The Andromeda Galaxy (M31) is our nearest giant galactic neighbour, at a distance of around 780 kpc, and is therefore one of the most well studied of all galaxies. Our birds-eye view of this galaxy means that many studies, such as classical novae surveys, are easier to carry out towards M31 than within our own Galaxy.
Despite its proximity, ground-based telescopes can resolve only the very brightest of its stars. The remainder cannot be seen individually but contribute to the smooth surface brightness distribution. This surface brightness increases rapidly towards the centre of M31 where the prominent bulge stellar population resides. The bulge is believed to have a triaxial geometry, as evidence by CO emission line studies and by the misalignment between the projected disk and bulge principal axes.
The aim of the Angstrom Project is to probe the geometry of the underlying mass distribution of the M31 bulge by looking for gravitational microlensing signatures from low mass stars. Low mass stars are the most common constituents of the stellar populations in the Milky Way and therefore should provide faithful tracers of the mass distribution in M31. We also aim to characterise the mass function of low mass stars in the M31 bulge. Whilst low-mass stars in M31 are far too faint to identify via direct detection methods, microlensing is capable of detecting them because their gravitational field may induce detectable transient distortions in the brightness of brighter background source stars. The duration of each distortion provides a statistical indicator of the mass of the interveining lensing star, whilst the frequency of microlensing is sensitive to the overall mass of the bulge. If more of the bulge mass comprises low mass stars then this provides an additional boost to the microlensing rate because there are more potential lensing objects per unit bulge mass.
The main difficulty in detecting microlensing events in M31 is that the source stars are usually unresolved. Microlensing of unresolved source stars is commonly referred to as pixel microlensing. Special difference imaging techniques are required to allow such events to be detected reliably. What one effectively observes is the peak of the underlying transient flux variation and this typically lasts only a few days for bulge microlensing involving low-mass stars. Because of this the bulge must be observed many times per night and this necessitates the use of several telescopes at widely separated longitude to allow 24-hour surveillance. This high cadence strategy should also allow exotic microlensing phenomena to be detected with greater efficiency, and will provide a new window on other transient sources or variable stars which may vary on very short timescales.
In short, the aims of the Angstrom Project are:
- to detect short-duration pixel microlensing events in the bulge of M31
- to use the event spatial distribution on the sky to map the geometry of the underlying bulge mass distribution
- to use the observed rate and the distribution of event timescales to measure the contribution of low-mass stars and sub-stellar objects to the bulge stellar population. Microlensing is the only technique capable of detecting them at such large distance from us.
- to detect exotic microlensing phenomena, such as finite-source events or binary lensing events, possibly including Super-Jupiter planetary systems within M31.
- to compile a unique catalogue of short-timescale (less than a few days) transients and variable stars within the bulge of M31.