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

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.

The OGLE View of Microlensing towards the Magellanic Clouds: Insights into Dark Matter Compact Halo Objects

The fourth installment of the Optical Gravitational Lensing Experiment (OGLE) microlensing series analyzes data collected from the Small Magellanic Cloud (SMC) as part of the OGLE-III campaign. The paper primarily focuses on investigating the presence and characteristics of Massive Compact Halo Objects (MACHOs) in the dark matter halo. The analysis yielded three convincing microlensing event candidates, leading to an optical depth of $\tau_{\rm SMC-OIII}=1.30\pm1.01~\times~10^{-7}$, a value consistent with expected contributions from the Galactic disk and SMC self-lensing.

The research scrutinizes event OGLE-SMC-03 closely, highlighting it as the first microlensing event attributed to a thick disk lens towards the SMC, suggesting that at least some of the microlensing events could originate from known stellar populations rather than hypothesized MACHOs. Detailed analysis combined data from both the Large and Small Magellanic Cloud events, strengthening the claim that most OGLE-detected events may be explained by self-lensing or disk lenses.

The apparent lack of substantial evidence for dark matter compact objects, particularly MACHOs, challenges prior expectations that such entities contribute significantly to the Galaxy's halo mass. This is consistent with the upper limit derived, which stipulates that similar lenses likely contribute only about 2% of the halo's total mass. This aligns within the expected number of black hole remnants, reinforcing theoretical frameworks delineating the halo's composition.

The implications of these findings extend beyond the immediate astronomical community. Practically, the reduced necessity for MACHOs as a dark matter component suggests a paradigm shift towards exploring alternative dark matter constituents, such as Weakly Interacting Massive Particles (WIMPs) or sterile neutrinos. Theoretically, these findings underscore the importance of recalibrating models of galactic halo dynamics, accounting for varying contributions from non-MACHO objects.

Future developments in gravitational lensing studies are anticipated as surveys like MOA and OGLE-IV enhance observation capabilities. These projects may yield better statistical data to discern rare lensing events, possibly elucidating the very nature of black holes or other compact objects in dark halos, thereby refining constraints on dark matter theories. This study stands as a testament to the evolving understanding of our galaxy's mass structure, fostering a deeper engagement with the intricacies of cosmic matter.