Publications by James Binney


Searching for an Intermediate Mass Black Hole in the Blue Compact Dwarf galaxy MRK 996

ArXiv (0)

A Georgakakis, YG Tsamis, BL James, A Aloisi

The possibility is explored that accretion on an intermediate mass black hole contributes to the ionisation of the interstellar medium of the Compact Blue Dwarf galaxy MRK996. Chandra observations set tight upper limits (99.7 per cent confidence level) in both the X-ray luminosity of the posited AGN, Lx(2-10keV)<3e40erg/s, and the black hole mass, <1e4/\lambda Msolar, where \lambda, is the Eddington ratio. The X-ray luminosity upper limit is insufficient to explain the high ionisation line [OIV]25.89\mu m, which is observed in the mid-infrared spectrum of the MRK996 and is proposed as evidence for AGN activity. This indicates that shocks associated with supernovae explosions and winds of young stars must be responsible for this line. It is also found that the properties of the diffuse X-ray emission of MRK996 are consistent with this scenario, thereby providing direct evidence for shocks that heat the galaxy's interstellar medium and contribute to its ionisation.


Locating the orbits delineated by tidal streams

ArXiv (0)

A Eyre, J Binney

We describe a technique that finds orbits through the Galaxy that are consistent with measurements of a tidal stream, taking into account the extent that tidal streams do not precisely delineate orbits. We show that if accurate line-of-sight velocities are measured along a well defined stream, the technique recovers the underlying orbit through the Galaxy and predicts the distances and proper motions along the stream to high precision. As the error bars on the location and velocities of the stream grow, the technique is able to find more and more orbits that are consistent with the data and the uncertainties in the predicted distances and proper motions increase. With radial-velocity data along a stream ~40deg long and <0.3deg wide on the sky accurate to ~1 km/s the precisions of the distances and tangential velocities along the stream are 4 percent and 5 km/s, respectively. The technique can be used to diagnose the Galactic potential: if circular-speed curve is actually flat, both a Keplerian potential and Phi(r) proportional to r are readily excluded. Given the correct radial density profile for the dark halo, the halo's mass can be determined to a precision of 5 percent.


Entropy Evolution of the Gas in Cooling Flow Clusters

ArXiv (0)

CR Kaiser, JJ Binney

We emphasise the importance of the gas entropy in studying the evolution of cluster gas evolving under the influence of radiative cooling. On this basis, we develop an analytical model for this evolution. We then show that the assumptions needed for such a model are consistent with a numerical solution of the same equations. We postulate that the passive cooling phase ends when the central gas temperature falls to very low values. It follows a phase during which an unspecified mechanism heats the cluster gas. We show that in such a scenario the small number of clusters containing gas with temperatures below about 1 keV is simply a consequence of the radiative cooling.


Dynamics of the Galaxy's Satellites

ArXiv (0)

J Binney

The Milky Way's satellites provide unique information about the density of the Galactic halo at large radii. The inclusion of even a few rather inaccurate proper motions resolves an ambiguity in older mass estimates in favour of higher values. Many of the satellites are concentrated into streams. The dynamics of the Magellanic Stream provided an early indication that the halo reaches out to beyond 100 kpc. Tidal forces between the Clouds are currently disturbing the Clouds' internal dynamics. One would expect this damage to worsen rapidly as the tidal field of the MW excites the eccentricity of the Clouds' mutual orbit. This process, which has yet to be completely modelled, is important for understanding the degree of self-lensing in searches for gravitational lensing events. The Sagittarius Dwarf galaxy very likely contributes significantly to the Galactic warp. The direction of the warp's line of nodes is incorrectly predicted by the simplest models of the Dwarf's orbit. More sophisticated models, in which a complex distribution of stripped dark matter is predicted, may be more successful.


Mass models of the Milky Way

ArXiv (0)

W Dehnen, J Binney

A parameterized model of the mass distribution within the Milky Way is fitted to the available observational constraints. The most important single parameter is the ratio of the scale length R_d* of the stellar disk to R0. The disk and bulge dominate v_c(R) at R<R0 only for R_d*/R0< 0.3. Since the only knowledge we have of the halo derives from studies like the present one, we allow it to contribute to the density at all radii. When allowed this freedom, however, the halo causes changes in assumptions relating to R << R0 to affect profoundly the structure of the best-fitting model at R >> R0. For example, changing the disk slightly from an exponential surface-density profile significantly changes the form of v_c(R) at R >> R0, where the disk makes a negligible contribution to v_c. Moreover, minor changes in the constraints can cause the halo to develop a deep hole at its centre that is not physically plausible. These problems call into question the proposition that flat rotation curves arise because galaxies have physically distinct halos rather than outwards-increasing mass-to-light ratios. The mass distribution of the Galaxy and the relative importance of its various components will remain very uncertain until more observational data can be used to constrain mass models. Data that constrain the Galactic force field at z > R and at R > R0 are especially important.

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