Figure 1.
The positions of three
new ultra-faint dwarf
galaxy candidates — DES
1, Eri III, and Tuc V (blue
dots) — with respect to
the Magellanic Clouds
(LMC and SMC). Some
additional new objects
are highlighted as yellow
dots. The underlying color
map traces the density
of neutral hydrogen
gas associated with the
Magellanic Clouds.
Figure 2.
Composite color images
of DES 1, Eri III, and Tuc
V. Both DES 1 and Eri III
are noticeable as dense
collections of stars in the
center of the fields. Tuc
V is a less-condensed
collection of stars and
therefore more difficult
to see.
Credit: Images prepared
by Jennifer Miller at
Gemini North
4
Probing Candidates with GMOS
As part of the ongoing Stromlo Milky Way
Satellite Survey — the deepest, most ex-
tended search for optically elusive satellite
galaxies and star clusters to date — our team
took advantage of exquisite observing con-
ditions at Gemini South to establish deep
Gemini Multi-Object Spectrograph (GMOS-S)
g’, r ‘ photometry for three of these ultra-faint
dwarf galaxy candidates: Dark Energy Survey
1 (DES 1), Eridanus III (Eri III) (DES J0222.7-
5217) and Tucana V (Tuc V) (DES J2337-6316).
They have been detected in the vicinity of
the Magellanic Clouds with DES 1 and Eri III
located about 80 and 87 kiloparsecs (kpc)
from the Sun, respectively, and Tuc V hav-
ing a distance comparable to the Magellanic
Clouds at 55 kpc. Figure 1 shows the posi-
tions of these three objects relative to the
Large and Small Magellanic Clouds.
Interestingly, all three objects had reported
half-light radii (≈10 pc [DES 1], ≈14 pc [Eri III],
and ≈17 pc [Tuc V]) that moved them into
GeminiFocus
the transition zone between star clusters and
dwarf galaxies. However, since these objects
reside at the limit of the initial detection pho-
tometry (g lim ≈ 23), this has introduced large
uncertainties into all of their parameters
— those generally used to discriminate be-
tween a baryon-dominated star cluster and
a dark-matter dominated dwarf galaxy. Our
new and deeper GMOS-S data allowed us to
refine their positions in the size-luminosity
plane and luminosity-metallicity parameter
space where there exist well-known rela-
tions between these parameters for dwarf
galaxies but not for star clusters.
Additionally, by probing several magnitudes
below the main sequence turn-off, we can
investigate the spatial distribution of main
sequence stars with different masses and
search for any evidence of mass segrega-
tion (as witnessed, for instance, in the star
cluster Kim 2). Evidence of mass segregation
can confirm a system as being purely bary-
onic and so may provide a unique opportu-
nity to resolve their origins with photometry.
Through these relations we can test the like-
lihood of their true nature as star clusters or
dwarf galaxies.
Figure 2 shows composite color images of
DES 1, Eri III, and Tuc V, while Figure 3 reveals
their corresponding color-magnitude dia-
gra ms based on GMOS-S photometry. As our
GMOS-S photometry allowed us to trace the
stellar populations 3-4 magnitudes deeper
than before (Figure 3), we could then accu-
rately determine a whole host of properties.
April 2018