P. J. Quinn

Possible Gravitational Microlensing Of A Star In The Large Magellanic Cloud

THERE is now abundant evidence for the presence of large quantities of unseen matter surrounding normal galaxies, including our own1,2. The nature of this ’dark matter‘ is unknown, except that it cannot be made of normal stars, dust or gas, as they would be easily detected. Exotic particles such as axions, massive neutrinos or other weakly interacting massive particles (collectively known as WIMPs) have been proposed3,4, but have yet to be detected. A less exotic alternative is normal matter in the form of bodies with masses ranging from that of a large planet to a few solar masses. Such objects, known collectively as massive compact halo objects5 (MACHOs), might be brown dwarfs or ‘jupiters’ (bodies too small to produce their own energy by fusion), neutron stars, old white dwarfs or black holes. Paczynski6 suggested that MACHOs might act as gravitational microlenses, temporarily amplifying the apparent brightness of background stars in nearby galaxies. We are conducting a microlensing experiment to determine whether the dark matter halo of our Galaxy is made up of MACHOs. Here we report a candidate for such a microlensing event, detected by monitoring the light curves of 1.8 million stars in the Large Magellanic Cloud for one year. The light curve shows no variation for most of the year of data taking, and an upward excursion lasting over 1 month, with a maximum increase of ∼2 mag. The most probable lens mass, inferred from the duration of the candidate lensing event, is ∼0.1 solar mass.

The macho project: 45 candidate microlensing events from the first-year Galactic bulge data

We report the detection of 45 candidate microlensing events in fields toward the Galactic bulge. These come from the analysis of 24 fields containing 12.6 million stars observed for 190 days in 1993. Many of these events are of extremely high signal-to-noise ratio and are remarkable examples of gravitational microlensing. The distribution of peak magnifications is shown to be consistent with the microlensing interpretation of these events. Using a subsample of 1.3 million {open_quotes}clump giant{close_quotes} stars whose distance and detection efficiency are well known, we find 13 events and estimate the microlensing optical depth toward the Galactic bulge as {tau}{sub bulge}=3.9{sub {minus}1.2}{sup +1.8}{times}10{sup {minus}6} averaged over an area of {approximately}12deg{sup 2} centered at Galactic coordinates l=2.55{degree} and b=3.64{degree}. This is similar to the value reported by the OGLE collaboration and is marginally higher than current theoretical models for {tau}{sub bulge}. The optical depth is also seen to increase significantly for decreasing {vert_bar}b{vert_bar}. These results demonstrate that obtaining large numbers of microlensing events toward the Galactic bulge is feasible, and that the study of such events will have important consequences for the structure of the Galaxy and its dark halo. {copyright} {ital 1997} {ital The American Astronomical Society}

First Observation of Parallax in a Gravitational Microlensing Event

We present the first detection of parallax effects in a gravitational microlensing event. Parallax in a gravitational microlensing event observed only from the Earth appears as a distortion of the (otherwise symmetrical) light curve arising from the motion of the Earth around the Sun. This distortion can be detected if the event duration is not much less than a year and if the projected velocity of the lens is not much larger than the orbital velocity of the Earth about the Sun. The event presented here has a duration (or Einstein diameter crossing time) of = 220 days and clearly shows the distortion resulting from the Earths motion. We find that the projected velocity of the lens is

Where Do the Disks of Spiral Galaxies End

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The MACHO Project Large Magellanic Cloud Variable Star Inventory. III. Multimode RR Lyrae Stars, Distance to the Large Magellanic Cloud, and Age of the Oldest Stars

-->= 75 ± 5 km s-1 at an angle of 28° ± 4° from the direction of increasing galactic longitude, as expected for a lens in the galactic disk. A likelihood analysis of this event yields estimates of the distance to and mass of the lens: Dlens = 1.7+1.1−0.7 kpc and M = 1.3+1.3−0.6 M☉. This suggests that the lens is a remnant such as a white dwarf or neutron star. It is possible, though less likely, that the lens is a main-sequence star. If so, we can add our upper limit on the observed flux from the lens to the analysis. This modifies the estimates to Dlens = 2.8+1.1−0.6 kpc and M = 0.6+0.4−0.2 M☉.

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

In spiral galaxies, the H I surface density declines with increasing radius to a point at which it is seen to truncate dramatically in the best observed cases. If the ambient radiation field is sufficiently strong, there exists a maximum radius beyond which the cold gas is unable to support itself against ionization. We have now succeeded in detecting ionized gas beyond the observed H I disk in spirals. Here we report on our findings for the Sculptor galaxy NGC 253. The H I disks in Sculptor galaxies extend to only about 1.2R25, although we have detected ionized gas to the limits of our survey, out to 1.4R25. This has important ramifications for spiral galaxies in that it now becomes possible to trace the gravitational potential beyond where the H I disk ends. The detections confirm that the rotation curve continues to rise in NGC 253, as it appears to do for other Sculptor galaxies, from the H I measurements, but there is a hint that the rotation curve may fall abruptly not far beyond the edge of the H I disk. If this is correct, it suggests that the dark halo of NGC 253 may be truncated near the H I edge and provides further support for the link between dark matter and H I. The line ratios are anomalous with [N II] ?6548 to H? ratios close to unity. While metallicities at these large radii are uncertain, such enhanced ratios, compared to solar-abundance H II regions ([N II] ?6548/H? = 0.05-0.2), are likely to require selective heating of the electron population without further ionization of N+. We discuss the most likely sources of ionization and heating, and the possible role of refractory element depletion (e.g., Ca, Si, and Fe) onto dust grains.

Real-Time Detection and Multisite Observations of Gravitational Microlensing

We report the discovery of 73 double-mode RR Lyrae (RRd) stars in fields near the bar of the Large Magellanic Cloud (LMC). The stars are detected among the MACHO database of short-period variables that currently contains about 7900 RR Lyrae stars. Fundamental periods (P0) for these stars are found in the range 0.46-0.55 days, and first-overtone-to-fundamental period ratios are found to be in the range 0.742 < P1/P0 < 0.748. The range in period ratios is unexpectedly large, and a significant fraction of our current sample has period ratios smaller than any previously discovered RRd variables. We present mean magnitudes, colors, and light-curve properties for all LMC RRd stars detected to date. We undertake a determination of the absolute magnitudes for these stars based primarily on pulsation theory and on the assumption that all observed stars are at the fundamental blue edge of the instability strip. Comparison of the calibrated MACHO V and RKC photometry with these derived absolute magnitudes yields an absorption-corrected distance modulus to the LMC of 18.48 ± 0.19 mag that is in good agreement with that found (18.5) through comparison of Galactic and LMC Cepheids. Exploring this luminosity calibration, we derive an increase in the distance modulus, and thus a reduction in the age found via isochrone fitting, for M15 of about 30% and discuss some implications for cosmology.

Is the Large Magellanic Cloud Microlensing Due to an Intervening Dwarf Galaxy

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☉.

MACHO Project Photometry of RR Lyrae Stars in the Sagittarius Dwarf Galaxy

The MACHO collaboration is carrying out an extensive survey to detect gravitational microlensing. We have recently demonstrated the ability to detect candidate microlensing events in real time, often well before the peak amplification of the event. This has made possible the acquisition of spectra over the course of an event, the invariance of which has lent strong support to the microlensing interpretation. This paper reports on photometric data that were acquired from two sites, Cerro Tololo Inter-American Observatory and Mount Stromlo, in response to the real time detection of a microlensing event. The superior photometry obtained from Chile and the complementary time coverage demonstrate the viability of mounting an aggressive campaign of microlensing follow-up observations.

On the Formation and Dynamics of Shells Around Elliptical Galaxies

The recent suggestion that the microlensing events observed toward the Large Magellanic Cloud are due to an intervening Sgr-like dwarf galaxy is examined. A search for foreground RR Lyrae in the MACHO photometry database yields 20 stars whose distance distribution follow the expected halo density profile. Cepheid and red giant branch clump stars in the MACHO database are consistent with membership in the LMC. There is also no evidence in the literature for a distinct kinematic population, for intervening gas, or for the turnoff of such a hypothetical galaxy. We conclude that if the lenses are in a foreground galaxy, it must be a particularly dark galaxy.