Does the Fornax dwarf spheroidal have a central cusp or core?

The dark matter dominated Fornax dwarf spheroidal has five globular clusters orbiting at ∼1 kpc from its centre. In a cuspy cold dark matter halo the globulars would sink to the centre from their current positions within a few Gyr, presenting a puzzle as to why they survive undigested at the present epoch. We show that a solution to this timing problem is to adopt a cored dark matter halo. We use numerical simulations and analytic calculations to show that, under these conditions, the sinking time becomes many Hubble times; the globulars effectively stall at the dark matter core radius. We conclude that the Fornax dwarf spheroidal has a shallow inner density profile with a core radius constrained by the observed positions of its globular clusters. If the phase space density of the core is primordial then it implies a warm dark matter particle and gives an upper limit to its mass of ∼0.5 keV, consistent with that required to significantly alleviate the substructure problem.     

On the Rayleigh–Taylor instability of radio bubbles in galaxy clusters

We consider the Rayleigh–Taylor instability in the early evolution of the rarefied radio bubbles (cavities) observed in many cooling-flow clusters of galaxies. The top of a bubble becomes prone to the Rayleigh–Taylor instability as the bubble rises through the intracluster medium (ICM). We show that while the jet is powering the inflation, the deceleration of the bubble–ICM interface is able to reverse the Rayleigh–Taylor instability criterion. In addition, the inflation introduces a drag effect which increases substantially the instability growth time. The combined action of these two effects considerably delays the onset of the instability. Later on, when the magnitude of the deceleration drops or the jet fades, the Rayleigh–Taylor and the Kelvin–Helmholtz instabilities set in and eventually disrupt the bubble. We conclude that the initial deceleration and drag, albeit unable to prevent the disruption of a bubble, may significantly lengthen its lifetime, removing the need to invoke stabilizing magnetic fields.     

Uncertainties of Synthetic Population Models

This paper investigates the uncertainties in the synthetic integrated colours of simple stellar populations, currently the most popular method of estimating the ages of unresolved stellar systems. Three types of uncertainties studied here originate from the stellar models, the population synthesis techniques, and from the stellar spectral libraries. Despite some skepticism, synthetic colours appear to be reliable age indicators as long as they are used for select age ranges. This revised version was published online in September 2006 with corrections to the Cover Date.     

Adiabatic scaling relations of galaxy clusters

The aim of this work is to show that, contrary to popular belief, galaxy clusters are not expected to be self-similar, even when the only energy sources available are gravity and shock-wave heating. In particular, we investigate the scaling relations between mass, luminosity and temperature of galaxy groups and clusters in the absence of radiative processes. Theoretical expectations are derived from a polytropic model of the intracluster medium and compared with the results of high-resolution adiabatic gasdynamical simulations. It is shown that, in addition to the well-known relation between the mass and concentration of the dark matter halo, the effective polytropic index of the gas also varies systematically with cluster mass, and therefore neither the dark matter nor the gas profiles are exactly self-similar. It is remarkable, though, that the effects of concentration and polytropic index tend to cancel each other, leading to scaling relations whose logarithmic slopes roughly match the predictions of the most-basic self-similar models. We provide a phenomenological fit to the relation between polytropic index and concentration, as well as a self-consistent scheme to derive the non-linear scaling relations expected for any cosmology and the best-fitting normalizations of the M – T , L – T and F – T relations appropriate for a Λ cold dark matter universe. The predicted scaling relations reproduce observational data reasonably well for massive clusters, where the effects of cooling and star formation are expected to play a minor role.     

Kinematics of the Open Cluster System in the Galaxy

Absolute proper motions and radial velocities of 202 open clusters in the solar neighborhood, which can be used as tracers of the Galactic disk, are used to investigate the kinematics of the Galaxy in the solar vicinity, including the mean heliocentric velocity components (u_1,u_2,u_3) of the open cluster system, the characteristic velocity dispersions (σ_1,σ_2,σ_3), Oort constants (A, B) and the large-scale radial motion parameters (C,D) of the Galaxy. The results derived from the observational data of proper motions and radial velocities of a subgroup of 117 thin disk young open clusters by means of a maximum likelihood algorithm are: (u_1,u2,u3) = (-16.1±1.0, -7.9±1.4,-10.4±1.5) kms~(-1), (σ_1,σ_2,σ_3) = (17.0 ±0.7,12.2±0.9,8.0±1.3) kms~(-1),(A,B) = (14.8±1.0,-13.0±2.7) kms~(-1) kpc~(-1) ,and(C,D) = (1.5±0.7,-1.2±1.5) kms~(-1) kpc~(-1) . A discussion on the results and comparisons with what was obtained by other authors is given.     

A Chandra X-ray observation of the globular cluster Terzan 1

We present a ∼19-ks Chandra Advanced CCD Imaging Spectrometer (ACIS)-S observation of the globular cluster Terzan 1. 14 sources are detected within 1.4 arcmin of the cluster centre with two of these sources predicted to be not associated with the cluster (background active galactic nuclei or foreground objects). The neutron star X-ray transient, X1732−304, has previously been observed in outburst within this globular cluster with the outburst seen to last for at least 12 yr. Here, we find four sources that are consistent with the ROSAT position for this transient, but none of the sources are fully consistent with the position of a radio source detected with the Very Large Array that is likely associated with the transient. The most likely candidate for the quiescent counterpart of the transient has a relatively soft spectrum and an unabsorbed 0.5–10 keV luminosity of  2.6 × 10³² erg s⁻¹  , quite typical of other quiescent neutron stars. Assuming standard core cooling, from the quiescent flux of this source we predict long (>400 yr) quiescent episodes to allow the neutron star to cool. Alternatively, enhanced core cooling processes are needed to cool down the core. However, if we do not detect the quiescent counterpart of the transient this gives an unabsorbed 0.5–10 keV luminosity upper limit of  8 × 10³¹ erg s⁻¹  . We also discuss other X-ray sources within the globular cluster. From the estimated stellar encounter rate of this cluster we find that the number of sources we detect is significantly higher than expected by the relationship of Pooley et al.     

New age estimates of M31 globular clusters from multicolour photometry

The large majority of extragalactic star cluster studies performed to date essentially use multicolour photometry, combined with theoretical stellar synthesis models, to derive ages, masses, extinction estimates and metallicities. M31 offers a unique laboratory for studies of globular cluster (GC) systems. In this paper, we obtain new age estimates for 91 M31 GCs, based on improved photometric data, updated theoretical stellar synthesis models and sophisticated new fitting methods. In particular, we used photometric measurements from the Two Micron All Sky Survey (2MASS), which, in combination with optical photometry, can partially break the well-known age–metallicity degeneracy operating at ages in excess of a few Gyr. We show robustly that previous age determinations based on photometric data were affected significantly by this age–metallicity degeneracy. Except for one cluster, the ages of our other sample GCs are all older than 1 Gyr. Their age distribution shows populations of young- and intermediate-age GCs, peaking at ∼3 and 8 Gyr, respectively, as well as the 'usual' complement of well-known old GCs, i.e. GCs of similar age as the majority of the Galactic GCs. Our results also show that although there is significant scatter in metallicity at any age, there is a notable lack of young metal-poor and old metal-rich GCs, which might be indicative of an underlying age–metallicity relationship among the M31 GC population.