May Symposium 2010 Jason Kalirai




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May Symposium 2010
Jason Kalirai, jkalirai@stsci.edu, & Massimo Robberto, robberto@stsci.edu
The 2010 May Symposium, “Stellar Populations in the Cosmological Context,” took place at the Institute on May 3–6, and attracted nearly 200 participants. The topic of the symposium was inspired by the enormous progress over the last two decades in two areas: the detailed study of nearby, resolved stellar populations, and the discovery and the characterization of high-redshift galaxies. Furthermore, the new panchromatic capabilities of Hubble’s Wide Field Camera 3 (WFC3) are now enabling a new leap forward in exciting research related to stellar populations across a diverse range of redshifts. Therefore, the premise of the symposium was that the physical processes and observed characteristics of local stellar populations, observed in a variety of environments, will become a fundamental tool for elucidating the formation, structure, and evolution of galaxies at all cosmic times and distances.
The symposium was a mix of 40 invited and contributed presentations, and 50 shorter presentations, where participants presenting posters were given an opportunity to advertise their work. Over 25% of all time during the meeting was reserved for extended discussions, including summaries at the end of each day led by various members of the scientific organizing committee. The symposium also included a special public talk one evening by astronaut Dr. John Grunsfeld (now the Institute’s Deputy Director) entitled “Hugging Hubble.” The banquet was held at the Maryland Science Center and included a screening of the new “Hubble 3D” IMAX movie.
The symposium started with a review of the latest observational and theoretical findings on the formation of stars and stellar populations. While star formation appears to be ubiquitous in the universe, the process is surprisingly inefficient. A variety of mechanisms (i.e., magnetic fields, turbulence, feedback) have been invoked to explain the high gas/star-mass ratio typically observed in star-forming regions. No one mechanism seems to be dominant, and indeed all may be relevant in certain environments or star-formation phases.
Observations of systems like 30 Doradus, in the Large Magellanic Cloud (LMC), provide strong evidence for star formation triggered by early generations of massive stars. The presence of massive stars is also often invoked for opposite cases, where molecular clouds have been disrupted and star formation therefore prevented. The timescale for massive stars to disrupt the parental cloud is apparently a critical factor for the evolution of star clusters. At the other end of the mass spectrum, new WFC3 observations of low-mass, pre-main-sequence stars in the LMC suggest that they continue to accrete for long timescales, thereby also providing an important tracer of the recent star formation.
Both individual stars and stars in clusters display simple initial mass functions (IMFs), with intriguing similarities in their power-law indices. While there are indirect indications of abnormal IMFs in some galaxies, there is no compelling evidence against a universal IMF in places where individual stars can be resolved and counted. The survival of clusters as bound systems may determine their currently observed mass distributions.
In globular clusters like Omega Cen and NGC 2808, an even more complex interplay between stars and gas, including feedback from the first evolved stars, has been invoked to explain the presence of multiple populations with different helium abundances. For over a century now, these systems have been treated as simple stellar populations that formed at the same time and that have the same chemical composition. Therefore these results have the potential to shatter our understanding of cluster formation and evolution. It remains to be explored whether the presence of multiple populations in clusters is the rule or the exception.

There are many nearby stars older than 10 Gyr, which must have formed at redshifts greater than two. (No genuine “first star”—Population III—has yet been found.) With iron abundances as low as [Fe/H] ~ –5, we can study the metallicities of these old stars in detail, and may discover important clues on the IMF in the early universe. For example, the high enhancement in alpha-elements of the lowest metallicity stars in the halo and dwarf galaxies provides strong evidence of enrichment from core-collapse supernovae.


Ultra-sensitive, wide-area imaging and spectroscopic observations have revealed abundant substructures in the halo of the Milky Way halo, as well as in the halos of other nearby galaxies, such as M31, M33, and NGC 891. These substructures, such as the Sagittarius stream, represent spectacular fossil records of past galaxy mergers and encounters, often traceable for hundreds of kiloparsecs. The overall structure of galaxies like the Milky Way—halo, bulge, and disk component—can be shaped by the merger history. It is an open question how closely such galactic cannibalism is linked to the shape and structure of galactic halos of dark matter. As rapidly as the observations are characterizing these substructures in nearby galaxies, new high-resolution computer simulations of galactic assembly are providing testable predictions in the cosmological context.
The knowledge gained from high-resolution studies of the kinematics, abundances, ages, and morphologies of various stellar populations in nearby clusters and galaxies is dramatically advancing our models of the evolution of stars. As galaxies are collections of stars themselves, the uncertainties in these models are necessarily passed along to studies that attempt to interpret the integrated light from distant galaxies using population synthesis techniques. This connection—moving from studies of detailed resolved stars to unresolved light—lies at the heart of many astrophysical topics under the rubric of galaxy formation and evolution. An example is provided by state-of-the-art Hubble imaging surveys that combine existing optical data from the Advanced Camera for Surveys with new, ultra-sensitive, high-resolution ultraviolet and infrared imaging from WFC3. Such surveys have discovered over a hundred galaxies at a redshift greater than seven, when the universe was only a few hundred million years old. Preliminary estimates of the masses, ages, and star-formation histories of these systems can be calculated by fitting the spectral energy distributions of these objects, using the theoretical models derived from local populations. Another example is provided by the study of galaxies at intermediate redshifts, when the cosmic star formation rate peaks. For these systems, the infrared grism of WFC3 is greatly improving our detailed knowledge of the stellar makeup of galaxies.
Despite the enormous progress that has been made on the study of stellar populations across cosmic time, some of the uncertainties in the fundamental properties such as age, metallicity, and mass remain large. New data sets can help remove degeneracies in modeling the light from these sources. Specifically, in the future, the James Webb Space Telescope and its infrared sensitivity will complement the current studies and push this science to the next level.
The web page for the 2010 May Symposium is http://www.stsci.edu/institute/conference/spring2010 . Included on the web page is a 100-page document with the “take home” message from each of the symposium presenters (under the “Conference Summaries” link). All of the May Symposium presentations have been webcast and are archived along with .ppt or .pdf presentations at this web site:

https://webcast.stsci.edu/webcast/index.xhtml


Figure 1. Poster for the 2010 May Symposium



Figure 2. NGC 5907 is an edge-on spiral galaxy similar to the Milky Way. In this ground-based image, presented by May Symposium participant David Martinez-Delgado, an extended tidal stream can be seen wrapping around the galaxy. This picture highlights galactic cannibalism whereby the stellar populations of a low-mass satellite are shredded off and accreted into the potential of a massive galaxy, forming a complex of arcing loops. (Martinez-Delgado, D., et al. 2008, Astrophysical Journal, 689, 184)


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