Institute of Astronomy

A new hydrodynamic code applicable to a space of an arbitrary number of dimensions is discussed and applied to a variety of polytropic stellar models. The principal feature of the method is the use of statistical techniques to recover analytical expressions for the physical variables from a known distribution of fluid elements. The equations of motion take the form of Newtonian equations for particles. Starting with a non-axisymmetric distribution of approximately 80 particles in three dimensions, the method is found to reproduce the structure of uniformly rotating and magnetic polytropes to within a few per cent. The method may be easily extended to deal with more complicated physical models.

When a rotating black hole is threaded by magnetic field lines supported by external currents flowing in an equatorial disc, an electric potential difference will be induced. If the field strength is large enough, the vacuum is unstable to a cascade production of electron–positron pairs and a surrounding force-free magnetosphere will be established. Under these circumstances it is demonstrated that energy and angular momentum will be extracted electromagnetically. As a further consequence it is shown that charge can never contribute significantly to the geometry of a rotating hole. The fundamental equations describing a stationary axisymmetric magnetosphere are derived and the details of the energy and angular momentum balance are discussed. A perturbation technique is developed which can be used to provide approximate solutions for slowly rotating holes. Solutions appropriate when the field lines threading the hole lie on conical and paraboloidal surfaces at large distances are described to illustrate this mechanism. These ideas are incorporated into a discussion of a model of active galactic nuclei containing a massive black hole surrounded by a magnetized accretion disc. In this model relativistic electrons can be accelerated at large distances from the hole and therefore will not incur serious losses, which is a defect of some existing models. In addition, if the field lines have paraboloidal shape, the energy will be beamed along antiparallel directions as observations of both compact and extended radio sources seem to require.

We suggest that most of the material in the Universe condensed at an early epoch into small ‘dark’ objects. Irrespective of their nature, these objects must subsequently have undergone hierarchical clustering, whose present scale we infer from the large-scale distribution of galaxies. As each stage of the hierarchy forms and collapses, relaxation effects wipe out its substructure, leading to a self-similar distribution of bound masses of the type discussed by Press & Schechter. The entire luminous content of galaxies, however, results from the cooling and fragmentation of residual gas within the transient potential wells provided by the dark matter. Every galaxy thus forms as a concentrated luminous core embedded in an extensive dark halo. The observed sizes of galaxies and their survival through later stages of the hierarchy seem inexplicable without invoking substantial dissipation; this dissipation allows the galaxies to become sufficiently concentrated to survive the disruption of their halos in groups and clusters of galaxies. We propose a specific model in which |$\Omega \simeq 0.2$|⁠, the dark matter makes up 80 per cent of the total mass, and half the residual gas has been converted into luminous galaxies by the present time. This model is consistent with the inferred proportions of dark matter, luminous matter and gas in rich clusters, with the observed luminosity density of the Universe and with the observed radii of galaxies; further, it predicts the characteristic luminosities of bright galaxies and can give a luminosity function of the observed shape.

We describe the goals, design, and implementation of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven year sky survey which began in May 2005. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H_2. The Large Area Survey, the Galactic Cluster Survey, and the Galactic Plane Survey cover approximately 7000 square degrees to a depth of K~18; the Deep Extragalactic Survey covers 35 square degrees to K~21, and the Ultra Deep Survey covers 0.77 square degrees to K~23. The prime aim of UKIDSS is to provide a long term astronomical legacy database; the design is however driven by a series of specific goals -- for example to find the nearest and faintest sub-stellar objects; to break the z=7 quasar barrier; to determine the epoch of re-ionisation; to determine the substellar mass function; to discover Population II brown dwarfs, if they exist; to measure the growth of structure from z=3 to the present day; to determine the epoch of spheroid formation; and to map the Milky Way through the dust, to several kpc. The data are being made available in a series of staged releases, the first of which (the "Early Data Release (EDR)") is described in Dye et al (2006). The data are immediately public to astronomers in all ESO member states, and available to the world after eighteen months. Before the formal survey began, UKIRT and the UKIDSS consortium collaborated in obtaining and analysing a series of small science verification (SV) projects to complete the commissioning of the camera. We show some results from these SV projects in order to demonstrate the likely power of the eventual complete survey.

The 2dF Galaxy Redshift Survey (2dFGRS) is designed to measure redshifts for approximately 250 000 galaxies. This paper describes the survey design, the spectroscopic observations, the redshift measurements and the survey data base. The 2dFGRS uses the 2dF multifibre spectrograph on the Anglo-Australian Telescope, which is capable of observing 400 objects simultaneously over a 28 diameter field. The source catalogue for the survey is a revised and extended version of the APM galaxy catalogue, and the targets are galaxies with extinction-corrected magnitudes brighter than b J 19:45. The main survey regions are two declination strips, one in the southern Galactic hemisphere spanning 808 158 around the SGP, and the other in the northern Galactic hemisphere spanning 758 108 along the celestial equator; in addition, there are 99 fields spread over the southern Galactic cap. The survey covers 2000 deg 2 and has a median depth of z 0:11. Adaptive tiling is used to give a highly uniform sampling rate of 93 per cent over the whole survey region. Redshifts are measured from spectra covering 3600-8000

We study dark matter halo density profiles in a high-resolution N-body simulation of a ΛCDM cosmology. Our statistical sample contains ∼5000 haloes in the range 1011−1014 h−1 M⊙, and the resolution allows a study of subhaloes inside host haloes. The profiles are parametrized by an NFW form with two parameters, an inner radius rs and a virial radius Rvir, and we define the halo concentration cvir=Rvirrs. First, we find that, for a given halo mass, the redshift dependence of the median concentration is cvir∝(1+z)−1. This corresponds to rs(z)∼constant, and is contrary to earlier suspicions that cvir does not vary much with redshift. The implications are that high-redshift galaxies are predicted to be more extended and dimmer than expected before. Secondly, we find that the scatter in halo profiles is large, with a 1σ Δ(log cvir)=0.18 at a given mass, corresponding to a scatter in maximum rotation velocities of ΔVmaxVmax=0.12. We discuss implications for modelling the Tully—Fisher relation, which has a smaller reported intrinsic scatter. Thirdly, subhaloes and haloes in dense environments tend to be more concentrated than isolated haloes, and show a larger scatter. These results suggest that cvir is an essential parameter for the theory of galaxy modelling, and we briefly discuss implications for the universality of the Tully—Fisher relation, the formation of low surface brightness galaxies, and the origin of the Hubble sequence. We present an improved analytic treatment of halo formation that fits the measured relations between halo parameters and their redshift dependence, and can thus serve semi-analytic studies of galaxy formation.

The Photodetector Array Camera and Spectrometer (PACS) is one of the three science instruments on ESA's far infrared and submillimetre observatory. It employs two Ge:Ga photoconductor arrays (stressed and unstressed) with 16×25 pixels, each, and two filled silicon bolometer arrays with 16×32 and 32×64 pixels, respectively, to perform integral-field spectroscopy and imaging photometry in the 60–210 <i>μ<i/>m wavelength regime. In photometry mode, it simultaneously images two bands, 60–85 <i>μ<i/>m or 85–125 <i>μ<i/>m and 125–210 <i>μ<i/>m, over a field of view of ~1.75'× 3.5', with close to Nyquist beam sampling in each band. In spectroscopy mode, it images a field of 47” × 47”, resolved into 5×5 pixels, with an instantaneous spectral coverage of ~1500 km s<sup>-1<sup/> and a spectral resolution of ~175 km s<sup>-1<sup/>. We summarise the design of the instrument, describe observing modes, calibration, and data analysis methods, and present our current assessment of the in-orbit performance of the instrument based on the performance verification tests. PACS is fully operational, and the achieved performance is close to or better than the pre-launch predictions.

We present a power-spectrum analysis of the final 2dF Galaxy Redshift Survey (2dFGRS), employing a direct Fourier method. The sample used comprises 221 414 galaxies with measured redshifts. We investigate in detail the modelling of the sample selection, improving on previous treatments in a number of respects. A new angular mask is derived, based on revisions to the photometric calibration. The redshift selection function is determined by dividing the survey according to rest-frame colour, and deducing a self-consistent treatment of k-corrections and evolution for each population. The covariance matrix for the power-spectrum estimates is determined using two different approaches to the construction of mock surveys, which are used to demonstrate that the input cosmological model can be correctly recovered. We discuss in detail the possible differences between the galaxy and mass power spectra, and treat these using simulations, analytic models and a hybrid empirical approach. Based on these investigations, we are confident that the 2dFGRS power spectrum can be used to infer the matter content of the universe. On large scales, our estimated power spectrum shows evidence for the 'baryon oscillations' that are predicted in cold dark matter (CDM) models. Fitting to a CDM model, assuming a primordial n s = 1 spectrum, h = 0.72 and negligible neutrino mass, the preferred parameters are m h = 0.168 0.016 and a baryon fraction b / m = 0.185 0.046 (1 errors). The value of m h is 1 lower than the 0.20 0.03 in our 2001 analysis of the partially

We present the results of a large library of cosmological N-body simulations, using power-law initial spectra. The non-linear evolution of the matter power spectra is compared with the predictions of existing analytic scaling formulae based on the work of Hamilton et al. The scaling approach has assumed that highly non-linear structures obey ‘stable clustering’ and are frozen in proper coordinates. Our results show that, when transformed under the self-similarity scaling, the scale-free spectra define a non-linear locus that is clearly shallower than would be required under stable clustering. Furthermore, the small-scale non-linear power increases as both the power spectrum index n and the density parameter Ω decrease, and this evolution is not well accounted for by the previous scaling formulae. This breakdown of stable clustering can be understood as resulting from the modification of dark matter haloes by continuing mergers. These effects are naturally included in the analytic ‘halo model’ for non-linear structure; we use this approach to fit both our scale-free results and also our previous cold dark matter data. This method is more accurate than the commonly used Peacock–Dodds formula and should be applicable to more general power spectra. Code to evaluate non-linear power spectra using this method is available from http://as1.chem.nottingham.ac.uk/~res/software.html. Following publication, we will make the power-law simulation data publically available through the Virgo website http://www.mpa-garching.mpg.de/Virgo/.

Abstract A new theorem on space-time singularities is presented which largely incorporates and generalizes the previously known results. The theorem implies that space-time singularities are to be expected if either the universe is spatially closed or there is an ‘object’ undergoing relativistic gravitational collapse (existence of a trapped surface) or there is a point p whose past null cone encounters sufficient matter that the divergence of the null rays through p changes sign somewhere to the past of p (i. e. there is a minimum apparent solid angle, as viewed from p for small objects of given size). The theorem applies if the following four physical assumptions are made: (i) Einstein’s equations hold (with zero or negative cosmological constant), (ii) the energy density is nowhere less than minus each principal pressure nor less than minus the sum of the three principal pressures (the ‘energy condition’), (iii) there are no closed timelike curves, (iv) every timelike or null geodesic enters a region where the curvature is not specially alined with the geodesic. (This last condition would hold in any sufficiently general physically realistic model.) In common with earlier results, timelike or null geodesic incompleteness is used here as the indication of the presence of space-time singularities. No assumption concerning existence of a global Cauchy hypersurface is required for the present theorem.

We present a rapid binary-evolution algorithm that enables modelling of even the most complex binary systems. In addition to all aspects of single-star evolution, features such as mass transfer, mass accretion, common-envelope evolution, collisions, supernova kicks and angular momentum loss mechanisms are included. In particular, circularization and synchronization of the orbit by tidal interactions are calculated for convective, radiative and degenerate damping mechanisms. We use this algorithm to study the formation and evolution of various binary systems. We also investigate the effect that tidal friction has on the outcome of binary evolution. Using the rapid binary code, we generate a series of large binary populations and evaluate the formation rate of interesting individual species and events. By comparing the results for populations with and without tidal friction, we quantify the hitherto ignored systematic effect of tides and show that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work. Tidal synchronism is important but, because orbits generally circularize before Roche lobe overflow, the outcome of the interactions of systems with the same semilatus rectum is almost independent of eccentricity. It is not necessary to include a distribution of eccentricities in population synthesis of interacting binaries; however, the initial separations should be distributed according to the observed distribution of semilatera recta rather than periods or semimajor axes.

We construct semi-analytic models for galaxy formation within the framework of a hierarchical clustering scenario for structure formation in the Universe. We use the algorithm of Kauffmann & White to generate ensembles of merging histories for present-day dark matter haloes with a wide range of circular velocities. A galaxy is assumed to form from gas which cools and turns into stars at the centre of a halo until that halo merges with a more massive object. At this time the galaxy loses its source of new gas and becomes a non-dominant object within a larger group or cluster. Our methods thus enable us to ‘look inside’ present dark matter haloes and investigate the formation, evolution and merging of the galaxies that they contain. We begin by investigating the properties of haloes with Vc = 220 km s–1, and use the observed properties of our Milky Way system to tune the free parameters that regulate star formation, hydrodynamic feedback from supernovae and the transformation of discs into spheroids by mergers. We then show that the same parameters lead to good agreement between the properties of galaxies in a Vc = 1000 km s–1 halo and observational data on the Virgo cluster of galaxies. This model correctly reproduces the observed trends in the luminosity, colour, gas content and morphology of galaxies. Turning to an investigation of the properties of the galaxy population as a whole, we highlight a problem that arises when applying this model to a ‘standard’ cold dark matter universe. If the zero-point of the Tully–Fisher relation is set by the properties of our Milky Way system, we find that standard CDM predicts too many haloes and results in a B-band luminosity density of the Universe that is a factor of 2 too high. The only apparent solution to this problem is to assume that many haloes remain observationally undetectable. We also compute the gas mass–luminosity relation for galaxies, the variation in galaxy morphology as a function of luminosity, star formation histories according to environment, the field galaxy luminosity function, and predictions for faint galaxy counts in the B and K bands. We conclude that, although it would be premature to attempt a detailed quantitative fit to specific cosmological models, the qualitative agreement between the data and the general picture that we present is already very encouraging.

Among ‘empirical’ methods of estimating oxygen abundances in extragalactic H ii regions, the use of the ratio of nebular lines of [O iii] and [N ii], first introduced by Alloin et al., is reappraised with modern calibration data and shown to have certain advantages over R23≡ ([O ii]+[O iii])/Hβ and N2 ≡[N ii]λ6583/Hα, particularly when applied to star-forming galaxies at high redshifts.

We present analytic formulae that approximate the evolution of stars for a wide range of mass M and metallicity Z. Stellar luminosity, radius and core mass are given as a function of age, M and Z, for all phases from the zero-age main sequence up to, and including, the remnant stages. For the most part we find continuous formulae accurate to within 5 per cent of detailed models. These formulae are useful for purposes such as population synthesis that require very rapid but accurate evaluation of stellar properties, and in particular for use in combination with N-body codes. We describe a mass-loss prescription that can be used with these formulae, and investigate the resulting stellar remnant distribution.

It is suggested that there may be a large number of gravitationally collapsed objects of mass 10–5 g upwards which were formed as a result of fluctuations in the early Universe. They could carry an electric charge of up to ± 30 electron units. Such objects would produce distinctive tracks in bubble chambers and could form atoms with orbiting electrons or protons. A mass of 1017 g of such objects could have accumulated at the centre of a star like the Sun. If such a star later became a neutron star there would be a steady accretion of matter by a central collapsed object which could eventually swallow up the whole star in about ten million years.

Powerful extragalactic radio sources comprise two extended regions containing magnetic field and synchrotron-emitting relativistic electrons, each linked by a jet to a central compact radio source located in the nucleus of the associated galaxy. These jets are collimated streams of plasma that emerge from the nucleus in opposite directions, along which flow mass, momentum, energy, and magnetic flux. Methods of using the observations diagnostically to infer the pressures, densities, and fluid velocities within jets are explained. The jets terminate in the extended radio components, where they interact strongly with the surrounding medium through a combination of shock waves and instabilities. Jets may expand freely, be confined by external gas pressure, or be pinched by toroidal magnetic fields. Shear flows are known to be Kelvin-Helmholtz unstable and thus may be responsible for some of the observed oscillation of jets about their mean directions and for creating the turbulence and shock waves needed to accelerate the relativistic electrons. Larger-scale bending may be caused by changes in the jet axis within the nucleus, gravitational interaction of the radio galaxy with a companion galaxy, or rapid motion of the source through dense intergalactic gas. The compact radio sources also exhibit a jet morphology and contain more direct clues as to the origins of jets; in particular, the variations sometimes observed imply bulk flows that are relativistic. It is widely believed that nuclear activity is ultimately ascribable to gas accreting onto a massive black hole. The accretion can proceed in several different fashions, depending upon whether or not the gas has angular momentum and whether or not the radiation emitted is sufficiently intense to influence the dynamics of the flow. Several distinct mechanisms for jet production in the context of black holes have been proposed. (Alternative mechanisms involving dense star clusters and massive spinning stars are also reviewed.) Supersonic jets may be collimated along the spin axis of a gas cloud surrounding the source of the lighter jet gas. Magnetic fields may be crucial in collimating jets, especially if they are wrapped around the jet by orbiting gas and can thereby collimate the outflow through the pinch effect. In fact, the spin energy of the black hole could also be extracted by magnetic torques, in which case the jet would contain electrons and positrons and carry a large electromagnetic Poynting flux. Statistical investigations of active galaxies also furnish valuable information on their nature and evolutionary behavior. The formation of particular kinds of sources appears to be correlated with environmental effects and cosmic epoch. In addition, the brightest compact radio sources on the sky, which probably involve relativistic motion, may be intrinsically faint objects beamed in our direction. There is now compelling evidence for the continuous fueling of extragalactic radio sources through jets emerging from the nucleus of the associated galaxy. The morphological classification of radio sources is interpreted in terms of the powers, speeds, and surroundings of jets. The ratio of the mass accretion rate to the mass of the hole may determine whether an active nucleus will be primarily a thermal object like an optical quasar or a nonthermal object like a radio galaxy. The authors outline a unified model of nuclear activity and assess what future progress may stem from observational developments (especially the proposed very long baseline array), experimental approaches (such as wind tunnel simulations), and theoretical studies (in particular, large-scale numerical hydrodynamical computing).

We quantify the complex interdependence of stellar binarity, the stellar mass-luminosity relation, the mass function, the colour-magnitude relation and Galactic disc structure, all of which must be understood when analysing star-count data and stellar luminosity functions. We derive a mass-Mv relation and a model for the change of stellar luminosity with changes in chemical abundance and age. Combination of this with detailed modelling of all astrophysical and observational contributions to the Malmquist scatter allows us to model star-count data without approximating Malmquist corrections. We show for the first time that a single mass function and normalization explain the stellar distribution towards both Galactic poles, as well as the distribution of stars within a distance of 5.2 pc of the Sun. The initial mass function can be approximated by ξ(m) ∝ m–α with α3 ≈ 2.7 for stars more massive than 1 Mʘ, α 2 ≈ 2.2 in the mass range 0.5 ≤ m ≤ 1 Mʘ and 0.70 < α 1 < 1.85 in the range 0.08 < m ≤ 0.5 Mʘ. If the stars at a distance of about 100 pc from the Galactic mid-plane have a metallicity smaller by about 0.1 dex than do the stars near the plane used to calibrate the mass-Mv and colour-magnitude relations, then both the stars within 5.2 pc of the Sun and the star-count data reaching to a photometric distance of 130 pc lead to a low-mass power-law index of α 1 ≈ 1.5. The change in the power index at 0.5 Mʘ may indicate a characteristic mass-scale in the star formation process. Our model is most consistent with the data if the proportion of binaries among ‘stars’ is larger than 50 per cent and if the component masses are uncorrelated. The possible decline of the proportion of binaries with increasing absolute magnitude, observed in the solar neighbourhood, is in agreement with our models. Two features in the luminosity function for low-mass stars are universal, being solely the result of stellar physics. These features are a flattening at Mv ≈ 7 and a conspicuous maximum at Mv ≈ 12. Binary stars cause the system luminosity function derived from photographic surveys to decrease uniformly with increasing magnitude relative to the single-star luminosity function, but both show the same general features. The solar neighbourhood mass density in main-sequence stars with masses between 0.08 and 100 Mʘ is ρ = 0.05 ± 0.01 Mʘ pc–3, of which main-sequence stars less massive than the Sun contribute about 80 per cent. These estimates explicitly include the effects of binaries. Unresolved binaries cause the apparent disc scaleheight to decrease with decreasing mass to a value of ≈ 150 pc for star-count data in the colour range 4 < V – I < 4.5 if all ‘stars’ are unresolved binaries with uncorrelated component masses. The vertical structure of the Galactic disc is not well described by a single exponential within a few hundred pc of the plane. Neglect of this can lead to a spurious apparent correlation between stellar mass and disc scaleheight.

Very minor revisions. Paper is 36 pages long, with 6 postscript figures. Also available on http://www-supernova.lbl.gov . Paper has been accepted by ApJ

The Simons Observatory (SO) is a new cosmic microwave background experiment being built on Cerro Toco in Chile, due to begin observations in the early 2020s. We describe the scientific goals of the experiment, motivate the design, and forecast its performance. SO will measure the temperature and polarization anisotropy of the cosmic microwave background in six frequency bands centered at: 27, 39, 93, 145, 225 and 280 GHz. The initial configuration of SO will have three small-aperture 0.5-m telescopes and one large-aperture 6-m telescope, with a total of 60,000 cryogenic bolometers. Our key science goals are to characterize the primordial perturbations, measure the number of relativistic species and the mass of neutrinos, test for deviations from a cosmological constant, improve our understanding of galaxy evolution, and constrain the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping ≈ 10% of the sky to a white noise level of 2 μK-arcmin in combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, r , at a target level of σ( r )=0.003. The large aperture telescope will map ≈ 40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope sky region and partially with the Dark Energy Spectroscopic Instrument. With up to an order of magnitude lower polarization noise than maps from the Planck satellite, the high-resolution sky maps will constrain cosmological parameters derived from the damping tail, gravitational lensing of the microwave background, the primordial bispectrum, and the thermal and kinematic Sunyaev-Zel'dovich effects, and will aid in delensing the large-angle polarization signal to measure the tensor-to-scalar ratio. The survey will also provide a legacy catalog of 16,000 galaxy clusters and more than 20,000 extragalactic sources.

The quasi-blackbody plus power-law spectra of many accreting black-hole sources suggests that relatively cold matter is surrounded by hard X-ray emitting plasma. Fluorescent iron lines are produced by X-irradiation of the cold gas. The shape and variability of these lines can be used to map the innermost regions around the black hole. In the case of a disc geometry for the cold gas, the effects of doppler-broadening and gravitational and transverse redshifts produce a characteristic line profile which depends upon inclination. We show here that the broad, iron emission line found in Cyg X-1 by Barr, White & Page is well modelled by fluorescent emission from the inner parts of an accretion disc inclined at ∼ 30 degrees. The mass of the central object and properties of the accretion flow can be determined by future higher resolution studies of this and similar sources, including Active Galaxies.