Deep SOAR follow-up photometry of two Milky Way outer-halo companions discovered with Dark Energy Survey


Abstract in English

We report the discovery of a new star cluster, DES 3, in the constellation of Indus, and deeper observations of the previously identified satellite DES J0222.7$-$5217 (Eridanus III). DES 3 was detected as a stellar overdensity in first-year Dark Energy Survey data, and confirmed with deeper photometry from the 4.1 metre Southern Astrophysical Research (SOAR) telescope. The new system was detected with a relatively high significance and appears in the DES images as a compact concentration of faint blue point sources. We determine that DES 3 is located at a heliocentric distance of $sim 76,mathrm{kpc}$ and it is dominated by an old ($simeq 9.8,mathrm{Gyr}$) and metal-poor ($mathrm{[Fe/H]}simeq -1.88$) population. While the age and metallicity values of DES 3 are similar to globular clusters, its half-light radius ($r_mathrm{h}sim 6.5,mathrm{pc}$) and luminosity ($M_V sim -1.9$) are more indicative of faint star clusters. Based on the apparent angular size, DES 3, with a value of $r_mathrm{h}sim 0.!^{prime}3$, is among the smallest faint star clusters known to date. Furthermore, using deeper imaging of DES J0222.7$-$5217 taken with the SOAR telescope, we update structural parameters and perform the first isochrone modeling. Our analysis yields the first age ($simeq 12.6,mathrm{Gyr}$) and metallicity ($mathrm{[Fe/H]}simeq -2.01$) estimates for this object. The half-light radius ($r_mathrm{h}sim 10.5,mathrm{pc}$) and luminosity ($M_Vsim -2.7$) of DES J0222.7$-$5217 suggest that it is likely a faint star cluster. The discovery of DES 3 indicates that the census of stellar systems in the Milky Way is still far from complete, and demonstrates the power of modern wide-field imaging surveys to improve our knowledge of the Galaxys satellite population.

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