The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs

TL;DR

This paper presents OGLE-III microlensing results towards the SMC, finding limited evidence for MACHOs as dark matter, and concludes most events are caused by known stellar populations, with minimal contribution from dark matter compact objects.

Contribution

It provides the final OGLE-III microlensing analysis of the SMC, identifying a thick disk lens candidate and refining constraints on MACHOs as dark matter.

Findings

Three microlensing candidates found towards the SMC.

Optical depth consistent with known stellar populations.

Black hole lensing contributes about 2% to halo mass.

Abstract

In this fourth part of the series presenting the Optical Gravitational Lensing Experiment (OGLE) microlensing studies of the dark matter halo compact objects (MACHOs) we describe results of the OGLE-III monitoring of the Small Magellanic Cloud (SMC). Three sound candidates for microlensing events were found and yielded the optical depth tau_SMC-OIII=1.30+-1.01 10^{-7}, consistent with the expected contribution from Galactic disk and SMC self-lensing. We report that event OGLE-SMC-03 is the most likely a thick disk lens candidate, the first of such type found towards the SMC. In this paper we also combined all OGLE Large and Small Magellanic Cloud microlensing results in order to refine the conclusions on MACHOs. All but one of OGLE events are most likely caused by the lensing by known populations of stars, therefore we concluded that there is no need for introducing any special dark matter compact objects in order to explain the observed events rates. Potential black hole event indicates that similar lenses can contribute only about 2 per cent to the total mass of the halo, which is still in agreement with the expected number of such objects.