An Inventory of Gravitational Microlenses Toward the Galactic Bulge |
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| Authors: | Cheongho Han & Kyongae Chang |
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| Institution: | Department of Astronomy and Space Science, Chungbuk National University, Chongju, Korea 361-763,;Department of Physics, Cheongju University, Chongju, Korea 360-764 |
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| Abstract: | Although microlensing experiments toward the Galactic bulge were originally initiated to check the feasibility of the experiments, they have now become an important tool which allows one to investigate Galactic matter composition. However, previous determination of the lens mass function was not based on the actual number of lenses even for the well-determined population of stellar lenses, but rather on arbitrarily assumed functional forms such as power laws, and thus the derived mass functions were subject to large uncertainties. In this paper, we take a different approach in which we first estimate the event rate distribution expected from observationally well-constrained populations of lenses, and then test other possible lens populations. By comparing the determined event rate distribution Γ( t E) for various mass function models of lens populations with the observed distribution, we find that stars and white dwarfs explain just ～ 50 per cent of the total observed events even including very faint stars just above the hydrogen-burning limit. Additionally, the expected time-scale distribution of events caused by these known populations of lenses deviates significantly from the observed distribution, especially in the short time-scale region. However, if the rest of the dynamical mass of the bulge (～ 2.1 × 1010 M) is composed of brown dwarfs, the expected event rate distribution matches the observation well. |
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| Keywords: | stars: low-mass brown dwarfs white dwarfs Galaxy: general dark matter gravitational lensing |
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