arXiv subjects

James Binney

Publications and source records attributed to James Binney.

At least 109 records · Page 6Linked to original sources

arXiv2007

Gaseous Haloes: Linking Galaxies to the IGM

In recent years evidence has accumulated that nearby spiral galaxies are surrounded by massive haloes of neutral and ionised gas. These gaseous haloes rotate more slowly than the disks and show inflow motions. They are clearly analogous to the High Velocity Clouds of the Milky Way. We show that these haloes cannot be produced by a galactic fountain process (supernova outflows from the disk) where the fountain gas conserves its angular momentum. Making this gas interact with a pre-existing hot corona does not solve the problem. These results point at the need for a substantial accretion of low angular momentum material from the IGM.

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arXiv2006

Clues from microquasars to the origin of radio-loudness of quasars

We analysed the long-term variability of four microquasars (GRS 1915+105, Cyg X-1, Cyg X-3, and Sco X-1) in radio and X rays. The results of our analysis indicate the existence of two distinct modes of energy output, which we refer to as the `coupled' mode and the `flaring' mode. The coupled mode is responsible for mildly fluctuating, flat-spectrum radio emission, coupled with the X-ray emission; the flaring mode produces powerful, steep-spectrum radio flares, with no significant counterpart in X rays. We find that the fraction of time spent by a typical microquasar in the flaring mode is similar to the fraction of quasars that are radio-loud. This is consistent with the hypothesis that radio-loudness of quasars is a function of the epoch at which the source is observed.

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arXiv2005

A Dynamical Model for the Extra-planar Gas in Spiral Galaxies

Recent HI observations reveal that the discs of spiral galaxies are surrounded by extended gaseous haloes. This extra-planar gas reaches large distances (several kpc) from the disc and shows peculiar kinematics (low rotation and inflow). We have modelled the extra-planar gas as a continuous flow of material from the disc of a spiral galaxy into its halo region. The output of our models are pseudo-data cubes that can be directly compared to the HI data. We have applied these models to two spiral galaxies (NGC891 and NGC2403) known to have a substantial amount of extra-planar gas. Our models are able to reproduce accurately the vertical distribution of extra-planar gas for an energy input corresponding to a small fraction (<4%) of the energy released by supernovae. However they fail in two important aspects: 1) they do not reproduce the right gradient in rotation velocity; 2) they predict a general outflow of the extra-planar gas, contrary to what is observed. We show that neither of these difficulties can be removed if clouds are ionized and invisible at 21cm as they leave the disc but become visible at some point on their orbits. We speculate that these failures indicate the need for accreted material from the IGM that could provide the low angular momentum and inflow required.

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arXiv2005

Radio-loud flares from microquasars and radio-loudness of quasars

The low-frequency power spectra of the X-ray and radio emission from four microquasars suggest that two distinct modes of energy output are at work: (i) the `coupled' mode in which the X-ray and radio luminosities are closely coupled and vary only weakly, and (ii) the `flaring' mode, which dramatically boosts the radio luminosity but makes no impact on the X-ray luminosity. The systems are in the flaring mode only a few percent of the time. However, flares completely dominate the power spectrum of radio emission, with the consequence that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg X-3, have radio power spectra that lie more than an order of magnitude above the corresponding X-ray power spectra. Of the four microquasars for which we have examined data, in only one, Cyg X-1, is the flaring mode seemingly inactive. While Cyg X-1 is a black-hole candidate, one of the three flaring sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes are driven by the accretion disk rather than black-hole spin. Radio imaging strongly suggests that the flaring mode involves relativistic jets. A typical microquasar is in the flaring mode a few percent of the time, which is similar to the fraction of quasars that are radio loud. Thus there may be no essential difference between radio-loud and radio-quiet quasars; radio loudness may simply be a function of the epoch at which the source is observed.

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arXiv2005

Time variability of AGN and heating of cooling flows

There is increasing evidence that AGN mechanical feedback is important in the energetics of cooling flows in galaxies and galaxy clusters. We investigate the implications of the variability of AGN mechanical luminosity L_m on observations of cooling flows and radio galaxies in general. It is natural to assume that l=ln(L_m/L_x) is a Gaussian process. Then L_m will be log-normally distributed at fixed cooling luminosity L_x, and the variance in a measure of L_m will increase with the time-resolution of the measure. We test the consistency of these predictions with existing data. These tests hinge on the power spectrum of l(t). Monitoring of Seyfert galaxies combined with estimates of the duty cycle of quasars imply flicker noise spectra, similar to those of microquasars. We combine a sample of sources in cooling flows that have cavities with the assumption that the average mechanical luminosity of the AGN equals L_x. Given that the mechanical luminosities are characterized by flicker noise, we find that their spectral amplitudes lie between the estimated amplitudes of quasars and the measured values for the radio luminosities of microquasars. The model together with the observation that powerful radio galaxies lie within a narrow range in optical luminosity, predicts the luminosity function of radio galaxies, in agreement with observations. Forthcoming radio surveys will test the prediction that the luminosity function turns over at about the smallest luminosities so far probed. [Abridged]

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arXiv2005

Rotation and anisotropy of galaxies revisited

The use of the tensor virial theorem (TVT) as a diagnostic of anisotropic velocity distributions in galaxies is revisited. The TVT provides a rigorous global link between velocity anisotropy, rotation and shape, but the quantities appearing in it are not easily estimated observationally. Traditionally use has been made of a centrally averaged velocity dispersion and the peak rotation velocity. Although this procedure cannot be rigorously justified, tests on model galaxies show that it works surprisingly well. With the advent of integral-field spectroscopy it is now possible to establish a rigorous connection between the TVT and observations. The TVT is reformulated in terms of sky-averages, and the new formulation is tested on model galaxies.

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arXiv2004

Modelling the Galaxy for GAIA

Techniques for the construction of dynamical Galaxy models should be considered essential infrastructure that should be put in place before GAIA flies. Three possible modelling techniques are discussed. Although one of these seems to have significantly more potential than the other two, at this stage work should be done on all three. A major effort is needed to decide how to make a model consistent with a catalogue such as that which GAIA will produce. Given the complexity of the problem, it is argued that a hierarchy of models should be constructed, of ever increasing complexity and quality of fit to the data. The potential that resonances and tidal streams have to indicate how a model should be refined is briefly discussed.

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arXiv2004

The Cosmological Context of Extraplanar Gas

I review evidence that galaxies form from gas that falls into potential wells cold, rather than from virialized gas, and that formation stops once an atmosphere of trapped virialized gas has accumulated. Disk galaxies do not have such atmospheres, so their formation is ongoing. During galaxy formation feedback is an efficient process, and the nuclear regions of disk galaxies blow winds. The cold infalling gas that drives continued star formation has a significant component of angular momentum perpendicular to that of the disk. Extraplanar gas has to be understood in the context set by nuclear outflows and cold skew-rotating cosmic infall.

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arXiv2004

Black holes, cuspy atmospheres, and galaxy formation

In cuspy atmospheres, jets driven by supermassive black holes (BHs) offset radiative cooling. The jets fire episodically, but often enough that the cuspy atmosphere does not move very far towards a cooling catastrophe in the intervals of jet inactivity. The ability of energy released on the sub-parsec scale of the BH to balance cooling on scales of several tens of kiloparsecs arises through a combination of the temperature sensitivity of the accretion rate and the way in which the radius of jet disruption varies with ambient density. Accretion of hot gas does not significantly increase BH masses, which are determined by periods of rapid BH growth and star formation when cold gas is briefly abundant at the galactic centre. Hot gas does not accumulate in shallow potential wells. As the Universe ages, deeper wells form, and eventually hot gas accumulates. This gas soon prevents the formation of further stars, since jets powered by the BH prevent it from cooling, and it mops up most cold infalling gas before many stars can form. Thus BHs set the upper limit to the masses of galaxies. The formation of low-mass galaxies is inhibited by a combination of photo-heating and supernova-driven galactic winds. Working in tandem these mechanisms can probably explain the profound difference between the galaxy luminosity function and the mass function of dark halos expected in the cold dark matter cosmology.

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arXiv2004

Cold filaments in galaxy clusters: effects of heat conduction

We determine the critical size l_crit of a filament of cold (T~10^4 K) gas that is in radiative equilibrium with X-ray emitting gas at temperatures T_out~10^6 - 10^8 K. Filaments smaller than l_crit will be rapidly evaporated, while longer ones will induce the condensation of the ambient medium. At fixed pressure P, l_crit increases as T_out^(11/4), while at fixed T_out it scales as 1/P. It scales as f^(1/2), where f is the factor by which the magnetic field depresses the thermal conductivity below Spitzer's benchmark value. For plausible values of f, l_crit is similar to the lengths of observed filaments. In a cluster such as Perseus, the value of l_crit increases by over an order of magnitude between the centre and a radius of 100 kpc. If the spectrum of seed filament lengths l is strongly falling with l, as is natural, then these results explain why filaments are only seen within a few kiloparsecs of the centres of clusters, and are not seen in clusters that have no cooling flow. We calculate the differential emission measure as a function of temperature for the interface between filaments and ambient gas of various temperatures. We discuss the implications of our results for the origin of the galaxy luminosity function.

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arXiv2003

Structural stability of cooling flows

Three-dimensional hydrodynamical simulations are used to investigate the structural stability of cooling flows that are episodically heated by jets from a central AGN. The radial profile of energy deposition is controlled by (a) the power of the jets, and (b) the pre-outburst density profile. A delay in the ignition of the jets causes more powerful jets to impact on a more centrally concentrated medium. The net effect is a sufficient increase in the central concentration of energy deposition to cause the post-outburst density profile to be less centrally concentrated than that of an identical cluster in which the outburst happened earlier and was weaker. These results suggest that the density profiles of cooling flows oscillate around an attracting profile, thus explaining why cooling flows are observed to have similar density profiles. The possibility is raised that powerful FR II systems are ones in which this feedback mechanism has broken down and a runaway growth of the source parameters has occurred.

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arXiv2003

Dark Matter in Galaxies: Conference Summary

The competition between CDM and MOND to account for the `missing mass' phenomena is asymmetric. MOND has clearly demonstrated that a characteristic acceleration $a_0$ underlies the data and understanding what gives rise to $a_0$ is an important task. The reason for MOND's success may lie in either the details of galaxy formation, or an advance in fundamental physics that reduces to MOND in a suitable limit. CDM has enjoyed great success on large scales. The theory cannot be definitively tested on small scales until galaxy formation has been understood because baryons either are, or possibly have been, dominant in all small-scale objects. MOND's predictive power is seriously undermined by its isolation from the rest of physics. In view of this isolation, the way forward is probably to treat CDM as an established theory to be used alongside relativity and electromagnetism in efforts to understand the formation and evolution of galaxies.

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arXiv2003

Heating cooling flows with jets

Active galactic nuclei are clearly heating gas in `cooling flows'. The effectiveness and spatial distribution of the heating are controversial. We use three-dimensional simulations on adaptive grids to study the impact on a cooling flow of weak, subrelativistic jets. The simulations show cavities and vortex rings as in the observations. The cavities are fast-expanding dynamical objects rather than buoyant bubbles as previously modelled, but shocks still remain extremely hard to detect with X-rays. At late times the cavities turn into overdensities that strongly excite the cluster's g-modes. These modes damp on a long timescale. Radial mixing is shown to be an important phenomenon, but the jets weaken the metallicity gradient only very near the centre. The central entropy density is modestly increased by the jets. We use a novel algorithm to impose the jets on the simulations.

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arXiv2003

On the origin of the galaxy luminosity function

Evidence is summarized that suggests that when a protogalaxy collapses, a fraction $f$ of its gas fails to heat to the virial temperature, where $f$ is large for haloes less massive than the value $M^*$ associated with $L^*$ galaxies. Stars and galaxies form only from the cool gas fraction. Hot gas is ejected from low-mass systems as in conventional semi-analytic models of galaxy formation. In high-mass systems it is retained but does not cool and form stars. Instead it builds up as a largely inert atmosphere, in which cooling is inhibited by an episodically active galactic nucleus. Cold gas frequently falls into galactic haloes. In the absence of a dense atmosphere of virial-temperature gas it builds up on nearly circular orbits and forms stars. When there is a sufficiently dense hot atmosphere, cold infalling gas tends to be ablated and absorbed by the hot atmosphere before it can form stars. The picture nicely explains away the surfeit of high-luminosity galaxies that has recently plagued semi-analytic models of galaxy formation, replacing them by systems of moderate luminosity from old stars and large X-ray luminosities from hot gas.

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arXiv2003

Discreteness effects in cosmological N-body simulations

An estimate of the convergence radius of a simulated CDM halo is obtained under the assumption that the peak phase-space density in the system is set by discreteness effects that operate prior to relaxation. The predicted convergence radii are approximately a factor 2 larger than those estimated for numerical convergence studies. A toy model is used to study the formation of sheets of the cosmic web, from which DM haloes form later. This model demonstrates the interplay between phase mixing and violent relaxation that must also be characteristic of spherical collapse. In the limit that sheets contain arbitrarily many particles, it seems that power-law profiles are established in both distance and energy. When only a finite number of particles is employed, relaxation is prematurely terminated and the power laws are broken. In a given simulation, the sheets with the highest peak phase-space densities are those that form from the longest waves. Hence simulations with little small-scale power are expected to form the cuspiest haloes.

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arXiv2003

Cooling Flows or Heating Flows?

It is now clear that AGN heat cooling flows, largely by driving winds. The winds may contain a relativistic component that generates powerful synchrotron radiation, but it is not clear that all winds do so. The spatial and temporal stability of the AGN/cooling flow interaction are discussed. Collimation of the winds probably provides spatial stability. Temporal stability may be possible only for black holes with masses above a critical value. Both the failure of cooling flows to have adiabatic cores and the existence of X-ray cavities confirm the importance of collimated outflows. I quantify the scale of the convective flow that the AGN Hydra would need to drive if it balanced radiative inward flow by outward flow parallel to the jets. At least in Virgo any such flow must be confined to r<~20 kpc. Hydrodynamical simulations suggest that AGN outbursts cannot last longer than ~25 Myr. Data for four clusters with well studied X-ray cavities suggests that heating associated with cavity formation approximately balances radiative cooling. The role of cosmic infall and the mechanism of filament formation are briefly touched on.

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arXiv2003

On the impossibility of advection dominated accretion

Using only the assumption that all interactions between particles in an accretion flow are electromagnetically mediated, it is shown that the time to establish equipartition between ions and electrons is shorter than the characteristic accretion time. Consequently, two-temperature fits to the spectra of accreting objects are unphysical, and models in which significant thermal energy is carried across the event horizon are effectively ruled out.

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arXiv2002

Two-Body Relaxation in Cosmological Simulations

It is logically possible that early two-body relaxation in simulations of cosmological clustering influences the final structure of massive clusters. Convergence studies in which mass and spatial resolution are simultaneously increased, cannot eliminate this possibility. We test the importance of two-body relaxation in cosmological simulations with simulations in which there are two species of particles. The cases of two mass ratios, sqrt(2):1 and 4:1, are investigated. Simulations are run with both a spatially fixed softening length and adaptive softening using the publicly available codes GADGET and MLAPM, respectively. The effects of two-body relaxation are detected in both the density profiles of halos and the mass function of halos. The effects are more pronounced with a fixed softening length, but even in this case they are not so large as to suggest that results obtained with one mass species are significantly affected by two-body relaxation. The simulations that use adaptive softening are less affected by two-body relaxation and produce slightly higher central densities in the largest halos. They run about three times faster than the simulations that use a fixed softening length.

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