MACHOs: Dark Matter & Microlensing Constraints

• Massive Compact Halo Objects (MACHOs) are non-luminous, compact objects in galactic halos that induce gravitational microlensing and serve as dark matter candidates.
• Research employs microlensing surveys, dynamical heating studies, and wide binary analysis to constrain MACHO abundance and mass distribution.
• Advances using Gaia data and FRB/GRB lensing techniques continue to refine MACHO parameters and explore remaining dark matter windows.

Massive Compact Halo Objects (MACHOs) are a class of astrophysical entities hypothesized as non-luminous constituents of galactic halos, originally posited as a baryonic or non-baryonic dark matter candidate. The MACHO paradigm encompasses a range of object types—including primordial black holes, white dwarfs, neutron stars, brown dwarfs, ultracompact minihalos, and various exotic compact objects. The defining features of MACHOs are their compactness (with characteristic radii much smaller than their Einstein radii at cosmological distances) and their capacity to induce gravitational microlensing of background sources. Over the last three decades, an intensive research program has sought to verify whether MACHOs constitute a significant fraction of the Galactic or cosmic dark matter. Investigation encompasses microlensing surveys, dynamical heating in stellar systems, wide-binary disruption, and lensing of fast radio bursts and gamma-ray bursts. Synthesis of constraints from these approaches currently places severe limits on MACHO dark matter abundance across a broad mass-spectrum.

1. MACHO Definition, Formation Channels, and Astrophysical Realizations

MACHOs are defined operationally as non-luminous or faint, compact objects that generate significant gravitational lensing but lack strong electromagnetic emission. Canonical candidates initially included baryonic relics such as faint stars (M dwarfs), brown dwarfs, neutron stars, stellar-mass black holes, and both hydrogen-rich (DA) and hydrogen-deficient (non-DA) white dwarfs. Due to nucleosynthetic and chemical abundance constraints, attention shifted to non-baryonic candidates such as primordial black holes (PBHs) and ultracompact minihalos (dense dark matter microhalos seeded by pregalactic perturbations or PBHs) (0908.0735, Yang et al., 2011).

PBHs form via collapse of horizon-scale overdensities with δ ≥ 0.3, while ultracompact minihalos may arise from lower-amplitude (δ ∼ 10^{-3}–10^{-1}) density perturbations during phase transitions (QCD, e± annihilation) that evade prompt collapse to black holes but still accrete dark matter by secondary infall. These structures acquire steep density profiles, e.g., ρ(r) ∝ r^{-2.25} (minihalo) or ρ(r) ∝ r^{-9/4} (minihalo:WIMP annihilation core), with implications for their lensing signatures and cosmological effects (0908.0735, Yang et al., 2011).

Baryonic MACHOs (e.g., white dwarfs) are constrained by both microlensing event rates and local white dwarf luminosity functions, as well as through chemical evolution models (Torres et al., 2010).

2. Microlensing Surveys and Constraints

The most powerful direct probe of MACHOs in the mass range 10^{-7}–30 M_⊙ is gravitational microlensing of background stars in the Large and Small Magellanic Clouds (LMC/SMC) and the Galactic bulge. Major collaborations (MACHO, EROS, OGLE) have monitored tens of millions of stars for characteristic transient amplification events, with detailed blending-corrected detection efficiency calculations (0905.2044, Wyrzykowski et al., 2011, Novati et al., 2013).

Microlensing computations hinge on the optical depth τ, the instantaneous probability a source is strongly lensed, given by

$$\tau = \frac{\pi}{2 N_* T_{\rm obs}} \sum_i \frac{t_{E,i}}{\epsilon(t_{E,i})}$$

where N_* is the monitored star count, T_obs the observation time, t_{E,i} the Einstein radius crossing time, and ε(t_{E,i}) the detection efficiency.

Current results yield the following constraints:

Survey	Mass (M_⊙)	f_halo (95% CL)
OGLE-II/III LMC	0.4	< 19% (0905.2044)
	0.01–0.2	< 10%
OGLE-III SMC	0.4	< 6% (Wyrzykowski et al., 2011)
EROS-2	0.001	< 12% (Garcia-Bellido et al., 31 Jan 2024)

Upper limits are substantially lower for subsolar masses due to the absence of events and improved stellar population corrections (e.g., blending). Some events may be self-lensing (lens and source in the LMC/SMC), as indicated by timescale distributions and the spatial clustering of events (0905.2044, Wyrzykowski et al., 2011, Novati et al., 2013).

The revision of rotation curves with Gaia DR3 data alters the inferred halo density profile, steepening the drop of ρ(r) at large radii (Garcia-Bellido et al., 31 Jan 2024). When this is taken into account, previous microlensing constraints are relaxed—scenarios with up to 100% of the halo in MACHOs become viable over a wide mass range (10^{-4}–100 M_⊙), barring a narrow interval around 0.01 M_⊙ where the maximum permissible fraction is ∼20% (Garcia-Bellido et al., 31 Jan 2024).

3. Dynamical Heating Constraints from Stellar Systems

The dynamical heating of star clusters and ultrafaint dwarf (UFD) galaxies by MACHOs provides robust limits for masses spanning 1 M_⊙ – 10^9 M_⊙ (Brandt, 2016, Zoutendijk et al., 2020, Graham et al., 1 Oct 2025). Continuous gravitational scattering increases stellar velocity dispersion and the half-light radius r_h. The heating timescale is inversely related to both the MACHO mass and the abundance, with the requirement that observed systems (e.g., the Eridanus II cluster and multiple UFDs) have not expanded excessively since their formation:

$$\frac{dr_h}{dt} = \frac{4\sqrt{2}\pi G f_{DM} m_a \ln\Lambda}{\sigma} \left[\alpha\left(\frac{M_*}{\rho r_h^2}\right) + 2\beta r_h\right]^{-1}$$

where m_a is MACHO mass, σ is 1D DM velocity dispersion, and α, β are profile-dependent coefficients (Brandt, 2016, Zoutendijk et al., 2020).

Applying this to Eridanus II and several UFDs yields:

• For a pure MACHO halo (f_MACHO = 1), MACHO mass must be m ≲ 5–10 M_⊙ (Brandt, 2016, Zoutendijk et al., 2020).
• Multiple UFDs provide concordant constraints in the 10–10^9 M_⊙ range (Graham et al., 1 Oct 2025).
• Exceptionally compact UFDs (e.g., Ursa Major III, should its nature as a UFD be confirmed) will push the bound to the subsolar regime and dramatically improve the upper limit for 1–10^5 M_⊙ (Graham et al., 1 Oct 2025).

These constraints are robust under variation of UFD structural parameters (half-light radius, σ_{*,los}, total halo mass), as well as under changes in the assumed dark matter density profile (cored Dehnen vs. cuspy NFW) (Graham et al., 1 Oct 2025).

4. Constraints from Wide Halo Binary Disruption

Gravitational perturbations from MACHOs with masses above a few M_⊙ threaten the survival of wide, low-binding-energy binaries in the stellar halo. Monte Carlo impulse-approximation simulations using large catalogs of wide halo binaries, combined with the calculation of evolved semiaxis distributions over 10 Gyr, yield:

• For the most halo-like binaries (e.g., systems spending <8% of their time in the disk), the 95% upper limit on MACHO mass is ≲5 M_⊙ (Monroy-Rodríguez et al., 2014).
• These limits are comparably stringent to those from dynamical heating of UFDs and Eridanus II clusters, and, when combined with microlensing lower limits (∼30 M_⊙), effectively close the allowed mass window for MACHO dark matter in these ranges (Monroy-Rodríguez et al., 2014).

5. Probes with Fast Radio Bursts, Gamma-Ray Bursts, and Gravitational Waves

The strong gravitational lensing of fast radio bursts (FRBs) and gamma-ray bursts (GRBs) by MACHOs in the 10–1000 M_⊙ window provides an alternative probe where classic microlensing is less sensitive. Key predictions are millisecond-scale time delays or interference patterns between lensed images (Muñoz et al., 2016, Wang et al., 2018, Ji et al., 2018, Katz et al., 2019):

• Lensing by MACHOs (M ≳ 20 M_⊙) produces doubled FRBs with a repeat interval of ~ few × (M/30 M_⊙) ms.
• The non-detection of FRB echoes in samples of 10^4 FRBs constrains the MACHO fraction to f_DM ≲ 0.08 for M ≳ 20 M_⊙ and down to f ≲ 0.001 for binary lenses in the 20–100 M_⊙ range (Muñoz et al., 2016, Wang et al., 2018).
• Strong lensing of GRB light curves by MACHOs imprints a characteristically delayed echo, detectable in autocorrelation if the detector’s signal-to-noise and time resolution are sufficient (Ji et al., 2018).

Moreover, compulsory use of interference patterns in FRB spectra can probe compact objects at cosmological distances in the 10^{-4}–0.1 M_⊙ range, offering sensitivity to otherwise invisible compact mini-halos (Katz et al., 2019).

6. Impact of Halo Models, Mass Functions, and Spatial Clustering

Constraints on the MACHO fraction are sensitive to the assumed Galactic halo profile, the MACHO mass function, and their spatial clustering:

• Replacement of the semi-isothermal halo with NFW or flexible power-law models alters the expected event rates and can relax microlensing constraints above 1 M_⊙, especially for triaxial or oblate halos (Calcino et al., 2018).
• Broadened (e.g., lognormal) mass functions can wash out sensitivity at specific masses; cluster-centric spatial distributions can shift constraints to higher effective lensing masses, weakening individual lens constraints (Calcino et al., 2018, Garcia-Bellido et al., 31 Jan 2024).
• Gaia DR3 rotation curve fits, showing a rapid density decline at R ≳ 19–20 kpc, allow for up to 100% of the halo in MACHOs over a wide range (10^{-4}–100 M_⊙) except in the most sensitive mass intervals around 0.01 M_⊙ (Garcia-Bellido et al., 31 Jan 2024).

7. Current Status and Remaining Windows

The union of microlensing, dynamical heating, wide binaries, and lensing of fast transient surveys restricts MACHOs to a minor role in the Galactic halo across most considered mass scales. Robust bounds typically are

• f_MACHO ≲ 10% for 10^{-7}–30 M_⊙ (microlensing/OGLE/EROS)
• f_MACHO ≲ 8–10% for 20–100 M_⊙ (FRB lensing binaries)
• f_MACHO ≲ 1 for masses outside sensitive ranges, with allowed regions strongly dependent on modeling assumptions and mass functions (Brandt, 2016, Graham et al., 1 Oct 2025, Garcia-Bellido et al., 31 Jan 2024).

Only specific mass intervals (notably 0.01–0.001 M_⊙) remain tightly constrained; outside these, new rotation curves and mass functions can render the 100% MACHO halo hypothesis viable, subject to confirmation with upcoming high-cadence, high-resolution lensing and dynamical surveys (Garcia-Bellido et al., 31 Jan 2024). The role of exceptionally compact UFDs, such as Ursa Major III, is critical for cementing or ruling out the remaining parameter space (Graham et al., 1 Oct 2025).

Summary Table: Principal MACHO Constraints by Mass Regime

Mass Regime	Dominant Constraint	95% CL f_MACHO Bound	Key Reference
10^{-7}–0.01 M_⊙	Microlensing (EROS/OGLE)	f_MACHO ≲ 10–12%	(0905.2044, Novati et al., 2013, Garcia-Bellido et al., 31 Jan 2024)
0.01–0.4 M_⊙	Microlensing, White Dwarfs	f_MACHO ≲ 10%	(0905.2044, Torres et al., 2010, Wyrzykowski et al., 2011)
1–10 M_⊙	UFD dynamical heating, wide binaries	f_MACHO ≲ 1; M_MACHO ≲ 5–10 M_⊙	(Monroy-Rodríguez et al., 2014, Brandt, 2016, Graham et al., 1 Oct 2025)
10–100 M_⊙	UFDs, FRB lensing	f_MACHO ≲ 8–10%	(Muñoz et al., 2016, Wang et al., 2018, Graham et al., 1 Oct 2025)
>10^5 M_⊙	Disk kinematics, UFD heating	f_MACHO ≲ 1; looser at high masses	(Graham et al., 1 Oct 2025)

Work continues on refining these boundaries, especially as new ultracompact systems are discovered and as next-generation surveys—the Large Synoptic Survey Telescope (LSST), CHIME/UTMOST, and sensitive X-ray/optical observatories—advance the sensitivity to faint or rare microlensing signatures, dynamical heating, and accretion "glow" mechanisms (Bai et al., 2020, Graham et al., 1 Oct 2025).

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References\ (0905.2044) The OGLE View of Microlensing towards the Magellanic Clouds. I. A Trickle of Events in the OGLE-II LMC data\ (Torres et al., 2010) White dwarfs with hydrogen-deficient atmospheres and the dark matter content of the Galaxy\ (Wyrzykowski et al., 2011) The OGLE View of Microlensing towards the Magellanic Clouds. IV. OGLE-III SMC Data and Final Conclusions on MACHOs\ (Novati et al., 2013) Microlensing towards the SMC: a new analysis of OGLE and EROS results\ (Monroy-Rodríguez et al., 2014) The end of the MACHO era- revisited: new limits on MACHO masses from halo wide binaries\ (Muñoz et al., 2016) Lensing of Fast Radio Bursts as a Probe of Compact Dark Matter\ (Brandt, 2016) Constraints on MACHO Dark Matter from Compact Stellar Systems in Ultra-Faint Dwarf Galaxies\ (Calcino et al., 2018) Updating the MACHO fraction of the Milky Way dark halo with improved mass models\ (Zoutendijk et al., 2020) The MUSE-Faint survey: I. Spectroscopic evidence for a star cluster in Eridanus 2 and constraints on MACHOs as a constituent of dark matter\ (Garcia-Bellido et al., 31 Jan 2024) Reanalysis of the MACHO constraints on PBH in the light of Gaia DR3 data\ (Graham et al., 1 Oct 2025) Robust bounds on MACHOs from the faintest galaxies