M31: A Unique Laboratory for Halo Microlensing

PI正在继续对M31中恒星的引力微透镜现象进行调查。这补充了对大麦哲伦星云和银河系凸起方向的MACHOs（大质量致密光晕天体）的调查。PI已经为这项工作构建了主要硬件，编写了在高度拥挤的领域寻找变星的软件，并在1.8米的梵蒂冈先进技术望远镜和1.3米的MDM望远镜上有大量的望远镜时间。用M31来寻找macho特别有趣，因为它是唯一一个可以用这种方式成功探测暗物质的外部螺旋星系。它与银河系的不同之处在于，通过光晕的视线可能与光晕微透镜的高光学深度（或每颗恒星的概率）有关。这有望产生比银河系更强的光晕透镜信号。此外，这个信号在整个星系的表面是可变的，这可以用来进一步约束晕暗物质分布模型。光晕质量的这种定位很可能也会导致更好地估计源-透镜-观察者的几何形状，从而得到作为透镜的物体的组成质量

AST 98-02984 Arlin P. Crotts The PI is continuing a survey of gravitational microlensing of stars in M31. This complements the surveys for MACHOs (massive compact halo objects) in the direction of the Large Magellanic Cloud and the Galactic Bulge. The PI has already constructed the major hardware for this effort, has written the software to find variable stars in highly crowded fields, and has large blocks of telescope time at the 1.8 meter Vatican Advanced Technology Telescope and the MDM 1.3 meter telescope. Using M31 to search for MACHOs is particularly interesting because it is the only external spiral galaxy that can be successfully probed for dark matter in this way. It is different from the Galaxy in that sightlines through the halo have associated with them a possibly high optical depth (or probability per star) of halo microlensing. This promises to produce a stronger halo lensing signal that the Galactic case. Furthermore, this signal is variable across the face of the galaxy, and this can be used to further constrain halo dark matter distribution models. It is likely that this localization of the halo mass will also lead to better estimates of the source-lens-observer geometry, and hence the component masses of the objects acting as lenses ***