Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds

TL;DR

EROS-2 conducts a large, bright-star microlensing survey toward the Magellanic Clouds to test whether machos comprise a major Milky Way halo component. By focusing on a bright, well-measured subsample and carefully modeling blending and detection efficiency, the study finds zero-to-one microlensing events where many would be expected if the halo were fully macho-dominated, yielding stringent upper limits on the optical depth and halo fraction across a broad macho-mass range. The results strongly constrain machos as the primary dark matter source in the mass window 0.6×10^-7 M_sun to 15 M_sun and are notably more restrictive than prior central-LMC measurements, contributing to a resolution of the MACHO-EROS discrepancy through broader sky coverage and reduced blending uncertainties. The work also underscores the role of self-lensing and halo structure in interpreting microlensing signals and highlights the value of combining bright-star and faint-star analyses in future surveys.

Abstract

The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called ``machos'') could be a major component of the dark matter halo of the Milky Way galaxy. To this end, EROS-2 monitored over 6.7 years $33\times10^6$ stars in the Magellanic clouds for microlensing events caused by such objects. In this work, we use only a subsample of $7\times10^6$ bright stars spread over $84 °^2$ of the LMC and $9 °^2$ of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent. Using this sample of bright stars, only one candidate event was found, whereas $\sim39$ events would have been expected if the Halo were entirely populated by objects of mass $M\sim0.4M_{\odot}$. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (\object{LMC}) due to such lenses is $τ<0.36\times10^{-7}$ (95%CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range $0.6\times10^{-7}M_\odot<M<15M_{\odot}$ are ruled out as the primary occupants of the Milky Way Halo.

Figures (20)

• Figure 1: Map of the EROS-2 LMC and SMC fields in equatorial coordinates. A total of 88 LMC and 10 SMC fields were monitored. The first number in each field is the field number and the second is the number of bright stars (as defined in Section \ref{['brightsec']}) in the field in units of $10^4$. The two shaded regions (the larger one centered on the LMC bar) are the $13.4\,\rm deg^2$ used by the MACHO collaboration to measure the optical depth macho57.
• Figure 2: The photometric precision as a function of $R_{\rm eros}$ for the dense field lm009 (top) and the sparse field lm048 (bottom). Each point represents the r.m.s. dispersion of the measurements for a single star after elimination of outliers. The vertical line shows the position of the bright-star magnitude cut (\ref{['brightdefeq']}) for the field, $R_{\rm eros}=18.23$ for lm009 CCDs 0-3 and $R_{\rm eros}=19.7$ for lm048 CCDs 0-3.
• Figure 3: The flux precision, $\Delta\phi/\phi$, for the Bright-Star Sample (2% of the sample are in the histograms). Each entry is the mean flux uncertainty for a star on its light curve. The top (bottom) histograms are for the $R_{\rm eros}$ ($B_{\rm eros}$) bands.
• Figure 4: The effects of blending on artificial stars added to the dense field lm009 (left) and the sparse field lm048 (right). The top two panels show the distribution of $\alpha_1$ using artificial stars that do not fall under a brighter pre-existing object. Stars further than 2 arcsec from the nearest pre-existing object are concentrated in the peak at $\alpha_1\sim 1$ while stars nearer to pre-existing objects form the tail at $\alpha_1 <1$. The middle two panels show, for objects within 2 arcsec of pre-existing objects, the correlation between $\alpha_1$ and the difference in magnitude between the artificial star and the pre-existing object. Artificial objects with $\alpha_1$ significantly below unity are associated with pre-existing objects of similar magnitude. The line shows the expected relation for two superimposed objects: $\alpha_1=1./(1.+10^{0.4\Delta R})$. The bottom two panels show the distribution of $(\alpha_1,\alpha_2)$ with $\alpha_2$ calculated from (\ref{['alphadeq']}) and the magnitude of the pre-existing object. The line shows the expected relation for two superimposed objects: $\alpha_1 + \alpha_2 =1$.
• Figure 5: The distribution of $u_1$ (solid line) and ($u_1+u_2$) dashed line for the sparse EROS field lm048 (top panel) the dense EROS field lm009 (middle panel) and the very dense HST field in lm009, ccd3 (bottom panel) as described in the text. The distributions of $u_1$ are all characterized by a peak near $u_1\sim1$ and a tail at $u_1<1$. With increasing star density, the mean of $u_1+u_2$ increases because of the increasing importance of secondary stars.