The origin and chemical evolution of the Galactic bulge/bar remains an unsolved problem. The Bulge Radial Velocity Assay (BRAVA) has found that the radial velocities of stars across the bulge are consistent with an N-body bar model. Yet over the same volume, a gradient in the chemical composition of stars is observed. If the bar was created via a purely dynamical process, no abundance gradient should be present. Maybe the bulge is a pseudo-bulge, and has an extended formation history, but evidence from both the turnoff age and abundance ratios of stars in the bulge are consistent with an old bulge that formed early and rapidly. Several studies now suggest that the bulge consists of multiple stellar subpopulations of differing age, metallicity, kinematics, and spatial extent. Studies continue to disagree on whether the abundances and kinematics of stars in the bulge are correlated. There is also the possibility that the disintegration of massive proto-clusters built the Galactic bulge. Within the last few years, high resolution spectroscopy of highly magnified, lensed bulge subdwarfs revealed what appears to be a stellar population that is metal rich, and 2-7 Gyr old- considerably younger than is inferred for the bulge field population from the analysis of color-magnitude diagrams. Two recent spectroscopic studies also suggest that the bulge is similar to and perhaps part of, the thick disk. Although there is no kinematic evidence for a 'classical' bulge at galactic latitudes below 8 degrees, infrared color-magnitude diagrams and some spectroscopic studies find evidence for a bulge-like population at high galactic latitude. Dr. Rich and his collaborators conduct a survey of the heavy elements across the inner bulge and bulge globular clusters to securely relate the bulge to the thick disk or the halo. Comparison of the composition of field and globular cluster stars helps to address whether the bulge formed from disintegrated globular clusters. Another part of this work is to undertake high dispersion spectroscopy of red giants at galactic latitudes above 20 degrees to assess whether the formation history of this population differs from that of the inner bulge. The research results are disseminated via refereed papers, colloquia, scientific meetings, and public lectures.
Intellectual Merit: One of the great questions of modern astrophysics concerns the origin of the Milky Way galaxy and its constituent parts. Our galaxy spans roughly 100,000 light years, and the appearance of the light is mostly that of a flat stellar system. The first image is an artist's conception of our Milky Way. We live inside our Galaxy of course, and can never view it from above (as Figure 1 shows). My research concerns the central yellowish region that is roughly football shaped- it used to be called (and still is often called) the "bulge" but is now known (thanks in part to my earlier NSF-funded work) to be a "bar". Thanks to a new generation of large telescopes and instruments partly funded by the NSF, we are able to actually study the stars in this system in great detail. Our project obtained the "spectra" of hundreds of stars in this region, which lies some 25,000 light years distant from the Sun. These spectra tell us a great deal- the speed with which the stars move, their temperatures, and their sizes (inferred though their surface gravities) and the abundance of iron. We compare the composition of these stars to the Sun. Stars in the bulge are very old- most are at least 10 billion years old- but many have an iron content that is at least as great, and sometimes up the three times as great, as our Sun. We also found that throughout the bulge, the red giant stars that we studied have about 3 times as much of what we call "alpha" elements- oxygen, silicon, magnesium, titanium- as does our Sun. This is interesting and something we were excited to learn about. Alpha elements are produced when stars 10 or more times as massive as our Sun explode. So discovering these alpha elements being elevated throughout the bulge confirmed our suspicion that our bulge/bar formed very quickly - in less than 1 billion years (and that's fast, when you consider that our disk is still forming stars 10 billion years later). We also discovered some stars with whopping amounts of heavy elements- rare earths like Europium. These may have been among the first stars to form in the bulge, and the one extreme case that we found was unique in our Milky Way. We are hoping to discover more such stars. In collaboration with a group from the University of Bologna, we studied the mysterious globular cluster Terzan 5. In my earlier NSF funded research, we had discovered that this 10 million Solar mass cluster is unique- unlike most globular star clusters, this one has stars that span a nearly factor ten range in in iron abundance. This work discovered that the cluster host stars that range from 1/10 the iron content of the Sun, to 5 times the iron content of the Sun. Only the supernova explosions of stars can produce iron and other metals, but we have no idea how this cluster was able to hold onto the gas expelled by the exploding stars, and then from subsequent generations of stars. It is an unsolved mystery. Our team helped to lay to rest some incorrect findings. It is generally true that the more metal rich a population of stars is in the bulge, the slower the stars move. We confirmed this to be the case with our work, as expected but contradicting some earlier studies. We also have compared our findings to models of how supernovae build up metals in the Milky Way. Our findings hint that the first stars to form in our galaxy were more massive than the young stars we see today. Our work used the nirspec spectrograph at the W.M. Keck Observatory in Hawaii, and the hydra spectrograph at the Cerro Tololo Observatory in Chile, which is part of the National Optical Astronomical Observatories, funded by NSF. Broader Impacts: In addition to publishing our findings in peer-reviewed journals and presenting them at scientific meetings, the PI worked with NSF Postdoctoral Fellow Christian Johnson, who now holds the prestigious Clay Fellowship at the Harvard/Smithsonian Center for Astrophysics. The PI, Rich, also wrote a 75 page review article for Planets, Stars, and Stellar Systems. This article will be used as a resource by students and postdoctoral scientists who do future work on the bulge. Rich also developed the careers of undergraduate students Christine Black (now at Dartmouth University for graduate school) and Tiffany Hsyu (now at UC Santa Cruz for Astronomy graduate school). Rich also gave 2 public lectures during the period of support.
