Astronomers have shown for the first time that even the smallest galaxies in the Universe have complex structures that indicate a complex history.

Using the Subaru Telescope, a team of astronomers from the National Astronomical Observatory of Japan, the Institute of Physics in Lithuania, the University of Durham, Paris Observatory, Kyoto University, Gunma Astronomical Observatory, and the University of Tokyo have discovered an extended halo of stars with a sharp cutoff in the dwarf irregular galaxy Leo A, a member of the Local Group of galaxies that includes the Milky Way.

Credit: National Astronomical Observatory of Japan

The discovery challenges current scenarios of galaxy formation by showing that instead of being the preservers of pristine building blocks that combined to form larger galaxies, dwarf irregular galaxies have their own history of build-up.

Understanding galaxy formation and evolution on time scales comparable to the age of the Universe is one of astronomy's greatest challenges. In the scenarios of standard cosmology, galaxies are built up via hierarchical merging: small primordial density fluctuations in the smooth distribution of matter in the early Universe grow and combine to form larger structures like the Milky Way. The most numerous type of galaxies in the universe -- dwarf irregular galaxies -- are supposed to preserve their properties unchanged over billions of years and represent pristine primeval building blocks. This is one reason why astronomers have recently been studying dwarf irregular galaxies with great interest.

The team led by Professors Nobuo Arimoto (National Astronomical Observatory of Japan) and Vladas Vansevicius (Institute of Physics, Lithuania) has studied Leo A -- an isolated and extremely gas rich dwarf irregular galaxy with only 0.01% of the mass of the Milky Way and a low fraction chemical elements produced by earlier generations of stars. These characteristics suggest that this galaxy has been evolving without significant interaction with other galaxies. This galaxy has been believed to have quite a simple structure, in contrast to large disk galaxies like the Milky Way. However, this view needs to be changed due to deep imaging of the outer regions of this galaxy with the Subaru Telescope.

Prior to these observations, Leo A was already known to have a large angular size (7' x 5'; at the distance of Leo A, 2.6 million light years, 1' in extent corresponds to 750 light years) and Subaru Telescope equipped with its Prime Focus Camera (Suprime-Cam) was an ideal instrument to study the stars at the galaxy's outer limits. A single exposure with Suprime-Cam covers a field of view of 34' x 27' with high sensitivity. The team acquired optical images of the dwarf irregular galaxy Leo A with three broad band filters in November 2001. In order to trace the entire extent of the old stars in Leo A, the team employed red giant branch (RGB) stars which are evolved low-mass stars with very high luminosity and are expected to represent well the extended structures of galaxies. They investigated inside an ellipse of semi-major axis a = 12' which fully covers the galaxy, and detected 1394 RGB stars distributed symmetrically and smoothly within this field.

The team found significantly larger disk structure (with a semi-major axis of 5.5') than previously known (3.5'). Moreover, the deep observations permitted the discovery of a new stellar component in dwarf irregular galaxies, which the team calls a "halo" (5.5'-7.5'), which has a less steep slope in the number density of RGB stars. The halo component ends at 8' from the center of the galaxy with a sharp cutoff in the RGB star distribution. The existence of such a halo structure in dwarf irregular galaxies had been unconfirmed before these observations.

The size of Leo A revealed by these new observations is twice as large as its previously accepted size, suggesting that even in the nearby universe we see galaxies only as "tips of icebergs" that are actually a few times more extended.

The newly discovered halo with a sharp stellar cutoff and the disk of the dwarf irregular galaxy Leo A closely resembles the structure as well as stellar and gaseous content found in large full-fledged disk galaxies like the Milky Way. The complicated structure of large massive galaxies has been believed to be a result of the merging of less massive galaxies like dwarf irregular ones.

However, this study clearly reveals that the dwarf irregular galaxy Leo A already has disk and halo components, and suggests complex build-up histories for even very low mass galaxies like Leo A, which are supposed to form directly from the primordial density fluctuations in the early universe, and challenges contemporary understanding of galaxy evolution. Professors N. Arimoto and V. Vansevicius believe Leo A is a "Rosetta stone" for understanding the process of galaxy formation and evolution and hope to confirm their conjecture with further studies of Leo A.

The scientific paper on this research has been accepted for publication in the August 20, 2004, Astrophysical Journal Letters (Volume 611, Number 2, L93).