Professor Justin Read

We present a novel positive potential-density pair expansion for modelling galaxies, based on the Miyamoto?Nagai disc. By using three sets of such discs, each one of them aligned along each symmetry axis, we are able to reconstruct a broad range of potentials that correspond to density profiles from exponential discs to 3D power-law models with varying triaxiality (henceforth simply ?twisted? models). We increase the efficiency of our expansion by allowing the scalelength parameter of each disc to be negative. We show that, for suitable priors on the scalelength and scaleheight parameters, these ?MNn discs? (Miyamoto?Nagai negative) have just one negative density minimum. This allows us to ensure global positivity by demanding that the total density at the global minimum is positive. We find that at better than 10 per cent accuracy in our density reconstruction, we can represent a radial and vertical exponential disc over 0.1?10 scalelengths/scaleheights with four MNn discs; a Navarro, Frenk and White (NFW) profile over 0.1?10 scalelengths with four MNn discs; and a twisted triaxial NFW profile with three MNn discs per symmetry axis. Our expansion is efficient, fully analytic, and well suited to reproducing the density distribution and gravitational potential of galaxies from discs to ellipsoids.

ACO 1703 is a cluster recently found to have a variety of strongly lensed objects: there is a quintuply imaged system at z = 0.888 and several other lensed objects from z = 2.2 to 3.0 (the cluster itself is at z = 0.28). It is not difficult to model the lens, as previous work has already done. However, lens models are generically nonunique. We generate ensembles of models to explore the nonuniqueness. When the full range of source redshifts is included, all models are close to Á r -1 out to 200 kpc. But if the quint is omitted, both shallower and steeper models (e.g., Á r -2) are possible. The reason is that the redshift contrast between the quint and the other sources gives a good measurement of the enclosed mass at two different radii, thus providing a good estimate of the mass profile in between. This result supports universal profiles and explains why single-model approaches can give conflicting results. The mass map itself is elongated in the northwest-southeast direction, like the galaxy distribution. An overdensity in both mass and light is also apparent to the southeast, which suggests mesostructure. © 2009. The American Astronomical Society. All rights reserved.

We describe the astrophysical and numerical basis of N-body simulations, both of collisional stellar systems (dense star clusters and galactic centres) and collisionless stellar dynamics (galaxies and large-scale structure). We explain and discuss the state-of-the-art algorithms used for these quite different regimes, attempt to give a fair critique, and point out possible directions of future improvement and development. We briefly touch upon the history of N-body simulations and their most important results. © Società Italiana di Fisica / Springer-Verlag 2011.

Due to their large dynamical mass-to-light ratios, dwarf spheroidal galaxies (dSphs) are promising targets for the indirect detection of dark matter (DM) in ³-rays. We examine their detectability by present and future ³-ray observatories. The key innovative features of our analysis are as follows: (i) we take into account the angular size of the dSphs; while nearby objects have higher ³-ray flux, their larger angular extent can make them less attractive targets for background-dominated instruments; (ii) we derive DM profiles and the astrophysical J-factor (which parametrizes the expected ³-ray flux, independently of the choice of DM particle model) for the classical dSphs directly from photometric and kinematic data. We assume very little about the DM profile, modelling this as a smooth split-power-law distribution, with and without subclumps; (iii) we use a Markov chain Monte Carlo technique to marginalize over unknown parameters and determine the sensitivity of our derived J-factors to both model and measurement uncertainties; and (iv) we use simulated DM profiles to demonstrate that our J-factor determinations recover the correct solution within our quoted uncertainties. Our key findings are as follows: (i) subclumps in the dSphs do not usefully boost the signal; (ii) the sensitivity of atmospheric Cherenkov telescopes to dSphs within ~20kpc with cored haloes can be up to ~50 times worse than when estimated assuming them to be point-like. Even for the satellite-borne Fermi-Large Area Telescope (Fermi-LAT), the sensitivity is significantly degraded on the relevant angular scales for long exposures; hence, it is vital to consider the angular extent of the dSphs when selecting targets; (iii) no DM profile has been ruled out by current data, but using a prior on the inner DM cusp slope 0 d³priord 1 provides J-factor estimates accurate to a factor of a few if an appropriate angular scale is chosen; (iv) the J-factor is best constrained at a critical integration angle ±c= 2rh/d (where rh is the half-light radius and d is the distance from the dwarf) and we estimate the corresponding sensitivity of ³-ray observatories; (v) the 'classical' dSphs can be grouped into three categories: well constrained and promising (Ursa Minor, Sculptor and Draco), well constrained but less promising (Carina, Fornax and Leo I), and poorly constrained (Sextans and Leo II); and (vi) observations of classical dSphs with the Fermi-LAT integrated over the mission lifetime are more pro

We use high resolution simulations of isolated dwarf galaxies to study the physics
of dark matter cusp-core transformations at the edge of galaxy formation: M200 =
107 109M .We work at a resolution ( 4 pc minimum cell size; 250M per particle)
at which the impact from individual supernovae explosions can be resolved, becoming
insensitive to even large changes in our numerical `sub-grid' parameters. We nd that
our dwarf galaxies give a remarkable match to the stellar light pro le; star formation
history; metallicity distribution function; and star/gas kinematics of isolated dwarf
irregular galaxies. Our key result is that dark matter cores of size comparable to the
stellar half mass radius r1=2 always form if star formation proceeds for long enough.
Cores fully form in less than 4 Gyrs for the M200 = 108M and 14 Gyrs for the
109M dwarf. We provide a convenient two parameter `coreNFW' tting function
that captures this dark matter core growth as a function of star formation time and
the projected stellar half mass radius.
Our results have several implications: (i) we make a strong prediction that if
CDM is correct, then `pristine' dark matter cusps will be found either in systems that
have truncated star formation and/or at radii r > r1=2; (ii) complete core formation
lowers the projected velocity dispersion at r1=2 by a factor 2, which is su cient to
fully explain the `too big to fail problem'; and (iii) cored dwarfs will be much more
susceptible to tides, leading to a dramatic scouring of the subhalo mass function inside
galaxies and groups.

Coalescing massive black hole binaries, formed during galaxy mergers, are expected to be a primary source of low-frequency gravitational waves. Yet in isolated gas-free spherical stellar systems, the hardening of the binary stalls at parsec-scale separations owing to the inefficiency of relaxation-driven loss-cone refilling. Repopulation via collisionless orbit diffusion in triaxial systems is more efficient, but published simulation results are contradictory. While sustained hardening has been reported in simulations of galaxy mergers with N