Keywords

We present new SDSS and Washington photometry of the young outer-halo stellar system Segue 3. Combined with archival VI-observations, our most consistent results yield $Z=0.006\pm 0.001$, $\mathrm{log}(\mathrm{Age})=9.42\pm 0.08$, ${(m-M)}_{0}=17.35\pm 0.08$, and $E(B-V)=0.09\pm 0.01$, with a high binary fraction of 0.39 ± 0.05 derived using the Padova models. We confirm that mass-segregation has occurred, supporting the hypothesis that this cluster is being tidally disrupted. A three-parameter King model yields a cluster radius of ${r}_{\mathrm{cl}}=0\mathop{.}\limits^{^\circ }017\pm 0\mathop{.}\limits^{^\circ }007$, a core radius of ${r}_{{\rm{c}}}=0\mathop{.}\limits^{^\circ }003\pm 0\mathop{.}\limits^{^\circ }001$, and a tidal radius of ${r}_{{\rm{t}}}=0\mathop{.}\limits^{^\circ }04\pm 0\mathop{.}\limits^{^\circ }02$. A comparison of Padova and Dartmouth model-grids indicates that the cluster is not significantly α-enhanced, with a mean $[\mathrm{Fe}/{\rm{H}}]=-{0.55}_{-0.12}^{+0.15}$ dex, and a population age of only 2.6 ± 0.4 Gyr. We rule out a statistically significant age spread at the main-sequence turnoff because of a narrow subgiant branch, and discuss the role of stellar rotation and cluster age, using Dartmouth and Geneva models: approximately 70% of the Seg 3 stars at or below the main-sequence turnoff have enhanced rotation. Our results for Segue 3 indicate that it is younger and more metal-rich than all previous studies have reported to date. From colors involving Washington C and SDSS-u filters, we identify several giants and a possible blue straggler for future follow-up spectroscopic studies, and we produce spectral energy distributions of previously known members and potential Segue 3 sources with Washington (CT₁), Sloan (ugri), and VI-filters. Segue 3 shares the characteristics of unusual stellar systems that have likely been stripped from external dwarf galaxies as they are being accreted by the Milky Way, or that have been formed during such an event. Its youth, metallicity, and location are all inconsistent with Segue 3 being a cluster native to the Milky Way.

We investigate the kinematic and photometric properties of the Segue 3 Milky Way companion using Keck/DEIMOS spectroscopy and Magellan/IMACS g- and r-band imaging. Using maximum likelihood methods to analyze the photometry, we study the structure and stellar population of Segue 3. We find that the half-light radius of Segue 3 is 26'' ± 5'' (2.1 ± 0.4 pc, for a distance of 17 kpc) and the absolute magnitude is a mere M_V = 0.0 ± 0.8 mag, making Segue 3 the least luminous old stellar system known. We find Segue 3 to be consistent with a single stellar population, with an age of 12.0^+1.5_{− 0.4} Gyr and an [Fe/H] of −1.7^+0.07_{− 0.27}. Line-of-sight velocities from the spectra are combined with the photometry to determine a sample of 32 stars which are likely associated with Segue 3. The member stars within three half-light radii have a velocity dispersion of 1.2 ± 2.6 km s⁻¹. Photometry of the members indicates that the stellar population has a spread in [Fe/H] of ≲ 0.3 dex. These facts, together with the small physical size of Segue 3, imply the object is likely an old, faint stellar cluster which contains no significant dark matter. We find tentative evidence for stellar mass loss in Segue 3 through the 11 candidate member stars outside of three half-light radii, as expected from dynamical arguments. Interpretation of the data outside of three half-light radii is complicated by the object's spatial coincidence with a previously known halo substructure, which may enhance contamination of our member sample.