A new look at microlensing limits on dark matter in the Galactic halo

Abstract

Context. The motivation for this paper is to review the limits set on the MACHO content of the Galactic halo by microlensing experiments in the direction of the Large Magellanic Cloud. This has been prompted by recent measurements of the Galactic rotation curve, which suggest that the limits have been biassed by the assumption of an over-massive halo. Aims. The paper first discusses the security of the detection efficiency calculations which are central to deriving the MACHO content of the Galactic halo. It then sets out to compare the rotation curves from various halo models with recent observations, with a view to establishing what limits can be put on an all-MACHO halo. Methods. The main thrust of the paper is to investigate whether lighter halo models which are consistent with microlensing by an all-MACHO halo are also consistent with recent measures of the Galactic rotation curve. In this case the population of bodies discovered by the MACHO collaboration would make up the entire dark matter content of the Galactic halo. Results. The main result of this paper is that it is easy to find low mass halo models consistent with the observed Galactic rotation curve, which also imply an optical depth to microlensing similar to that found by the MACHO collaboration. This means that all-MACHO halos cannot be ruled out on the basis of their observations. Conclusions. Limits placed on the MACHO content of the Galactic halo from microlensing surveys in the Magellanic Clouds are inconsistent and model dependent, and do not provide a secure basis for rejecting an all-MACHO halo.

Cosmic microwave background limits on accreting primordial black holes

“…For comparison, we also show the CMB bound previously derived by ROM (thin dashed curve), as well as various dynamical constraints: micro-lensing constraints from the EROS [15] (purple curve) and MACHO [14] (blue curve) collaborations (but see Ref. [53] for caveats), limits from Galactic wide binaries [17], and ultra-faint dwarf galaxies [54] (in all cases we show the most conservative limits provided in the referenced papers).…”

The signature of primordial black holes in the dark matter halos of galaxies

“…The case for compact bodies as dark matter received a severe setback with the publication of results from microlensing surveys in the Galactic halo (Alcock et al 2000;Tisserand et al 2007;Wyrzykowski et al 2011), which put limits on any population of solar mass compact bodies. However, more recent measures of the structure of the Galactic halo, and reassessment of other aspects of the surveys, have now thrown doubt on the reliability of the limits on dark matter in the form of compact bodies set by these groups (Hawkins 2015;Green 2017;Calcino et al 2018).…”

“…The limits on dark matter particles come from a wide variety of sources, including direct detection (Aprile et al 2012;Akerib et al 2014), annihilation signals (Ackermann et al 2014) and cosmic microwave background (CMB) measurements (Madhavacheril et al 2014). The search for dark matter in other forms has received little attention, and has largely been confined to the Galactic halo (Alcock et al 2000;Tisserand et al 2007;Wyrzykowski et al 2011), with inconclusive and apparently contradictory results (Hawkins 2015). In these searches, the idea is to detect a population of compact bodies in the halo of the Milky Way by observing the rate at which stars in the background Magellanic Clouds are microlensed.…”

Disentangling the Potential Dark Matter Origin of LIGO’s Black Holes

“…Hawkins (2015) acknowledges that the models that did allow for 100% MACHO dark matter may not be an accurate depiction of the Milky Way, but suggests that ruling out MACHO dark matter is premature. Green (2017) also explicitly explored this model dependency for the tighter EROS2 constraints and tracked changes resulting from the variation of several parameters, confirming Hawkins' claim that current constraints are extremely model dependent.…”