Microlensing results toward the Large Magellanic Cloud and implications for galactic dark matter

Details Microlensing results toward the Large Magellanic Cloud and implications for galactic dark matter Microlensing results toward the Large Magellanic Cloud and implications for galactic dark matter

Nov 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........6v&link_type=abstract

Thesis (PhD). UNIVERSITY OF CALIFORNIA, SAN DIEGO, Source DAI-B 61/05, p. 2579, Nov 2000, 189 pages.

Computer Science

2

Scientific paper

Dark matter in the form of massive compact halo objects (MACHOs) may make a non-trivial contribution to the total mass of the Galaxy. The presence of MACHOs in the halo of the Galaxy can be probed for with the technique of microlensing. Microlensing is characterized by the transient, achromatic brightening of a background star due to gravitational deflection of its light by a massive ``lens'' (the MACHO) passing near our line of sight to the source. By monitoring the luminosity of millions of stars nightly it is possible to observe these chance alignments and to estimate the MACHO contribution to the Galaxy. Data are presented on a search for microlensing towards the Large Magellanic Cloud (LMC). Analysis of 5.7-years of photometry on 11.9 million stars in the LMC reveals 13-17 microlensing events. A detailed treatment of the survey's detection efficiency shows that this is significantly more than the ~2 to 4 events expected from lensing by known stellar populations. The timescales (t̂) of the events range from 34 to 230 days. The microlensing optical depth towards the LMC is estimated for events with 2 < t̂ < 400 days to be t4002=1.2+0.4- 0. 3×10-7 , with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8% to 50%. A 100% MACHO halo is ruled out at the 95% C.L. for all halo models except the most extreme. The most likely MACHO mass is between 0.15 Msolar and 0.9 Msolar ), depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9+4- 3×1010 Msolar , independent of the halo model. A detailed treatment of potential backgrounds to microlensing is given and a new source of background, supernovae in galaxies behind the LMC, is discussed.

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