Dynamical modelling of dwarf spheroidal galaxies using Gaussian-process emulation

We present a novel and efficient method for fitting dynamical models of stellar kinematic data for dwarf spheroidal galaxies (dSph). Our approach is based on Gaussian-process emulation (GPE), which is a sophisticated form of curve fitting that requires fewer training data than alternative methods. We use a set of validation tests and diagnostic criteria to assess the performance of the emulation procedure. We have implemented an algorithm in which both the GPE procedure and its validation are fully automated. Applying this method to synthetic data, with fewer than 100 model evaluations we are able to recover a robust confidence region for the three-dimensional parameter vector of a toy model of the phase-space distribution function of a dSph. Although the dynamical model presented in this paper is low-dimensional and static, we emphasize that the algorithm is applicable to any scheme that involves the evaluation of computationally expensive models. It therefore has the potential to render tractable previously intractable problems, for example, the modelling of individual dSphs using high-dimensional, time-dependent N-body simulations

Dark matter in disc galaxies - II. Density profiles as constraints on feedback scenarios

The disparity between the density profiles of galactic dark matter haloes predicted by dark matter only cosmological simulations and those inferred from rotation curve decomposition, the so-called cusp–core problem, suggests that baryonic physics has an impact on dark matter density in the central regions of galaxies. Using a Markov Chain Monte Carlo analysis of galactic rotation curves we constrain density profiles and an estimated minimum radius for baryon influence, r[subscript: 1], which we couple with a feedback model to give an estimate of the fraction of matter within that radius that must be expelled to produce the observed halo profile. We examine the rotation curves of eight galaxies taken from the THINGS (The HI Nearby Galaxy Survey) data set and determine constraints on the radial density profiles of their dark matter haloes. For some of the galaxies, both cored haloes and cosmological ρ ∝ r[superscript: −1] cusps are excluded which requires finely tuned baryonic feedback. For galaxies which exhibit extended cores in their haloes (e.g. NGC 925), the use of a split power-law halo profile yields models without the unphysical, sharp features seen in models based on the Einasto profile. We have found there is no universal halo profile which can describe all the galaxies studied here

Star clusters as building blocks for dSph galaxy formation

We study numerically the formation of dSph galaxies. Intense starbursts, e.g., in gas-rich environments, typically produce a few to a few hundred young star clusters within a region of just a few hundred pc. The dynamical evolution of these star clusters may explain the formation of the luminous component of dwarf spheroidal (dSph) galaxies. Here, we perform a numerical experiment to show that the evolution of star cluster complexes in dark-matter haloes can explain the formation of the luminous components of dSph galaxies. © International Astronomical Union 2010

Too small to succeed: The difficulty of sustaining star formation in low-mass haloes

Published by Oxford University Press on behalf of the Royal Astronomical Society.We present high-resolution simulations of an isolated dwarf spheroidal (dSph) galaxy between redshifts z ~ 10 and z ~ 4, the epoch when several Milky Way dSph satellites experienced extended star formation, in order to understand in detail the physical processes which affect a low-mass halo's ability to retain gas. It is well established that supernova feedback is very effective at expelling gas from a 3 × 107 M· halo, the mass of a typical redshift 10 progenitor of a redshift 0 halo with mass ~109 M·. We investigate the conditions under which such a halo is able to retain sufficient high-density gas to support extended star formation. In particular, we explore the effects of: an increased relative concentration of the gas compared to the dark matter; a higher concentration dark matter halo; significantly lower supernova rates; enhanced metal cooling due to enrichment from earlier supernovae. We show that disc-like gas distributions retain more gas than spherical ones, primarily due to the shorter gas cooling times in the disc. However, a significant reduction in the number of supernovae compared to that expected for a standard initial mass function is still needed to allow the retention of highdensity gas. We conclude that the progenitors of the observed dSphs would only have retained the gas required to sustain star formation if their mass, concentration and gas morphology were already unusual for those of a dSph-mass halo progenitor by a redshift of 10

A novel JEANS analysis of the Fornax dwarf using evolutionary algorithms: mass follows light with signs of an off-centre merger

Dwarf galaxies, among the most dark matter dominated structures of our Universe, are excellent test-beds for dark matter theories. Unfortunately, mass modelling of these systems suffers from the well-documented mass-velocity anisotropy degeneracy. For the case of spherically symmetric systems, we describe a method for non-parametric modelling of the radial and tangential velocity moments. The method is a numerical velocity anisotropy "inversion", with parametric mass models, where the radial velocity dispersion profile, σ 2 π , is modelled as a B-spline, and the optimization is a three-step process that consists of (i) an evolutionary modelling to determine the mass model form and the best B-spline basis to represent σ 2 π ; (ii) an optimization of the smoothing parameters and (iii) a Markov chain Monte Carlo analysis to determine the physical parameters. The mass-anisotropy degeneracy is reduced into mass model inference, irrespective of kinematics. We test our method using synthetic data. Our algorithm constructs the best kinematic profile and discriminates between competing dark matter models. We apply our method to the Fornax dwarf spheroidal galaxy. Using a King brightness profile and testing various dark matter mass models, our model inference favours a simple mass-follows-light system. We find that the anisotropy profile of Fornax is tangential (β(r) < 0) and we estimate a total mass of M tot = 1.613 +0.050 -0.075 × 10 8 M ⊙ , and a mass-to-light ratio of υ V = 8.93 +0.32 -0.47 (M ⊙ /L ⊙ ). The algorithm we present is a robust and computationally inexpensive method for non-parametric modelling of spherical clusters independent of the mass-anisotropy degeneracy