The MACHO Project: Microlensing Optical Depth toward the Galactic Bulge from Difference Image Analysis

By Charles R. Alcock, R. Allsman, D. Alves, T. Axelrod, A. Becker, D. Bennett, K. Cook, A. J. Drake, Kenneth C. Freeman, M. Geha and 14 more. 2000

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We present the microlensing optical depth toward the Galactic bulge based on the detection of 99 events found in our Difference Image Analysis (DIA) survey. This analysis encompasses 3 yr of data, covering ~17 million stars in ~4 deg2, to a source-star baseline magnitude limit of V = 23. The DIA technique improves the quality of photometry in crowded fields, and allows us to detect more microlensing events with faint source stars. We find that this method increases the number of detection events by 85% compared with the standard analysis technique. DIA light curves of the events are presented, and the microlensing fit parameters are given. The total microlensing optical depth is estimated to be τtotal = 2.43 × 10-6, averaged over eight fields centered at l = 2.°68 and b = -3.°35. For the bulge component, we find τbulge = 3.23 × 10-6, assuming a 25% stellar contribution from disk sources. These optical depths are in good agreement with the past determinations of the MACHO and OGLE groups, and are higher than predicted by contemporary Galactic models. We show that our observed event timescale distribution is consistent with the distribution expected from normal mass stars, if we adopt the Scalo stellar mass function as our lens mass function. However, we note that since there is still disagreement about the exact form of the stellar mass function, there is uncertainty in this conclusion. Based on our event timescale distribution, we find no evidence for the existence of a large population of brown dwarfs in the direction of the Galactic bulge.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

• Quantitative Interpretation of Quasar Microlensing Light Curves (2003, background): “Nevertheless, these mass estimates are consistent with previous results for this system (e.g. Lewis & Irwin 1996, Wyithe et al. 2000b) and Galactic microlensing studies (e.g. Alcock et al. 2000).”
• Spiral arms, bar shape and bulge microlensing in the Milky Way (2001, background): “They give an optical depth for all sources, based on 99 events from 3 years of data, of t26 ¼ 2:43þ0:3920:38 averaged over eight fields centred at l ¼ 28:68, b ¼ 238:35 (Alcock et al. 2000a).”
• Revisiting the cosmic cooling crisis (2001, background): “Earlier microlensing results which suggested that there might be a large population of dark objects in the Galaxy halo are no longer compelling, following a reanalysis of the MACHO project data (Alcock et al. 2000).”
• Variable stars in the Magellanic Clouds: results from OGLE and SIRIUS (2003, background): “Thanks to the gravitational microlensing search projects (OGLE, e.g., Udalski, Kubiak & Szymański 1997; MACHO, e.g., Alcock et al. 2000; EROS, e.g., Afonso et al. 1999; MOA, e.g., Bond et al. 2001), a large number of variable stars have been found in the Magellanic Clouds.”
• Insights on the Milky Way bulge formation from the correlations between kinematics and metallicity (2010, background): “…distribution (e.g. Blitz & Spergel 1991), photometric data (e.g. Stanek et al. 1994; Babusiaux & Gilmore 2005; Benjamin et al. 2005), OH/IR and SiO-maser kinematics (e.g. Habing et al. 2006), stellar kinematics (e.g. Rangwala et al. 2009) and microlensing surveys (e.g. Alcock et al. 2000).”
• Microlensing Optical Depth toward the Galactic Bulge from Microlensing Observations in Astrophysics Group Observations during 2000 with Difference Image Analysis (2002, methodology): “From 2000 we introduced the Difference Image Analysis (DIA) (Crotts 1992; Phillips & Davis 1995; Alard & Lupton 1998; Alard 2000; Alcock et al. 1999, 2000; Woźniak 2000; Bond 1 PrincetonUniversity Observatory, Princeton, NJ 08544-1001.”
• Stellar Contribution to the Galactic Bulge Microlensing Optical Depth (2003, methodology): “On the one hand, MACHO made several new estimates of the bulge optical depth, one based on a differenceimage analysis of a large number of primarily faint sources, yielding ¼ 3:2 0:5 10 6 (Alcock et al. 2000), and another based on clump giants, yielding ¼ 2:0 0:4 10 6 (Popowski et al. 2001).”
• A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192 (2008, background): “All six of the planets detected by microlensing of main sequence source stars were detected in events with durations more than 2.5 times longer than the measured median event timescale of tE = 16 days (Alcock et al. 2000; Sumi et al. 2003, 2006).”
• Real-time difference imaging analysis of moa galactic bulge observations during 2000 (2001, background): “Alcock et al. (2000) proposed classifying microlensing events found by DIA as either classical or ‘pixel lensing’ events following the definition by Gould (1996) of pixel lensing events as microlensing of unresolved sources.”
• Simulation of a Space-based Microlensing Survey for Terrestrial Extrasolar Planets (2000, background): “Only about3×10−6 of Galactic bulge stars are microlensed at any given time (Udalski et al. 1994; Alcock et al. 1997; Alcock et al. 2000b), and only∼ 2% of earth-mass planets orbiting these stars will be in the right position to be detected (Bennett & Rhie 1996).”
• Order and Chaos in the Local Disc Stellar Kinematics Induced by the Galactic Bar (2001, background): “…(Binney etal.1991;Englm aier& G erhard 1999;Fux 1999;W einer & Sellwood 1999),large m icrolensing opticaldepths towardsthe G alactic bulge (Paczynskietal.1994;K uijken 1997;G yuk 1999;Alcock et al.2000) and possibly inner stellarkinem atics(Sevensteretal.1999;seealso G erhard 1999forarecentreview).”
• Vvv dr1: the first data release of the milky way bulge and southern plane from the near-infrared eso public survey vista variables in the via lactea (2011, methodology): “In addition, we complement the existing bulge microlensing surveys such as MACHO (Alcock et al. 2000) and OGLE (Udalski et al. 1993), which observe in optical bands, with limited or no colour information.”