Discovery and Spectroscopic Characterization of a Distant, Compact Milky Way Satellite in Gemini

Abstract

We present the discovery of a compact Milky Way satellite in the constellation of Gemini. This system was discovered by cross-matching detections from two independent search algorithms applied to Blanco/DECam data from the third data release of the DECam Local Volume Exploration survey (DELVE DR3), and confirmed with deeper imaging from Gemini/GMOS-N. Based on these data, we determine that the system is an ultra-faint ($M_V=-2.1_{-0.6}^{+0.4}$), compact ($r_{1/2}=8.6_{-1.2}^{+1.4}$ pc) system located at a heliocentric distance of $120_{-6}^{+7}$ kpc. These physical properties place the system in the regime of ambiguous, ultra-faint compact Milky Way halo satellites that cannot be confidently classified as dwarf galaxies or star clusters from morphology alone; we therefore name the system DELVE 8/Gemini I. From medium-resolution Keck/DEIMOS spectroscopy, we securely identify four members including two blue horizontal branch stars, confirming the system as a bound satellite moving at a mean radial velocity of $v_{\mathrm{hel}}=-82.7_{-3.9}^{+3.7}{\mathrm{km}\mathrm{s}}^{-1}$. We also use these spectra to place an upper limit of $[Fe/H]\lesssim -2.5$ on the metallicity of DELVE 8/Gemini I's brightest star, supporting the classification of the system as either an ancient star cluster or ultra-faint dwarf galaxy. The discovery of faint, distant systems similar to DELVE 8/Gemini I is expected to become more common with upcoming surveys.

14 pages, 5 figures, 2 tables; To be submitted to AAS Journals