Project goal



Project goal







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Data analysis pipeline

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The main goal of the project is to characterize the dark matter component of galactic halos in form of MACHOs (Massive Astrophysical Compact Halo Objects) by using the microlensing effect [1]. The first surveys aimed at the detection of microlensing events have been carried out towards the Magellanic Clouds, so as to probe the MACHO content within the Galactic halo. The results obtained up to now are not conclusive. The MACHO collaboration [2] reported the detection of 1317 microlensing candidates concluding that a significant mass fraction (~20%) of the Galactic halo is in form of compact object of mass of ~0.5 M, while the EROS collaboration [3] have found no microlensing candidate events and accordingly have put a rather strict upper limit for this contribution (below 10% in the mass range 0.11 M). The nature and the location of the detected candidate microlensing events remain debated (e.g. [4]) and new observational campaigns are currently under way [5].

The contradictory results obtained towards the Magellanic Clouds challenge to probe the MACHO distribution along different line of sights. Beyond the Galaxy, M31 represents the next more suitable target for microlensing searches [6]. First, looking at it from outside, we can probe the full extent of the M31 halo, whereas this is not possible for the Galactic one. Furthermore, the line of sight towards M31 allows one to probe the Galactic halo along a different direction with respect to that towards the Magellanic Clouds. Finally, the inclination of the M31 disc is expected to give a clear signature in the spatial distribution for microlensing events due to lenses in the M31 halo. Because of the distance, the sources of microlensing events are not resolved object, a regime usually referred to as "pixel-lensing" [7].

Observational campaigns towards M31 have been succesfully carried out [8], but, also because of the low cross-section of the process, until now the detection of only a few candidate microlensing events along this line of sight has been reported [9]. A main difficulty in the analysis arises because of the degeneracy in the parameter space of the events, such that it is not straightforward to disentagle the relevant physical information (the lens mass, that is related to the event characteristic duration) from the other parameters such as distances and relative velocities of the lenses and magnitude of the sources. It is then difficult to draw conclusions on the physical issue of interest, the MACHO distribution, and indeed the few analyses getting to a tentative conclusion are in contradiction one with each other. The POINTAGAPE collaboration have reported the detection of 5 candidate microlensing events and claimed for an evidence of a MACHO contribution towards M31, with a lower limit on the halo mass fraction in form of MACHOs of about 20% in the mass range preferred by the Galactic searches (0.11 M) [10]. On the other hand, the MEGA collaboration, even if reporting the detection of a rather large number of candidate events, 14, have claimed that MACHOs are not needed to explain the detected signal and have put an upper limit on the halo fraction of about 30% in the same mass range [11].

The contradictory outputs of the POINT-AGAPE and the MEGA analyses put in evidence a further aspect of microlensing searches towards M31, namely the relevance played by the expected "self-lensing" signal, namely microlensing events for which both the lens and the source belong to the same M31 luminous population (either bulge or disc). A correct modeling of the luminous components of the probed galaxy is therefore essential to probe theMACHO distribution. The self-lensing signal can then be viewed as either a noisy background one must get rid of or a unique tool to investigate the luminous M31 component characteristics (mass function, luminosity function). Indeed, the ANGSTROM collaboration is carrying out a microlensing campaign right towards the M31 bulge, with a rather small field of view of about 44, with the specific aim of probing the self-lensing signal [12].

The scientific issue of the dark matter distribution in form of MACHOs, in particular as for the line of sight towards M31, is still an open question. Our observational programme enters this framework. Our aim is to study the microlensing signal towards M31: both the expected self-lensing signal and the possible MACHO lensing signal.



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E. Kerins et al., MNRAS 365, 1099 (2006)





















Editor of this page: Luigi Mancini  Last update: 07/01/2009