2003/07/15 by AGAPE Collaboration, E. Kerins, J. An +16
Physics and Astronomy · #Andromeda Galaxy #Anisotropy #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Asymmetry #Brown dwarf #Flattening #Galaxies: Formation, Evolution, Phenomena #Galaxy #Gravitational microlensing #Halo #Massive compact halo object #Milky Way #Optics #Physics #Stars #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1086/379060
published as Astrophys.J.598:993-999,2003 · latex, including 6 figures. Accepted for publication in ApJ
arxiv created 2003/07/15 · openalex publication_date 2003/12/01 · arxiv updated 2012/08/27 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/06
The POINT-AGAPE collaboration is currently searching for massive compact halo objects (MACHOs) toward the Andromeda galaxy (M31). The survey aims to exploit the high inclination of the M31 disk, which causes an asymmetry in the spatial distribution of M31 MACHOs. Here, we investigate the effects of halo velocity anisotropy and flattening on the asymmetry signal using simple halo models. For a spherically symmetric and isotropic halo, we find that the underlying pixel lensing rate in far-disk M31 MACHOs is more than 5 times the rate of near-disk events. We find that the asymmetry is further increased by about 30% if the MACHOs occupy radial orbits rather than tangential orbits, but it is substantially reduced if the MACHOs lie in a flattened halo. However, even for halos with a minor- to major-axis ratio of q = 0.3, the number of M31 MACHOs in the far side outnumber those in the near side by a factor of ~2. There is also a distance asymmetry, in that the events on the far side are typically farther from the major axis. We show that, if this positional information is exploited in addition to number counts, then the number of candidate events required to confirm asymmetry for a range of flattened and anisotropic halo models is achievable, even with significant contamination by variable stars and foreground microlensing events. For pixel lensing surveys that probe a representative portion of the M31 disk, a sample of around 50 candidates is likely to be sufficient to detect asymmetry within spherical halos, even if half the sample is contaminated, or to detect asymmetry in halos as flat as q = 0.3, provided less than a third of the sample comprises contaminants. We also argue that, provided its mass-to-light ratio is less than 100, the recently observed stellar stream around M31 is not problematic for the detection of asymmetry.