The MACHO Project: Microlensing Results from 5.7 Years of Large Magellanic Cloud Observations

By Charles R. Alcock, R. Allsman, D. Alves, T. Axelrod, A. Becker, D. Bennett, K. H. Cook, N. Dalal, A. J. Drake, Kenneth C. Freeman and 15 more. 2000

755 citations recorded by the source corpus.

We report on our search for microlensing toward the Large Magellanic Cloud (LMC). Analysis of 5.7 yr of photometry on 11.9 million stars in the LMC reveals 13-17 microlensing events. A detailed treatment of our detection efficiency shows that this is significantly more than the ~2-4 events expected from lensing by known stellar populations. The timescales () of the events range from 34 to 230 days. We estimate the microlensing optical depth toward the LMC from events with 2 < < 400 days to be τ = 1.2 × 10-7, with an additional 20% to 30% of systematic error. The spatial distribution of events is mildly inconsistent with LMC/LMC disk self-lensing, but is consistent with an extended lens distribution such as a Milky Way or LMC halo. Interpreted in the context of a Galactic dark matter halo, consisting partially of compact objects, a maximum-likelihood analysis gives a MACHO halo fraction of 20% for a typical halo model with a 95% confidence interval of 8%-50%. A 100% MACHO halo is ruled out at the 95% confidence level for all except our most extreme halo model. Interpreted as a Galactic halo population, the most likely MACHO mass is between 0.15 and 0.9 M☉, depending on the halo model, and the total mass in MACHOs out to 50 kpc is found to be 9 × 1010 M☉, independent of the halo model. These results are marginally consistent with our previous results, but are lower by about a factor of 2. This is mostly due to Poisson noise, because with 3.4 times more exposure and increased sensitivity to long-timescale events, we did not find the expected factor of ~4 more events. In addition to a larger data set, this work also includes an improved efficiency determination, improved likelihood analysis, and more thorough testing of systematic errors, especially with respect to the treatment of potential backgrounds to microlensing. We note that an important source of background are supernovae (SNe) in galaxies behind the LMC.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/2bdba7bcc228f8baf6ca505810f812c4490dfdee

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Citations in the corpus

Dynamical Family Properties and Dark Halo Scaling Relations of Giant Elliptical Galaxies (2000, background): “Near-maximal M/L in ellipticals are in line with results for the Milky Way (Gerhard (1999)) and barred galaxies (Debattista & Sellwood (1998), Weiner, Sellwood & Williams (2000)), where independent dynamical constraints on the luminous mass favour near-maximal disks, while the situation is less clea”
Modified Newtonian dynamics as an alternative to dark matter (2002, background): “Now, more than a decade later, a significant baryonic contribution to the halos of galaxies in the form of “machos” or low mass stars seems to have been ruled out (Alcock et al 2000).”
The gravitational wave signal from the Galactic disk population of binaries containing two compact objects. (2001, background): “The most likely MACHO mass is between 0.15 and 0.9M , depending on the halo model, and the total mass in MACHO’s out to 50 kpc is 9+4−3×1010M , independent of the halo model (Alcock et al. 2000).”
The stellar halo of the Galaxy (2008, background): “However, this scenario appears unlikely, as the microlensing surveys are unable to assign more than 8% of the matter to compact dark objects (Tisserand et al. 2007, although see Alcock et al. (2000) for a different while earlier result).”
The Mass of the Milky Way: Limits from a newly assembled set of halo objects (2002, background): “5 we discuss implications for the origin of the halo microlensing events toward the LMC and the mass estimate of the Local Group, and consider the prospects for more obtaining more accurate estimates of the total mass of the Galaxy in the near future.”
Dynamical Blueprints for Galaxies (2008, methodology): “In analyzing their 5.7 yr data set, the MACHO experiment considered a wide range of Galactic models (Alcock et al. 2000).”
The Cosmic Energy Inventory (2004, background): “One possibility is that the observations of microlensing toward the Magellanic Clouds have detected massive compact dark objects (Alcock et al. 2000; Afonso et al. 2003).”
THE CATALINA SURVEYS PERIODIC VARIABLE STAR CATALOG (2014, background): “High-amplitude delta Scutis (HADS, AL Velorum stars) have amplitudes greater than 0.1 mags (Alcock et al 2000c), while low-amplitude delta Scutis (LADS) have smaller amplitudes.”
Galactic Stellar and Substellar Initial Mass Function (2003, background): “The MACHO observations (Alcock et al. 2000) yield a microlensing optical depth, based on 13 to 17 events, τ = 1.2+0.4−0.3×10−7, with a total mass in the objects within 50 kpc M50 = 9 +4 −3×1010M⊙, i.e. .”
A new look at the large-scale H I structure of the Large Magellanic Cloud (2002, background): “1999) and H II regions (Oey 1996), molecular cloud astrophysics (Johansson et al. 1994), and for providing background stars with which to study possible microlenses in the Galactic halo (Alcock et al. 2000).”
SMARTS OPTICAL AND INFRARED MONITORING OF 12 GAMMA-RAY BRIGHT BLAZARS (2012, background): “(More speculatively, achromatic flares could be caused by gravitational microlensing in the halo of our galaxy, as in the MACHO experiment (Alcock et al. 2000), since these blazar jets are effectively point sources, but the probability of such an event should be vanishingly small.)”
LSST: From Science Drivers to Reference Design and Anticipated Data Products (2008, background): “Given the age 30 of the Galactic halo, a significant fraction of the mass in this component may reside in these remnant stars (e.g., Alcock et al. 2000; Tisserand et al. 2007) and therefore their discovery directly constrains the Galactic mass budget.”