The impact of spheroid stars for Macho microlensing surveys of the Andromeda Galaxy

The Andromeda Galaxy (M31) is an important test case for a number of microlensing surveys looking for massive compact halo objects (Machos). A long-standing theoretical prediction is that the high inclination of the M31 disk should induce an asymmetry in the spatial distribution of M31 Macho events, whilst the distribution of variable stars and microlensing events in the M31 disk should be symmetric. We examine the role of stars in the M31 visible spheroid as both lenses and sources to microlensing events. We compute microlensing event number density maps and estimate pixel-lensing rates and event durations for three-component models of M31 which are consistent with the observed rotation curve, surface brightness profile and dynamical mass estimates. Three extreme models are considered: a massive spheroid model; a massive disk model; and a massive Macho halo model. An important consequence of the spheroid is that, even if Machos are absent in M31, an asymmetric spatial signature is still generally expected from stellar lensing alone. The relative mass-to-light ratio of the spheroid and disk populations controls which of these signatures dominates the overall stellar spatial distribution. We find that the inclusion of the spheroid weakens the M31 Macho spatial asymmetry by about 20-30% over a disk-only asymmetry for the models considered. We also find for our models that Machos dominate over most of the far disk provided they contribute at least ~25% of the halo dark matter density. The presence of the spheroid also has beneficial consequences for M31 lensing surveys. The stellar asymmetry is likely to be important in distinguishing between a spheroidal Macho halo or a highly flattened halo or dark matter dominated disk, since spatial asymmetries of opposing signs are expected in these cases. (Abridged)Comment: 10 pages. Accepted for publication in MNRA

Social enterprise as a model for developing Aboriginal lands

AbstractCommon property (communal land) is often viewed negatively with some claiming that communal land ownership and the absence of private property rights more generally have been insurmountable barriers to Indigenous enterprise. This paper provides a brief overview of common property resources and explores how Aboriginal common property is being used by some Aboriginal groups to develop social enterprises that provide benefits to remote communities, the environment and wider Australia. It notes that while some conservation and philanthropic organisations recognise this and have begun to work with and invest in these enterprises, government support often remains risk averse

Microlensing Halo Models with Abundant Brown Dwarfs

All previous attempts to understand the microlensing results towards the Large Magellanic Cloud (LMC) have assumed homogeneous present day mass functions (PDMFs) for the lensing populations. Here, we present an investigation into the microlensing characteristics of haloes with spatially varying PDMFs and anisotropic velocity dispersion tensors. One attractive possibility -- suggested by baryonic dark cluster formation in pregalactic and protogalactic cooling flows -- is that the inner halo is dominated by stellar mass objects, whereas low mass brown dwarfs become more prevalent on moving outwards. The contribution to the microlensing rate must be dominated by dark remnants (of about 0.5 solar masses) to recover the observed timescales of the microlensing experiments. But, even though stellar remnants control the rate, they do not dominate the mass of the baryonic halo, and so the well-known enrichment and mass budget problems are much less severe. Using a simple ansatz for the spatial variation of the PDMF, models are constructed in which the contribution of brown dwarfs to the mass of the baryonic halo is 55 % and to the total halo is 30 %. An unusual property of the models is that they predict that the average timescale of events towards M31 is shorter than the average timescale towards the LMC. This is because the longer line of sight towards M31 probes more of the far halo where brown dwarfs are the most common constituent.Comment: 17 pages, 1 figure, in press at The Astrophysical Journal (Letters