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