Constraints on Intervening Stellar Populations Toward the Large Magellanic Cloud

Abstract

The suggestion by Zaritsky & Lin that a vertical extension of the red clump feature in color-magnitude diagrams of the Large Magellanic Cloud (LMC) is consistent with a significant population of foreground stars to the LMC that could account for the observed microlensing optical depth has been challenged by various investigators. We respond by (1) examining each of the challenges presented and (2) presenting new photometric and spectroscopic data. We conclude that although the CMD data do not mandate the existence of a foreground population, they are entirely consistent with a foreground population associated with the LMC that contributes significantly (~ 50%) to the observed microlensing optical depth. From our new data, we conclude that <~ 40% of the VRC stars are young, massive red clump stars because (1) synthetic color-magnitude diagrams created using the star formation history derived indepdently from HST data suggest that < 50% of the VRC stars are young, massive red clump stars, (2) the angular distribution of the VRC stars is more uniform than that of the young (age < 1 Gyr) main sequence stars, and (3) the velocity dispersion of the VRC stars in the region of the LMC examined by ZL is inconsistent with the expectation for a young disk population. Each of these arguments is predicated on assumptions and the conclusions are uncertain. Therefore, an exact determination of the contribution to the microlensing optical depth by the various hypothesized foreground populations, and the subsequent conclusions regarding the existence of halo MACHOs, requires a detailed knowledge of many complex astrophysical issues, such as the IMF, star formation history, and post-main sequence stellar evolution. (abridged)

Scheduled for publication in AJ in May 1999

References in corpus (15)

• A Prescription for Building the Milky Way's Halo from Disrupted Satellites
• The disruption of nearby galaxies by the Milky Way
• A Digital Photometric Survey of the Magellanic Clouds: First Results From One Million Stars
• Stellar Populations in Three Outer Fields of the LMC
• The survival of the Sgr dwarf galaxy and the flatness of the rotation curve of the Galaxy
• Microlensing of tidal debris on the Magellanic great circle
• Evidence for an Intervening Stellar Population Toward the Large Magellanic Cloud
• Observational discovery of the AGB-bump in densely populated color-magnitude diagrams of galaxies and star clusters
• The LMC Microlensing Events: Evidence for a Warped and Flaring Milky Way Disk?
• On the Distribution of Dust in the Large Magellanic Cloud
• Is the LMC Microlensing Due to an Intervening Dwarf Galaxy?
• Red Clump Morphology as Evidence Against a New Intervening Stellar Population as the Primary Source of Microlensing Toward the LMC
• Magellanic Cloud Gravitational Microlensing Results: What Do They Mean?
• Re-examination of the possible tidal stream in front of the LMC
• Red Clump Stars as a Tracer of Microlensing Optical Depth

Cited by in corpus (11)

• Non-Baryonic Dark Matter - Observational Evidence and Detection Methods
• Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of the LMC
• Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and the Intrinsic Elongation of the LMC
• Binary Microlensing Events from the MACHO Project
• The Rotation Curve of the Large Magellanic Cloud and the Implications for Microlensing
• Self-Lensing Models of the LMC
• Microlensing towards the Large Magellanic Cloud
• A Radial Velocity Survey for LMC Microlensed Sources
• A New Argument Against An Intervening Stellar Population Toward the LMC
• Extinction bias of microlensed stars towards the LMC and the fraction of machos in the halo
• Probing the mass function of halo dark matter via microlensing