The MACHO Project Large Magellanic Cloud Microlensing Results from the First Two Years and the Nature of the Galactic Dark Halo

Abstract

The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the Ðrst 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (D1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with days to be based upon our eight event sample. This exceeds the 2 \ tü \ 200 q 2 200 \ 2.9~0 .9 1.4 ] 10~7 optical depth, expected from known stars, and the di †erence is to be compared q backgnd \ 0.5 ] 10~7, with the optical depth predicted for a "" standard ÏÏ halo composed entirely of MACHOs : q halo \ 4.7 To compare with Galactic halo models, we perform likelihood analyses on the full eight-event ] 10~7. sample and a six-event subsample (which allows for two events to be caused by a nonhalo "" background ÏÏ). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of which is about half of the "" standard halo ÏÏ value. We also Ðnd a most prob-2.0~0 .71.2 ] 1011 M _ , able MACHO mass of although this value is strongly model dependent. In addition, the 0.5~0 .2 0.3 M _ , absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs : objects from 10~4 to 0.03 contribute of the "" standard ÏÏ dark halo.

Monte Carlo simulations of the halo white dwarf population

“…The suggestion of the MACHO team (Alcock et al 1997(Alcock et al , 2000 that white dwarfs contribute significantly to the mass budget of the Galactic halo has motivated a large number of observational searches (Knox et al 1999;Ibata et al 1999;Oppenheimer et al 2001;Majewski & Siegel 2002;Nelson et al 2002) for these elusive white dwarfs. Also several theoretical works (Reylé et al 2001;Flynn et al 2003) have analyzed this possibility.…”

Do Low Luminosity Stars Matter?

“…Microlensing experiments like MACHO (Alcock et al (1997); Alcock et al (2000)), EROS (Afonso et al (2000); Lasserre et al (2000)) and OGLE (Udalski et al (1998)), found that the events detected towards the galactic bulge and the LMC were consistent with the lensing objects being small (in size) stars, with a mass of ∼ 0.6M , a very good match to a WD star. These faint objects could account for about 20 % to 30 % of the dark matter in the galactic halo, in this case the prime candidates are faint Halo WDs.…”

“…The search, in this case, was motivated by the results of the microlensing experiments (MACHO (Alcock et al (1997); Alcock et al (2000)), EROS (Afonso et al (2000); Lasserre et al (2000)) and OGLE (Udalski et al (1998)), that suggested that up to 30% of the Halo baryonic dark matter could be in the form of WDs. So far, the observed density of matter in halo WDs has only an upper limit of ρ halo w d < 4 × 10 −5 M pc −3 (Harris et al (2006)).…”

Astrophysical implications of gravitational microlensing of gravitational waves

“…Alcock et al 1997a;De Paolis et al 1996;Melchior et al 1998). These observations have led to the conclusion that about 50% of the galactic dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects) with mass m 0.4 M .…”

“…We assume that halo lens objects have a mass 0.4 M and constitute a fraction f 0.5 of the halo dark matter as shown by microlensing observations (see e.g. Alcock et al 1997a…”