University of Minnesota
School of Physics & Astronomy


Starbursts in Dwarf Galaxies

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Kristen McQuinn
Annie Bartels

Kristen McQuinn, Research Associate at the School of Physics and Astronomy, studies star bursts in dwarf galaxies. A starburst is a period of relatively active star-making in the normally quiet realm of dwarf galaxies. Starbursts are thought to be more common in the early universe, but Astrophysicists still do not understand the evolutionary connection between starbursts and their host galaxies.

By studying a sample of 20 dwarf systems in the nearby universe, McQuinn hopes to pin down starbursts as drivers of galaxy evolution.
The first step toward getting a complete picture of starbursts was to ask: what makes a starburst? How long does it last? Are the new stars in little clusters and pockets all over the galaxy? “We used to think that starbursts were just in individual clusters and that they didn’t last very long (10 million years). We are finding that is not the case. Starbursts are spread over the whole galaxy and cumulatively they are lasting 400-500 million years rather than 10 million years.” McQuinn studied data from the Hubble space telescope to measure the brightness and the color of these individual stars. From the brightness and color one can estimate their age. “I liken it to taking a picture of small town USA. You get everyone lined up on main street. You can pick out the babies, the elderly, the teenagers, and the adults in the population. “By age dating the stellar populations, McQuinn is able to figure out which stars were born in each epic of time, going back 14 billion years.

Once McQuinn had this detailed population history, she was able to make a map of the various ages of stars in the dwarf galaxy. "You can pick out stars that were formed 50 million years ago and make a ‘map’ of the various ages of stars in that dwarf galaxy." McQuinn is then able to characterize the length of the star formation event and the distribution. "The answer seems to be that while some of them are concentrated, they are more widely distributed than we thought." A wide distribution implies something about what triggered the starburst. If it is happening all over the galaxy roughly simultaneously, it suggests that something is globally shaping the gas in this galaxy to start it forming so many stars. McQuinn says that this research has helped rule out a lit fuse scenario where one pocket of star formation starts, the stars blow up in super novae and their shockwaves trigger star formation in an adjacent area. Her research found a new paradigm where the star formation was much longer lasting and spread throughout the galaxy.

One of the questions McQuinn’s research hopes to answer is whether or not this is a phase of evolution that all dwarf galaxies go through or if it is only happening in a sub-sample of dwarfs. It is not well understood what drives high levels of star formation of dwarf galaxies. With spiral galaxies, it is well known that there are spiral density waves that move around the galaxy. The gas is crunched up like cars in a traffic jam and the stars are formed in spiral arms. There are no spiral waves in a dwarf galaxy so understanding the starbursts can help us understand the bigger question of how stars are formed in dwarf galaxies.

McQuinn is currently planning data runs at two telescopes to try to analyze the gas in these galaxies. She will be looking for evidence of one galaxy passing close to a dwarf galaxy that might have triggered star formation. That gravitational interaction would be subtle but it is enough to shake up the gas in the dwarf galaxy and form stars. In order to find this evidence McQuinn needs very deep observations of the gas. They will be taking data at Park Telescope in Australia and at Greenbank telescope in West Virginia to investigate both the northern and southern hemisphere sources, in her previously studied sample of 20 starburst galaxies.

McQuinn says that if they can find evidence of interactions with other galaxies it will show that these starburst events are not internal to the dwarf galaxy but environmentally dependent on its location and its nearest neighbors. "It is really where the research is going these days, not just on one system but where does the system sit in its larger environment. It is a question of nature versus nurture."

Her research also aims to understand how the starbursting galaxy effects the environment around it. Her team is in the midst of analyzing data from three different observing runs: at the flagship 4 meter telescope at Kit Peak National Observatory; at the Bok 2.3 meter telescope; and the WIYN 3.5 meter Telescope. They are looking for outflows of hot ionized gas caused by star formation in dwarf galaxies. McQuinn is looking for a complete picture of the process of galaxy evolution. One scenario she is testing is whether a gravitational interaction is responsible for shaking up the gas, causing the starburst. Some of this newly formed material might be blown out into the interstellar medium by a galactic wind. McQuinn says the goal of the whole research program is to create a holistic picture of dwarf galaxy evolution from the burst trigger, the temporal and special characteristics of the burst, to the end of the life cycle of the burst.

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