arXiv subjects

M. I. Wilkinson

Publications and source records attributed to M. I. Wilkinson.

62 records · Page 4Linked to original sources

arXiv2003

Tracing the Star Stream Through M31 Using Planetary Nebula Kinematics

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disk of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ~20 objects whose kinematics are inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.

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arXiv2002

New Mass Estimators for Tracer Populations

We introduce the tracer mass estimator. This is a new and simple way to estimate the enclosed mass from the projected positions and line of sight velocities of a tracer population (such as globular clusters, halo stars and planetary nebulae). Like the projected mass estimator, it works by averaging (projected distance) x (radial velocity squared)/ G over the sample. However, it applies to the commonest case of all, when the tracer population does not follow the overall dark matter density. The method is verified against simulated datasets drawn from Monte Carlo realisations of exact solutions of the collisionless Boltzmann equation and applied to the recent M31 globular cluster data set of Perrett et al. (2002), as well as to M31's satellite galaxies.

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arXiv2002

Mass segregation in rich clusters in the Large Magellanic Cloud - III. Implications for the initial mass function

The distribution of core radii of rich clusters in the Large Magellanic Cloud (LMC) systematically increases in both upper limit and spread with increasing cluster age. Cluster-to-cluster variations in the stellar initial mass function (IMF) have been suggested as an explanation. We discuss the implications of the observed degree of mass segregation in our sample clusters for the shape of the initial mass function. Our results are based on Hubble Space Telescope/WFPC2 observations of six rich star clusters in the LMC, selected to include three pairs of clusters of similar age, metallicity, and distance from the LMC centre, and exhibiting a large spread in core radii between the clusters in each pair. All clusters show clear evidence of mass segregation: (i) their luminosity function slopes steepen with increasing cluster radius, and (ii) the brighter stars are characterized by smaller core radii. For all sample clusters, both the slope of the luminosity function in the cluster centres and the degree of mass segregation are similar to each other, within observational errors of a few tenths of power-law slope fits to the data. This implies that their INITIAL mass functions must have been very similar, down to ~0.8 - 1.0 M(sun). We therefore rule out variations in the IMF of the individual sample clusters as the main driver of the increasing spread of cluster core radii with cluster age.

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arXiv2001

Dark Matter in Dwarf Spheroidals I: Models

This paper introduces a new two-parameter family of dwarf spheroidal (dSph) galaxy models. The density distribution has a Plummer profile and falls like the inverse fourth power of distance in projection, in agreement with the star-count data. The first free parameter controls the velocity anisotropy, the second controls the dark matter content. The dark matter distribution can be varied from one extreme of mass-follows-light through a near-isothermal halo with flat rotation curve to the other extreme of an extended dark halo with harmonic core. This family of models is explored analytically in some detail -- the distribution functions, the intrinsic moments and the projected moments are all calculated. For the nearby Galactic dSphs, samples of hundreds of discrete radial velocities are becoming available. A technique is developed to extract the anisotropy and dark matter content from such data sets by maximising the likelihood function of the sample of radial velocities. This is constructed from the distribution function and corrected for observational errors and the effects of binaries. Tests on simulated data sets show that samples of 1000 discrete radial velocities are ample to break the degeneracy between mass and anisotropy in the nearby dSphs. Interesting constraints can already be placed on the distribution of the dark matter with samples of 160 radial velocities (the size of the present-day data set for Draco).

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arXiv2001

Dark Matter in Dwarf Spheroidals II: Observations and Modelling of Draco

We present stellar radial velocity data for the Draco dwarf spheroidal galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our dataset consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude V=19.5 with a mean velocity precision of 2 km/s. We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo with gently rising rotation curve. There is a <2 sigma signature of rotation about the long axis, inconsistent with tidal disruption being the source of the rising dispersion. By comparing our dataset with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood. We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law. We are able to rule out both a mass-follows-light distribution and an extended halo with a harmonic core at the 2.5 to 3 sigma significance level, depending on the details of our assumptions about Draco's stellar binary population. Our modelling lends support to the idea that the dark matter in dwarf spheroidals is distributed in the form of massive, nearly isothermal haloes.

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arXiv2000

Dynamical Mass Estimates for the Halo of M31 from Keck Spectroscopy

The last few months have seen the measurements of the radial velocities of all of the dwarf spheroidal companions to the Andromeda galaxy (M31) using the spectrographs (HIRES and LRIS) on the Keck Telescope. This paper summarises the data on the radial velocities and distances for all the companion galaxies and presents new dynamical modelling to estimate the mass of extended halo of M31. The best fit values for the total mass of M31 are between 7 and 10 x 10^{11} solar masses, depending on the details of the modelling. The mass estimate is accompanied by considerable uncertainty caused by the smallness of the dataset; for example, the upper bound on the total mass is roughly 24 x 10^{11} solar masses, while the lower bound is about 3 x 10^{11} solar masses. These values are less than the most recent estimates of the most likely mass of the Milky Way halo. Bearing in mind all the uncertainties, a fair conclusion is that the M31 halo is roughly as massive as that of the Milky Way halo. There is no dynamical evidence for the widely held belief that M31 is more massive -- it may even be less massive.

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arXiv2000

The Mass of the Andromeda Galaxy

This paper argues that the Milky Way galaxy is probably the largest member of the Local Group. The evidence comes from estimates of the total mass of the Andromeda galaxy (M31) derived from the three dimensional positions and radial velocities of its satellite galaxies, as well as the projected positions and radial velocities of its distant globular clusters and planetary nebulae. The available dataset comprises ten satellite galaxies, seventeen distant globular clusters and nine halo planetary nebulae with radial velocities. We find the halo of Andromeda has a mass of about 12 x 10^11 solar masses, together with a scalelength of roughly 90 kpc and a predominantly isotropic velocity distribution. For comparison, our earlier estimate for the Milky Way halo is 19 x 10^11 solar masses. Though the error bars are admittedly large, this suggests that the total mass of M31 is probably less than that of the Milky Way. We verify the robustness of our results to changes in the modelling assumptions and to errors caused by the small size and incompleteness of the dataset. Future prospects using radial velocity surveys of globular clusters and planetary nebulae, as well as microarcsecond astrometry from the upcoming satellite missions, are discussed.

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arXiv1999

The Present and Future Mass of the Milky Way Halo

A simple model for the Milky Way halo is presented. It has a flat rotation curve in the inner regions, but the density falls off sharply beyond an outer edge. This truncated, flat rotation curve (TF) model possesses a rich family of simple distribution functions which vary in velocity anisotropy. The model is used to estimate the total mass of the Milky Way halo using the latest data on the motions of satellite galaxies and globular clusters at Galactocentric radii greater than 20 kpc. This comprises a dataset of 27 objects with known distances and radial velocities, of which 6 also possess measured proper motions. Unlike earlier investigations, we find entirely consistent maximum likelihood solutions unaffected by the presence or absence of Leo I, provided both radial and proper motion data are used. The availability of the proper motion data for the satellites is crucial as, without them, the mass estimates with and without Leo I are inconsistent at the 99 % confidence level. All these results are derived from models in which the velocity normalisation of the halo potential is taken as 220 km/s. A detailed analysis of the uncertainties in our estimate is presented, including the effects of the small dataset, possible incompleteness or correlations in the satellite galaxy sample and the measurement errors. The most serious uncertainties come from the size of the dataset, which may cause a systematic underestimate by a factor of two, and the measurement errors, which cause a scatter in the mass of the order of a factor of two. We conclude that the total mass of the halo is 1.9 x 10^12 solar masses, while the mass within 50 kpc is 5.4 x 10^11 solar masses. Future prospects using radial velocity surveys of blue horizontal branch stars and microarcsecond astrometry from the upcoming satellite missions are discussed.

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