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D75

Dwarf galaxy DDO 75. Image by Lowell intern Lauren Hill.

In Depth

According to simulations, the first stars formed in dark matter mini-haloes a few 100 million years after the Big Bang. There was no dust, no heavy elements, no magnetic fields, and no mess from previous stars. These first stars are believed to have polluted the inter-galactic medium with elements heavier than helium (which astronomers call metals), necessary for the efficient formation of stars. At extremely low metallicities, star formation should be very difficult. This thirst for heavy elements should have critical consequences to the star formation history of galaxies. The production of heavy elements is no longer considered a passive result of galaxy evolution, but rather a crucial driver of galaxy evolution. The conditions of the early universe are not replicated exactly in nearby galaxies. But dwarfs are the most metal poor galaxies in the nearby universe. With a handle on the star-formation processes and characteristics in dwarfs, the LITTLE THINGS team can look at the role that heavy elements plays.

The LITTLE THINGS survey, or Local Irregulars That Trace Luminosity Extremes (LITTLE) and The HI (neutral hydrogen) Nearby Galaxy Survey (THINGS), brings together deep, high spatial and high-spectral resolution HI-line maps with optical, ultraviolet, and infrared data of the 41 dwarf-irregular galaxies, covering nearly the full range of galactic parameters seen in dwarf galaxies. At the heart of this project are data that show the distribution of HI gas in the galaxies. These data were collected with the National Radio Astronomy Observatory?s Very Large Array (VLA) near Socorro, NM, and they show clouds, shells, and turbulent structures that are important for star formation.

The team made sure that they were producing a first-class data set that is timeless in its usefulness to their survey and to the community. The work of calibrating and mapping the VLA HI data took three-and-a-half years, and involved dealing with complications arising from the transition from the VLA to the Expanded VLA (new electronics and fiber-optic cables) and the use of a new algorithm for constructing the images from the VLA data. However, as of December 2011, data reductions were completed, and at the end of January 2012, the data were made available to the public from the NRAO web site.

Sixteen different and varied science studies are currently underway by the LITTLE THINGS team with this exquisite data set, and more are planned. Early results include the discovery that the star-forming disks of most dwarf irregular galaxies are shrinking from the outside-in over time, in contrast to the standard paradigm of inside-out disk growth for spiral galaxies. Here are a few more results: 1) The team has determined from the rotation curve (the speed at which the stars and gas at each radii are orbiting the center of the galaxy) the baryonic (things we can see) mass and dark matter mass of LITTLE THINGS galaxies, and compared them with those of dwarf galaxies from new cosmological simulations. The galaxies are consistent with the simulations, implying a particular distribution of the dark matter at the center of the galaxies. 2) In spiral and dwarf galaxies the starlight declines with radius because the stellar mass density drops off. But the team has found that in many, the starlight abruptly begins falling off more steeply in the outer reaches of the stellar disk. The team doesn't yet know why this happens but the place where this change happens seems to be about the same stellar brightness in all the dwarfs. 3) The higher the density of the gas, the higher the rate at which the gas is forming new stars.

Dr. Hunter has studied star formation in irregular galaxies since her graduate school days; in fact, she received an award from the Astronomical Society of the Pacific for her thesis work on the star-forming properties of this type of galaxies. Dr. Hunter is also Lowell's Deputy Director for Science and runs the observatory's Navajo-Hopi Astronomy Outreach Program.

Also on the LITTLE THINGS team are Lowell's Dr. Kim Herrmann, a post-doctoral fellow, and Hong-Xin Zhang, a pre-doctoral fellow.

A special session concerning LITTLE THINGS research results was held in January at the American Astronomical Society?s 219th meeting in Austin, TX.

For more on LITTLE THINGS, click here.

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