Showing papers in "Monthly Notices of the Royal Astronomical Society in 1994"

A recipe for galaxy formation

Abstract: We present a detailed prescription for how galaxy formation can be modelled in hierarchical theories of structure formation. Our model incorporates the formation and merging of dark matter halos, the shock heating and radiative cooling of baryonic gas gravitationally confined in these halos, the formation of stars regulated by the energy released by evolving stars and supernovae, the merging of galaxies within dark matter halos, and the spectral evolution of the stellar populations that are formed. The procedure that we describe is very flexible and can be applied to any hierarchical clustering theory. We explore the effects of varying the stellar initial mass function, star formation rates and galaxy merging. The results we compare with an extensive range of observational data, including the B and K galaxy luminosity functions, galaxy colours, the Tully-Fisher relation and galaxy number counts.These data strongly constrain the models and enable the relative importance of each of the physical process to be assessed. We present a broadly successful model defined by a plausible choice of parameters. This fiducial model produces a much more acceptable luminosity function than most previous studies. This is achieved through a modest rate of galaxy mergers and strong suppression of star formation in halos of low circular velocity. However, it fails to produce galaxies as red as many observed ellipticals and, compared with the observed Tully-Fisher relation, the model galaxies have circular velocities which are too large. ** uuencoded compressed postscript file containing all text and figures.**

Reconstructing the linear power spectrum of cosmological mass fluctuations

Abstract: We describe an attempt to reconstruct the initial conditions for the formation of cosmological large-scale structure. The power spectrum of the primordial fluctuations is affected by bias, nonlinear evolution and redshift-space distortions, but we show how these effects can be corrected for analytically. Using eight independent datasets, we obtain excellent agreement in the estimated linear power spectra given the following conditions. First, the relative bias factors for Abell clusters, radio galaxies, optical galaxies and IRAS galaxies must be in the ratios 4.5:1.9:1.3:1. Second, the data require redshift-space distortion: $\Omega~{0.6}/b_{\ss I} = 1.0 \pm 0.2$. Third, low values of $\Omega$ and bias are disfavoured. The shape of the spectrum is extremely well described by a CDM transfer function with an apparent value of the fitting parameter $\Omega h =0.25$. Tilted models predict too little power at 100 Mpc wavelengths.

Merger rates in hierarchical models of galaxy formation – II. Comparison with N-body simulations

Abstract: We have made a detailed comparison of the results of N-body simulations with the analytical description of the merging histories of dark matter haloes presented in Lacey & Cole, which is based on an extension of the Press-Schechter method. We find the analytical predictions for the halo mass function, merger rates and formation times to be remarkably accurate. The N-body simulations used 1283 particles and were of self- similar clustering, with Ω=1 and initial power spectra P(k)∝kn with spectral indices n =−2, −1,0. The analytical model is, however, expected to apply for arbitrary Ω and more general power spectra. Dark matter haloes were identified in the simulations using two different methods and at a range of overdensities. For haloes selected at mean overdensities ∼100-200, the analytical mass function was found to provide a good fit to the simulations with a collapse threshold close to that predicted by the spherical collapse model, with a typical error of ≲30 per cent over a range of 103 in mass, which is the full dynamical range of our N-body simulation. This was insensitive to the type of filtering used. Over a range of 102- 103 in mass, there was also good agreement with the analytical predictions for merger rates including their dependence on the masses of the two haloes involved and the time interval being considered, and for formation times, including the dependence on halo mass and formation epoch. The analytical Press-Schechter mass function and its extension to halo lifetimes and merger rates thus provide a very useful description of the growth of dark matter haloes through hierarchical clustering, and should provide a valuable tool in studies of the formation and evolution of galaxies and galaxy clusters.

Opacities for stellar envelopes

Abstract: We define stellar envelopes to be those regions of stellar interiors in which atoms exist and are not markedly perturbed by the plasma environment. Availability of accurate and extensive atomic data is a prime requirement for the calculation of envelope opacities. For envelopes we adopt the criterion of mass density ρ≤0.01 g cm −3 . We present radiative Rosseland mean opacities for envelopes obtained using atomic data calculated in an international collaboration referred to as the Opacity Project, or OP. Equations of state are calculated using an occupation-probability formalism. To a good approximation, ionization equilibria and level populations in envelopes depend only on the temperature T and electron density N e and are insensitive to chemical mixtures

Elemental abundances for a sample of southern galctic planetary nebulae

Abstract: We present spectrophotometric observations of a sample of 80 southern galactic planetary nebulae (PN), and derive elemental abundances for 68 objects, supplementing the optical observations with UV data in 25 cases. We define Type I PN as those objects that have experienced envelope-burning conversion to nitrogen of dredgedup primary carbon. Such nebulae are recognised by their having nitrogen abundances that exceed the total C + N abundance of H II regions in the same galaxy. In our own galaxy, this criterion corresponds to N/O > 0.8. In the current sample, 11 nebulae having N/O > 0.8 are thereby classified as Type I. For these Type I PN, no evidence is found for oxygen depletion, compared with non-Type I PN. No trend is found between the N/O and O/H ratios for the entire sample, and the mean O/H ratios for the non-Type I and Type I PN are the same within the errors; O/H=(4.93±2.22)×10−4 by number for 42 non-Type I PN and O/H=(4.42±1.44)×10−4 for 11 Type I PN. Also, no difference is found between the oxygen abundances in the PN in this sample and the oxygen abundances in galactic H II regions. Hence we find no evidence for the ON cycle (which is predicted to operate during the second dredge-up) to have significantly altered the surface abundances of the progenitor stars, even for the Type I PN. The helium abundances derived for the non-Type I PN are in accord with those predicted by BeckerI the carbon has been enhanced by He-burning processed material brought up by the third dredge-up.

The Contribution of the obscuring torus to the x-ray spectrum of Seyfert galaxies: A Test for the unification model

Abstract: The presence of an obscuring torus around the nucleus of a Seyfert galaxy, as supposed in the popular unification scheme, can strongly modify the X-ray spectrum for both type 1 and type 2 Seyfert galaxies. We study this problem by means of Monte Carlo simulations, finding that, if the torus is Compton-thick, it can scatter at small angles a significant fraction of the nuclear radiation, and contribute to the continuum of Seyfert 1 galaxies above ∼10 keV, and to the fluorescence iron line at 6.4 keV. At large inclination angles and for large torus column densities, the spectrum is attenuated by photoabsorption and Compton scattering, while the iron fluorescence line produced by the torus can have large equivalent widths

Magnetic field dragging in accretion discs

Abstract: We consider a thin accretion disc of half-thickness H, vertically threaded by a magnetic field. The field is due to contributions from both the disc current and an external current (giving rise to a uniform external field). We derive an integro-differential equation for the evolution of the magnetic field, subject to magnetic diffusivity η and disc accretion with radial velocity v r . The evolution equation is solved numerically, and a steady state is reached. The evolution equation depends upon a single, dimensionless parameter D=2η/(3H|v r |)=(R/H)(η/ν), where the latter equality holds for a viscous disc having viscosity ν. At the disc surface, field lines are bent by angle i from the vertical, such that tan i=1.52D −1

Simulations of dissipative galaxy formation in hierarchically clustering universes – II. Dynamics of the baryonic component in galactic haloes

Abstract: We present self-consistent 3D simulations of the formation of virialized systems containing both gas and dark matter. Using a fully Lagrangian code based on the smooth particle hydrodynamics technique and a tree data structure, we follow the evolution of regions of comoving radius 2-3 Mpc with proper inclusion of the tidal effects of surrounding material. Initial conditions at high redshifts assume an Einstein-de Sitter universe, a biased cold dark matter perturbation spectrum (b=2.5), and a baryonic mass fraction of 10 per cent. The gas is initially cold and radiates in the manner expected for a plasma of primordial composition. We neglect star formation and associated processes. We find that most of the gas settles rapidly into centrifugally supported discs at the centres of small dark matter clumps

The Preferential Formation of High-Mass Stars in Shocked Interstellar Gas Layers

Abstract: Gravitationally unstable, shocked layers of interstellar gas are produced by cloud-cloud collisions and by expanding nebulae around massive stars. We show that the resulting fragments are likely to be of high mass, and initially well separated (i.e. weakly bound to one another, if at all). This result may explain why dynamically active regions tend to have a high efficiency of massive star formation, and why they tend to relax quickly into a self-propagating mode which generates sequences of OB subgroups. These tendencies are manifested on many scales, from local star-forming regions like Orion, through regions like 30 Doradus in the LMC, to the most IR-luminous starburst galaxies. We also show that, for a wide range of input parameters, gravitational fragmentation of a shocked layer occurs when the column density of hydrogen nuclei through the accumulating layer reaches a value. This may be one reason for the mass-radius relation for molecular cloud clumps first noted by Larson.

The variability of optically selected quasars

Abstract: The variability properties of a sample of 300 optically selected quasars near the South Galactic Pole (SGP) have been studied over a baseline of 16 years using a series of twelve UK Schmidt Telescope B J plates at seven epochs. In this sample there is a correlation between variability and both quasar luminosity and redshift, at greater than the 99 per cent signiicance level. Quasars of high luminosity show signiicantly less variability than those with low luminosity, the rms variation ranging from about 0.25 mag at M B = ?23 to 0.1 mag at M B = ?28. The observed trends can be explained by an intrinsic dependence of quasar variability on luminosity combined with the eeects of time dilation, and have implications for quasar samples selected by variability or multicolour techniques. The change in the slope of the n(m) relation at the break can be less pronounced in variability-selected samples. Our results can be used to test models for the origin and reprocessing of the continuum emission in quasars.

Spectroscopy of $A0620-00$: the mass of the black hole and an image of its accretion disc

Abstract: We present spectra of the black hole binary A0620-00 taken at Hα and Hβ to study the distribution of emission-line flux from its accretion disc. Using the spectra of Hα, we have measured the K-type companion star's radial velocity semi-amplitude and rotational broadening to be K 2 =433±3 km s −1 and v sin i=83±5 km s −1 . Accounting for the Roche-lobe geometry of the companion, the rotational broadening gives the mass ratio q=M 2 /M 1 =0.06 7±0.01. The disc contributes 6±3 per cent of the light at Hα and 17±3 per cent at Hβ, with the K star providing the rest. The masses of the compact object and K star derived from our values of K 2 and q are M 1 =(3.09±0.09)sin −3 i M ○. and M 2 =(0.21±0.04)sin −3 i M ○.

The origin of the Magellanic Stream

Abstract: SHORTENED ABSTRACT: We present numerical investigations designed to critically test models of the origin of the Magellanic Stream. The most developed model is the tidal model which fails to reproduce several of its characteristic properties. We suggest an alternative model for the origin of the Stream which can explain all of its observed features and dynamics, as well as provide a strong constraint on the distribution of gas within the halo of the Milky Way. We propose that the Stream consists of material which was ram-pressure stripped from the Magellanic System during its last passage through an extended ionized disk of the Galaxy. This collision took place some 500 million years ago at a galacto-centric distance of about 65 kpc, and swept $\sim 20$\% of the least bound HI into the Stream. The gas with the lowest column density lost the most orbital angular momentum, and is presently at the tip of the Stream, having fallen to a distance of $\sim 20$ kpc from the Milky Way attaining a negative velocity of 200 \kms. To prevent the stripped material from leading the Magellanic Clouds and attaining too large an infall velocity, we postulate the existence of an extended dilute halo of diffuse ionized gas surrounding the Milky Way. If the halo gas is at the virial temperature of the potential well of the Milky Way, its thermal emission would contribute $\sim$ 40\% of the observed diffuse background radiation in the 0.5-1.0 keV (M) band, consistent with recent ROSAT measurements as well as pulsar dispersion measures. Ram pressure stripping

Delayed GeV emission from cosmological gamma-ray bursts: impact of a relativistic wind on external matter

Abstract: Sudden collapse of a compact object, or coalescence of a compact binary, can generate an unsteady relativistic wind that lasts for a few seconds. The wind is likely to carry a high magnetic field; and its Lorentz factor depends on the extent to which it is 'loaded' with baryons. If the Lorentz factor is $\sim 100$, internal dissipation and shocks in this wind produce a non-thermal gamma-ray burst, detectable in the range $0.1\MeV \siml E_\gamma \siml 0.1-1\GeV$ out to cosmological distances. The cooled wind ejecta would subsequently be decelerated by the external medium. The resultant blast wave and reverse shock can then give rise to a second burst component, mainly detectable in the GeV range, with a time delay relative to the MeV burst ranging from minutes to hours.

Clustering in the 1.2-Jy IRAS Galaxy Redshift Survey – II. Redshift distortions and $\xi (r_p, \pi)$

Abstract: We examine the effect of redshift space distortions on the galaxy two-point correlation function ξ(r p , π) as a function of separations parallel (r p ) and perpendicular (π) to the line of sight. Modelling of ξ(r p , π) measured from a full-sky redshift survey of IRAS galaxies allows us to characterize the moments of the velocity distribution function of pairs of galaxies. We are guided in our parametrization of models by results from numerical simulations of cold dark matter (CDM) models. It is essential that the models for ξ(r p , π) contain the effects of both the first and second moments of the velocity distribution function, as they distort the redshift space correlations in opposing directions

On the nature of non-thermal radiation from cosmological γ-ray bursters

Abstract: Relativistic electron-positron winds with strong magnetic fields are considered as a source of radiation for cosmological γ-ray bursters. Such a wind is generated by a millisecond pulsar with a very strong magnetic field. An electron-positron plasma near the pulsar is optically thick and in quasi-thermodynamic equilibrium. It is shown that most of the radiation from the pulsar wind is non-thermal and is generated in the following way. Kinetic energy, which is released in the process of deceleration of the neutron star rotation, is transformed mainly to magnetic field energy. The magnetic field is frozen in the outflowing plasma if the distance to the pulsar is smaller than ∼10 13 cm

High-mass-loss AGB Stars in the South Galactic Cap

Abstract: Observations are presented for 61 Miras in the South Galactic Cap (b<−30°) which were selected on the basis of their IRAS 12/25-μm flux ratios as high-mass-loss candidates. JHKL photometry (over 1 500 observations) was obtained for all of the stars, optical spectra for 49 and OH-maser detections for four. 58 of the Miras are oxygen-rich, and three are carbon stars with thick shells. Periods have been determined for all of the stars for which they were not already known - they range from 166 to 700 d. The carbon Mira IZ Peg (AFGL 3099) shows two periods, of 488 and 345 d, respectively. Distances and mass-loss rates have been determined for all 61 stars

Resonant Oscillations and Tidal Heating in Coalescing Binary Neutron Stars

Abstract: Tidal interaction in a coalescing neutron star binary can resonantly excite the g-mode oscillations of the neutron star when the frequency of the tidal driving force equals the intrinsic g-mode frequencies. We study the g-mode oscillations of cold neutron stars using recent microscopic nuclear equations of state, where we determine self-consistently the sound speed and Brunt-V\"ais\"al\"a frequency in the nuclear liquid core. The properties of the g-modes associated with the stable stratification of the core depend sensitively on the pressure-density relation as well as the symmetry energy of the dense nuclear matter. The frequencies of the first ten g-modes lie approximately in the range of $10-100$ Hz. Resonant excitations of these g-modes during the last few minutes of the binary coalescence result in energy transfer and angular momentum transfer from the binary orbit to the neutron star. The angular momentum transfer is possible because a dynamical tidal lag develops even in the absence of fluid viscosity. However, since the coupling between the g-mode and the tidal potential is rather weak, the amount of energy transfer during a resonance and the induced orbital phase error are very small. Resonant excitations of the g-modes play an important role in tidal heating of binary neutron stars. Without the resonances, viscous dissipation is effective only when the stars are close to contact. The resonant oscillations result in dissipation at much larger orbital separation. The actual amount of tidal heating depends on the viscosity of the neutron star. Using the microscopic viscosity, we find that the binary neutron stars are heated to a temperature $\sim 10^8$ K before they come into contact.

Reionization and thermal evolution of a photoionized intergalactic medium.

Abstract: In most models of the reionization of the intergalactic medium, sources of UV radiation create ionized regions around them which expand until they overlap. The gas is impulsively heated when it enters one of these ionized regions. We discuss the various physical effects that determine the temperature of the gas after the passage of the ionization front. Sources with different spectra may heat the surrounding gas to different temperatures, creating large-scale inhomogeneities of the gas temperature in the Universe. The presence of helium is particularly important, since it allows the post-reionization temperature to reach values as high as ∼ 5×10 4 K. Subsequent adiabatic heating as gas collapses into haloes can raise the temperature even more, explaining large b-parameters in Lyα clouds in the minihalo model