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Slicing Into Nearby Galaxies

Guillermo Blanc

Guillermo Blanc (left) in the control room of the Hobby-Eberly Telescope during a VIRUS-P test run with astronomers Gary Hill and Niv Drory. [Damond Benningfield]

University of Texas at Austin graduate student Guillermo Blanc is talking dissection. But he’s not a biologist, he’s an astronomer. Blanc is using a new instrument at McDonald Observatory to dissect nearby galaxies to learn how stars form, and in the process he is generating a flood of new information that will benefit other scientists’ work.

Blanc is mapping star formation in 30 nearby spiral galaxies using the VIRUS-P instrument on the Observatory’s 107-inch (2.7-meter) Harlan J. Smith Telescope. His project is called VENGA, the VIRUS-P Exploration of Nearby Galaxies.

“We are going to dissect these galaxies in every possible way,” he says. “The tendency for galaxy studies today is to look at galaxies farther away, and farther back in time. But to understand those, you really need to first understand galaxies in detail. The best way to do that is to look at nearby galaxies. It allows you to interpret the data on distant galaxies.”

His first VENGA target is M51, the Whirlpool Galaxy. The study is helping to determine the drivers for star formation in galaxies. Previous studies disagreed about the role played by gas in setting the rate at which stars form. Blanc’s study, done in collaboration with Texas astronomers Amanda Heiderman, Karl Gebhardt, Neal Evans, and Joshua Adams, shows that the amount of molecular gas (atoms of gas that have bonded together to form molecules) is the key factor in determining how many stars are formed.

dots

VIRUS-P measured the intensity of a tracer of star formation at 246 points across the central region of the Whirlpool galaxy (M51). Red dots indicate higher levels of star formation; the blue and black dots indicate lower levels of star formation. [Guillermo Blanc/McDonald Observatory]

“We confirm that the star formation rate correlates very well with the amount of molecular gas present in different regions inside galaxies. These two quantities are expected to be correlated since this is gas in giant molecular clouds, which are the birth places of stars. These new type of observations allow us not only to observe this correlation, but also to measure precisely how these two quantities relate to each other,” he says.

He also found that the efficiency of star formation in this galaxy is very low — only one percent of the available gas is transformed into stars in a characteristic time. Additionally, Blanc’s studies reveal that for a given amount of gas, the rate of star formation can vary by a factor of three, meaning there might be other important drivers of star formation. Blanc plans to investigate these in future studies.

VIRUS-P is the key to Blanc’s studies of star formation. For each of its 30 target galaxies, VENGA will obtain spectra at many points in the galaxy, from the center to the outermost regions of the galactic disk. With VIRUS-P’s array of 246 optical fibers, the speed and direction of motion of both stars and gas at 246 different points inside the galaxy, as well such quantities as the star formation rate, the amount of light that is absorbed by clouds of dust, and the chemical composition of gas and stars, can be measured simultaneously in a single telescope pointing.

The instrument is “ideal for studying nearby galaxies,” Blanc says. “These galaxies are so nearby they are huge on the sky, so this is the most efficient instrument there is to study them.”

The galaxies in the VENGA survey come in a wide range of shapes. Some have large central bulges of stars, others almost none. Some have a bar running through the center, while others don’t. The galaxies also have different levels of star formation activity. All of this data will be of interest to many astronomers, and the team plans to make it freely available.

The VENGA team is composed of a large group of astronomers from Texas and Germany’s Max Planck Institute for Extraterrestrial Physics. The data will help these astronomers study the different types of galactic bulges and the evolutionary paths galaxies took to form them, the chemical compositions of galaxies’ bulges versus that of their disks, and the role that galactic bars have in driving gas toward the centers of galaxies.

Additional Resources

VENGA Home page

The Whirlpool Galaxy

Whirlpool galaxy

A Hubble Space Telescope view of the Whirlpool galaxy. Star formation is concentrated in the bright red blobs at the edges of the spiral arms. [NASA/Hubble Heritage Team (STScI/AURA)]