AST-0707563/0707426/0707468/0707835 Hunter/Elmegreen/Simpson/Young
This is a collaborative project led by Dr Deidre Hunter of Lowell Observatory, along with Dr Bruce Elmegreen at the IBM Thomas J. Watson Research Center, Dr Caroline Simpson of Florida International University, and Dr Lisa Young from the New Mexico Institute of Mining and Technology.
The team will obtain and analyze high angular and velocity resolution neutral hydrogen interferometric data of dwarf irregular galaxies and combine these with their extensive optical, ultra-violet, and infra-red images, in order to determine how these galaxies form star-forming gas clouds that propel their evolution. This study will lead to an improved understanding of star formation in all disk galaxies, by addressing a number of questions: what regulates star formation in small galaxies; what is the relative importance of sequential triggering; what regulates turbulence; how important is triggering by random turbulence; what is happening in the far outer parts of dwarf galaxies; what aspect of star formation changes in blue compact dwarf galaxies? These questions need to be answered because dwarf galaxies are the most common galaxy, the most pristine chemically, and the type most closely connected to the earliest star-forming systems in the universe.
High school students, undergraduates, graduate students, and post-doctoral researchers have been, and will continue to be, involved in this journey of scientific discovery. Many of these participants have in the past been women, and some were also from under-represented groups. The team will continue this preference. The investigators are all also involved in continuing public and K-12 outreach activities.
Dwarf galaxies, the most numerous but tiniest galaxies in the universe, are the closest analogs in the nearby universe to the low mass dark matter haloes that formed after the Big Bang and in which the first stars formed. Yet, we do not understand the processes that lead to star formation even in simple dwarfs. With 400 hours of VLA time and hundreds of nights at Lowell Observatory and other observatories, we assembled a comprehensive data set on a sample of nearby gas-rich dwarf galaxies. These data trace their current star formation, older stellar populations, gas content, and dynamics, in order to understand what drives star formation and the evolution of dwarf irregulars. We have learned that in dwarfs, unlike the inner parts of spiral disks, the gas density is not a predictor of star formation rate. For a given gas density, there can be a range of a factor of 100 in the rate of star formation. So although gas is necessary to form clouds from which stars are born, its mere presense at a reasonable density is not sufficient to make that happen. Surprisingly, however, the star formation rates at very low gas densities are higher than predicted. The star formation rate does correlate well with the density of older stars. However, we do not know if this is due to older stars helping to form clouds for star formation or due to other environmental characteristics. The role of turbulence in bringing gas together into a cloud for star-formation is not clear. We found that the area over which star formation is taking place is shrinking with time in dwarfs, contrary to the inside-out growth seen in spirals. Dwarfs have disks in which the stellar light drops off exponentially, but the drop often has an abrupt change in slope in the outer disk. Furthermore, the brightness of the starlight at this break is the same in dwarfs and in spirals, implying something fundamental is happening at the break in all star-forming galaxies. We find that outer stellar disks of dwarfs go on in radius for a surprisingly long ways, and into regions where the gas density is far too low to understand how new stars can form out there. We are using the Herschel far-infrared space telescope launched by the Europeans and the ALMA array of millimeter radio telescopes to study the structure of molecular clouds at very low abundances, such as would have been prevelant in the early universe. We find thick shells and small cores compared to the Milky Way and as predicted by theory. However, it is not clear yet what difference this makes to the star formation itself. The shape of dwarf galaxies appears to depend on the mass of the galaxy, with smaller mass dwarfs being puffier and larger mass dwarfs being more like thick disks. Interactions of dwarfs with other dwarfs can significantly alter the galaxies, and this may have affected far more dwarfs than we had previously realized. Many of the LITTLE THINGS team participated in the Lowell Observatory Navajo-Hopi Astronomy Outreach Program to work with 5th-8th grade teachers and their classes at Hopi and Navajo Nation schools. The purpose is to help teachers get students excited about science. We also gave public talks and worked with high school, undergraduate, and graduate students.
