Macho Research Articles

The Meanings of Macho: Being a Man in Mexico City

The Meanings of Macho: Being. Man in Mexico City. MATTHEW C. GUTMANN. Berkeley: University of California Press, 1996. xiv. 330 pp., maps, notes, glossary, bibliography, index.

Primordial black hole formation in supergravity

We study a double inflation model (a preinflation + a new inflation) in supergravity and discuss the formation of primordial black holes which may be identified with massive compact halo objects (MACHOs) observed in the halo of our galaxy. The preinflation drives an inflaton for the new inflation close to the origin through supergravity effects and the new inflation naturally occurs. If the total e-fold number of the new inflation is smaller than $\sim 60$, both inflations produce cosmologically relevant density fluctuations. If the coherent inflaton oscillation after the preinflation continues until the beginning of the new inflation, density fluctuations on small cosmological scales can be set suitably large to produce black holes MACHOs of masses $\sim 1 M_{\odot}$ in a wide region of parameter space in the double inflation model.

Gravitational microlensing—A report on the MACHO project

There is abundant evidence that the mass of the Universe is dominated by dark matter of unknown form. The MACHO project is one of several teams searching for the dark matter around our Galaxy in the form of massive compact halo objects (MACHOs). If a compact object passes very close to the line of sight to a background star, the gravitational deflection of light causes an apparent brightening of the star, i.e., a gravitational ``microlensing'' event. Such events will be very rare, so millions of stars must be monitored for many years. We describe our search for microlensing using a very large CCD camera on the dedicated 1.27 m telescope at Mt. Stromlo, Australia: currently some 14 events have been discovered towards the Large Magellanic Cloud. The lack of short-timescale events excludes planetary mass MACHOs as a major contributor to the dark matter, but the observed long events (durations 1--6 months) suggest that a major fraction may be in fairly massive objects $\ensuremath{\sim}0.5{\mathrm{M}}_{\ensuremath{\bigodot}}.$ It is currently difficult but not impossible to explain these events by other lens populations; we discuss some prospects for clarifying the nature of the lenses.

Search for Machos by the MOA Collaboration

Search for Machos by the MOA Collaboration

Massive compact halo objects viewed from a cosmological perspective: contribution to the baryonic mass density of the universe

We estimate the contribution of massive compact halo objects (MACHOs) and their stellar progenitors to the mass density of the Universe. If the MACHOs that have been detected reside in the halo of our Galaxy, then a simple extrapolation of the Galactic population (out to 50 kpc) of MACHOs to cosmic scales gives a cosmic density ϱ MACHO=(1–5)×10 9 hM ⊙Mpc −3, which in terms of the critical density corresponds to Ω MACHO = (0.0036-0.017)h −1 . Should the MACHO halo extend out to much further than 50 kpc, then Ω MACHO would only be larger. Such a mass density is comparable to the cosmic baryon density implied by Big Bang nucleosynthesis. If we take the central values of the estimates, then MACHOs dominate the baryonic content of the Universe today, with Ω MACHO Ω B ∼0.7 h . However, the cumulative uncertainties in the density determinations only require that Ω MACHO Ω B ≥ 1 6 hf gal′ , where the fraction of galaxies that contain MACHOs f gal>;0.17 and h is the Hubble constant in units of 100 km s −1 Mpc −1. Our best estimate for Ω MACHO is hard to reconcile with the current best estimates of the baryonic content of the intergalactic medium indicated by measurements of the Lyman-α forest; however, measurements of Ω Lyα are at present uncertain, so that such a comparison may be premature. If the MACHOs are white dwarfs resulting from a single burst of star formation (without recycling), then their main sequence progenitors would have been at least twice more massive: Ω ∗ = (0.007-0.034)h −1 . Thus, far too much gaseous baryonic material would remain in the Galaxy unless there is a Galactic wind to eject it. Indeed a MACHO population of white dwarfs and the gas ejected from their main sequence progenitors accounts for a significant fraction of all baryons. This fact must be taken into account when attempting to dilute the chemical by-products of such a large population of intermediate mass stars. We stress the difficulty of reconciling the MACHO mass budget with the accompanying carbon production in the case of white dwarfs. In the simplest picture, even if the excess carbon is ejected from the Galaxy by a Galactic wind, measurements of carbon abundances in Lyman α forest lines with values 10 −2 solar require that only about 10 −2 of all baryons can have passed through the white dwarf progenitors. Such a fraction can barely be accommodated by our estimates of Ω MACHO and would be in conflict with Ω ∗ .

Microlensing constraints on MACHO masses from the double quasar Q0957+561

Gravitational lensing can be used to determine whether the dark matter in galactic halos is in the form of objects of stellar or sub-stellar mass. Searches for these MACHOs (MAssive Compact Halo Objects) have become popular in our own Milky Way from studies of the rare events of a MACHO magnifying a star in the Large Magellanic Cloud. Here we show that we can also address this question by investigating well sampled light curves of multiply imaged quasars. The light bundles pass through (halos of) foreground galaxies and are subject to microlensing by MACHOs there. The recently obtained well-sampled light curves for the two images of the double quasar Q0957+561A and B are very similar to each other, except for an overall time delay and scaling factor. This allows us to put limits on the amount of microlensing that took place in this system. By comparison with intensive numerical simulations we can exclude a halo consisting of objects in the mass range 10 −7 to 10 −3 M ⊙ for a quasar size of 10 14 cm. For quasar sizes as large as 3 × 10 15 cm we can exclude only the mass interval 10 −4 to 10 −3 M ⊙. A halo consisting of objects with 10 −2 or 10 −1 M ⊙ cannot be ruled out yet, but it should produce MACHO induced fluctuations in future observations.

Halo Dark Clusters of Brown Dwarfs and Molecular Clouds

The discovery of Massive Astrophysical Compact Halo Objects (MACHOs) in microlensing experiments makes it compelling to understand their physical nature, as well as their formation mechanism. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs, and the present paper deals with this option. According to a recently proposed scenario, brown dwarfs are clumped along with cold molecular clouds into dark clusters -- in several respects similar to globular clusters -- which form in the outer part of the galactic halo. Here, we analyze the dynamics of these dark clusters and we address the possibility that a sizable fraction of MACHOs can be binary brown dwarfs. Moreover, we point out that Ly-$\alpha$ absorption systems naturally fit within the present picture.

Primordial black hole formation in a double inflation model in supergravity

It has been recently pointed out that the initial value problem in new inflation models is naturally solved by supergravity effects if there exists a preinflation before the new inflation. We study this double inflation model in detail and find that density fluctuations on small cosmological scales are much larger than those on large scales due to the peculiar property of the new inflation. We show that this results in the production of primordial black holes which have $\ensuremath{\sim}{1M}_{\ensuremath{\bigodot}}$ masses in a certain parameter region of the double inflation model. We stress that these black holes may be identified with massive compact halo objects observed in the halo of our galaxy.

Gravitational lensing of type Ia supernovae by galaxy clusters

We propose a method to remove the mass-sheet degeneracy that arises when the mass of galaxy clusters is inferred from gravitational shear. The method utilizes high-redshift standard candles that undergo weak lensing. Natural candidates for such standard candles are type Ia supernovae (SNe Ia). When corrected with the light-curve shape (LCS), the peak magnitude of SNe Ia provides a standard candle with an uncertainty in apparent magnitude of Δm≃0.1–0.2. Gravitational magnification of a background SN Ia by an intervening cluster would cause a mismatch between the observed SN Ia peak magnitude compared with that expected from its LCS and redshift. The average detection rate for SNe Ia with a significant mismatch of ≥2Δm behind a cluster at z≃0.05–0.15 is about 1–2 supernovae per cluster per year at J, I, R≲25–26. Since SNe are point-like sources for a limited period, they can experience significant microlensing by massive compact halo objects (MACHOs) in the intracluster medium. Microlensing events caused by MACHOs of ∼10−4 M⊙ are expected to have time-scales similar to that of the SN light curve. Both the magnification curve by a MACHO and the light curve of a SN Ia have characteristic shapes that allow us to separate them. Microlensing events caused by MACHOs of smaller mass can unambiguously be identified in the SN light curve if the latter is continuously monitored. The average number of identifiable microlensing events per nearby cluster (z≲0.05) per year is ∼0.02 (f/0.01), where f is the fraction of the cluster mass in MACHOs of masses 10−7<Mmacho/M⊙<10−4.

Binary brown dwarfs in the Galactic halo?

Microlensing events towards the Large Magellanic Cloud entail that a sizable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are a viable candidate for MACHOs. Various reasons strongly suggest that a large amount of MACHOs should actually consist of binary brown dwarfs. Yet, this circumstance looks in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with a more accurate photometric observation can resolve binary brown dwarfs.

Probing the Mass Fraction of MACHOs in Extragalactic Halos

Current microlensing searches calibrate the mass fraction of the Milky Way halo that is in the form of massive astrophysical compact halo objects (MACHOs). We show that surveys like the Sloan Digital Sky Survey (SDSS) can probe the same quantity in halos of distant galaxies. Microlensing of background quasars by MACHOs in intervening galaxies would distort the equivalent width distribution of the quasar emission lines by an amplitude that depends on the projected quasar-galaxy separation. For a statistical sample of ~105 quasars as expected in the SDSS, this distortion is detectable at the 2 σ level out to a quasar-galaxy impact parameter of several tens of kpc, as long as extragalactic halos are made of MACHOs. Detection of this signal would test whether the MACHO fraction inferred for the Milky Way halo is typical of other galaxies.

Binary brown dwarfs in the Galactic halo?

A B STR A CT Microlensing events towards the Large Magellanic Cloud imply that a sizeable fraction of dark matter is in the form of MACHOs (Massive Astrophysical Compact Halo Objects), presumably located in the halo of the Galaxy. Within the present uncertainties, brown dwarfs are viable candidates for MACHOs. Various considerations strongly suggest that a large number of MACHOs should actually consist of binary brown dwarfs. Yet this circumstance appears to be in flat contradiction with the fact that MACHOs have been detected as unresolved objects so far. We show that such an apparent paradox does not exist within a model in which MACHOs are clumped into dark clusters along with cold molecular clouds, since dynamical friction on these clouds makes binary brown dwarfs very close. Moreover, we argue that future microlensing experiments with more accurate photometric observations can resolve binary brown dwarfs.

Microlensing of Quasars by Stars in Their Damped Lyα Absorbers

The damped Ly? absorbers (DLAs) in quasar spectra are believed to be the progenitors of present-day disk galaxies. We examine the probability for microlensing of background quasars by stars in their DLAs. Microlensing by an individual star should magnify the continuum but not the broad emission lines of the quasars. Consequently, the equivalent width distribution of microlensed quasars would be distorted. We model a representative spiral galaxy as a closed system composed of a bulge, a disk, and a halo, and we evolve the mass fraction of stars in the disk based on the observed metallicity of DLAs at high redshifts. The microlensing signatures are stronger if the halo of the galaxy is made of massive compact halo objects (MACHOs). In this case, the distortion imprinted by microlensing on the equivalent width distribution of quasar emission lines can be detected with high significance in a sample of ~10 DLAs with H I column densities N 1021 cm-2 and absorption redshifts zabs 1. About one-tenth of all quasars with DLAs (N 1020 cm-2) might show excess variability on timescales shorter than 5 yr. A search for these signals would complement microlensing searches in local galaxies and calibrate the MACHO mass fraction in galactic halos at high redshifts.

Microlensing of tidal debris on the Magellanic great circle

Increasing evidence suggests that the Galactic halo is lumpy on kpc scales as a result of the accretion of at least a dozen small galaxies [Large and Small Magellanic Clouds (LMC/SMC), Sgr, Fornax, etc.]. Faint stars in such lumpy structures can significantly microlense a background star with an optical depth of 10−7–10−6, which is comparable to the observed value to the LMC. The observed microlensing events towards the LMC can be explained by a tidal debris tail from the progenitor of the Magellanic Clouds and Magellanic Stream. The LMC stars can either lense stars in the debris tail a few kpc behind the LMC, or be lensed by stars in the part of the debris tail in front of the LMC. The models are consistent with an elementary particle dominated Galactic halo without massive compact halo objects (MACHOs). They also differ from Sahu's LMC-self-lensing model by predicting a higher optical depth and event rate and lower concentration of events to the LMC centre.

Galactic versus Extragalactic Pixel Lensing Events toward M31

A new type of gravitational microlensing experiment toward a field where stars are not resolved is being developed observationally and theoretically: pixel lensing. When the experiment is carried out toward the M31 bulge area, events may be produced both by massive compact halo objects (MACHOs) in our Galactic halo and by lenses in M31. We estimate that ~10%-15% of the total events are caused by Galactic halo MACHOs assuming an all-MACHO halo. If these Galactic events could be identified, they would provide us with an important constraint on the shape of the halo. We test various observ-ables that can be used for the separation of Galactic halo/M31 events. These observables include the Einstein timescale, the effective duration of an event, and the flux at the maximum amplification, but they cannot be used to separate the two populations of events. However, we find that most high maximum-flux Galactic halo events can be isolated through a satellite-based measurement of the flux difference caused by the parallax effect. For the detection of the flux difference, it is required to monitor events with an exposure time of ~20 minutes by a 0.5 m telescope mounted on a satellite. Such observations could be carried out as a minor component of a mission aimed primarily at events seen toward the Galactic bulge and the Large Magellanic Cloud. In addition, proper motion can be used to isolate Galactic halo/M31 events, but for only ~5% of high signal-to-noise ratio M31 events and only 1% of Galactic halo events.

The MACHO Project: Limits on Planetary Mass Dark Matter in the Galactic Halo from Gravitational Microlensing

The MACHO project has been monitoring about 10 million stars in the Large Magellanic Cloud (LMC) in the search for gravitational microlensing events caused by massive compact halo objects (MACHOs) in the halo of the Milky Way. In our standard analysis, we have searched this data set for well-sampled, long-duration microlensing light curves, detected several microlensing events consistent with MACHOs in the 0.1 M☉ ≲ m ≲ 1.0 M☉ mass range, and set limits on the abundance of objects with masses 10−5 M☉ ≲ m ≲ 10−1 M☉. In this paper, we present a different type of analysis involving the search for very short timescale brightenings of stars, which is used to set strong limits on the abundance of lower mass MACHOs. Our analysis of the first 2 years of data toward the LMC indicates that MACHOs with masses in the range 2.5 × 10–7 M☉ < m < 5.2 × 10−4 M☉ cannot make up the entire mass of a standard spherical dark halo. Combining these results with those from the standard analysis, we find that the halo dark matter cannot be comprised of objects with masses 2.5 × 10−7 M☉ < m < 8.1 × 10−2 M☉.

The MACHO project constraints on low mass machos in the galactic halo

The MACHO project has been monitoring about ten million stars in the Large Magellanic Cloud in the search for gravitational microlensing events caused by massive compact halo objects (Machos) in the halo of the Milky Way. The standard analysis for well sampled, long duration microlensing is sensitive to objects with masses 10 −5 ≲ m ≲ 1M ⊙ . However, a different analysis method sensitive to short duration events extends the sensitivity of the experiment to lower masses. Combining the results of the analyses of the first two years of data from the LMC shows that Machos with masses in the range 2.5 × 10 −7 m −2 M ⊙ cannot make up the entire mass of a standard spherical dark halo

The MACHO project 2nd year LMC microlensing results and dark matter implications

The MACHO Project is searching for galactic dark matter in the form of massive compact halo objects (Machos). Millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are photometrically monitored in an attempt to detect rare gravitational microlensing events caused by otherwise invisible Machos. Analysis of two years of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing events, far more than the $\sim1$ event expected from lensing by low-mass stars in known galactic populations. From these eight events we estimate the optical depth towards the LMC from events with $2 < \that < 200$ days to be $\tau_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}$. This exceeds the optical depth of $0.5\ten{-7}$ expected from known stars and is to be compared with an optical depth of $4.7\ten{-7}$ predicted for a ``standard'' halo composed entirely of Machos. The total mass in this lensing population is $\approx 2^{+1.2}_{-0.7} \ten{11} \msun$ (within 50 kpc from the Galactic center). Event timescales yield a most probable Macho mass of $0.5^{+0.3}_{-0.2}\msun$, although this value is quite model dependent.

GAMMA-RAY BURSTS FROM HALO NEUTRON STARS CROSSING DARK CLUSTERS

Two classes of neutron stars may exist in the galactic halo: high velocity neutron stars originating from the disk and injected in the halo and neutron stars originating from globular clusters (via type II supernovae and/or accretion induced collapse of white dwarfs). Moreover, the halo dark matter is likely in the form of dark clusters made of Massive Astrophysical Compact Halo Objects (MACHOs) and cold molecular clouds, expected to be formed in the outer part of the galaxy. We suggest that halo neutron stars may emit gamma-ray bursts crossing dark clusters. This assumption allows us to explain all the observed properties of gamma-ray bursts (rate, isotropy, cumulative peak flux distribution), including the different spectral properties of the two classes of short and long bursts. Several methods to test this model, independently on observations of gamma-ray bursts, are discussed.

Monte Carlo Simulations of MACHO Parallaxes from a Satellite

Three ongoing microlensing experiments have found more candidate events than expected from the known stars. These experiments measure only one parameter of the massive compact halo objects (machos), the magnification time scale of the events. More information is required to understand the nature of the machos. A satellite experiment has been proposed to measure their projected transverse speed $\tilde{v} = v/(1-z)$, where $v$ is the macho transverse speed and $z$ its distance divided by the distance of the source. Measurement of $\tilde{v}$ would determine whether the machos were in the Galactic disk, Galactic halo, or in the Large Magellanic Cloud (LMC). We simulate events observed toward the LMC by the Earth and by a satellite in an Earth like heliocentric orbit. To leading order, such an experiment determines $\tilde{v}$ up to a two fold degeneracy. More precise measurements break the degeneracy. We show that with photometric precisions of 3\% to 4\% and approximately 1 observation per day, $\tilde{v}$ can be measured with a maximum error of 20\% for 70\% to 90\% of events similar to the ones reported by the EROS and MACHO collaborations. The projected transverse velocity is known with the same maximum error for 60\% to 75\% of these events. This 20\% maximum error is not a 1 $\sigma$ error but is mostly due to degeneracy between two possible solutions, each one being localized to much better than 20\%. These results are obtained with an Earth-satellite separation of 1 AU, and are improved by a larger separation.