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Hubble Finds Structure in an Unstructured Galaxy

A black background dotted with stars and distant galaxies. The distribution of stars increases toward image center, forming the loose oval shape of the irregular galaxy Leo A that stretches horizontally across the image. Three foreground stars shine brightly – one located at center-top, and two others in the lower-right quadrant of the image. Each holds four bright barbs (called diffraction spikes) that extend outward from the star.

The galaxy Leo A resides about 2.6 million light-years away.

NASA, ESA, A. del Pino Molina (CEFCA), K. Gilbert and R. van der Marel (STScI), A. Cole (University of Tasmania); Image Processing: Gladys Kober (NASA/Catholic University of America)

This NASA Hubble Space Telescope image features the nearby dwarf irregular galaxy Leo A, located some 2.6 million light-years away. The relatively open distribution of stars in this diminutive galaxy allows light from distant background galaxies to shine through.

Astronomers study dwarf galaxies like Leo A because they are numerous and may offer clues to how galaxies grow and evolve. Dwarf galaxies are small and dim making the most distant members of this galaxy type difficult to study. As a result, astronomers point their telescopes toward those that are relatively near to our own Milky Way galaxy, like Leo A.

Leo A is one of the most isolated galaxies in our Local Group of galaxies. Its form appears as a roughly spherical, sparsely populated mass of stars with no obvious structural features like spiral arms.

The data that created this image come from four Hubble observing programs. Three of these looked at star formation histories of relatively nearby dwarf galaxies. The fourth sought to better determine the mass of our Local Group by looking at the motions of dwarf galaxies just outside of the Local Group.

The Hubble observations that looked at star formation found distinct structural differences in the age and distribution of stars in the galaxy. Most of the younger stars are located in the middle of the galaxy, while the number of older stars increases as you move outward from the center. Hubble observations also suggest that the galaxy’s halo of stars is about one-third larger than previous estimates. This distribution suggests that star formation in Leo A occurred from the outside-in, or that older stars efficiently migrated to the outskirts of Leo A in the early stages of its evolution.