On sky distribution of all known Milky Way satellite candidates with respect to the Magellanic Clouds and the neutral hydrogen gas of the Magellanic stream. For more details we refer to Nidever et al. (2010). The three candidates discussed in this study are highlighted in cyan.

False color RGB image of DES1 which is the small overdensity of stars in the centre of this field. The arrows in the lower right corner have a length of 15 arcseconds.

By measuring the brightness of about a dozen stars, lingering just outside of our galaxy, a team of astronomers believe they have solved a nearby intergalactic mystery. The researchers exposed the identities of three ultra-faint dwarf galaxy candidates using the Gemini South telescope. The team reports that the objects appear to be loose clusters of stars, not dwarf galaxies as some had previously believed. This finding has profound ramifications on the quantity of cold dark matter around our Milky Way and, by implication, other galaxies.

Using the Gemini Multi-Object Spectrograph (GMOS) at the Gemini South telescope in Chile, an international research team led by Dr. Blair C. Conn of the Australian National University studied three ultra-faint dwarf galaxy candidates, and found they were not as expected.

The three ultra-faint dwarf galaxy suspects, DES1, Eridanus III, and Tucana V, located in the vicinity of the Magellanic Clouds, were studied using a wide array of classification techniques. For each, fundamental properties including age, mass, luminosity, metallicity (ratio of heavier elements) and distance were determined. Based upon these parameters, the objects have instead been classified as star clusters.

While the brightness and metallicity are consistent with that of ultra-faint dwarf galaxies, their size and structure reveal their true nature. DES1 and Eri III are, according to the researchers, old, small, and highly elliptical stellar populations with very low metallicity.  Tuc V displays a low-level excess of stars at various locations across the GMOS field without a well-defined center. This suggests that Tuc V is either a star cluster in a late stage of dissolution, or a grouping of stars associated with the Small Magellanic Cloud (SMC) halo.

Classification of these faint objects as star clusters implies that they are not dominated by dark matter, as dwarf galaxies typically are, “and so we are still trying to define ultra-faint dwarf galaxies. Where are these smallest galaxies, what are their properties and how many are there? Answering these questions will help complete the census of Milky Way satellites and let us understand the history of our galaxy.”, says Conn.

Conn and his team are looking into the “Missing Satellites” problem which was originally identified almost two decades ago. Based on what is called the hierarchical formation scenario, many astronomers expected a large number of dwarf satellite galaxies, each containing a high fraction of dark matter, surrounding larger galaxies like our Milky Way. However, too few such satellites have been found to account for the expected amounts of dark matter.  Thus, classifying these ultra-faint objects is crucial to our understanding of dark matter in the Universe.

Watch for a feature article on this result in the April issue of GeminiFocus.

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