Showing papers in "Monthly Notices of the Royal Astronomical Society in 1998"

The formation of galactic discs

Abstract: We study the population of galactic discs expected in current hierarchical clustering models for structure formation. A rotationally supported disc with exponential surface density profile is assumed to form with a mass and angular momentum which are fixed fractions of those of its surrounding dark halo. We assume that haloes respond adiabatically to disc formation, and that only stable discs can correspond to real systems. With these assumptions the predicted population can match both present-day discs and the damped Lya absorbers in QSO spectra. Good agreement is found provided that: (i) the masses of discs are a few per cent of those of their haloes; (ii) the specific angular momenta of discs are similar to those of their haloes; (iii) present-day discs were assembled recently (at z # 1). In particular, the observed scatter in the size–rotation velocity plane is reproduced, as are the slope and scatter of the Tully–Fisher (TF) relation. The zero-point of the TF relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values derived from disc dynamics. High-redshift discs are predicted to be small and dense, and could plausibly merge together to form the observed population of elliptical galaxies. In many (but not all) currently popular cosmogonies, discs with rotation velocities exceeding 200 kms ¹1 can account for a third or more of the observed damped Lya systems at z , 2:5. Half of the lines of sight to such systems are predicted to intersect the absorber at r * 3h ¹1 kpc and about 10 per cent at r > 10h ¹1 kpc. The cross-section for absorption is strongly weighted towards discs with large angular momentum and therefore large size for their mass. The galaxy population associated with damped absorbers should thus be biased towards low surface brightness

A unifying view of the spectral energy distributions of blazars

Abstract: ABSTRA C T We collect data at well-sampled frequencies from the radio to the g-ray range for the following three complete samples of blazars: the Slew survey, the 1-Jy samples of BL Lacs and the 2-Jy sample of flat-spectrum radio-loud quasars (FSRQs). The fraction of objects detected in g-rays (E * 100 MeV) is ,17, 26 and 40 per cent in the three samples respectively. Except for the Slew survey sample, g-ray detected sources do not differ either from other sources in each sample, or from all the g-ray detected sources, in terms of the distributions of redshift, radio and X-ray luminosities or of the broad-band spectral indices (radio to optical and radio to X-ray). We compute average spectral energy distributions (SEDs) from radio to g-rays for each complete sample and for groups of blazars binned according to radio luminosity, irrespective of the original classification as BL Lac or FSRQ. The resulting SEDs show a remarkable continuity in that (i) the first peak occurs in different frequency ranges for different samples/luminosity classes, with most luminous sources peaking at lower frequencies; (ii) the peak frequency of the g-ray component correlates with the peak frequency of the lower energy one; (iii) the luminosity ratio between the high and low frequency components increases with bolometric luminosity. The continuity of properties among different classes of sources and the systematic trends of the SEDs as a function of luminosity favour a unified view of the blazar phenomenon: a single parameter, related to luminosity, seems to govern the physical properties and radiation mechanisms in the relativistic jets present in BL Lac objects as well as in FSRQs. The general implications of this unified scheme are discussed while a detailed theoretical analysis, based on fitting continuum models to the individual spectra of most g-ray blazars, is presented in a separate paper.

A theoretical unifying scheme for gamma-ray bright blazars

Abstract: ABSTRA C T The phenomenology of g-ray bright blazars can be accounted for by a sequence in the source power and intensity of the diffuse radiation field surrounding the relativistic jet. Correspondingly, the equilibrium particle distribution peaks at different energies. This leads to a trend in the observed properties: an increase of the observed power corresponds to: (i) a decrease in the frequencies of the synchrotron and inverse Compton peaks, and (ii) an increase in the ratio of the powers of the high- and low-energy spectral components. Objects along this sequence would be observationally classified respectively as high-frequency BL Lac objects, lowfrequency BL Lac objects, high-polarization quasars and low-polarization quasars. The proposed scheme is based on the correlations among the physical parameters derived in the present paper by applying to 51 g-ray loud blazars two of the most accepted scenarios for the broad-band emission of blazars, namely the synchrotron self-Compton and external Compton models. This also explains the observational trends presented by Fossati et al., dealing with the spectral energy distributions of all blazars. This gives us confidence that our scheme applies to all blazars as a class.

The ROSAT Brightest Cluster Sample — I. The compilation of the sample and the cluster log N—log S distribution

Abstract: We present a 90 per cent flux-complete sample of the 201 X-ray-brightest clusters of galaxies in the northern hemisphere (δ ≥ 0°), at high Galactic latitudes (|b| ≥ 20°), with measured redshifts z ≤ 0.3 and fluxes higher than 4.4 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All-Sky Survey data and is the largest X-ray-selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X-ray-brightest Zwicky clusters and other clusters selected from their X-ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non-cluster X-ray sources. X-ray fluxes are computed using an algorithm tailored for the detection and characterization of X-ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1σ error). We find the cumulative log N–log S distribution of clusters to follow a power law κSα with α = 1.31+0.06−0.03 (errors are the 10th and 90th percentiles) down to fluxes of 2 × 10−12 erg cm−2 s−1, i.e. considerably below the BCS flux limit. Although our best-fitting slope disagrees formally with the canonical value of −1.5 for a Euclidean distribution, the BCS log N–log S distribution is consistent with a non-evolving cluster population if cosmological effects are taken into account. Our sample will allow us to examine large-scale structure in the northern hemisphere, determine the spatial cluster–cluster correlation function, investigate correlations between the X-ray and optical properties of the clusters, establish the X-ray luminosity function for galaxy clusters, and discuss the implications of the results for cluster evolution.

On the formation of massive stars

Abstract: A B STR A CT We present a model for the formation of massive (Mz10 M>) stars through accretion-induced collisions in the cores of embedded dense stellar clusters. This model circumvents the problem of accreting on to a star whose luminosity is sufficient to reverse the infall of gas. Instead, the central core of the cluster accretes from the surrounding gas, thereby decreasing its radius until collisions between individual components become sufficient. These components are, in general, intermediate-mass stars that have formed through accretion on to low-mass protostars. Once a sufficiently massive star has formed to expel the remaining gas, the cluster expands in accordance with this loss of mass, halting further collisions. This process implies a critical stellar density for the formation of massive stars, and a high rate of binaries formed by tidal capture.

Stellar evolution models for Z = 0.0001 to 0.03

Abstract: We have calculated a grid of empirically well tested evolutionary tracks with masses M between 0.5 and 50 M⊙, spaced by approximately 0.1 in log M, and with metallicities Z = 0.0001, 0.0003, 0.001, 0.004, 0.01, 0.02 and 0.03. We use a robust and fast evolution code with a self-adaptive non-Lagrangian mesh, which employs the mixing-length theory but treats convective mixing as a diffusion process, solving simultaneously for the structure and the chemical composition. The hydrogen and helium abundances are chosen as functions of the metallicity: X = 0.76 − 3.0ZY = 0.24 + 2.0Z. Two sets of models were computed, one without and one with a certain amount of enhanced mixing or ‘overshooting’. This amount has been empirically chosen by means of various sensitive tests for overshooting: (1) the luminosity of core helium burning (blue loop) giants of well-known mass, (2) the width of the main sequence as defined by double-lined eclipsing binaries with well-measured masses and radii, and (3) the shape and implied stellar distribution of isochrones of various open clusters. The first two tests have been the subject of previous papers, the third test is discussed in this paper. On the basis of these tests, we recommend the use of the overshooting models for masses above about 1.5M ⊙. We describe here the characteristics of the models, the procedure for constructing isochrones for arbitrary age and metallicity from the models, and the performance of these isochrones for several intermediate-age and old open clusters. All original models are available in electronic form and we describe the means by which they may be obtained.

Dark matter haloes within clusters

Abstract: We examine the properties of dark matter haloes within a rich galaxy cluster using a high-resolution simulation that captures the cosmological context of a cold dark matter universe. The mass and force resolution permit the resolution of 150 haloes with circular velocities larger than 80 km s−1 within the cluster virial radius of 2 Mpc (with Hubble constant H0 = 50 km s−1 Mpc−1). This enables an unprecedented study of the statistical properties of a large sample of dark matter haloes evolving in a dense environment. The cumulative fraction of mass attached to these haloes varies from close to zero per cent at 200 kpc to 13 per cent at the virial radius. Even at this resolution the overmerging problem persists; haloes that pass within 100–200 kpc of the cluster centre are tidally disrupted. Additional substructure is lost at earlier epochs within the massive progenitor haloes. The median ratio of apocentric to pericentric radii is 6:1, so that the orbital distribution is close to isotropic, circular orbits are rare and radial orbits are common. The orbits of haloes are unbiased with respect to both position within the cluster and the orbits of the smooth dark matter background, and no velocity bias is detected. The tidal radii of surviving haloes are generally well-fitted using the simple analytic prediction applied to their orbital pericentres. Haloes within clusters have higher concentrations than those in the field. Within the cluster, halo density profiles can be modified by tidal forces and individual encounters with other haloes that cause significant mass loss —‘galaxy harassment’. Mergers between haloes do not occur inside the cluster virial radius.

Studies of ultracompact H ii regions — II. High-resolution radio continuum and methanol maser survey

Abstract: High spatial resolution radio continuum and 6.67-GHz methanol spectral line data are presented for methanol masers previously detected by Walsh et al. (1997). Methanol maser and/or radio continuum emission is found in 364 cases towards IRAS-selected regions. For those sources with methanol maser emission, relative positions have been obtained to an accuracy of typically 0.05 arcsec, with absolute positions accurate to around 1 arcsec. Maps of selected sources are provided. The intensity of the maser emission does not seem to depend on the presence of a continuum source. The coincidence of water and methanol maser positions in some regions suggests there is overlap in the requirements for methanol and water maser emission to be observable. However, there is a striking difference between the general proximity of methanol and water masers to both cometary and irregularly shaped ultracompact (UC) H ii regions, indicating that, in other cases, there must be differing environments conducive to stimulating their emission. We show that the methanol maser is most likely present before an observable UC H ii region is formed around a massive star and is quickly destroyed as the UC H ii region evolves. There are 36 out of 97 maser sites that are linearly extended. The hypothesis that the maser emission is found in a circumstellar disc is not inconsistent with these 36 maser sites, but is unlikely. It cannot, however, account for all other maser sites. An alternative model which uses shocks to create the masing spots can more readily reproduce the maser spot distributions.

Evolutionary synthesis of stellar populations: a modular tool

Abstract: A new tool for the evolutionary synthesis of stellar populations is presented, which is based on three independent matrices, giving respectively (1) the fuel consumption during each evolutionary phase as a function of stellar mass, (2) the typical temperatures and gravities during such phases, and (3) the colours and bolometric corrections as functions of gravity and temperature. The modular structure of the code allows one easily to assess the impact on the synthetic spectral energy distribution of the various assumptions and model ingredients, such as, for example, uncertainties in stellar evolutionary models, the mixing length, the temperature distribution of horizontal branch stars, asymptotic giant branch mass loss, and colour–temperature transformations. The so-called ‘AGB phase transition’ in Magellanic Cloud clusters is used to calibrate the contribution of the thermally pulsing asymptotic giant branch phase to the synthetic integrated luminosity. As an illustrative example, solar-metallicity (Y = 0.27, Z = 0.02) models, with ages ranging between 30 Myr and 15 Gyr and various choices for the slope of the initial mass function, are presented. Synthetic broad-band colours and the luminosity contributions of the various evolutionary stages are compared with Large Magellanic Cloud and Galactic globular cluster data. In all these cases, a good agreement is found. Finally, the evolution is presented of stellar mass-to-light ratios in the bolometric and UBVRK passbands, in which the contribution of stellar remnants is accounted for.

Evidence for substructure in lens galaxies

Abstract: We discuss whether one should expect that multiply imaged QSOs can be understood with ‘simple’ lens models that contain only a few parameters. Whereas for many lens systems such simple mass models yield a remarkably good description of the observed properties, there are some systems which are notoriously difficult to understand quantitatively. We argue that at least in one case (B 1422+231) these difficulties are not (solely) due to a ‘wrong’ parametrization of the lens model, but that the discrepancy between observed and model-predicted flux ratios is due to substructure in the lens. As in microlensing for optical fluxes, such substructure can distort also the radio flux ratios predicted by ‘simple’ mass models, in particular for highly magnified images, without appreciably changing image positions. Substructure also does not change the time delay significantly, and therefore has little effect on the determination of the Hubble constant using time delays. We quantify these statements with several simple scenarios for substructure, and propose a strategy to model lens systems in which substructure is suspected.

Gamma-ray bursts from internal shocks in a relativistic wind : temporal and spectral properties

Abstract: We construct models for gamma-ray bursts where the emission comes from internal shocks in a relativistic wind with a highly non uniform distribution of the Lorentz factor. We follow the evolution of the wind using a very simplified approach where a large number of layers interact by direct collisions but where all pressure waves have been suppressed. We suppose that the magnetic field and the electron Lorentz factor reach large equipartition values in the shocks. Synchrotron photons emitted by the relativistic electrons have a typical energy in the gamma-ray range in the observer frame. Synthetic bursts are constructed as the sum of the contributions from all the internal elementary shocks and their temporal and spectral properties are compared to the observations. We reproduce the diversity of burst profiles, the ``FRED'' shape of individual pulses and the short time scale variability. Synthetic bursts also satisfy the duration-hardness relation and individual pulses are found to be narrower at high energy, in agreement with the observations. These results suggest that internal shocks in a relativistic wind may indeed be at the origin of gamma-ray bursts. A potential problem is however the relatively low efficiency of the dissipation process. If the relativistic wind is powered by accretion from a disc to a stellar mass black hole it implies that a substantial fraction of the available energy is injected into the wind.

Depolarization and Faraday effects in galaxies

Abstract: Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto-ionic medium and relativistic electrons are non-uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non-uniformity of the magneto-ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line-of-sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies. We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations. We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.

Early star formation and the evolution of the stellar initial mass function in galaxies

Abstract: It has frequently been suggested in the literature that the stellar IMF in galaxies was top-heavy at early times. This would be plausible physically if the IMF depended on a mass-scale such as the Jeans mass that was higher at earlier times because of the generally higher temperatures that were present then. In this paper it is suggested, on the basis of current evidence and theory, that the IMF has a universal Salpeter-like form at the upper end, but flattens below a characteristic stellar mass that may vary with time. Much of the evidence that has been attributed to a top-heavy early IMF, including the ubiquitous G-dwarf problem, the high abundance of heavy elements in clusters of galaxies, and the high rate of formation of massive stars in high-redshift galaxies, can be accounted for with such an IMF if the characteristic stellar mass was several times higher during the early stages of galaxy evolution. However, significant variations in the mass-to-light ratios of galaxies and large amounts of dark matter in stellar remnants are not as easily explained in this way, because they require more extreme and less plausible assumptions about the form and variability of the IMF. Metal-free ‘population III’ stars are predicted to have an IMF that consists exclusively of massive stars, and they could help to account for some of the evidence that has been attributed to a top-heavy early IMF, as well as contributing importantly to the energetics and chemical enrichment of the early Universe.

A ROSAT study of the cores of clusters of galaxies — I. Cooling flows in an X-ray flux-limited sample

Abstract: ABSTRA C T This is the first part of a study of the detailed X-ray properties of the cores of nearby clusters. We have used the flux-limited sample of 55 clusters listed by Edge et al., and archival and proprietary data from the ROSAT observatory. In this paper an X-ray spatial analysis based on the surface-brightness-deprojection technique is applied to the clusters in the sample with the aim of studying their cooling flow properties. We determine the fraction of cooling flows in this sample to be 70‐90 per cent, and estimate the contribution of the flow region to the cluster Xray luminosity. We show that the luminosity within a strong cooling flow can account for up to 70 per cent of a cluster X-ray bolometric luminosity. Our analysis indicates that about 40 per cent of the clusters in the sample have flows depositing more than 100 M( yr π1 throughout the cooling region, and that these possibly have been undisturbed for many Gyr, confirming that cooling flows are the natural state of cluster cores. New cooling flows in the sample are presented, and previously ambiguous ones are clarified. We have constructed a catalogue of some intracluster medium properties for the clusters in this sample. The profiles of the mass deposited from cooling flows are analysed, and evidence is presented for the existence of breaks in some of the profiles. Comparison is made to recent optical and radio data. We crosscorrelate our sample with the Green Bank, NVSS and FIRST surveys, and with the volumelimited sample of brightest cluster galaxies presented by Lauer & Postman. Although weak trends exist, no strong correlation between optical magnitude or radio power of the brightest cluster galaxy and the strength of the flow is found.

Probing the Universe with the Lyα forest — I. Hydrodynamics of the low-density intergalactic medium

Abstract: We introduce an efficient and accurate alternative to full hydrodynamic simulations, Hydro-PM (HPM), for the study of the low column density Lyman-alpha forest (NHI <∼ 10 14 cm−2). It consists of a Particle-Mesh (PM) solver, modified to compute, in addition to the gravitational potential, an effective potential due to the gas pressure. Such an effective potential can be computed from the density field because of a tight correlation between density and pressure in the low density limit (δ <∼ 10), which can be calculated for any photo-reionization history by a method outlined in Hui & Gnedin (1997). Such a correlation exists, in part, because of minimal shock-heating in the low density limit. We compare carefully the density and velocity fields as well as absorption spectra, computed using HPM versus hydrodynamic simulations, and find good agreement. We show that HPM is capable of reproducing measurable quantities, such as the column density distribution, computed from full hydrodynamic simulations, to a precision comparable to that of observations. We discuss how, by virtue of its speed and accuracy, HPM can enable us to use the Lyman-alpha forest as a cosmological probe. We also discuss in detail the smoothing of the gas (or baryon) fluctuation relative to that of the dark matter on small scales due to finite gas pressure: (1) It is shown the conventional wisdom that the linear gas fluctuation is smoothed on the Jeans scale is incorrect for general reionization (or reheating) history; the correct linear filtering scale is in general smaller than the Jeans scale after reheating, but larger prior to it. (2) It is demonstrated further that in the mildly nonlinear regime, a PM solver, combined with suitable pre-filtering of the initial conditions, can be used to model the low density IGM. But such an approximation is shown to be less accurate than HPM, unless a non-uniform pre-filtering scheme is implemented.

A hyperluminous galaxy at z = 2.8 found in a deep submillimetre survey

Abstract: We present a detailed study of SMM 02399–0136, a hyperluminous, active galaxy selected from a submm survey of the distant Universe. This galaxy is the brightest source in the fields of six rich, lensing clusters, with a total area of 0.01 deg2, that we have mapped with a sensitivity of ∼ 2 mJy beam−1 at 850 μm. We identify a compact optical counterpart with an apparent magnitude of B ∼ 23 and a low surface-brightness companion ∼ 3 arcsec away. Our spectroscopy shows that both components have the same redshift; z = 2.803 ± 0.003. The emission line widths, FWHM ≃ 1000–1500 km s−1, and line ratios, along with the compact morphology and high luminosity (MB ≃ −24.0) of the galaxy, indicate that SMM 02399–0136 contains a rare dust-embedded, narrow-line or type-2 active galactic nucleus (AGN). The source is gravitationally lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5, and our detailed lens model allows us to accurately correct for this. Taking the amplification into account, we estimate that SMM 02399–0136 is intrinsically a factor of five times more luminous than IRAS F 10214+4724. Its far-infrared (FIR) and Hα luminosities and low surface-brightness radio emission are indicative of an interaction-induced starburst, and the star formation rate (SFR) could be several thousand solar masses per year. This assumes that the starburst is the dominant source of energy, but we cannot yet determine reliably the relative contributions of the starburst and the buried AGN. A dust mass of 5–7 × 108 M⊙ is indicated by our data for a dust temperature of 40–50 K, independent of the dominant energy source. We estimate the possible space density of such luminous submm sources, and find that while a large population of these obscured sources could be detected in future wide-field submm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies such as SMM 02399–0136 and F 10214+4724 cannot easily be detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.

A new measure for cosmic shear

Abstract: Cosmic shear, i.e. the distortion of images of high-redshift galaxies through the tidal gravitational field of the large-scale matter distribution in the Universe, offers the opportunity to measure the power spectrum of the cosmic density fluctuations without any reference to the relation of dark matter to luminous tracers. We consider here a new statistical measure for cosmic shear, the aperture mass Map(θ), which is defined as a spatially filtered projected density field and which can be measured directly from the image distortions of high-redshift galaxies. By selecting an appropriate spatial filter function, the dispersion of the aperture mass is a convolution of the power spectrum of the projected density field with a narrow kernel, so that 〈M2ap(θ)〉 provides a well-localized estimate of the power spectrum at wavenumbers s ∼ 5/θ. We calculate 〈M2ap〉 for various cosmological models, using the fully non-linear power spectrum of the cosmic density fluctuations. The non-linear evolution yields a significant increase of 〈M2ap〉 relative to the linear growth on scales below ∼ 0°.5. The third-order moment of Map can be used to define a skewness, which is a measure of the non-Gaussianity of the density field. We present the first calculation of the skewness of cosmic shear in the framework of the quasi-linear theory of structure growth. We show that it yields a sensitive measure of the cosmological model; in particular, it is independent of the normalization of the power spectrum. Several practical estimates for 〈M2ap〉 are constructed and their dispersions calculated. On scales below a few arcminutes, the intrinsic ellipticity distribution of galaxies is the dominant source of noise, whereas on larger scales the cosmic variance becomes the most important contribution. We show that measurements of Map in two adjacent apertures are virtually uncorrelated, which implies that an image with side-length L can yield [L/(2θ)]2 mutually independent estimates for Map. We show that one square degree of a high-quality image is sufficient to detect the cosmic shear with the Map-statistic on scales below ∼ 10 arcmin, and to estimate its amplitude with an accuracy of ∼ 30 per cent on scales below ∼ 5 arcmin.

P3M‐SPH simulations of the Lyα forest

Abstract: We investigate the importance of several numerical artefacts such as lack of resolution on spectral properties of the Lyα forest as computed from cosmological hydrodynamic simulations in a standard cold dark matter universe. We use a new simulation code which is based on a combination of a hierarchical particle–particle–particle–mesh (P3M) scheme for gravity and smoothed particle hydrodynamics (SPH) for gas dynamics. We have performed extensive comparisons between this new code and a modified version of the HYDRA code of Couchman et al. and find excellent agreement. We have also rerun the TREESPH simulations of Hernquist et al. using our new codes and find very good agreement with their published results. This shows that results from hydrodynamical simulations that include cooling are reproducible with different numerical algorithms. We then use our new code to investigate several numerical effects, such as resolution, on spectral statistics deduced from Voigt profile fitting of lines by running simulations with gas particle masses of 1.4 × 108, 1.8 × 107, 2.2 × 106 and 2.1 × 105 M⊙. When we increase the numerical resolution the mean effective hydrogen optical depth converges and so does the column density distribution. However, higher resolution simulations produce narrower lines and consequently the b parameter (velocity width) distribution has only marginally converged in our highest resolution run. Obtaining numerical convergence for the mean He II transmission is demanding. When progressively smaller haloes are resolved at better resolution, a larger fraction of low-density gas contracts to moderate overdensities in which He ii is already optically thick, and this increases the net transmission, making it difficult to simulate He ii reliably. Our highest resolution simulation gives a mean effective optical depth in He ii 5 per cent lower than the simulation with eight times lower mass resolution, illustrating the degree to which the He ii optical depth has converged. In contrast, the hydrogen mean optical depth for these runs is identical. As many properties of the simulated Lyα forest depend on resolution, one should be careful when deducing physical parameters from a comparison of the simulated forest with the observed one. We compare predictions from our highest resolution simulation in a cold dark matter universe, with a photoionizing background inferred from quasars as computed by Haardt &38; Madau, with observations. The simulation reproduces both the H I column density and b parameter distribution when we assume a high baryon density, ΩB h2 ≳ 0.028. In addition we need to impose a higher intergalactic medium (IGM) temperature than predicted within our basic set of assumptions. We argue that such a higher temperature could be caused by differences between the assumed and true reionization history. The simulated H I optical depth is in good agreement with observations, but the He ii optical depth is lower than observed. Fitting the He ii optical depth requires a larger jump, ∼ 14, between the photon flux at the H I and He ii edge than is present in the Haardt &38; Madau spectrum.

Towards gravitational wave asteroseismology

Abstract: We present new results for pulsating neutron stars. We have calculated the eigenfrequencies of the modes that one would expect to be the most important gravitational wave sources: the fundamental fluid f mode, the first pressure p mode and the first gravitational wave w mode, for twelve realistic equations of state. From these numerical data we have inferred a set of 'empirical relations' between the mode frequencies and the parameters of the star (the radius R and the mass M). Some of these relations prove to be surprisingly robust, and we show how they can be used to extract the details of the star from observed modes. The results indicate that, should the various pulsation modes be detected by the new generation of gravitational wave detectors that come online in a few years, the mass and the radius of neutron stars can be deduced with errors no larger than a few per cent.

Tidal friction in triple stars

Abstract: Tidal friction in close binaries, with periods of a few days, is expected to circularize the orbit on a time-scale long compared with human observation but shorter than, or comparable to, the lifetimes of main-sequence stars. In a hierarchical triple star, however, the perturbing effect of the distant third star may decircularize the inner orbit significantly on a time-scale of the order of days (as in λ Tau) or centuries (as in β Per). If the inner pair is observed to be semidetached, however, it is plausible to assume that the eccentricity is small. This may be because tidal friction is operating on a comparably short time-scale, and so it is in principle amenable to observation. We attempt to determine a lower limit to the strength of tidal friction in λ Tau and β Per, on the basis of this consideration. Tidal friction will also lead to a secular transfer of angular momentum from the inner orbit to the outer orbit. Too rapid a transfer may lead to orbital shrinkage that is fast compared with the nuclear time-scales of the inner systems, and this can also be ruled out on observational grounds. Thus we may be able to set an upper as well as a lower limit to the strength of tidal friction, on the basis of observations. In a young hierarchical triple, provided that the orbits are fairly nearly orthogonal, tidal friction can serve to reduce the inner orbital period from months to days within a fairly short period of time, of order P2out/Pin. This may be a significant mechanism for producing young short-period binaries.

Chemical enrichment and the origin of the colour-magnitude relation of elliptical galaxies in a hierarchical merger model

Abstract: In this paper, we present a model of the formation and chemical enrichment of elliptical galaxies that differs from the conventional picture in two fundamental ways. (i) Ellipticals do not form in a single monolithic collapse and burst of star formation at high redshift. Instead, most of their stars form at modest rates in disc galaxies, which then merge to form the ellipticals. (ii) Galaxies do not undergo ‘closed-box’ chemical evolution. Instead, metals can be transferred between the stars, cold gas and hot gas haloes of the galaxies. It is assumed that metals are ejected out of disc galaxies during supernova explosions and that these metals enter the hot gas component. The fact that metals are more easily ejected from small galaxies leads to the establishment of a mass–metallicity relation for the disc systems. Large ellipticals are more metal-rich because they are formed from the mergers of larger discs. We use semi-analytic techniques to follow the formation, evolution and chemical enrichment of cluster elliptical galaxies in a merging hierarchy of dark matter haloes. The inclusion of the new metallicity-dependent spectral synthesis models of Bruzual & Charlot enables us to compute the colours, line indices and mass-to-light ratios of these galaxies. We find that with physically realistic parameters and with the assumption that feedback is efficient, even in massive galaxies, we are able to reproduce the slope and scatter of the colour–magnitude and the Mg2–σ relations for cluster ellipticals. Bright field ellipticals have the same metallicities but are younger than their cluster counterparts, and thus exhibit more scatter in the equivalent widths of their Balmer absorption lines. We are not able to match the increase in M/L for bright ellipticals if we assume their mass to measure purely the total quantity of stars in these objects. We also study the evolution of these relations to high redshift. We show that the luminosity–metallicity relation does not change with redshift, but the mean stellar age of the galaxies scales with the age of the Universe. This is why the evolution of cluster ellipticals appears to be well described by simple passive evolution. Finally, we study the enrichment history of the intracluster gas. Our models predict that more than 80 per cent of the metals were ejected by galaxies with circular velocities less than 250 km s−1 at redshifts greater than 1. The metallicity of the intracluster medium is thus predicted to evolve very little out to z>1.

Transients from initial conditions: a perturbative analysis

Abstract: The standard procedure to generate initial conditions in numerical simulations of structure formations is to use the Zel’dovich approximation (ZA). Although the ZA correctly reproduces the linear growing modes of density and velocity perturbations, non-linear growth is inaccurately represented, particularly for velocity perturbations because of the ZA failure to conserve momentum. This implies that it takes time for the actual dynamics to establish the correct statistical properties of density and velocity fields. We extend the standard formulation of non-linear perturbation theory (PT) to include transients as non-linear excitations of decaying modes caused by the initial conditions. These new non-linear solutions interpolate between the initial conditions and the late-time solutions given by the exact non-linear dynamics. To quantify the magnitude of transients, we focus on higher order statistics of the density contrast δ and velocity divergence Θ, characterized by the Sp and Tp parameters. These describe the non-Gaussianity of the probability distribution through its connected moments 〈δp〉c ≡ Sp〈δ2〉p−1, 〈Θp〉c ≡ Tp 〈Θ2〉p−1. We calculate Sp(a) and Tp(a) to leading order in PT with top-hat smoothing as a function of the scale factor a. We find that the time-scale of transients is determined, at a given order p, by the effective spectral index neff. The skewness factor S3 (T3) attains 10 per cent accuracy only after a ≈ 6 (a ≈ 15) for neff ≈ 0, whereas higher (lower) neff demands more (less) expansion away from the initial conditions. These requirements become much more stringent as p increases, always showing slower decay of transients for Tp than Sp. For models with density parameter Ω ≠ 1, the conditions above apply to the linear growth factor; thus an Ω = 0.3 open model requires roughly a factor of 2 larger expansion than a critical density model to reduce transients by the same amount. The predicted transients in Sp are in good agreement with numerical simulations. More accurate initial conditions can be achieved by using second-order Lagrangian PT (2LPT), which reproduces growing modes up to second order and thus eliminates transients in the skewness parameters. We show that for p > 3 this scheme can reduce the required expansion by more than an order of magnitude compared to the ZA. Setting up 2LPT initial conditions requires only minimal, inexpensive changes to ZA codes. We suggest simple steps for its implementation.

Mass segregation in young stellar clusters

Abstract: We investigate the evolutionary effect of dynamical mass segregation in young stellar clusters. Dynamical mass segregation acts on a time-scale of order the relaxation time of a cluster. Although some degree of mass segregation occurs earlier, the position of massive stars in rich young clusters generally reflects the cluster's initial conditions. In particular, the positions of the massive stars in the Trapezium cluster in Orion cannot be due to dynamical mass segregation, but indicate that they formed in, or near, the centre of the cluster. Implications of this for cluster formation and for the formation of high-mass stars are discussed.

Multifrequency polarimetry of 300 radio pulsars

Abstract: Polarimetric observations of 300 pulsars have been conducted with the 76-m Lovell telescope at Jodrell Bank at radio frequencies centred around 230, 400, 600, 920, 1400 and 1600 MHz. More than half of the pulsars have no previously published polarization profiles and this compilation represents about three times the sum of all previously published pulsar polarization data. A selection of integrated polarization profiles is provided. Tables of pulse widths and the degree of both linear and circular polarization are given for all pulsars, and these act as an index for all the data, which are available by anonymous ftp in numerical and graphical form.

The light curves of soft X‐ray transients

Abstract: We show that the light curves of soft X-ray transients (SXTs) follow naturally from the disc instability picture, adapted to take account of irradiation by the central X-ray source during the outburst. Irradiation prevents the disc from returning to the cool state until central accretion is greatly reduced. This happens only after most of the disc mass has been accreted by the central object, on a viscous time-scale, accounting naturally for the exponential decay of the outburst on a far longer time-scale (τ20–40 d) than seen in dwarf novae, without any need to manipulate the viscosity parameter α. The accretion of most of the disc mass in outburst explains the much longer recurrence time of SXTs compared with dwarf novae. This picture also suggests an explanation of the secondary maximum seen in SXT light curves about 50–75 d after the start of each outburst, since central irradiation triggers the thermal instability of the outer disc, adding to the central accretion rate one viscous time later. The X-ray outburst decay constant τ should on average increase with orbital period, but saturate at a roughly constant value ∼40 d for orbital periods longer than about a day. The bolometric light curve should show a linear rather than an exponential decay at late times (a few times τ). Outbursts of long-period systems should be entirely in the linear decay regime, as is observed in GRO J1744−28. UV and optical light curves should resemble the X-rays but have decay time-scales up to 2–4 times longer.

Semi‐analytic modelling of galaxy evolution in the IR/submm range

Abstract: This paper proposes a new semi-analytic modelling of galaxy properties in the IR/submm wavelength range, which is explicitly set in a cosmological framework. We start from a description of the non-dissipative and dissipative collapses of primordial perturbations, and add star formation, stellar evolution and feedback, as well as the absorption of starlight by dust and its re-emission in the IR and submm. This type of approach has had some success in reproducing the optical properties of galaxies. We hereafter propose a simple extension to the IR/submm range. The growth of structures is followed according to the standard cold dark matter model. We assume that star formation proceeds either in a ‘quiescent’ mode, e.g., as in discs, or in a ‘burst’ mode with 10 times shorter time-scales. In order to reproduce the current data on the evolution of the comoving cosmic star formation rate and gas densities, we need to introduce a mass fraction involved in the ‘burst’ mode strongly increasing with redshift, probably reflecting the increase of interaction and merging activity. We estimate the IR/submm luminosities of these ‘mild starburst’ and ‘luminous UV/IR galaxies’, and we explore how much star formation could be hidden in heavily extinguished, ‘ultraluminous IR galaxies’ by designing a family of evolutionary scenarios which are consistent with the current status of the ‘cosmic constraints’, as well as with the IRAS 60-μm luminosity function and faint counts, but with different high-z IR luminosity densities. However, these scenarios generate a cosmic infrared background whose spectrum falls within the ±1σ range of the isotropic IR component detected by Puget et al. and revisited by Guiderdoni et al. We give predictions for the faint galaxy counts and redshift distributions at IR and submm wavelengths. The submm range is very sensitive to the details of the evolutionary scenarios. As a result, the ongoing and forthcoming observations with ISO and SCUBA (and later with SIRTF, SOFIA, FIRST and PLANCK) will put strong constraints on the evolution of galaxies at z∼1 and beyond.

Extragalactic source counts and contributions to the anisotropies of the cosmic microwave background: predictions for the Planck Surveyor mission

Abstract: We present predictions for the counts of extragalactic sources, the contributions to fluctuations and their angular power spectrum in each channel foreseen for the Planck Surveyor (formerly COBRAS/SAMBA) mission. The contribution to fluctuations owing to clustering of both radio and far-IR sources is found to be generally small in comparison with the Poisson term; however the relative importance of the clustering contribution increases and may eventually become dominant if sources are identified and subtracted down to faint flux limits. The central Planck frequency bands are expected to be ‘clean’: at high galactic latitude (|b| > 20°), where the reduced galactic noise does not prevent the detection of the extragalactic signal, only a tiny fraction of pixels is found to be contaminated by discrete extragalactic sources. Moreover, the ‘flat’ angular power spectrum of fluctuations resulting from extragalactic sources substantially differs from that of primordial fluctuations; therefore, the removal of contaminating signals is eased even at frequencies where point sources give a sizeable contribution to the foreground noise.

Resolving the discrepancy between X-ray and gravitational lensing mass measurements for clusters of galaxies

Abstract: We present a detailed comparison of mass measurements for clusters of galaxies using ASCA and ROSAT X-ray data and constraints from strong and weak gravitational lensing. Our results, for a sample of 13 clusters (including six with massive cooling flows, five without cooling flows, and two intermediate systems), provide a consistent description of the distribution of gravitating matter in these systems. For the six cooling-flow clusters, which are the more dynamically relaxed systems, the X-ray and strong gravitational lensing mass measurements show excellent agreement. The core radii for the mass distributions are small, with a mean value (using a simple isothermal parametrization) of ∼ 50h−150 kpc. These results imply that thermal pressure dominates over non-thermal processes in the support of the X-ray gas against gravity in the central regions of the cooling-flow clusters, and that the hydrostatic assumption used in the X-ray mass determinations is valid. For the non-cooling-flow clusters, the masses determined from the strong-lensing data exceed the X-ray values by factors of 2−4. However, significant offsets between the X-ray and lensing centres are observed, indicating that the X-ray and strong-lensing data are probing different lines of sight through the clusters. These offsets, and the generally complex dynamical states of the clusters inferred from their X-ray morphologies, lensing data and galaxy distributions, suggest that the gravitational potentials in the central regions of the non-cooling-flow systems are evolving rapidly, and that the assumption of hydrostatic equilibrium involved in the X-ray mass measurements is likely to have broken down. The discrepancies between the X-ray and strong-lensing mass measurements may be reconciled if the dynamical activity has caused the X-ray analyses to overestimate the core radii of the dominant mass clumps in the clusters. Substructure and line-of-sight alignments of material towards the cluster cores may also contribute to the discrepancies since they will increase the probability of detecting gravitational arcs in the clusters and can enhance the lensing masses, without significantly affecting the X-ray data. On larger spatial scales, comparisons of the X-ray mass results with measurements from weak gravitational lensing show excellent agreement for both cooling-flow and non-cooling-flow clusters. Our method of analysis accounts for the effects of cooling flows on the X-ray data. We highlight the importance of this and show how the inappropriate use of simple isothermal models in the analysis of X-ray data for clusters with massive cooling flows will result in significant underestimates of the virial temperatures and masses of these systems.

Gamma‐ray bursts from stellar remnants: probing the Universe at high redshift

Abstract: ABSTRA C T A gamma-ray burst (GRB) releases an amount of energy similar to that of a supernova explosion, which combined with its rapid variability suggests an origin related to neutron stars or black holes. Since these compact stellar remnants form from the most massive stars not long after their birth, GRBs should trace the star formation rate in the Universe; we show that the GRB flux distribution is consistent with this. Because of the strong evolution of the star formation rate with redshift, it follows that the dimmest known bursts have z , 6, much above the value usually quoted and beyond the most distant quasars. This explains the absence of bright galaxies in well-studied GRB error boxes. The increased distances imply a peak luminosity of 8:3 〈 10 51 erg s π1 and a rate density of 0.025 per million years per galaxy. These values are 20 times higher and 150 times lower, respectively, than are implied by fits with nonevolving GRB rates. This means either that GRBs are caused by a much rarer phenomenon than mergers of binary neutron stars, or that their gamma-ray emission is often invisible to us due to beaming. Precise burst locations from optical transients will discriminate between the various models for GRBs from stellar deaths, because the distance between progenitor birth place and burst varies greatly among them. The dimmest GRBs are then the most distant known objects, and may probe the Universe at an age when the first stars were forming.

Chemical composition of the Orion nebula derived from echelle spectrophotometry

Abstract: We present echelle spectroscopy in the 3500- to 7060-?… range for two positions of the Orion nebula. The data were obtained using the 2.1-m telescope at Observatorio Astron??mico Nacional in San Pedro M??rtir, Baja California. We have measured the intensities of about 220 emission lines, in particular 81 permitted lines of C+, N+, N++, O0, O+, Ne0, Si+, Si++ and S+, some of them produced by recombination only and others mainly by fluorescence. We have determined electron temperatures, electron densities and ionic abundances using different continuum and line intensity ratios. We derived the He, C and O abundances from recombination lines and find that the C/H and O/H values are very similar to those derived from B stars of the Orion association, and that these nebular values are independent of the temperature structure. We have also derived abundances from collisionally excited lines. These abundances depend on the temperature structure; accurate t2 values have been derived comparing the O II recombination lines with the [O III] collisionally excited lines. The gaseous abundances of Mg, Si and Fe show significant depletions, implying that a substantial fraction of these atoms is tied up in dust grains. The derived depletions are similar to those found in warm clouds of the Galactic disc, but are not as large as those found in cold clouds. A comparison of the solar and Orion chemical abundances is made.