The velocities, positions, and chemical composition of the stars in our Galaxy have "locked in" the memory of the properties of the gas clouds which formed them. By selecting RR Lyrae stars (pulsating variable stars which are also found in globular clusters) to study, I ensure that I am looking at stars that were born at least 10-12 billion years ago, when the Galaxy itself was forming. Using telescopes in Chile and Arizona, I have obtained data on four samples of RR Lyrae stars, each in a different region of the Galaxy. The regions I am studying are:
The Solar Neighborhood sample gives us a large (300 stars), well-studied sample to use as a benchmark. The motions and compositions of these stars are consistent with their having formed in small dwarf galaxies long ago. The dwarf galaxies then merged with the forming Milky Way, mixing into the halo.
By comparing the properties of the stars in different regions, I hope to place more constraints on pictures of how our Galaxy formed. In particular, the study of globular cluster motions suggests that the inner part of the halo may have formed by the collapse and spin-up of a single large gas cloud, while the outer halo broke into many fragments that formed autonomous dwarf galaxies, which later merged with the growing Galaxy. Comparison of the orbital properties of stars in the different Galactic regions will test this hypothesis.
Here is a nice movie (and caption) taken from a computer simulation showing the formation of a galaxy.
I am also interested in the star formation history of nearby galaxies, such as the newly-discovered Sagittarius Dwarf (Sgr). Ata Sarajedini and I have worked on determining the ages and compositions of the Sgr stars, with special emphasis on the massive globular cluster M54. We have determined that the four globular clusters in Sgr formed about 14 billion years ago. There then appeared to be a hiatus in star formation. Most of the "field" stars in Sgr formed between 11 and 5 billion years ago, though a trickle of star formation continued until about 500 million years ago. Xiaosong Liu, a masters student at BGSU, is developing computer code to artificially simulate different age/metallicity/formation-rate scenarios for comparison with our observations. He hopes to make a more quantitative description of the star formation strength as a function of time in the field of Sgr.
This ties in tightly with my study of Galactic formation, since Sgr appears to be a perfect example of the dwarf galaxy "fragments" that merged together billions of years ago to form the halo of our Galaxy. If Sgr had merged with the Galaxy 12 billion years ago, the clusters would have been completely formed and would today be indistinguishable from the other MW globulars. The gas in Sgr would have drained into the disk of the forming Galaxy and contributed to the formation of generations of disk stars. Simulations suggest that Sgr will merge with our galaxy in the next billion years or so.
In a related project, I have obtained Hubble Space Telescope images of the dwarf elliptical galaxy NGC 5206. Our images resolve the brightest red giant stars in this galaxy and confirm that it lies at a distance of NNN million light years, in the Centraurus Group of galaxies that neighbors our own Local Group. At this center of this galaxy lies an enormous star cluster. Ken Freeman and others have suggested that the globular clusters in our own Galaxy are similar "galactic nuclei" which were born in dwarf galaxies, but these galaxies merged with the Milky Way billions of years ago. The nuclear clusters survived and went into orbit around our galaxy, while the bodies of the galaxy were shredded by the tidal effects of the Galaxy and the gas and stars where incororated indiviually into the disk and halo of our own Galaxy. Our observations show that the nucleus of NGC 5206 is much more massive than any globular clusters in our Galaxy, so unless the nuclei are eroded significantly by tidal effects, globular clusters are probably not accreted galactic nuclei. However, we discovered a half dozen smaller star clusters sprinkled throughout NGC 5206 whose colors, sizes and luminosities are indistinguishable from those of globular clusters in the Milky Way. It seems likely that many Milky Way globulars are captured during the merger of small dwarf galaxies
I have also worked on discovering RR Lyrae stars (RRL) in several globular clusters and monitoring their brightness and color variations over time. The globular M54 contained over 60 RRL, which enabled us to determine the distance and reddening of the cluster with high accuracy. We also have observations of a dozen metal-rich globular clusters which we are searching for the presence of RRL. RRL are typically scarce in these clusters -- by finding more, we hope to learn about how these unusual stars form.
We are using pictures taken with the Hubble Space Telescope to measure the distance to the nearby globular cluster M5. We find and measure the brightness of white dwarf stars in this cluster. The predicted luminosities of these stars is relatively accurate, so we get an accurate distance to the cluster. M5 contains a large number of RR Lyrae stars -- taking their apparent brightness from the literature and combining it with our accurate distance enables us to determine the luminosity of the RR Lyrae stars (like the "wattage" of a light bulb). This work complements my past and present work on determining the RR Lyrae luminosity using a method called statistical parallax. An accurate knowledge of the RR Lyrae luminosity makes them "standard candles" -- by simply measuring the apparent brightness of an RR Lyrae, you have determined its distance.
Together with students at BGSU, I am monitoring the color variations of many nearby, low-reddening field RR Lyrae variables. Our goal is to better calibrate the intrinsic color of the stars, and to improve their use as tools for measuring the amount of reddening due to interstellar dust. Here is more information on the BGSU Variable Star Project.