Galaxy evolution. Galactic paleontology

Individual low-mass stars have very long lives, comparable to the age of the universe, and can thus be used to probe ancient star formation. At present, such stars can be identified and studied only in the Milky Way and in the very closest of our neighboring galaxies, which are predominantly small dwarf galaxies. These nearby ancient stars are a fossil record that can provide detailed information about the physical processes that dominated the epoch of galaxy formation and subsequent evolution.     

Galaxy disruption in a halo of dark matter

The relics of disrupted satellite galaxies have been found around the Milky Way and Andromeda, but direct evidence of a satellite galaxy in the early stages of disruption has remained elusive. We have discovered a dwarf satellite galaxy in the process of being torn apart by gravitational tidal forces as it merges with a larger galaxy's dark matter halo. Our results illustrate the morphological transformation of dwarf galaxies by tidal interaction and the continued buildup of galaxy halos.     

Energy Conditions in the Epoch of Galaxy Formation

The energy conditions of Einsteinian gravity (classical general relativity) do not require one to fix a specific equation of state. In a Friedmann-Robertson-Walker universe where the equation of state for the cosmological fluid is uncertain, the energy conditions provide simple, model-independent, and robust bounds on the behavior of the density and look-back time as a function of red shift. Current observations suggest that the "strong energy condition" was violated sometime between the epoch of galaxy formation and the present. This implies that no possible combination of "normal" matter is capable of fitting the observational data.     

Genesis of the heaviest elements in the Milky Way Galaxy

We review the origin and evolution of the heavy elements, those with atomic numbers greater than 30, in the early history of the Milky Way. There is a large star-to-star bulk scatter in the concentrations of heavy elements with respect to the lighter metals, which suggests an early chemically unmixed and inhomogeneous Galaxy. The relative abundance patterns among the heavy elements are often very different from the solar system mix, revealing the characteristics of the first element donors in the Galaxy. Abundance comparisons among several halo stars show that the heaviest neutron-capture elements (including barium and heavier) are consistent with a scaled solar system rapid neutron-capture abundance distribution, whereas the lighter such elements do not conform to the solar pattern. The stellar abundances indicate an increasing contribution from the slow neutron-capture process (s-process) at higher metallicities in the Galaxy. The detection of thorium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy.     

ASTRONOMY: Enhanced: Monsters at the Heart of Galaxy Formation

Supermassive black holes (BHs) have long been implicated in explaining active galactic nuclei such as quasars. In his Perspective, Kormendy discusses recent work that suggests a larger role for these objects. The Hubble Space Telescope has provided many new detections of BHs, and these new results indicate that they play an important role in the galaxy formation processes.     

A Transient Radio Source near the Center of the Milky Way Galaxy

In late December 1990, a new radio source appeared near the center of our galaxy rivaling the intensity of Sgr A(*) (the compact radio source at the galactic center). Following its first detection, the flux density of the galactic center transient (GCT) increased rapidly to a maximum 1 month later, and then declined gradually with a time scale of about 3 months. Surprisingly, the GCT maintained a steep radio spectrum during both its rising and decay phases. The neutral hydrogen (HI) absorption shows similar absorption to that in front of Sgr A(*); this indicates that the GCT lies near the galactic center. Furthermore, both HI and OH observations show an additional deep absorption at +20 kilometers per second with respect to the local standard of rest. Thus, the GCT is either embedded in or located behind a molecular cloud moving with that velocity. The cloud can be seen on infrared images. Its opacity is shown to be inadequate to conceal a supernova near the galactic center. It is argued that the GCT was probably transient radio emission from synchrotron-radiating plasma associated with an x-ray binary system.     

The geometric distance and proper motion of the Triangulum Galaxy (M33)

We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, we obtained a distance of 730 +/- 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 +/- 59 kilometers per second relative to the Milky Way. These measurements promise a method to determine dynamical models for the Local Group and the mass and dark-matter halos of M31, M33, and the Milky Way.     

Spectra, Variability, Size, and Polarization of H2O Microwave Emission Sources in the Galaxy

Radio spectral line radiation of water molecules at a wavelength of 1.35 centimeters has been measured from eight sources in the galaxy. The sources are less than I arc minute in diameter, have extremely high brightness temperatures, and show many spectral features. Some spectral features are slightly polarized and some have changed greatly in intensity in a few weeks time.