for a large fraction (∼50 per cent) the SED turns over into the Rayleigh–Jeans regime at wavelengths longward of 90 μm. For comparison, we also fit semi-empirical templates based on local galaxy data; however, these underestimate the far-infrared SED shape by a factor of at least 2 and in extreme cases up to 10 for the majority (∼70 per cent) of the sources. Further investigation of SED characteristics reveals that the mid-infrared (70/24 μm) continuum slope is decoupled from various galaxy properties such as the total infrared luminosity and far-infrared peak, quantified by the L ₁₆₀/ L ₇₀ ratio. In view of these results, we propose that these high-redshift galaxies have different properties to their local counterparts, in the sense that large amounts of dust cause heavy obscuration and are responsible for an additional cold emissive component, appearing as a far-infrared excess in their SEDs.