Evalyn I. Gates

The Local Halo Density

For almost 20 years models of the Galaxy have included a dark halo responsible for supporting a substantial fraction of the local rotation velocity and a flat rotation curve at large distances. Estimates of the local halo density range from 2 × 10-25 g cm-3 to 10 × 10-25 g cm-3. By careful modeling of the Galaxy, taking account of the evidence that dark halos are flattened and recent microlensing data, we arrive at a more quantitative estimate, 9.2+ 3.8−3.1 × 10-25 g cm-3. Microlensing toward the LMC indicates that only a small fraction, less than ~30%, can be in the form of MACHOs, which is consistent with the idea that most of the halo consists of cold dark matter particles.

Cold Dark Matter

Motivated by inflation, the theory of big-bang nucleosynthesis, and the quest for a deeper understanding of fundamental forces and particles, a paradigm for the development of structure in the universe has evolved. It holds that most of the matter exists in the form of slowly moving elementary particles left over from the earliest moments—cold dark matter—and that the small density inhomogeneities that seed structure formation arose from quantum fluctuations around 10−34 seconds after the big bang. A flood of observations, from determinations of the Hubble constant to measurements of the anisotropy of cosmic background radiation, are now testing the cold dark matter paradigm.

Cold + hot dark matter and the cosmic microwave background

We examine the cosmic microwave background power spectrum for adiabatic models with a massive neutrino component. We present the results of a detailed numerical evolution of cold+hot dark matter (CHDM) models and compare these results with the standard cold dark matter (CDM) spectrum. The difference is of order 5{percent}{endash}10{percent} for 400{lt}{ital l}{lt}1000 for currently popular CHDM models. Using semi-analytic approximations, we also discuss the relevant physics involved. Finally, we remark on the ability of future experiments to differentiate between these models. An all-sky experiment with a beam size smaller than 30{prime} can distinguish between CHDM and CDM if other cosmological parameters are known. Even allowing other parameters to vary, it may be possible to distinguish CDM from CHDM. {copyright} {ital 1996 The American Astronomical Society.}

Discovery of New Ultracool White Dwarfs in the Sloan Digital Sky Survey

We report the discovery of five new white dwarfs in the Sloan Digital Sky Survey. Four are ultracool, exhibiting strong collision-induced absorption (CIA) from molecular hydrogen and are similar in color to the three previously known coolest white dwarfs, SDSS J1337+00, LHS 3250, and LHS 1402. The fifth star shows milder CIA flux suppression and has a color and spectral shape similar to WD 0346+246. All five new white dwarfs are faint (g > 18.9) and have significant proper motions. One of the new ultracool white dwarfs, SDSS J0947, appears to be in a binary system with a slightly warmer (Teff ~ 5000 K) white dwarf companion.

Microlensing and halo cold dark matter.

We discuss the implications of the more than 50 microlensing events seen by the EROS, MACHO, and OGLE collaborations for the composition of the halo of our galaxy. The event rates indicate that the halo mass fraction in MACHOs is less than 30\%, consistent with expectations for a universe whose primary component is cold dark matter. We caution that the uncertainties are such that a larger MACHO fraction cannot yet be excluded.

Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system—its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310–00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.

Gravitational microlensing and the galactic halo

By means of extensive galactic modeling we study the implications of the MACHO, EROS, and OGLE microlensing results for the composition of the dark halo of the Galaxy as well as for other properties of the Galaxy. While microlensing toward the LMC most directly probes the halo MACHO mass fraction, microlensing toward the bulge is almost equally important as it strongly constrains the Galactic model. We take the Galaxy to be comprised of luminous and dark disk components, a bulge, and a dark halo consisting of both MACHOs and cold dark matter with each component being described by several observationally motivated parameters. We pare down an initial model space of millions of galactic models to viable models, those which are consistent with the observational data, including rotation curve, local projected mass density, and microlensing rates toward the LMC and bulge. On the basis of a conservative, minimal set of observational constraints an all-MACHO halo cannot yet be excluded, although in most viable models of the Galaxy the halo MACHO fraction is between 0% and 30%, consistent with expectations for a universe whose primary component is cold dark matter. An all-MACHO halo is required to be light, and when data on the local escape velocity and satellite-galaxy proper motions, which probe the extent of the dark halo, are taken into account, models which have a high MACHO mass fraction are ruled out. We also explore the possibility that there are no MACHOs in the halo. Finally, we point out several important tests that could definitively exclude an all-MACHO halo, e.g., evidence that the optical depth for microlensing is less than 1.5\ifmmode\times\else\texttimes\fi{}

Solar neutrino data, solar model uncertainties and neutrino oscillations

{10}^{\mathrm{\ensuremath{-}}7}

A New (Old) Component of the Galaxy as the Origin of the Observed Large Magellanic Cloud Microlensing Events

toward the LMC or greater than 3\ifmmode\times\else\texttimes\fi{}

No need for MACHOS in the halo

{10}^{\mathrm{\ensuremath{-}}6}