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Astron. Astrophys. 324, 843-856 (1997)

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AGAPE: a search for dark matter towards M 31 by microlensing effects on unresolved stars *

R. Ansari 1, M. Aurière 2, P. Baillon 3, A. Bouquet 4, 5, G. Coupinot 2, Ch. Coutures 6, C. Ghesquière 5, Y. Giraud-Héraud 5, P. Gondolo ** 4, 7, J. Hecquet 8, J. Kaplan 4, 5, Y. Le Du 5, A.L. Melchior *** 5, M. Moniez 1, J.P. Picat 8 and G. Soucail 8

1 Laboratoire de l'Accélérateur Linéaire, Université Paris-Sud, F-91405 Orsay, France
2 Observatoire Midi-Pyrénées, unité associée au CNRS (UMR 5572), F-62500 Bagnères de Bigorre, France
3 CERN, CH-1211 Genève 23, Switzerland
4 Laboratoire de Physique Théorique et Hautes Energies, Universités Paris 6 and Paris 7, unité associée au CNRS (URA 280), 2, Place Jussieu, F-75251 Paris Cedex 05, France
5 Laboratoire de Physique Corpusculaire, Collège de France, Laboratoire associé au CNRS-IN2P3 (URA 6441), 11 place Marcelin Berthelot, F-75231 Paris Cedex 05, France
6 SPP/DAPNIA, CEN Saclay, F-91191 Gif-sur-Yvette, France
7 Department of Physics, University of Oxford, 1 Keble Road, Oxford, OX1 3NP, United Kingdom
8 Observatoire Midi-Pyrénées, unité associée au CNRS (UMR 5572), 14 avenue Belin, F-31400 Toulouse, France

Received 13 June 1996 / Accepted 9 December 1996

Abstract

M 31 is a very tempting target for a microlensing search of compact objects in galactic haloes. It is the nearest large galaxy, it probably has its own dark halo, and its tilted position with respect to the line of sight provides an unmistakable signature of microlensing. However most stars of M 31 are not resolved and one has to use the "pixel method": monitor the pixels of the image rather than the stars. AGAPE is the implementation of this idea. Data have been collected and treated during two autumns of observation at the 2 metre telescope of Pic du Midi. The process of geometric and photometric alignment, which must be performed before constructing pixel light curves, is described. Seeing variations are minimised by working with large super-pixels ([FORMULA]) compared with the average seeing. A high level of stability of pixel fluxes, crucial to the approach, is reached. Fluctuations of super-pixels do not exceed 1.7 times the photon noise which is 0.1% of the intensity for the brightest ones. With such stable data, 10 microlensing events are expected for a full "standard halo". With a larger field, a regular and short time sampling and a long lever arm in time, the pixel method will be a very efficient tool to explore the halo of M 31.

Key words: Galaxy: halo – cosmology: observation – cosmology: dark matter – cosmology: gravitational lensing – galaxies: M 31 – methods: observational

* Based on data collected with the 2m Telescope Bernard Lyot (TBL) operated by INSU-CNRS and Pic du Midi Observatory (USR 5026). The experiment was funded by IN2P3 and INSU of CNRS
** Supported in part by the European Community (EC contract no. CHRX-CT93-0120)
*** Supported in part by Fondation Singer Polignac

Send offprint requests to: J. Kaplan, kaplan@cdf.in2p3.fr

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998

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