2 research outputs found
A Candidate M31/M32 Intergalactic Microlensing Event
We report the discovery of a microlensing candidate projected 2'54" from the
center of M32, on the side closest to M31. The blue color (R-I= 0.00 +/- 0.14)
of the source argues strongly that it lies in the disk of M31, while the
proximity of the line of sight to M32 implies that this galaxy is the most
likely host of the lens. If this interpretation is correct, it would confirm
previous arguments that M32 lies in front of M31. We estimate that of order one
such event or less should be present in the POINT-AGAPE data base. If more
events are discovered in this direction in a dedicated experiment, they could
be used to measure the mass function of M32 up to an unknown scale factor. By
combining microlensing observations of a binary-lens event with a measurement
of the M31-M32 relative proper motion using the astrometric satellites SIM or
GAIA, it will be possible to measure the physical separation of M31 and M32,
the last of the six phase-space coordinates needed to assign M32 an orbit.Comment: Submitted to ApJ Letters. 13 pages, 2 figure
Theory of pixel lensing towards M31 -- II. The velocity anisotropy and flattening of the MACHO distribution
The POINT-AGAPE collaboration is currently searching for massive compact halo
objects (MACHOs) towards 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 increased further by about 30%
if the MACHOs occupy radial orbits rather than tangential orbits, but is
substantially reduced if the MACHOs lie in a flattened halo. However, even for
haloes with a minor-to-major axis ratio q = 0.3, the numbers of M31 MACHOs in
the far-side outnumber those in the near-side by a factor of ~2. We show that,
if 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 which probe a representative portion of the M31 disk, a
sample of around 50 candidates is likely to be sufficient to detect asymmetry
within spherical haloes, even if half the sample is contaminated, or to detect
asymmetry in haloes 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. (Abstract slightly abridged.)Comment: latex, including 6 figures. Accepted for publication in Ap