Distance To Nearest Galaxy Measured

Nearly 200,000 light-years from Earth, the Large Magellanic Cloud, a satellite galaxy of the Milky Way, floats in space, in a long and slow dance around our galaxy. Vast clouds of gas within it slowly collapse to form new stars. In turn, these light up the gas clouds in a riot of colors, visible in this image from the NASA/ESA Hubble Space Telescope.  The Large Magellanic Cloud (LMC) is ablaze with star-forming regions. From the Tarantula Nebula, the brightest stellar nursery in our cosmic neighborhood, to LHA 120-N 11, part of which is featured in this Hubble image, the small and irregular galaxy is scattered with glowing nebulae, the most noticeable sign that new stars are being born.

Image Credit: ESA/NASA/Hubble

Astronomers survey the scale of the Universe by first measuring the distances to close-by objects (for example Cepheid variable stars studied by Wendy Freedman, director of the Carnegie Observatories, and her collaborators) and then using observations of these objects in more distant galaxies to pin down distances further and further out in the Universe. But this chain is only as accurate as its weakest link. Up to now finding a precise distance to the LMC has proved elusive. Because stars in this galaxy are used to fix the distance scale for more remote galaxies, an accurate distance is crucially important.

“Because the LMC is close and contains a significant number of different stellar distance indicators, hundreds of distance measurements using it have been recorded over the years,” Thompson said. “Unfortunately, nearly all the determinations have systemic errors, with each method carrying its own uncertainties.”

The international collaboration worked out the distance to the Large Magellanic Cloud by observing rare close pairs of stars, known as eclipsing binaries. These pairs are gravitationally bound to each other, and once per orbit, as seen from Earth, the total brightness from the system drops as each component eclipses its companion. By tracking these changes in brightness very carefully, and also measuring the orbital speeds of the stars, it is possible to work out how big the stars are, how massive they are, and other information about their orbits. When this is combined with careful measurements of the apparent brightness, remarkably accurate distances can be determined.

This vibrant image from NASA’s Spitzer Space Telescope shows the Large Magellanic Cloud, a satellite galaxy to our own Milky Way galaxy. The Large Magellanic Cloud, located 160,000 light-years from Earth, is one of a handful of dwarf galaxies that orbit the Milky Way.

Credit: NASA/JPL-Caltech/STScI 

This method has been used before in taking measurements to the LMC, but with hot stars. As such, certain assumptions had to be made and the distances were not as accurate as desired. This new work, led by Grzegorz Pietrzynski of the Universidad de Concepcion in Chile and Warsaw University Observatory in Poland, used 16-years-worth of observations to identify a sample of intermediate mass binary stars with extremely long orbital periods, perfect for measuring precise and accurate distances.

The team observed eight of these binary systems over eight years, gathering data at Las Campanas Observatory and the European Southern Observatory. The LMC distance calculated using these eight binary stars is purely empirical, without relying on modeling or theoretical predictions. The team refined the uncertainty in the distance to the LMC down to 2.2 percent. This new measurement can be used to decrease the uncertainty in calculations of the Hubble constant to 3 percent, with prospects of improving this to a 2 percent uncertainty in a few years as the sample of binary stars is increased.

This work was supported by BASAL Centro de Astrofisica y Tecnologias Afines (CATA), the Polish Ministry of Science, the Foundation for Polish Science (FOCUS, TEAM), the Polish National Science Centre, and the GEMINI-CONICYT fund. The OGLE project has received funding from the European Research Council “Advanced Grant” program.

The Carnegie Institution for Science is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.

 Ian Thompson
Carnegie Institution