GeminiFocus April 2018 - Page 8

Magellanic Stream (see Figure 1). However, both systems are much more distant than the Magellanic Clouds themselves. As mea- sured from the Galactic center, DES 1 and Eri III are 74 and 91 kpc distant, which are 37% and 69% further away, respectively, than the Magellanic Clouds. However, they have simi- lar angular separations (23.9° vs. 22.3°) and 3D distances (31.7 kpc vs. 41.0 kpc) to the Small Magellanic Cloud. Figure 5. Two diagrams demonstrating how candidates DES 1 and Eri III (red dots) compare to Milky Way globluar star clusters (open circles) and other satellite galaxy candidates, known dwarf galaxies (large and bright), and confirmed star clusters (small and faint) (all black dots). How do DES 1 and Eri III compare with known satellites of the Milky Way? Figure 5 shows data for our two candidates compared to the size-luminosity and metallicity-luminos- ity relations of known dwarf galaxies (large and bright), globular star clusters, and con- firmed star clusters (small and faint). We can see that although non-globular cluster ob- jects show a general trend in the diagrams, at the small and faint end of the scale (lower left corner) these objects are exclusively star clusters. It is only above sizes of around r h = 20 pc that objects are more unequivocally dwarf galaxies. In the size-luminosity space, DES 1 and Eri III are located much closer to the star clusters. In the metallicity-luminosity space, they once again are found just outside the 1-sig- ma trend line, although the errors show that they are not inconsistent with a dwarf galaxy population, though rather unlikely mem- bers. This seems to be a common trait for these new objects, as they share many prop- erties with both star clusters and dwarf gal- axies. As for DES 1 and Eri III, the majority of evidence points to them as being star clus- ters associated with the Magellanic Clouds. That raises even more interesting questions. Did they fall into the Milky Way halo with the Magellanic Clouds or another dwarf galaxy? Were they stripped off in the same event that is currently disrupting the Magellanic Clouds themselves? What are the other ob- jects in the same region of sky? What About Tuc V? Under analysis, Tuc V presented another in- teresting challenge. While we found an ex- cess of stars in the color-magnitude diagram, the object was not centrally concentrated like the other two candidates. We could not Left: The size-luminosity characteristics of both DES 1 and Eri III are closer to star clusters than dwarf galaxies, which become more definite when their half- radius is roughly ≥20 pc (gray strip) Right: Although the metallicity-luminosity characteristics of DES 1 and Eri III are more consistent with the Milky Way dwarf galaxy population, they are borderline objects, just outside the 1-sigma trend line (dotted lines), and probably not members. 6 GeminiFocus April 2018