Geza Gyuk

Self-lensing Models of the Large Magellanic Cloud

All of the proposed explanations for the microlensing events observed toward the LMC have diffi- culties. One of these proposed explanations, LMC self-lensing, which invokes ordinary LMC stars as the long sought-after lenses, has recently gained considerable popularity as a possible solution to the micro- lensing conundrum. In this paper, we carefully examine the full range of LMC self-lensing models, including for the —rst time the contribution of the LMC bar in both sources and lenses. In particular, we review the pertinent observations made of the LMC and show how these observations place limits on such self-lensing models. We —nd that, given current observational constraints, no purely LMC disk models are capable of producing optical depths as large as that reported in the MACHO collaboration 2 year analysis. We also introduce a new quantitative measure of the central concentration of the micro- lensing events and show that it discriminates well between disk/bar self-lensing and halo microlensing. Besides pure disk/bar, we also consider alternative geometries and present a framework which encom- passes the previous studies of LMC self-lensing. We discuss which model parameters need to be pushed in order for such models to succeed. For example, like previous workers, we —nd that an LMC halo geometry may be able to explain the observed events. However, since all known LMC tracer stellar populations exhibit disklike kinematics, such models will have difficulty being reconciled with obser- vations. For SMC self-lensing, we —nd predicted optical depths diUering from previous results, but more than sufficient to explain all observed SMC microlensing. In contrast, for the LMC we —nd a self-lensing optical depth contribution between 0.47 ) 10~8 and 7.84 ) 10~8, with 2.44 ) 10~8 being the value for the set of LMC parameters most consistent with current observations. Subject headings: dark mattergalaxies: halosgalaxies: kinematics and dynamics ¨ gravitational lensingMagellanic CloudsAll of the proposed explanations for the microlensing events observed towards the LMC have difficulties. One of these proposed explanations, LMC self-lensing, which invokes ordinary LMC stars as the long sought-after lenses, has recently gained considerable popularity as a possible solution to the microlensing conundrum. In this paper, we carefully examine the set of LMC self-lensing models. In particular, we review the pertinent observations made of the LMC, and show how these observations place limits on such self-lensing models. We find that, given current observational constraints, no purely LMC disk models are capable of producing optical depths as large as that reported in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider alternate geometries, and present a framework which encompasses the previous studies of LMC self-lensing. We discuss which model parameters need to be pushed in order for such models to succeed. For example, like previous workers, we find that an LMC halo geometry may be able to explain the observed events. However, since all known LMC tracer stellar populations exhibit disk-like kinematics, such models will have difficulty being reconciled with observations. For SMC self-lensing, we find predicted optical depths differing from previous results, but more than sufficient to explain all observed SMC microlensing. In contrast, for the LMC we find a self-lensing optical depth contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the set of LMC parameters most consistent with current observations.

A New (Old) Component of the Galaxy as the Origin of the Observed Large Magellanic Cloud Microlensing Events

We suggest a new component of the Milky Way galaxy that can account for both the optical depth and the event durations obtained by the MACHO microlensing survey toward the Large Magellanic Cloud. This component is consistent with recent evidence for a significant population of faint white dwarf stars, detected in a proper-motion study of the Hubble Deep Field, which cannot be accounted for by stars in the disk or spheroid. This new component, which represents less than 4% of the total dark halo mass, consists of (mostly) old white dwarf stars distributed in a highly extended (very thick) disk configuration. It extends beyond the traditional thin and thick disks, but well within the dark, roughly spherical cold dark matter (CDM) halo. We explore the distribution of proper motions expected for white dwarfs in this model and the ability of future surveys to differentiate between models. Finally, we argue that such a component is reasonable, natural, and consistent with a variety of observations, and many of the problems associated with a significant halo population of white dwarfs are ameliorated.

Optical Depth from Realistic Microlensing Models of M31

We provide a set of microlensing optical depth maps for M31. Optical depths toward Andromeda were calculated on the basis of a four-component model of the lens and source populations: disk and bulge sources lensed by bulge, M31 halo, and Galactic halo lenses. We confirm the high optical depth and the strong optical depth gradient along the M31 minor axis due to a dark halo of lenses and also discuss the magnitude of the self-lensing due to the bulge. We explore how the shape of the optical depth maps to M31 vary with the halo parameters core radius and flattening.

Star Counts and the Warped and Flaring Milky Way Disk

This paper reports the star count predictions of warped and flaring models of the outer Milky Way disk. These have been suggested as possible locations of the lenses responsible for the microlensing events toward the Large Magellanic Cloud (LMC). Three such models are investigated in detail, and the theoretical predictions are confronted with Hubble Space Telescope star count data in seven low-latitude fields (30° < |b| < 40°). If the warped and flaring disk population has the same characteristics as disk stars in the solar neighborhood, then the models can be unambiguously ruled out. Metallicity gradients are well known in disk galaxies and may cause the outer disk population to differ in colors and luminosity from the local population. This effect is studied using a simple Ansatz for the variation in the color-magnitude relation with position, and while it is shown to lead to better agreement with the star counts, upper limits on the contribution of the warped and flaring disk to the optical depth are still below the value measured toward the LMC. Only if the warp is very asymmetric or if the luminosity function changes strongly with Galactocentric radius can the models be made consistent with the star counts.

First microlensing candidates from the MEGA survey of M31

We present the first M31 candidate microlensing events from the Microlensing Exploration of the Galaxy and Andromeda (MEGA) survey. MEGA uses several telescopes to detect microlensing towards the nearby Andromeda galaxy, M31, in order to establish whether massive compact objects are a significant contribution to the mass budget of the dark halo of M31. The results presented here are based on observations with the Isaac Newton Telescope on La Palma, during the 1999/00 and 2000/01 observing seasons. In this data set, 14 variable sources consistent with microlensing have been detected, 12 of which are new and 2 have been reported previously by the POINT-AGAPE group. A preliminary analysis of the spatial and timescale distributions of the candidate events support their microlensing nature. We compare the spatial distributions of the candidate events and of long-period variable stars, assuming the chances of finding a long-period variable and a microlensing event are comparable. The spatial distribution of our candidate microlensing events is more far/near side asymmetric than expected from the detected long-period variable distribution. The current analysis is preliminary and the asymmetry not highly significant, but the spatial distribution of candidate microlenses is suggestive of the presence of a microlensing halo.