GeminiFocus April 2018 | Page 6

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