Showing papers in "Monthly Notices of the Royal Astronomical Society in 1992"

The existence and shapes of dust lanes in galactic bars

Abstract: I study the flow of gas in and around the bars or ovals of model barred galaxies and show that it is intimately linked to the properties of the periodic orbits. Simulations show that the density of the gas in and around the bar region is low, except for the centre and two narrow lanes which are the loci of shocks. Such shocks form if the x 1 periodic orbits have either loops or large curvature values at their apocentres. The form of the shock loci depends on a number of parameters characterizing the bar and disc potentials. In order for shocks to be offset from the bar major axis towards its leading side, the x 2 and x 3 families must not only exist but also cover a sufficient extent along the bar major axis

Relativistic fireballs: energy conversion and time-scales

Abstract: The expansion energy of a relativistic fireball can be reconverted into radiation when it interacts with an external medium. For expansion with Lorentz factors greater than or approximately equal to 1000 into a typical galactic environment, the corresponding time-scale in the frame of the observer is of the order of seconds. This mechanism would operate in any cosmological scenario of gamma-ray bursts involving initial energies of order a percent of a stellar rest mass, and implies photon energies and time-scales compatible with those observed in gamma-ray bursts.

Precision photometry of early-type galaxies in the Coma and Virgo clusters: a test of the universality of the colour–magnitude relation – II. Analysis

Abstract: A detailed comparison of the colours of early-type galaxies in the Virgo and Coma clusters have been undertaken. The data set presented by the authors (1992, MNRAS, 254) is used to investigate the universality of the colour-magnitude correlation. This work reassesses the anomaly noted by Aaronson, Persson & Frogel (1981, ApJ, 245). Following these authors, we initially compare the relative distances of the two clusters derived from the L-σ and U-V and V-K colour-magnitude correlations. However, in contrast to their result, we find that all three relations give compatible relative distance moduli. A similar result is found by correlating the colours and velocity dispersions with photometric diameters

Suppressing the formation of dwarf galaxies via photoionization

Abstract: In hierarchical clustering theories, some sort of feedback mechanism is required to prevent most of the baryonic material collapsing into subgalactic objects at high redshifts. It is argued that a photoionizing background of the magnitude suggested by applying the Gunn-Peterson constraint to high-redshift quasars, would strongly suppress the cooling of a hydrogen-helium plasma and so inhibit the formation of dwarf galaxies. The effectiveness of this mechanism depends on the spectrum of the photoionizing radiation and so galaxy formation could depend on the proximity of protogalactic perturbations to unusual sources of hard photons such as luminous quasars

The Primordial helium abundance from observations of extragalactic H-II regions

Abstract: The pre-galactic (assumed primordial) helium mass fraction Y p is determined to be 0.228 ± 0.005 (s.e.) or Y p < 0.242 with 95 per cent confidence, taking reasonably likely systematic errors into account. This is based on INT and AAT observations of emission lines from H II galaxies combined with selected data from the literature relating to extragalactic H II regions in general, discussed in a consistent analysis taking into account the known corrections and likely sources of error

The relation between abundance gradients and the physical properties of spiral galaxies

Abstract: Some correlations of the behaviour of chemical abundance gradients with other physical properties of spiral galaxies are established. The sample is some 30 galaxies for which good published spectrophotometry of H II regions is available. The central abundances of spirals are correlated with their mass, barred galaxies have shallow gradients, and non-barred spirals show a correlation of gradient slope with morphological type. The correlation of abundance with mass surface density is confirmed and there are indications that the effective yield may vary as a function of metallicity

On the likelihood ratio for source identification.

Abstract: We discuss the general use of the likelihood ratio technique for source identification, as applied to, for example, IRAS galaxies and radio sources, and provide an analytic treatment which avoids the approximations given in previous discussions. In particular, a precise formula is derived for the probability that any candidate is the correct identification; this can be significantly different from published results and also remains valid in the case of multiple candidates

On dwarf elliptical galaxies and the faint blue counts

Abstract: The nature of the faint blue objects seen in deep images of the sky has been a source of great puzzlement. Their high surface density and weak clustering argues against their being either the progenitors or the merging components of the present-day bright galaxies. The faint blue counts are interpreted as being due to dwarf elliptical galaxies undergoing their initial starburst at z∼1. The starburst epoch is determined by the decline in the UV background, which results in the stably confined photoionized gas in dark haloes of M ∼ 10 9 M ⊙ being able to cool and settle in the centre of the haloes and undergo star formation

COBE Background radiation anisotropies and large scale structure in the universe

Abstract: The microwave background anisotropies detected by the COBE DMR experiment provide our first detailed information about primordial fluctuations. Their properties suggest that the Universe is flat, and had Gaussian initial fluctuations with a scale-invariant spectrum. We discuss the constraints imposed on such theoretical models by the COBE measurements, by observations of galaxy clustering, and by the observed streaming motions of galaxies. When normalized to match the COBE results, models with Ω = 1 and with more large-scale power than the standard cold dark matter (CDM) model predict lower streaming motions than are observed, but agree well with the dynamics of clustering on smaller scales

Morphology of bar orbits

Abstract: I study the periodic orbits in a barred galaxy model potential, concentrating on the families that are relevant to the gas flow within the bar. This model has four free parameters, namely its central concentration and the axial ratio, quadrupole moment and pattern speed of its bar. It will be used in a companion paper to study the gas flow in and around the bar. Here I compare its properties to those of barred galaxies. I then describe a typical characteristic diagram and the different families of periodic orbits that constitute it. I discuss the existence and extent of the families x 2 and x 3 , for realistic values of the free parameters of the model, and compare two different definitions of the inner Lindblad resonance in strong bar models

General relativistic electric potential drops above pulsar polar caps

Abstract: We study the general relativistic electrodynamics of an isolated, rotating, magnetic neutron star. We consider the region of a neutron star magnetosphere with steady, space charge limited flow along open magnetic field lines. The explicit solutions to the Maxwell equations are obtained. Being the simplest, this model enables one to carry out analytically a general relativistic treatment, and to demonstrate the influence of the effects of General Relativity on the creation of an electric field in the afore-mentioned region.

Interferometer phase calibration sources – I. The region $35^{\circ} \leq \delta \leq 75^{\circ} $

Abstract: A catalogue of 800 compact radio sources in the declination range 35° ≤ δ ≤ 75°, whose positions have been measured to an rms accuracy of ∼12 milliarcsec with the VLA, is presented. They are primarily intended for use as phase reference sources for the Jodrell Bank MERLIN but they will also be suitable phase calibrators for the VLA and the VLBI networks

The Baryon content of the Universe

Abstract: We estimate the baryon mass density of the Universe due to the stars in galaxies and the hot gas in clusters and groups of galaxies. The galaxy contribution is computed by using the Efstathiou, Ellis & Peterson luminosity function, together with van der Marel and Persic & Salucci’s mass-to-light versus luminosity relationships. We find stars ≃ 0.002. For clusters and groups we use the Edge et al. X -ray luminosity function, and Edge & Stewart and Kriss, Cioffi & Canizares’ (gas mass )-luminosity relations. We find gas ≃ 0.001. The total amount of visible baryons is then b ≃ 0.003, i.e. less than 10 per cent of the lower limit predicted by standard primordial nucleosynthesis, implying that the great majority of baryons in the Universe are unseen.

A high-frequency survey of the southern Galactic plane for pulsars

Abstract: Results of an HF survey designed to detect young, distant, and short-period pulsars are presented. The survey detected a total of 100 pulsars, 46 of which were previously unknown. The periods of the newly discovered pulsars range between 47 ms and 2.5 ms. One of the new discoveries, PSR 1259-63, is a member of a long-period binary system. At least three of the pulsars have ages less than 30,000 yr, bringing the total number of such pulsars to 12. The majority of the new discoveries are distant objects with high dispersion measures, which are difficult to detect at low frequencies. This demonstrates that the survey has reduced the severe selection effects of pulse scattering, high Galactic background temperature, and dispersion broadening, which hamper the detection of such pulsars at low radio frequencies. The pulsar distribution in the southern Galaxy is found to extend much further from the Galactic center than that in the north, probably due to two prominent spiral arms in the southern Galaxy.

Destruction and survival of polycyclic aromatic hydrocarbons in active galaxies

Abstract: Infrared spectra of dusty galactic environments often contain emission features attributed to polycyclic aromatic hydrocarbons or PAHs, which can be considered to be very small grains or very large molecules. Although IR spectra of starburst galaxies almost always show these emission features, similar spectra of active galaxies are usually featureless. Even in those active galaxies that do exhibit PAH emission, the PAHs still appear to be eradicated from the nuclear region. This dichotomy suggests that PAHs are destroyed by the intense hard radiation field from an AGN. Laboratory experiments show that certain PAHs are, in fact, so effectively destroyed by individual EUV and X-ray photons that they cannot survive even at kiloparsec distances from active nuclei. Regions within active galaxies that do show PAH emission must therefore be shielded from the central X-ray source by a substantial column density of X-ray absorbing gas.

The starburst model for active galactic nuclei : the broad-line region as supernova remnants evolving in a high-density medium

Abstract: It is shown that the broad permitted and semipermitted emission lines of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN) can be generated by strongly radiative (rapidly cooling) supernova remnants expected to occur in the central regions of early-type galaxies undergoing a starburst. Supernova remnants interacting with a circumstellar medium with a density of about n ∼ 10 7 cm -3 become strongly radiative while still expanding at several thousand km/s, and miss the adiabatic Sedov track. Radiative cooling becomes important well before the thermalization of the ejecta is completed, and the shocked matter undergoes a fast condensation behind both the outgoing forward shock and the reverse shock. Two concentric, high-density, and fast-moving thin shells are then formed

The Edinburgh-Durham Southern Galaxy Catalogue. IV : The cluster catalogue

Abstract: The Edinburgh-Durham Cluster Catalogue (EDCC) is presented. This is the first machine-based, objectively selected sample of clusters of galaxies. It consists of 737 clusters or groups of all richnesses, over 0.5 sr of sky, centred on the South Galactic Pole (SGP). The primary galaxy data set for the cluster survey is the Edinburgh-Durham Southern Galaxy Catalogue (EDSGC). The EDCC was constructed using an automatic peak-finding algorithm and is complete to m 10 (b j ) = 18.75

Lagrangian theory of gravitational instability of Friedman-Lemaitre cosmologies and the 'Zel'dovich approximation'

Abstract: The aim of this paper is to clarify the connection between the so-called Zel'dovich approximation and perturbative solutions of the Euler-Poisson system for the motion of a self-gravitating dust continuum, evaluated in the Lagrangian picture of fluid dynamics. Solutions of the Lagrangian equations, linearized at an isotropically expanding background, are derived. This approximation is investigated; it contains the Zel'dovich approximation as well as a generalized form of it as subclasses, and allowed treatment of the non-linear evolution of vortical perturbations consistently within the framework of self-gravitating motions. In contrast to the prediction of the standard linear approximation, vorticity is coupled to the density enhancement and is amplified in the present approximation

A simple approach to the evolution of twisted accretion discs

Abstract: A simple set of equations is introduced which governs the time evolution of a twisted accretion disc. The time evolution is governed by two «viscosities», one governing shear within the plane of the disc and the other governing shear perpendicular to the disc (brought about by non-planarity of the disc). It is shown that these equations can be put in numerical difference form so that angular momentum is locally conserved. Numerical simulations are given for two simple cases, the evolution of a simple twist in a steady Keplerian accretion disc, and the effect of precession about an axis which is not aligned with the disc on the inner regions of the disc (the Bardeen-Petterson effect)

Properties of cooling flows in a flux-limited sample of clusters of galaxies

Abstract: An analysis of the mass flow rates of cooling flows in an X-ray flux-limited sample of clusters of galaxies, in which most of the clusters have been observed with both imaging and spectroscopic detectors, is presented. Using high-quality images and constraints from broad-band spectra, it is found that the fraction of clusters with central cooling times less than the Hubble time is high (at least 70 per cent and possibly 90 per cent), indicating that cooling flows are a common and long-lived phenomenon. The fraction is higher than found in previous studies, due to our consideration of the effects of the spatial resolution and the signal-to-noise ratio of the images

Direct diagnosis of infall in collapsing protostars – I. The theoretical identification of molecular species with broad velocity distributions

Abstract: We have developed chemical models, including depletion on to grains, for the collapsing envelopes of protostars. Our primary goal has been to identify molecular species having broad velocity distributions, so that observations of their line profiles can be employed to diagnose the dynamics of protostar formation, and to yield information on the collapse age and depletion rate. Previously obtained data for NH3 emission lines, from a number of dark cores with embedded protostars, have shown no clear evidence for systematic collapse. Results from our model indicate that the collapse initiates very soon after the formation of an isothermal pressure-balanced sphere. The collapse is preceded by a period of exceptional quiescence. We demonstrate here that as depletion reduces the fractional abundance of NH3 in the accelerating gas of an infalling envelope and prevents the formation of broad wings on NH3 emission lines, the fractional abundances of some other species initially increase as the heavy elements become depleted, leading those species to have emission lines broader than those of NH3. CH is an observable species which, for a large variety of conditions, possesses a fairly constant or increasing fractional abundance in collapsing envelope gas in which depletion is occurring. High angular resolution observations of CH would be very desirable, but would require appropriate instrumentation on an array, or perhaps the Arecibo telescope. When the water abundance is sufficiently high in the outer envelope, so that many molecular ions are removed primarily in reactions with it rather than by dissociative recombination, the abundances of HCO+, N2H+ and H2S rise when collapse and depletion occur. Millimetre lines from these species can be observed with single-dish telescopes at higher angular resolution than can NH3, resulting in their lines having broader wings in spatially resolved sources than those of NH3, even when their abundances relative to NH3 do not increase substantially in the infall. HCO is a particularly interesting species, because its abundance ratio relative to HCO+ increases with growing density if the fractional abundance of gas-phase heavy metals remains constant. Also, HCO line emission can be observed with high angular resolution.

Optimal estimates of line-of-sight velocity distributions from absorption line spectra of galaxies: nuclear discs in elliptical galaxies

Abstract: Methods for extracting line-of-sight velocity distributions (LOSVDs) from the broadening of absorption lines in early-type galaxies have been developed. These methods operate exclusively in pixel space, they use the full information content of the data, they permit a rigorous treatment of the errors, and they relax the standard a priori assumption that the LOSVD is a Gaussian. Two problems have been considered : optimal noise filtering for model-independent extraction of the LOSVD, subject only to the constraint that it be bandwidth-limited; and optimal parameter estimation when the LOSVD can be assumed to belong to a parametrized family of models, for example, single or double Gaussians

Quasar evolution and the growth of black holes

Abstract: A 'minimalist' model of AGN evolution is analyzed that links the measured luminosity function to an elementary description of black hole accretion. The observed luminosity function of bright AGN is extrapolated and simple prescriptions for the growth and luminosity of black holes are introduced to infer quasar birth rates, mean fueling rates, and relict black hole distribution functions. It is deduced that the mean accretion rate scales as (M exp -1./5)(t exp -6.7) and that, for the most conservative model used, the number of relict black holes per decade declines only as M exp -0.4 for black hole masses between 3 x 10 exp 7 and 3 x 10 exp 9 solar masses. If all sufficiently massive galaxies pass through a quasar phase with asymptotic black hole mass a monotonic function of the galaxy mass, then it is possible to compare the space density of galaxies with estimated central masses to that of distant quasars.

Simulating the formation of a cluster of galaxies

Abstract: Simulations (P 3 M + SPH) containing both dark matter and gas, of the formation of a cluster of galaxies are presented. An efficient potential solver, SPH algorithm and cooling implementation make for an extremely fast scheme. The numerical two-body relaxation time is shown to constrain severely the resolution which can be attained. The cluster in our simulations forms by the merger of three large groups at which time relaxation creates an isothermal mass distribution with no central core. Subsequent infall of small groups does not disturb this structure but gives an outer radius which grows uniformly with time

A study of FRII radio galaxies with $z \lt 0.15$ – I. High-resolution maps of eight sources at 3.6 cm

Abstract: VLA maps of total intensity and fractional polarization at 3.6 cm are presented for eight radio galaxies, part of a sample of 30 nearby FRII sources with P 178 > 2 × 10 25 W/(Hz × sr). High-sensitivity images of the hotspots (with resolutions of 0.25 and 0.75 arcsec and average noise levels of ∼30 μJy) show strikingly different and complex structures. Jets are found in six of the sources

Accretion disc viscosity: a simple model for a magnetic dynamo.

Abstract: We develop here a simple physical model for the manner in which a magnetic dynamo might operate in an accretion disc and so provide an effective (magnetic) viscosity. In contrast to other dynamo models, the mechanism we discuss does not depend on the existence of some hydrodynamical small-scale turbulent flow hypothesized to be already present in a non-magnetic disc. Rather, the model we present depends on three well-established physical processes: the Parker instability, the Balbus-Hawley instability and magnetic field reconnection. The model gives rise to finite but non-stationary magnetic field configurations

Solar pulsational stability – I. Pulsation-mode thermodynamics

Abstract: It is not currently known what excites solar five-minute oscillations. Of the two most plausible possibilities, thermal overstability and stochastic excitation by turbulent convection, the single most important discriminating factor is the intrinsic stability of the pulsation modes. In view of this fact, the problem of the linear stability of model solar envelopes is addressed. A time-dependent, non-local mixing-length prescription is employed for convection, and the Eddington approximation to radiative transfer. The calculations reveal that low-degree acoustic modes are damped. Moreover, the theoretical damping rates compare well with measurements of solar oscillation line widths. Turbulent pressure fluctuations play a critical role in stabilizing the pulsations

Variability studies of Seyfert galaxies – II. Spectroscopy

Abstract: Spetroscopic observations of 36 southern Seyferts are presented. Most of these also have been monitored photometrically over a period of four years (Winkler et al., this issue). The objects are classified, line fluxes and widths are measured and peculiarities of the spectra are discussed. Many of the objects have been observed more the once and changes in the line strengths and profiles are examined. Comparisons are also made with the spectra published by other investigators

Pumping the interstellar methanol masers

Abstract: The pumping of interstellar methanol masers is studied, covering a range of excitation conditions. The Class I (Class A) masers result from collisional excitation followed by spontaneous radiative decay. Masers appear in low-frequency transitions where the upper state is strongly favoured by large Einstein A coefficients, associated with high-frequency transitions, or by slower loss rates. Absorption (anti-inversion) is seen when the lower state is so favoured. The Class II (Class B) masers appear when there is a source of continuum radiation warmer than the gas kinetic temperature

On the true shapes of galaxies

Abstract: We analyse the distribution of axial ratios of images in the APM Bright Galaxy Survey (APMBGS). The sample analysed contains a total of 20 399 galaxies (2135 ellipticals, 4782 S0s, 13 482 spirals). For these objects the APM survey provides homogeneous measurements of image characteristics with great accuracy. We apply analytic inversion to infer the true distribution of axial ratios for oblate and prolate models. We also apply Monte Carlo techniques to explore triaxial models.