Showing papers in "The Astrophysical Journal in 1991"
TL;DR: In this article, a linear analysis is presented of the instability, which is local and extremely powerful; the maximum growth rate which is of the order of the angular rotation velocity, is independent of the strength of the magnetic field.
Abstract: A broad class of astronomical accretion disks is presently shown to be dynamically unstable to axisymmetric disturbances in the presence of a weak magnetic field, an insight with consequently broad applicability to gaseous, differentially-rotating systems. In the first part of this work, a linear analysis is presented of the instability, which is local and extremely powerful; the maximum growth rate, which is of the order of the angular rotation velocity, is independent of the strength of the magnetic field. Fluid motions associated with the instability directly generate both poloidal and toroidal field components. In the second part of this investigation, the scaling relation between the instability's wavenumber and the Alfven velocity is demonstrated, and the independence of the maximum growth rate from magnetic field strength is confirmed.
4,265 citations
TL;DR: In this article, the formation of galaxies by gas condensation within massive dark halos is studied, where the structure grows through hierarchical clustering of a mixture of gas and dissipationless dark matter.
Abstract: Analytic methods for studying the formation of galaxies by gas condensation within massive dark halos are presented. The present scheme applies to cosmogonies where structure grows through hierarchical clustering of a mixture of gas and dissipationless dark matter. The simplest models consistent with the current understanding of N-body work on dissipationless clustering, and that of numerical and analytic work on gas evolution and cooling are adopted. Standard models for the evolution of the stellar population are also employed, and new models for the way star formation heats and enriches the surrounding gas are constructed. Detailed results are presented for a cold dark matter universe with Omega = 1 and H(0) = 50 km/s/Mpc, but the present methods are applicable to other models. The present luminosity functions contain significantly more faint galaxies than are observed.
2,228 citations
TL;DR: In this paper, the authors proposed a solution based on the theory of the excursion sets of F(r, R sub f), the four-dimensional initial density perturbation field smoothed with a continuous hierarchy of filters of radii.
Abstract: It is pointed out that most schemes for determining the mass function of virialized objects from the statistics of the initial density perturbation field suffer from the cloud-in-cloud problem of miscounting the number of low-mass clumps, many of which would have been subsumed into larger objects. The paper proposes a solution based on the theory of the excursion sets of F(r, R sub f), the four-dimensional initial density perturbation field smoothed with a continuous hierarchy of filters of radii R sub f.
1,826 citations
TL;DR: In this article, the dynamical and radiative consequences of disk accretion onto magnetic T T Tauri stars (TTS) were examined using the Ghosh and Lamb model, and it was shown that a prolonged disk accuration phase is compatible with the low rotation rates measured in these stars if they possess a kilogauss strength field that disrupts the disk at a distance of a few stellar radii from the center.
Abstract: The dynamical and radiative consequences of disk accretion onto magnetic T Tauri stars (TTS) are examined using the Ghosh and Lamb model. It is shown that a prolonged disk accretion phase is compatible with the low rotation rates measured in these stars if they possess a kilogauss strength field that disrupts the disk at a distance of a few stellar radii from the center. It is estimated that a steady state in which the net torque exerted on the star is zero can be attained on a time scale that is shorter than the age of the youngest visible TTS. Although the disk does not develop an ordinary shear boundary layer in this case, one can account for the observed UV excess and Balmer emission in terms of the shocks that form at the bottom of the high-latitude magnetic accretion columns on the stellar surface. This picture also provides a natural explanation of some of the puzzling variability properties of stars like DF Tau and RY Lup. YY Ori stars are interpreted as magnetic TTS in which the observer's line of sight is roughly parallel to an accretion column. 37 refs.
1,030 citations
TL;DR: In this article, the dynamics of gas are modeled in a merger of two disk/halo galaxies of equal mass using a hybrid N-body/gasdynamics code.
Abstract: The dynamics of gas are modeled in a merger of two disk/halo galaxies of equal mass using a hybrid N-body/gasdynamics code. Violent tidal forces acting on the disks draw out extended tails and trigger the formation of central bars. As such bars form, gas in the inner half of each disk loses most of its angular momentum through gravitational torques and falls into a compact cloud within the center of the galaxy. These nuclear gas clouds merge when their parent galaxies do, resulting in the rapid assembly of about 5 billion solar masses of gas, which may plausibly be identified with the large central clouds seen in CO observations of galaxies such as NGC 520. Violent star formation in such central gas clouds, which seems inevitable, offers a likely explanation for luminous IRAS galaxies and may contribute significantly to the central stellar populations of merger remnants. If some of the nuclear gas can continue to lose angular momentum, it may be able to fuel or even form a central black hole, resulting in a radio galaxy or possibly even a quasar. 47 refs.
968 citations
TL;DR: In this article, a two-phase accretion disk model with thermal Comptonization is presented, with emphasis on thermal Comptonisation as the basic mechanism for producing the X-ray power-law continuum of Seyfert galaxies within the framework of the inner emission region.
Abstract: Attention is given to a two-phase accretion disk model, with emphasis on thermal Comptonization as the basic mechanism for producing the X-ray power-law continuum of Seyfert galaxies within the framework of the new picture of the inner emission region. A substantial fraction, f, of the gravitational power is assumed to be dissipated via buoyancy and reconnection of the magnetic fields in a hot tenuous 'corona' surrounding the disk's main body. Coupled thermal balance equations for the two phases yield the temperature of the hot phase and the slope of the Comptonized component self-consistently as a function of f and tau, the optical depth of the hot phase. It is found that for f approximately equal to 1 and tau less than 1, the temperature of the hot phase adjusts so as to maintain alpha approximately equal to 1. For small tau the temperature of the hot phase is sufficient for the production of electron-positron pairs to be important. Pair production contributes to the optical depth and limits the maximum temperature allowed.
951 citations
TL;DR: In this article, the emission-line and continuum properties of a set of broad absorption line QSOs and 29 normal BALs (i.e., non-BALs) are compared.
Abstract: The emission-line and continuum properties of a set of 25 broad absorption line QSOs (BALs) and 29 normal QSOs (i.e., non-BALs) are compared. This sample is augmented by an additional 17 BALs. A balnicity index is defined in order to separate the non-BALs from the BALs as objectively as possible, as well as to provide a measure of the strength of the broad absorption line features. It is found that the emission-line properties and the continua of non-BALs and BALs are remarkably similar.
936 citations
TL;DR: In this article, the density profiles and shapes of dark halos are studied using the results of N-body simulations of the gravitational collapse of density peaks, using from 3×10 4 to 3× 10 5 particles.
Abstract: The density profiles and shapes of dark halos are studied using the results of N-body simulations of the gravitational collapse of density peaks. The simulations use from 3×10 4 to 3×10 5 particles, which allow density profiles and shapes to be well resolved. The core radius of a typical dark halo is found to be no greater than the softening radius, e=1.4 kpc. The density profiles can be fitted with an analytical model with an effective power law which varies between −1 in the center to −4 at large radii. The dark halos have circular velocity curves which behave like the circular velocity contribution of the dark component of spiral galaxies inferred from rotation curve decompositions.
846 citations
TL;DR: The apparent optical depth (OOP) method as mentioned in this paper is a procedure for analyzing the absorption lines of an ionized gas toward HD 64760, where the absorption-line data are directly converted into a form that provides direct scientific interpretations of the physical conditions in the interstellar absorbing medium as a function of velocity.
Abstract: Attention is given to the apparent optical depth method, a procedure for analyzing interstellar absorption lines. Observed absorption-line profiles are converted into profiles of apparent optical depth, and apparent column density per unit velocity. By comparing the latter for a given interstellar species having two or more absorption lines which differ in the product, the presence or absence of unresolved saturated structure in the profiles can be directly inferred. The method is illustrated using absorption-line data from the IUE satellite for the highly ionized gas toward HD 64760. Additional illustrations and a study of the method's accuracy are provided through a series of numerical simulations of multicomponent interstellar absorption situations. The method is compared to the standard growth curve method for deriving interstellar column densities. The principal value of the apparent optical depth method is that the absorption-line data are directly converted into a form that provides for direct scientific interpretations of the physical conditions in the interstellar absorbing medium as a function of velocity.
775 citations
TL;DR: In this article, an extensive homogeneous grid of H II region models is presented, where the behavior of strong oxygen lines can be modeled by taking proper account of the softening of the ionizing spectra produced by stars of increasing metallicity.
Abstract: An extensive, homogeneous grid of H II region models, is presented. It is found that the behavior of the strong oxygen lines can be modeled by taking proper account of the softening of the ionizing spectra produced by stars of increasing metallicity. This provides a calibration of the abundance indicating line ratio ([OII] λ3727 + [OIII] λλ4959, 5007)/Hβ which is comparable in accuracy to that obtainable by direct methods when a temperature-sensitive line ratio is available. No metallicity dependence of the IMF is required to explain the systematic softening of stellar spectra with increasing metallicity.
757 citations
TL;DR: In this paper, the authors present a detailed analysis of the profiles of lines emitted from an accretion disk around a rotating black hole for a number of line emissivity laws, with an emphasis on cases where most of the flux originates from the inner regions of the disk.
Abstract: The iron Kα line is detected in Galactic X-ray sources and in some active galactic nuclei (AGNs). This line is likely to be produced by fluorescence from a cold X-ray-irradiated gas. Previous studies of line profiles have concentrated on the case of a disk around a nonrotating black hole. This paper presents calculations of profiles of lines emitted from an accretion disk around a rotating black hole. The calculations were done for a number of different line emissivity laws, with an emphasis on cases where most of the flux originates from the inner regions of the disk
TL;DR: In this paper, it is estimated that about 10 percent of all lensing episodes of the Galactic bulge stars will strongly display the binary nature of the lens, even if the companion is a planet.
Abstract: Almost all stars are in binary systems. When the separation between the two components is comparable to the Einstein ring radius corresponding to the combined mass of the binary acting as a gravitational lens, then an extra pair of images can be created, and the light curve of a lensed source becomes complicated. It is estimated that about 10 percent of all lensing episodes of the Galactic bulge stars will strongly display the binary nature of the lens. The effect is strong even if the companion is a planet. A massive search for microlensing of the Galactic bulge stars may lead to a discovery of the first extrasolar planetary systems.
TL;DR: In this article, the abundances of the light elements were recalculated within the framework of primordial nucleosynthesis in the standard hot big band model, in order to estimate the primordial abundances.
Abstract: The abundances of D, He-3, He-4, and Li-7, are presently recalculated within the framework of primordial nucleosynthesis in the standard hot big band model, in order to estimate the primordial abundances of the light elements. A comparison between theory and experiment demonstrates the consistency of standard model predictions; the baryon density parameter is constrained on the basis of a nucleon-to-photon ratio of 2.8-4.0. These bounds imply that the bulk of the baryons in the universe are dark, requiring that the universe be dominated by nonbaryonic matter.
TL;DR: In this article, the solar coronal magnetic field is proposed to be in a self-organized critical state, thus explaining the observed power-law dependence of solar-flare-occurrence rate on flare size which extends over more than five orders of magnitude in peak flux.
Abstract: The solar coronal magnetic field is proposed to be in a self-organized critical state, thus explaining the observed power-law dependence of solar-flare-occurrence rate on flare size which extends over more than five orders of magnitude in peak flux. The physical picture that arises is that solar flares are avalanches of many small reconnection events, analogous to avalanches of sand in the models published by Bak and colleagues in 1987 and 1988. Flares of all sizes are manifestations of the same physical processes, where the size of a given flare is determined by the number of elementary reconnection events. The relation between small-scale processes and the statistics of global-flare properties which follows from the self-organized magnetic-field configuration provides a way to learn about the physics of the unobservable small-scale reconnection processes. A simple lattice-reconnection model is presented which is consistent with the observed flare statistics. The implications for coronal heating are discussed and some observational tests of this picture are given.
TL;DR: A very high signal-to-noise ratio (S/N of about 400) composite spectrum of the rest-frame ultraviolet and optical region of high luminosity quasars is presented in this article.
Abstract: A very high signal-to-noise ratio (S/N of about 400) composite spectrum of the rest-frame ultraviolet and optical region of high luminosity quasars is presented. The spectrum is derived from 718 individual spectra obtained as part of the Large Bright Quasar Survey. The moderate resolution, 4A or less, and high signal-to-noise ratio allow numerous weak emission features to be identified. Of particular note is the large equivalent-width of the Fe II emission in the rest-frame ultraviolet and the blue continuum slope of the composite. The primary aim of this paper is to provide a reference spectrum for use in line identifications, and a series of large-scale representations of the composite spectrum are shown. A measure of the standard deviation of the individual quasar spectra from the composite spectrum is also presented. 12 refs.
TL;DR: In the course of a search for solar-like oscillations in bright late-type stars, Doppler variability was observed in the F5 subgiant Procyon as mentioned in this paper.
Abstract: In the course of a search for solar-like oscillations in bright late-type stars, Doppler variability was observed in the F5 subgiant Procyon The variations have frequencies within a 11 mHz range centered at 09 mHz, and a total rms amplitude within that range of 25 m/s Observations of Arcturus and scattered sunlight made with the same equipment during the same time interval show no such variation, indicating that the variations seen on Procyon are of stellar origin The Doppler signal seen is entirely consistent with solar-like p-modes on Procyon, with maximum mode amplitudes of about 50 cm/s and periods around 20 minutes Several statistical tests support the identification for the signal with narrow-band oscillations, but none does so conclusively Assuming that the signal does arise from p-modes, there is evidence that the frequency splitting nu(0) is 71 micro-Hz The data do not permit a definite estimate of this quantity, and other values of nu(0) fit the observations about equally well In order to clarify the sources of ambiguity in this and similar observations, the data acquisition, reduction, and interpretation are described 32 refs
TL;DR: In this paper, the authors consider the dynamical evolution of an accretion disk undergoing Keplerian shear flow in the presence of a weak magnetic field and consider some nonlinear consequences of this instability.
Abstract: We consider the dynamical evolution of an accretion disk undergoing Keplerian shear flow in the presence of a weak magnetic field. A linear perturbation analysis presented in a companion paper shows that such a flow is dynamically unstable; here we consider some nonlinear consequences of this instability. We solve the equations of compressible magnetohydrodynamics using a two-dimensional finite-difference code. The Keplerian disk is threaded with a weak magnetic field that has a magnetic energy density much less than the thermal pressure
TL;DR: In this paper, the mixing-length theory of turbulent convection in stellar atmospheres is improved theoretically by adding one of two new expressions (one with no free parameters) for the mixing length.
Abstract: Improvements of the mixing-length theory (MLT) of turbulent convection in stellar atmospheres are developed theoretically. It is pointed out that inaccuracies are introduced into MLT by the approximating assumptions of a single large eddy (rather than many eddies of different sizes) and of incompressibility. In the proposed new model, the full spectrum of turbulent eddies is determined using more recent turbulence models (e.g., the eddy-damped quasi-normal Markovian model of Orszag, 1977), and a new formula for the convective flux is obtained which gives values up to 10 times greater than those of the MLT at high convective efficiencies. The problem of compressibility is addressed by adding one of two new expressions (one with no free parameters) for the mixing length. Numerical results from simulations of a solar-type star and a 0.8-solar-mass globular-cluster star are presented in tables and graphs and discussed in detail; the agreement with observations is found to be better than with the MLT.
TL;DR: The 40 ultraluminous galaxies in the IRAS Bright Galaxy Sample of sources stronger than S = 5.24 Jy at lambda = 60 microns were mapped with approximately 0.25 arcsec resolution at 8.44 GHz.
Abstract: The 40 ultraluminous galaxies in the IRAS Bright Galaxy Sample of sources stronger than S = 5.24 Jy at lambda = 60 microns were mapped with approximately 0.25 arcsec resolution at 8.44 GHz. Twenty-five contain diffuse radio sources obeying the FIR-radio correlation; these are almost certainly starburst galaxies. Fourteen other galaxies have nearly blackbody FIR spectra with color temperatures between 60 and 80 K so their (unmeasured) FIR angular sizes must exceed approximately 0.25 arcsec, yet they contain compact (but usually resolved) radio sources smaller than this limit. The unique radio and FIR properties of these galaxies can be modeled by ultraluminous nuclear starbursts so dense that they 67 are optically thick to free-free absorption at about 1.49 GHz and dust absorption at about 25 microns. Only one galaxy (UGC 08058 = Mrk 231) is a dominated by a variable radio source too compact to be an ultraluminous starburst; it must be powered by a 'monster'.
TL;DR: In this paper, the authors used submillimeter continuum observations of 29 pre-main-sequence objects in Taurus and Orion to study the wavelength dependence of particle emission and found that in most cases the emission is optically thin at wavelengths near 1 mm, so the measured incides should be close to the true values.
Abstract: Submillimeter continuum observations of 29 pre-main-sequence objects in Taurus and Orion are used to study the wavelength dependence of particle emission. These objects are mostly T Tauri stars whose long-wavelength emission is thought to originate in circumstellar disks. The flux densities imply power-law frequency distributions with spectral indices between 2 and 3 in almost all cases. If the emission is optically thin, the particle emissivities have power-law indices between −1 and 1; otherwise, these values are lower limits. It is argued that in most cases the emission is optically thin at wavelengths near 1 mm, so the measured incides should be close to the true values
TL;DR: In this paper, some 158 micron (CII) fine structure line observations from a sample of fourteen gas rich galaxies are reported. And the results confirm and generalize previous basic results that the (cII) line is bright amounting to approximately 0.1 to 1 percent of the Far Infra Red (FIR) luminosity of the nuclear regions of galaxies.
Abstract: Some 158 micron (CII) fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the (CII) line is bright amounting to approximately 0.1 to 1 percent of the Far Infra Red (FIR) luminosity of the nuclear regions of galaxies; the (CII) line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the (CII) line tracks the FIR continuum in a manner consistent with the PDR models; the integrated (CII) to isotope (12)CO (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the (CII) line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.
TL;DR: In this paper, two different methods, classical and Bayesian, for determining confidence intervals involving Poisson-distributed data are compared and reasons for preferring the Bayesian over the classical method are given.
Abstract: Two different methods, classical and Bayesian, for determining confidence intervals involving Poisson-distributed data are compared. Particular consideration is given to cases where the number of counts observed is small and is comparable to the mean number of background counts. Reasons for preferring the Bayesian over the classical method are given. Tables of confidence limits calculated by the Bayesian method are provided for quick reference.
TL;DR: Optical constants for some samples of amorphous carbon are presented for the energy range 4.1 × 10 −3 to 3.5 × 10 3 eV, using a self-consistent method which combines extinction data in the infrared with other measurements of optical constants at higher energy as discussed by the authors.
Abstract: Optical constants for some samples of amorphous carbon are presented for the energy range 4.1 × 10 −3 to 3.5 × 10 3 eV, using a self-consistent method which combines extinction data in the infrared with other measurements of optical constants at higher energy. By construction, the optical constants satisfy the Kramers-Kronig relations. The effects of shape and clustering on the optical constants are examined and found to be significant. The CDE shape distribution mimics qualitatively the effects of compact clusters. Other data showing how the optical properties depend on the degree of hydrogenation and on the type of bonding in the amorphous carbon are discussed
TL;DR: In this article, the ideal-MHD equations are used to show that a coronal current filament can suddenly lose equilibrium if its magnetic energy exceeds a critical value, and this imbalance ejects the filament upwards.
Abstract: The ideal-MHD equations are used to show that a coronal current filament can suddenly lose equilibrium if its magnetic energy exceeds a critical value. The loss of equilibrium in the configuration results from an imbalance between magnetic tension and compression, and this imbalance ejects the filament upwards. Near the critical value, the equilibrium configuration develops a vertical current sheet attached to the photosphere at the point directly below the filament. When equilibrium is lost, field lines anchored to the photosphere are stretched upwards, and the current sheet rapidly grows longer. Without reconnection in the current sheet, the filament travels only a short distance before reaching a new equilibrium, and the net magnetic energy released is less than 1 percent of the stored magnetic energy. However, with reconnection, the filament travels upwards indefinitely, and all of the stored energy is released.
TL;DR: The first well-defined, homogeneous radio sample of 34 BL Lac objects selected from a large-area survey of sources brighter than 1 Jy at 5 GHz is presented in this paper.
Abstract: The first well-defined, homogeneous radio sample of 34 BL Lac objects selected from a large-area survey of sources brighter than 1 Jy at 5 GHz is presented. Extensive optical spectroscopy reveals weak emission lines in roughly 3/4 of the 34 BL Lac objects in the sample. Optical imaging reveals that the nearby BL Lac objects are not stellar, and that the surface brightness distribution of the surrounding fuzz is consistent with the host galaxies being bright ellipticals. The results suggest that gravitational lensing may affect three of the 17 high-redshift BL Lac objects in the sample. The observations do not support a microlensing scenario for the low-redshift objects.
TL;DR: In this paper, radio observations of 60 bright IRAS galaxies with redshifts of 1500-25,000 km/sec were reported, with a mean ratio of H2 to warm dust of 540 + or - 290, corresponding to a total gas/dust ratio of 900-1100.
Abstract: Radio observations of 60 bright IRAS galaxies with redshifts of 1500-25,000 km/sec are reported. Data obtained in the 1-0 line of CO using the 12-m NRAO radio telescope during 1985-1988 are presented in extensive tables, graphs, and line profiles and analyzed along with similar data on 29 less distant IRAS bright galaxies (Tinney et al., 1990). The galaxies are found to have H2 masses of (1-60) x 10 to the 9th solar mass and a mean ratio of H2 to warm dust of 540 + or - 290, corresponding to a total gas/dust ratio of 900-1100. The discrepancy between this value and that for the Galaxy (about 150) is tentatively attributed to the presence of undetected cold dust or errors in interpreting the IR data. The mechanisms which might be responsible for the high ratios of IR luminosity to H2 mass (2-220 solar luminosity per solar mass) are discussed.
TL;DR: In this paper, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white- dwarfs that can ignite nuclear burning at significantly lower central densities than in the previous models.
Abstract: Recent discovery of an unexpectedly large number of low-mass binary pulsars (LMBPs) in globular clusters has instigated active discussions on the evolutionary origin of binary pulsars. Prompted by the possibility that at least some of LMBPs originate from accretion-induced collapse (AIC) of white dwarfs, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white dwarfs that can ignite explosive nuclear burning at significantly lower central densities than in the previous models. Even with low critical densities, AIC is still much more likely than explosion for both types of white dwarfs. Possible regions for AIC are presented in a diagram of mass accretion rate vs initial mass of the white dwarfs.
TL;DR: In this paper, a class of 1D hydrodynamical evolutionary sequences for gas flows in elliptical galaxies was presented, which employ model galaxies whose virialized density distributions are constrained to lie on their fundamental plane.
Abstract: The novel class of 1D hydrodynamical evolutionary sequences for gas flows in elliptical galaxies presented employs model galaxies whose virialized density distributions are constrained to lie on their fundamental plane. The models evolve through as many as three consecutive (wind, outflow, inflow) evolutionary stages. These evolutionary sequences indicate that the X-ray-faint galaxies are still in their wind phase, while the bulk of ellipticals are in the outflow phase, and a few of the brightest galaxies may already have undergone transition to the inflow regime. The effects of interactions with the intracluster medium, ram pressure, encounters, and merging events are qualitatively discussed. 114 refs.
TL;DR: The first detection of very strong maser emission in the 6.6 GHz 5 1 → 6 0 A + transition of methanol toward a large number of star-forming regions is reported in this article.
Abstract: The first detection of very strong maser emission in the 6.6 GHz 5 1 → 6 0 A + transition of methanol toward a large number of star-forming regions is reported. Essentially all 6.6 GHz maser sources found are associated with interstellar OH masers. Toward a number of sources, the 6.6 GHz emission has flux densities of several thousand Jy, exceeding that of any known OH maser. Since all known masers in the 12 GHz 2 0 → 3 −1 E transition observationally accessible to us have been found to have 6.6 GHz counterparts, the latter line is undoubtedly a Class II methanol maser transition. In all cases the 6.6 GHz luminosity exceeds the 12 GHz luminosity, often by large factors
TL;DR: In this paper, the authors use self-similar scaling laws to find that the neutral evolution in the optical spectrum is quite well with what is expected, but that the negative X-ray evolution is very hard to reconcile with the idea that state of the gas in clusters is the result of gravitational clustering.
Abstract: In hierarchical models for structure formation like CDM, the mass scale of clustering evolves rapidly and clusters in the past are predicted to be less massive, but denser and more numerous. The indication from optical searches is that the cluster population is roughly stable in that high velocity dispersion clusters seem to be about as abundant at z ~ 0.5 as at the present while X-ray surveys show a strong negative evolution, there being many fewer highluminosity clusters in the past compared to the present. Using self-similar scaling laws I find that the neutral evolution in the optical agrees quite well with what is expected, but that the negative X-ray evolution is very hard to reconcile with the idea that state of the gas in clusters is the result of gravitational clustering. One way to remedy this discrepancy is to assume that the gas we now see in clusters was heated at some earlier epoch by non-gravitational processes. If one discards the optical observations, an alternative is to invoke primordial fluctuations with a much redder spectrum, and I discuss how one might discriminate between these theories with future observations.